Concrete- An artificial stone-like material used for various structural purposes. It is made by
mixing cement and various aggregates, such as sand, pebbles, gravel, shale, etc., with water and allowing the mixture to harden by hydration.
Here are just a few facts to help convince you that the topic of concrete deserves to become a part of your science curriculum:
• Concrete is everywhere!! Roads, sidewalks, houses, bridges, skyscrapers, pipes, dams, canals, missile silos, and nuclear waste containment. There are even concrete canoes and
Frisbee competitions.
• It is strong, inexpensive, plentiful, and easy to make. But more importantly, it’s versatile. It can be molded to just about any shape.
• Concrete is friendly to the environment. It’s virtually all natural. It’s recyclable.
• It is the most frequently used material in construction.
• Slightly more than a ton of concrete is produced every year for each person on the planet, approximately 6 billion tons per year.
• By weight, one-half to two-thirds of our infrastructures are made of concrete such as: roads, bridges, buildings, airports, sewers, canals, dams, and subways.
• Approximately 60% of our concrete highways need repair and 40% of our concrete highway bridges are structurally deficient or functionally obsolete.
• Large cities lose up to 30% of their daily water supply due to leaks in concrete water pipes.
• It has been estimated that the necessary repairs and improvements to our infrastructures will cost $3.3 trillion over a nineteen-year period. $1 trillion of that is needed for repairing
the nation’s concrete.
• Cement has been around for at least 12 million years and has played an important role in history.
Brainstorming Activity 2: Why is Concrete Important?
Objective: Students will create a list of the importance of concrete and explain how it affects their lives.
Procedure: 1. "Why concrete is important?" In a large group students will create a list of the importance of studying concrete.
2. Upon completion of their list, students will develop acronyms for concrete based on their list of concrete's importance. (See example below.)
3. Students will discuss the implications that would occur if we could no longer make concrete. (i.e. increasing levels of CO2 production or federal regulations)
Brainstorming Activity 3: Applications of Concrete
Objective: Students will create a list of the past, present, and future applications of concrete and how these applications affect their lives and lifestyles.
Procedure:
In small groups, the students will list applications for concrete:
1. In the past:
Students will create a list of past applications for concrete that has influenced their lives and/or lifestyles.
2. Currently:
Students will describe common applications of concrete that they encounter daily. Label these as present applications of concrete.
3. In the future:
Students will create a list of future applications of concrete by predicting how concrete will affect their lives in the future.
4. Students will present their lists to the class in the form of a collage or a mobile displaying the correlation between their lives and lifestyles with the applications of concrete throughout their lives.
APPLICATIONS OF CONCRETE
Past, Present, and Future
roads sidewalks houses
bricks/blocks bridges walls
beams foundations floors
sewer pipes water mains computer chip backing **
canals missile silos containment of nuclear waste
dams churches automobile brake lining **
caskets monuments solidification of hazardous wastes
tombs indoor furniture garden ornaments
swimming pools airport runways sailing boats
canoes barges subways
tunnels parking garages patio bricks
holding tanks cement “overshoes” sculptures
flower pots & planters chimneys mantels
ballast bath tubs grave vaults
bank vaults basements lamp posts
telephone poles electric light poles Frisbees
headstones steps fence posts
business/credit cards ** fertilizer bone replacement **
insulating tiles/bricks corn silos park benches
parking stones roof tiles water troughs
water tanks curb & gutters nuclear reactor containment structures
artificial rocks office buildings parking lots
railroad ties airports monorails
picnic tables swimming pools break waters
wharves & piers bird baths barbecue pits
stadium seats fountains lunar bases **
** Denotes future applications.
The History of Concrete:
A Timeline
Cement has been around for at least 12 million years. When the earth itself was undergoing intense geologic changes natural, cement was being created. It was this natural cement that humans first put to use. Eventually, they discovered how to make cement from other materials.
12,000,000 BC Reactions between limestone and oil shale during spontaneous
combustion occurred in Israel to form a natural deposit of cement
compounds. The deposits were characterized by Israeli geologists in the
1960’s and 70’s.
3000 BC Used mud mixed with straw to bind dried bricks. They also used gypsum
Egyptians mortars and mortars of lime in the pyramids.
Chinese Used cementitious materials to hold bamboo together in their boats and in
the Great Wall.
800 BC Used lime mortars which were much harder than later Roman mortars.
Greeks, Crete
& Cyprus
300 BC Used bitumen to bind stones and bricks.
Babylonians
& As Syrians
300 BC - 476 AD Used pozzolana cement from Pozzuoli, Italy near Mt. Vesuvius to build the
Romans Appian Way, Roman baths, the Coliseum and Pantheon in Rome, and
the Pont du Gard aqueduct in south France. They used lime as a
cementitious material. Pliny reported a mortar mixture of 1 part lime to 4
parts sand. Vitruvius reported a 2 parts pozzolana to 1 part lime. Animal
fat, milk, and blood were used as admixtures (substances added to cement
to increase the properties.) These structures still exist today!
1200 - 1500 The quality of cementing materials deteriorated. The use of burning lime
The Middle Ages and pozzolan (admixture) was lost, but reintroduced in the 1300’s.
1678 Joseph Moxon wrote about a hidden fire in heated lime that appears upon
the addition of water.
1779 Bry Higgins was issued a patent for hydraulic cement (stucco) for exterior
plastering use.
1780 Bry Higgins published “Experiments and Observations Made With the
View of Improving the Art of Composing and Applying Calcereous
Cements and of Preparing Quicklime.”
1793 John Smeaton found that the calcination of limestone containing clay gave
a lime which hardened under water (hydraulic lime). He used hydraulic
lime to rebuild Eddystone Lighthouse in Cornwall, England which he had
been commissioned to build in 1756, but had to first invent a material that
would not be affected by water. He wrote a book about his work.
1796 James Parker from England patented a natural hydraulic cement by
calcining nodules of impure limestone containing clay, called Parker’s
Cement or Roman Cement.
1802 In France, a similar Roman Cement process was used.
1810 Edgar Dobbs received a patent for hydraulic mortars, stucco, and plaster,
although they were of poor quality due to lack of kiln precautions.
1812 -1813 Louis Vicat of France prepared artificial hydraulic lime by calcining synthetic mixtures of limestone and clay.
1818 Maurice St. Leger was issued patents for hydraulic cement.
Natural Cement was produced in the USA. Natural cement is limestone that naturally has the appropriate amounts of clay to make the same type of concrete as John Smeaton discovered.
1820 - 1821 John Tickell and Abraham Chambers were issued more
hydraulic cement patents.
1822 James Frost of England prepared artificial hydraulic lime like Vicat’s and
called it British Cement.
1824 Joseph Aspdin of England invented portland cement by burning finely
ground chalk with finely divided clay in a lime kiln until carbon dioxide
was driven off. The sintered product was then ground and he called it
portland cement named after the high quality building stones quarried at
Portland, England.
1828 I. K. Brunel is credited with the first engineering application of
portland cement, which was used to fill a breach in the Thames Tunnel.
1830 The first production of lime and hydraulic cement took place in Canada.
1836 The first systematic tests of tensile and compressive strength took place in
Germany.
1843 J. M. Mauder, Son & Co. were licensed to produce patented portland
cement.
1845 Isaac Johnson claims to have burned the raw materials of portland cement
to clinkering temperatures.
1849 Pettenkofer & Fuches performed the first accurate chemical analysis of
portland cement.
1860 The beginning of the era of portland cements of modern composition.
1862 Blake Stonebreaker of England introduced the jaw breakers to crush
clinkers.
1867 Joseph Monier of France reinforced William Wand’s (USA) flower pots
with wire ushering in the idea of iron reinforcing bars (re-bar).
1871 David Saylor was issued the first American patent for portland cement. He
showed the importance of true clinkering.
1880 J. Grant of England show the importance of using the hardest and densest portions of the clinker. Key ingredients were being chemically analyzed.
1886 The first rotary kiln was introduced in England to replace the vertical shaft
kilns.
1887 Henri Le Chatelier of France established oxide ratios to prepare the
proper amount of lime to produce portland cement. He named the
components: Alite (tricalcium silicate), Belite (dicalcium silicate), and
Celite (tetracalcium aluminoferrite). He proposed that hardening is caused by the formation of crystalline products of the reaction between cement and
water.
1889 The first concrete reinforced bridge is built.
1890 The addition of gypsum when grinding clinker to act as a retardant to the setting of concrete was introduced in the USA. Vertical shaft kilns were replaced with rotary kilns and ball mills were used for grinding cement.
1891 George Bartholomew placed the first concrete street in the USA in
Bellefontaine, OH. It still exists today!
1893 William Michaelis claimed that hydrated metasilicates form a gelatinous
mass (gel) that dehydrates over time to harden.
1900 Basic cement tests were standardized.
1903 The first concrete high rise was built in Cincinnati, OH.
1908 Thomas Edison built cheap, cozy concrete houses in Union, NJ.
They still exist today!
1909 Thomas Edison was issued a patent for rotary kilns.
1929 Dr. Linus Pauling of the USA formulated a set of principles for the
structures of complex silicates.
1930 Air entraining agents were introduced to improve concrete's resistance to freeze/thaw damage.
1936 The first major concrete dams, Hoover Dam and Grand Coulee Dam, were built. They still exist today!
1956 U.S. Congress annexed the Federal Interstate Highway Act.
1967 First concrete domed sport structure, the Assembly Hall, was constructed at The University of Illinois, at Urbana-Champaign.
1970's Fiber reinforcement in concrete was introduced.
1975 CN Tower in Toronto, Canada, the tallest slip-form building, was constructed.
Water Tower Place in Chicago, Illinois, the tallest building was constructed.
1980's Superplasticizers were introduced as admixtures.
1985 Silica fume was introduced as a pozzolanic additive.
The "highest strength" concrete was used in building the Union
Plaza constructed in Seattle, Washington.
1992 The tallest reinforced concrete building in the world was constructed at
311 S. Wacker Dr., Chicago, Illinois.
Scientific Principles
What is in This Stuff?
The importance of concrete in modern society cannot be underestimated. Look around you and you will find concrete structures everywhere such as buildings, roads, bridges, and dams. There is no escaping the impact concrete makes on your everyday life. So what is it?
Concrete is a composite material which is made up of a filler and a binder. The binder (cement paste) "glues" the filler together to form a synthetic conglomerate. The constituents used for the binder are cement and water, while the filler can be fine or coarse aggregate. The role of these constituents will be discussed in this section.
Cement, as it is commonly known, is a mixture of compounds made by burning limestone and clay together at very high temperatures ranging from 1400 to 1600 ˚C.
Although there are other cements for special purposes, this module will focus solely on portland cement and its properties. The production of portland cement begins with the quarrying of limestone, CaCO3. Huge crushers break the blasted limestone into small pieces. The crushed limestone is then mixed with clay (or shale), sand, and iron ore and ground together to form a homogeneous powder. However, this powder is microscopically heterogeneous. (See flowchart.)
Figure 1: A flow diagram of Portland Cement production.
The mixture is heated in kilns that are long rotating steel cylinders on an incline. The kilns may be up to 6 meters in diameter and 180 meters in length. The mixture of raw materials enters at the high end of the cylinder and slowly moves along the length of the kiln due to the constant rotation and inclination. At the low end of the kiln, a fuel is injected and burned, thus providing the heat necessary to make the materials react. It can take up to 2 hours for the mixture to pass through the kiln, depending upon the length of the cylinder.
Figure 2: Schematic diagram of rotary kiln.
As the mixture moves down the cylinder, it progresses through four stages of transformation. Initially, any free water in the powder is lost by evaporation. Next, decomposition occurs from the loss of bound water and carbon dioxide. This is called calcination. The third stage is called clinkering. During this stage, the calcium silicates are formed. The final stage is the cooling stage.
The marble-sized pieces produced by the kiln are referred to as clinker. Clinker is actually a mixture of four compounds which will be discussed later. The clinker is cooled, ground, and mixed with a small amount of gypsum (which regulates setting) to produce the general-purpose portland cement.
Water is the key ingredient, which when mixed with cement, forms a paste that binds the
aggregate together. The water causes the hardening of concrete through a process called hydration. Hydration is a chemical reaction in which the major compounds in cement form chemical bonds with water molecules and become hydrates or hydration products. Details of the hydration process are explored in the next section. The water needs to be pure in order to prevent side reactions from occurring which may weaken the concrete or otherwise interfere with the hydration process. The role of water is important because the water to cement ratio is the most critical factor in the production of "perfect" concrete. Too much water reduces concrete strength, while too little will make the concrete unworkable. Concrete needs to be workable so that it may be consolidated and shaped into different forms (i.e.. walls, domes, etc.). Because concrete must be both strong and workable, a careful balance of the cement to water ratio is required when making concrete.
Aggregates are chemically inert, solid bodies held together by the cement. Aggregates come in various shapes, sizes, and materials ranging from fine particles of sand to large, coarse rocks. Because cement is the most expensive ingredient in making concrete, it is desirable to minimize the amount of cement used. 70 to 80% of the volume of concrete is aggregate keeping the cost of the concrete low. The selection of an aggregate is determined, in part, by the desired characteristics of the concrete. For example, the density of concrete is determined by the density of the aggregate. Soft, porous aggregates can result in weak concrete with low wear resistance, while using hard aggregates can make strong concrete with a high resistance to abrasion.
Aggregates should be clean, hard, and strong. The aggregate is usually washed to remove any dust, silt, clay, organic matter, or other impurities that would interfere with the bonding reaction with the cement paste. It is then separated into various sizes by passing the material through a series of screens with different size openings.
Refer to Demonstration 1
Table 1: Classes of Aggregates
class examples of aggregates used uses
ultra-lightweight vermiculite lightweight concrete which
ceramic spheres can be sawed or nailed, also
perlite for its insulating properties
lightweight expanded clay used primarily for making shale or slate lightweight concrete for crushed brick structures, also used for its insulating properties.
normal weight crushed limestone used for normal concrete
sand projects
river gravel
crushed recycled concrete
heavyweight steel or iron shot used for making high density
steel or iron pellets concrete for shielding against
nuclear radiation
Refer to Demonstration 2
The choice of aggregate is determined by the proposed use of the concrete. Normally sand, gravel, and crushed stone are used as aggregates to make concrete. The aggregate should be well-graded to improve packing efficiency and minimize the amount of cement paste needed. Also, this makes the concrete more workable.
Refer to Demonstration 3
Properties of Concrete
Concrete has many properties that make it a popular construction material. The correct proportion of ingredients, placement, and curing are needed in order for these properties to be optimal.
Good-quality concrete has many advantages that add to its popularity. First, it is economical when ingredients are readily available. Concrete's long life and relatively low maintenance requirements increase its economic benefits. Concrete is not as likely to rot, corrode, or decay as other building materials. Concrete has the ability to be molded or cast into almost any desired shape. Building of the molds and casting can occur on the work-site which reduces costs.
Concrete is a non-combustible material which makes it fire-safe and able withstand high temperatures. It is resistant to wind, water, rodents, and insects. Hence, concrete is often used for storm shelters.
Concrete does have some limitations despite its numerous advantages. Concrete has a relatively low tensile strength (compared to other building materials), low ductility, low strength-to-weight ratio, and is susceptible to cracking. Concrete remains the material of choice for many applications regardless of these limitations.
Hydration of Portland Cement
Concrete is prepared by mixing cement, water, and aggregate together to make a workable paste. It is molded or placed as desired, consolidated, and then left to harden. Concrete does not need to dry out in order to harden as commonly thought.
The concrete (or specifically, the cement in it) needs moisture to hydrate and cure (harden). When concrete dries, it actually stops getting stronger. Concrete with too little water may be dry but is not fully reacted. The properties of such a concrete would be less than that of a wet concrete. The reaction of water with the cement in concrete is extremely important to its properties and reactions may continue for many years. This very important reaction will be discussed in detail in this section.
Portland cement consists of five major compounds and a few minor compounds. The composition of a typical portland cement is listed by weight percentage in Table 2.
Cement Compound Weight Percentage Chemical Formula
Tricalcium silicate 50 % Ca3SiO5 or 3CaO.SiO2
Dicalcium silicate 25 % Ca2SiO4 or 2CaO.SiO2
Tricalcium aluminate 10 % Ca3Al2O6 or 3CaO .Al2O3
Tetracalcium aluminoferrite 10 % Ca4Al2Fe2O10 or
4CaO.Al2O3.Fe2O3
Gypsum 5 % CaSO4.2H2O
Table 2: Composition of portland cement with chemical composition and weight percent.
When water is added to cement, each of the compounds undergoes hydration and contributes to the final concrete product. Only the calcium silicates contribute to strength. Tricalcium silicate is responsible for most of the early strength (first 7 days). Dicalcium silicate, which reacts more slowly, contributes only to the strength at later times. Tricalcium silicate will be discussed in the greatest detail.
The equation for the hydration of tricalcium silicate is given by:
Tricalcium silicate + Water--->Calcium silicate hydrate+Calcium hydroxide + heat
2 Ca3SiO5 + 7 H2O ---> 3 CaO.2SiO2.4H2O + 3 Ca(OH)2 + 173.6kJ
Upon the addition of water, tricalcium silicate rapidly reacts to release calcium ions, hydroxide ions, and a large amount of heat. The pH quickly rises to over 12 because of the release of alkaline hydroxide (OH-) ions. This initial hydrolysis slows down quickly after it starts resulting in a decrease in heat evolved.
The reaction slowly continues producing calcium and hydroxide ions until the system becomes saturated. Once this occurs, the calcium hydroxide starts to crystallize. Simultaneously, calcium silicate hydrate begins to form. Ions precipitate out of solution accelerating the reaction of tricalcium silicate to calcium and hydroxide ions. (Le Chatlier's principle). The evolution of heat is then dramatically increased.
The formation of the calcium hydroxide and calcium silicate hydrate crystals provide "seeds" upon which more calcium silicate hydrate can form. The calcium silicate hydrate crystals grow thicker making it more difficult for water molecules to reach the unhydrated tricalcium silicate. The speed of the reaction is now controlled by the rate at which water molecules diffuse through the calcium silicate hydrate coating. This coating thickens over time causing the production of calcium silicate hydrate to become slower and slower.
Figure 3: Schematic illustration of the pores in calcium silicate through different stages of hydration.
The above diagrams represent the formation of pores as calcium silicate hydrate is formed. Note in diagram (a) that hydration has not yet occurred and the pores (empty spaces between grains) are filled with water. Diagram (b) represents the beginning of hydration. In diagram (c), the hydration continues. Although empty spaces still exist, they are filled with water and calcium hydroxide. Diagram (d) shows nearly hardened cement paste. Note that the majority of space is filled with calcium silicate hydrate. That which is not filled with the hardened hydrate is primarily calcium hydroxide solution. The hydration will continue as long as water is present and there are still unhydrated compounds in the cement paste.
Dicalcium silicate also affects the strength of concrete through its hydration. Dicalcium silicate reacts with water in a similar manner compared to tricalcium silicate, but much more slowly. The heat released is less than that by the hydration of tricalcium silicate because the dicalcium silicate is much less reactive. The products from the hydration of dicalcium silicate are the same as those for tricalcium silicate:
Dicalcium silicate + Water--->Calcium silicate hydrate + Calcium hydroxide +heat
2 Ca2SiO4 + 5 H2O---> 3 CaO.2SiO2.4H2O + Ca(OH)2 + 58.6 kJ
The other major components of portland cement, tricalcium aluminate and tetracalcium aluminoferrite also react with water. Their hydration chemistry is more complicated as they involve reactions with the gypsum as well. Because these reactions do not contribute significantly to strength, they will be neglected in this discussion. Although we have treated the hydration of each cement compound independently, this is not completely accurate. The rate of hydration of a compound may be affected by varying the concentration of another. In general, the rates of hydration during the first few days ranked from fastest to slowest are:
tricalcium aluminate > tricalcium silicate > tetracalcium aluminoferrite > dicalcium silicate.
Refer to Demonstration 4
Heat is evolved with cement hydration. This is due to the breaking and making of chemical bonds during hydration. The heat generated is shown below as a function of time.
Figure 4: Rate of heat evolution during the hydration of portland cement
The stage I hydrolysis of the cement compounds occurs rapidly with a temperature increase of several degrees. Stage II is known as the dormancy period. The evolution of heat slows dramatically in this stage. The dormancy period can last from one to three hours. During this period, the concrete is in a plastic state which allows the concrete to be transported and placed without any major difficulty. This is particularly important for the construction trade who must transport concrete to the job site. It is at the end of this stage that initial setting begins. In stages III and IV, the concrete starts to harden and the heat evolution increases due primarily to the hydration of tricalcium silicate. Stage V is reached after 36 hours. The slow formation of hydrate products occurs and continues as long as water and unhydrated silicates are present.
Refer to Demonstration 5
Strength of Concrete
The strength of concrete is very much dependent upon the hydration reaction just discussed. Water plays a critical role, particularly the amount used. The strength of concrete increases when less water is used to make concrete. The hydration reaction itself consumes a specific amount of water. Concrete is actually mixed with more water than is needed for the hydration reactions. This extra water is added to give concrete sufficient workability. Flowing concrete is desired to achieve proper filling and composition of the forms. The water not consumed in the hydration reaction will remain in the microstructure pore space. These pores make the concrete weaker due to the lack of strength-forming calcium silicate hydrate bonds. Some pores will remain no matter how well the concrete has been compacted.
Figure 5: Schematic drawings to demonstrate the relationship between the water/cement ratio and porosity.
The empty space (porosity) is determined by the water to cement ratio. The relationship between the water to cement ratio and strength is shown in the graph that follows.
Figure 6: A plot of concrete strength as a function of the water to cement ratio.
Low water to cement ratio leads to high strength but low workability. High water to cement ratio leads to low strength, but good workability.
The physical characteristics of aggregates are shape, texture, and size. These can indirectly affect strength because they affect the workability of the concrete. If the aggregate makes the concrete unworkable, the contractor is likely to add more water which will weaken the concrete by increasing the water to cement mass ratio.
Time is also an important factor in determining concrete strength. Concrete hardens as time passes. Why? Remember the hydration reactions get slower and slower as the tricalcium silicate hydrate forms. It takes a great deal of time (even years!) for all of the bonds to form which determine concrete's strength. It is common to use a 28-day test to determine the relative strength of concrete.
Concrete's strength may also be affected by the addition of admixtures. Admixtures are substances other than the key ingredients or reinforcements which are added during the mixing process. Some admixtures add fluidity to concrete while requiring less water to be used. An example of an admixture which affects strength is superplasticizer. This makes concrete more workable or fluid without adding excess water. A list of some other admixtures and their functions is given below. Note that not all admixtures increase concrete strength. The selection and use of an admixture are based on the need of the concrete user.
SOME ADMIXTURES AND FUNCTIONS
TYPE FUNCTION
AIR ENTRAINING improves durability, workability, reduces bleeding, reduces freezing/thawing problems
(e.g. special detergents)
SUPERPLASTICIZERS increase strength by decreasing water needed for workable concrete
(e.g. special polymers)
RETARDING delays setting time, more long term strength, offsets adverse high temp. weather
(e.g. sugar )
ACCELERATING speeds setting time, more early strength, offsets adverse low temp. weather
(e.g. calcium chloride)
MINERAL ADMIXTURES improves workability, plasticity, strength
(e.g. fly ash)
PIGMENT adds color
(e.g. metal oxides)
Table 3: A table of admixtures and their functions.
Durability is a very important concern in using concrete for a given application. Concrete provides good performance through the service life of the structure when concrete is mixed properly and care is taken in curing it. Good concrete can have an infinite life span under the right conditions. Water, although important for concrete hydration and hardening, can also play a role in decreased durability once the structure is built. This is because water can transport harmful chemicals to the interior of the concrete leading to various forms of deterioration. Such deterioration ultimately adds costs due to maintenance and repair of the concrete structure. The contractor should be able to account for environmental factors and produce a durable concrete structure if these factors are considered when building concrete structures.
Concrete Summary
Concrete is everywhere. Take a moment and think about all the concrete encounters you have had in the last 24 hours. All of these concrete structures are created from a mixture of cement and water with added aggregate. It is important to distinguish between cement and concrete as they are not the same. Cement is used to make concrete!
(cement + water) + aggregate = concrete
Cement is made by combining a mixture of limestone and clay in a kiln at 1450˚ C. The product is an intimate mixture of compounds collectively called clinker. This clinker is finely ground into the powder form. The raw materials used to make cement are compounds containing some of the earth’s most abundant elements, such as calcium, silicon, aluminum, oxygen, and iron.
Water is a key reactant in cement hydration. The incorporation of water into a substance is known as hydration. Water and cement initially form a cement paste that begins to react and harden (set). This paste binds the aggregate particles through the chemical process of hydration. In the hydration of cement, chemical changes occur slowly, eventually creating new crystalline products, heat evolution, and other measurable signs.
cement + water = hardened cement paste
The properties of this hardened cement paste, called binder, control the properties of the concrete. It is the inclusion of water (hydration) into the product that causes concrete to set, stiffen, and become hard. Once set, concrete continues to harden (cure) and become stronger for a long period of time, often up to several years.
The strength of the concrete is related to the water to cement mass ratio and the curing conditions. A high water to cement mass ratio yields a low strength concrete. This is due to the increase in porosity (space between particles) that is created with the hydration process. Most concrete is made with a water to cement mass ratio ranging from 0.35 to 0.6.
Aggregate is the solid particles that are bound together by the cement paste to create the synthetic rock known as concrete. Aggregates can be fine, such as sand, or coarse, such as gravel. The relative amounts of each type and the sizes of each type of aggregate determines the physical properties of the concrete.
sand + cement paste = mortar
mortar + gravel = concrete
Sometimes other materials are incorporated into the batch of concrete to create specific characteristics. These additives are called admixtures. Admixtures are used to: alter the fluidity (plasticity) of the cement paste; increase (accelerate) or decrease (retard) the setting time; increase strength (both bending and compression); or to extend the life of a structure. The making of concrete is a very complex process involving both chemical and physical changes. It is a material of great importance in our lives.
References
Abercrombie, S. Ferrocement: Building with Cement, Sand, and Wire Mesh. Schocken Books,
NY, 1977.
Bye, G. C. Portland Cement: Composition, Production and Properties. Pergamon Press, NY,
1983.
Hewlett, P. C., and Young, J. F. “Physico-Chemical Interactions Between Chemical Admixtures
and Portland Cement,” Journal of Materials Education. Vol. 9, No. 4, 1987.
Introduction to Concrete Masonry. Instructor's Edition, Associated General Contractors of America, Washington D.C., Oklahoma State Department of Vocational and Technical Ed., Stillwater, 1988.
Kosmatka, Steven H., and Panarese, William C. Design and Control of Concrete Mixtures, Thirteenth edition, Portland Cement Association, 1988.
Materials Science of Concrete I, II, III. edited by Jan P. Skalny, American Ceramic Society, Inc,
Westerville, OH, 1989.
Mindess, S., and Young, J.F. Concrete. Prentice-Hall, Inc., Englewood Cliffs, NJ, 1981.
Mitchell, L. Ceramics: Stone Age to Space Age. Scholastic Book Services, NY, 1963.
Rixom, M. R., and Mailuaganam, N. P. Chemical Admixtures for Concrete. R. & F.N. Spon, NY, 1986.
Roy, D. Instructional Modules in Cement Science. Pennsylvania State University, PA, 1985.
Sedgwick, J. “Strong But Sensitive” The Atlantic Monthly Vol. 267, No. 4, April 1991,
pp 70-82.
Weisburd, S. “Hard Science” Science News Vol. 134, No. 2, July 9, 1988, pp 24-26.
Resources
Portland Cement Association
5420 Old Orchard Rd.
Skokie, IL 60077
Tel. 708-966-6200
Fax 708-966-9781
NSF Center for Science and Technology of Advanced Cement-Based Materials
Northwestern University
Evanston, IL 60208-4400
Tel. 708-491-8569
Fax 708-467-1078
Materials Resources: cement, sand, and, admixtures - contact your local concrete dealer. Check the yellow pages.
Admixture sources: Axim Concrete Technology
ESSROC Co.
7230 Northfield Rd.
Walton Hills, OH
Tel. 216-966-0444
Fax 216-439-6773
Master Builders
23700 Chagrin Blvd.
Cleveland, OH 44122
Tel. 216-831-5500
W.R. Grace
62 Whittmore Ave.
Cambridge, MA 02140-1692
Tel. 617-876-1400
Note: These companies supply admixtures to your local ready-mix concrete companies. For quantities needed for your labs, it is best to contact a local concrete supplier.
Aberdeen’s Concrete Construction is a periodical published monthly. This would be a good resource for articles on applications and improvements in concrete. 1-800-323-3550 for subscriptions ($24 for a year). Their address:
426 S. Westgate St.
Addison, IL 60101
Tel. 708-543-0870
Fax 708-543-3112
You may want to contact them requesting complimentary copies or subscriptions.
Master Materials and Equipment Grids
Material |
Demo 1 |
Demo 2 |
Demo 3 |
Demo 4 |
Demo 5 |
sand |
HIS |
HIS |
HIS |
|
|
kitty litter |
DS |
|
|
|
|
glass container and lid (pop bottle) |
G |
G |
|
|
|
3 % NaOH solution |
|
LE |
|
|
|
cement |
|
|
HIS |
HIS |
HIS |
pea gravel |
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HIS |
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thermometer |
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LE |
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insulated container with lid |
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DS |
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drinking straws |
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G |
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plastic cup |
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DS |
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glass petri dishes |
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LE |
pH hydrion paper or universal indicator |
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LE |
spatula |
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LE |
KEY FOR TABLE: |
H = HARDWARE
G = GROCERY |
Material |
Lab 1 |
Lab 2 |
Lab 3 |
Lab 4 |
Lab 5 |
Lab 6 |
baking soda |
G |
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air filled balloon |
DS |
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corn starch |
G |
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flour |
G |
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sulfur |
LE |
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pepper |
G |
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sugar |
G |
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perlite (or Styrofoam beads) |
H |
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H |
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clay |
DS |
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iron filings |
LE |
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test tube |
LE |
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magnet |
LE |
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magnifying glass |
LE |
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wooden splints |
LE |
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cement |
HIS |
HIS |
HIS |
HIS |
HIS |
HIS |
sand |
HIS |
HIS |
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HIS |
HIS |
HIS |
gravel |
HIS |
HIS |
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HIS |
HIS |
HIS |
banana split dishes |
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O |
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trust spacers to make beams |
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HIS |
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mixing containers (sm. buckets, |
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DS |
DS |
DS |
DS |
DS |
cylinder molds (paper or plastic |
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O |
O |
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disposable gloves |
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G |
G |
G |
G |
G |
mixing utensils |
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DS |
DS |
100 ml graduated cylinder |
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LE |
LE |
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balance |
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LE |
LE |
LE |
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drying oven |
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LE |
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refrigerator |
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LE |
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Ziploc bags |
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DS |
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pH paper or universal indicator |
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LE |
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sealable container for 1 cylinder |
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DS |
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petri dish |
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LE |
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barge mold |
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O |
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spreading utensil |
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DS |
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cargo (washers or weights) |
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H |
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finishing tools |
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DS |
form for flower pot |
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DS |
8' and 2' 2x4's |
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HIS |
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large gate hinge |
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H |
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sheet metal |
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H |
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copper pipe caps |
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H |
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rubber stoppers or washers |
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H |
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2 - C-clamps (6'' or larger) |
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H |
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PVC pipe |
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HIS |
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hydraulic jack |
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see lab |
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dowel rod |
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DS |
releasing agents |
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G |
wire mesh |
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H |
Demonstration 1:
Making a Silt Test
Objective: The purpose of this demonstration is to determine the viability of an aggregate based on a silt test.
Materials and Supplies:
Sample aggregate (sand and kitty litter work well for comparison)
Glass container with lid
Water
Ruler
Procedure:
1. Place 5 cm of aggregate in the container.
2. Fill the container with water so the water level is 2 cm above the aggregate.
3. Shake vigorously for 1 minute, making the last few shakes in a swirling motion to level off the aggregate.
4. It is suggested that this demonstration be done twice, once with sand and once with kitty litter to obtain various results.
5. Allow the container to stand for an hour, or until the liquid above the aggregate is clear.
6. The layer that appears above the aggregate is referred to as silt. Measure the
silt layer. If this layer is more than 3 mm thick, the aggregate is not suitable for concrete work unless excess silt is removed by washing.
C. Expected Results:
The sand leaves a 3mm layer and therefore is a viable aggregate. Kitty litter, which is clay, leaves a thicker layer and is not a suitable aggregate.
Demonstration 2
Conducting an Organic Matter Test
Objective: The purpose of this demonstration is to determine the viability of an aggregate based on the amount of organic matter present.
Materials Needed:
Sand
A 50:50 mixture of sand and dirt
Glass container (10 oz. juice jar or similar size) with lid
A 3% solution of sodium hydroxide NaOH (made by dissolving 9 grams of sodium hydroxide, household lye, or caustic soda, in 300 mL of water, preferably distilled).
Procedure:
1. Fill container with sand to the 150 mL mark.
2. Add 120 ml of 3 % NaOH solution.
3. Shake thoroughly for 1 or 2 minutes and allow to stand for 24 hours.
4. Repeat the procedure using the sand-dirt mixture.
5. Indicate the color of the liquid remaining on the top of the aggregate.
The color of the liquid will indicate whether or not the aggregate contains too much organic matter. A colorless liquid indicates a clean aggregate, free from organic matter. A straw-colored solution, not darker than apple-cider vinegar, indicates some organic matter bur not enough to be seriously objectionable. Darker colors mean that it contains too much organic matter and should not be used unless it is washed and tested again.
Expected Results:
The sand leaves a colorless liquid. The sand-dirt mixture produces a yellow to orange liquid.
Demonstration 3:
Effect of Aggregate on Workability of Concrete
Objective: The purpose of this demonstration is to determine the effect of different aggregates on the workability of the resulting concrete.
Materials and Supplies:
cement two containers labeled:
water "limestone chip aggregate"
small limestone chips(pea gravel) "sand aggregate"
sand
Procedure:
1. Add 1 part of cement and 1/2 part water to each container. Suggested amounts include 50 grams of concrete and 25 ml of water.
2. Mix to make the cement paste.
3. To the appropriate container, add 3 parts (150g) of limestone chips and mix.
4. To the second container, add 3 parts (150 g) of sand and mix.
5. Using gloved hands, knead the concrete to determine its workability.
6. Which concrete mixture is more workable? Why?
Expected Results:
The sand aggregate is more workable, because the smaller particles facilitate flow. The larger particles of the gravel hinder it.
Demonstration 4
It's Heating Up!
Objective: The purpose of this demonstration is to track the temperature changes that occur during the curing process of concrete.
Materials and Supplies:
fresh cement-- use at least 100 grams for best results
thermometer
insulated container with a cover
drinking straw
plastic cup to hold cement
Suggestions:
1. Use 150 grams of cement and 75 mL of water in a 6 ounce yogurt container or other plastic container.
2. Use an insulated 1 quart drinking mug or place the sample in a plastic bottle which is set inside a child’s thermos. The thermometer is inserted into a one-holed rubber stopper which fits the thermos. Alternatively, the sample bottle can be placed in a box filled with Styrofoam. Another option would be to use a coffee can. The space inside the can could be packed with insulation, and the outside could be wrapped in pipe insulation. A hole can be cut in the coffee can lid to accommodate the thermometer.
Procedure:
1. Fill the container with fresh concrete, using aggregate is not necessary.
2. Fold over an inch of the drinking straw and tape closed. Insert thermometer into straw.
3. Place the filled container into an insulated container. Insert the drinking straw housing the thermometer into the center of the concrete. Place the lid securely on the container.
4. Record the temperature every 5 minutes for 20 minutes. Most of the change will occur in the first 15 minutes but will continue throughout the whole curing period.
Optional Procedure:
1. Repeat experiment using an admixture, such as calcium chloride, which speeds up the process (Use 2 % of CaCl2 by weight of cement )
2. Attach the apparatus to a computer thermocouple that will record the temperature changes for the class over a day.
Expected Results:
You should see an increase for 4 hours, most of which is observed within the first 15-20 minutes. The larger the mass of concrete the higher the temperature rise. 500 grams of concrete should give a rise of about 10˚C if well insulated. 150 grams of cement gave a 4 ˚C change.
Demonstration 5
pH of Cement
Objective: The purpose of this demonstration is to show the reaction of water with cement and the accompanying change in pH.
Materials and Supplies:
distilled water
cement
petri dish
overhead
universal indicator or pH hydrion paper
spatula
Procedure:
1. Fill a Petri dish half full of distilled water.
2. If using universal indicator, place a drop in the water and mix. If solution is not yellow, add a drop of dilute acid to make it yellow. If using the pH paper, lay 2 strips on the bottom of the dish so the diameter of the dish is covered.
3. Place the dish on the overhead.
4. Into the solution or on top of the pH paper, place a spatula full of cement.
5. Observe the change.
Expected Results:
As the cement mixes with the water, hydroxide ions are formed, thus changing the indicator solution to blue. Emphasis should be placed on the fact that a chemical reaction is occurring not a dissolving process.
The Basic Mix:
A general teacher's guide for concrete preparation
The physical properties of density and strength of concrete are determined, in part, by the proportions of the three key ingredients, water, cement, and aggregate. You have your choice of proportioning ingredients by volume or by weight. Proportioning by volume is less accurate, however due to the time constraints of a class time period this may be the preferred method.
A basic mixture of mortar can be made using the volume proportions of 1 water : 2 cement : 3 sand. Most of the student activities can be conducted using this basic mixture. Another “old rule of thumb” for mixing concrete is 1 cement : 2 sand : 3 gravel by volume. Mix the dry ingredients and slowly add water until the concrete is workable. This mixture may need to be modified depending on the aggregate used to provide a concrete of the right workability. The mix should not be too stiff or too sloppy. It is difficult to form good test specimens if it is too stiff. If it is too sloppy, water may separate (bleed) from the mixture.
Remember that water is the key ingredient. Too much water results in weak concrete. Too little water results in a concrete that is unworkable.
Suggestions:
1. If predetermined quantities are used, the method used to make concrete is to dry blend solids and then slowly add water (with admixtures, if used).
2. It is usual to dissolve admixtures in the mix water before adding it to the concrete. Superplasticizer is an exception.
3. Forms can be made from many materials. Cylindrical forms can be plastic or paper tubes, pipe insulation, cups, etc. The concrete needs to be easily removed from the forms. Pipe insulation from a hardware store was used for lab trials. This foam-like material was easy to work with and is reusable with the addition of tape. The bottom of the forms can be taped, corked, set on glass plates, etc. Small plastic weighing trays or Dairy Queen banana split dishes can be used as forms for boats or canoes.
4. If compression tests are done, it may be of interest to spread universal indicator over the broken face and note any color changes from inside to outside. You may see a yellowish surface due to carbonation from CO2 in the atmosphere. The inside may be blue due to calcium hydroxide.
5. To answer the proverbial question, “Is this right?” a slump test may be performed. A slump test involves filling an inverted, bottomless cone with the concrete mixture. A Styrofoam or paper cup with the bottom removed makes a good bottomless cone. Make sure to pack the concrete several times while filling the cone. Carefully remove the cone by lifting it straight upward. Place the cone beside the pile of concrete. The pile should be about 1/2 to 3/4 the height of the cone for a concrete mixture with good workability.
(SEE DIAGRAM)
6. To strengthen samples and to promote hydration, soak concrete in water (after it is set).
7. Wet sand may carry considerable water, so the amount of mix water should be reduced
to compensate.
8. Air bubbles in the molds will become weak points during strength tests. They can be eliminated by:
i. packing the concrete.
ii. Tapping the sides of the mold while filling the mold.
iii. "rodding" the concrete inside the mold with a thin spatula.
9. Special chemicals called “water reducing agents” are used to improve workability at low water to cement ratios and thus produce higher strengths. Most ready-mix companies use these chemicals, which are known commercially as superplasticizers. They will probably be willing to give you some at no charge.
10. You can buy a bag of cement from your local hardware store. A bag contains 94 lb. (40kg) of cement. Once the bag has been opened, place it inside a garbage bag (or two) that is well sealed from air. This will keep the cement fresh during the semester. An open bag will pick up moisture and the resulting concrete may be weaker. Once cement develops lumps, it must be discarded. The ready mix company in your area may give you cement free of charge in a plastic pail.
Experiment 1
What's the Matter
Introduction to the Physical Properties of Matter
Objective: The objective of this lab is to identify different classes of matter based on physical properties. This lab introduces the key ingredients of concrete. It provides a deeper understanding of the physical properties of concrete.
Scientific Principles:
Matter is divided into the four basic states of solid, liquid, gas, and plasma. Matter is classified based on composition. Homogeneous matter is matter that appears the same throughout a mixture. Heterogeneous matter is matter that has differing appearances throughout the mixture. The concept map below shows the relationship between some of the primary classes of matter.
Matter is identified by its characteristic physical properties. Physical properties are those that can be determined without altering the composition of the substance, such as, color, odor, density, strength, elasticity, magnetism, and solubility. Chemical properties are descriptions of the substance and its reactions with other substances to create new substances with new properties. These chemical properties are identified through chemical reactions. Evidence of a chemical reaction possibly occurring can be seen through a color change, temperature change, evolution of a gas, and the formation of a new substance. This lab will only focus on the physical properties of matter.
Time: 45-50 minutes
Materials and Supplies:
test tubes
magnet
magnifying glass
water
wooden splints
Different samples of matter (any of the following):
baking soda
balloon filled with air
iron filings
flour
sulfur
corn starch
sugar
vermiculite or perlite
Styrofoam beads
salt
pepper
cement
aggregates
clay
General Safety Guidelines:
• Wear safety goggles.
• Some of the materials may cause skin irritation. Wear gloves.
Procedure:
1. Examine each sample. Record color, odor, and relative particle size in the data table. Use a magnifying glass if necessary.
2. With a magnet, test each sample for magnetic properties. Record whether the sample is magnetic or not.
3. Test the solubility in water of each sample by adding 5 mL of water to a small test tube. Add some of the sample to the water. Flick the test tube with your finger to help mix the sample in the water. (Note: If mixing does not occur, use a wooden splint.) Record observations.
Data and Calculations:
Fill in the data table based on your observations.
Sample |
Color |
Odor |
Particle Size |
Magnetic |
Solubility |
State of Matter |
Class of Matter |
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Questions:
1 a. How do you determine which sample is the most soluble?
b. List the samples from highest to lowest solubility.
2. Which of the samples would be classified as a mixture?
3. What physical properties of matter were tested in this lab?
4. What physical properties of concrete would be important to consider when making a structure?
Why?
Notes for Teacher:
• This is a version of a common classification experiment conducted in many chemistry and general science courses. It is included in this module to provide a means for the introduction of concrete and its key ingredients.
• The total number of samples is left to the teacher’s discretion. Sand, water, gravel, cement, and concrete should be samples of matter to be tested. This will provide for the introduction of the topic concrete and its key ingredients.
• Be sure the magnet is protected from directly picking up any of the magnetic fragments in the samples. The magnet may be covered with tape or students may place a piece of paper between the magnet and the sample.
• It is strongly recommended that the teacher do a trial run of this experiment before using it with the students.
• This experiment could be used as a lead in to density by asking, “Of the materials that didn’t dissolve in water, which was the most dense and least dense?”
• Note that when cement is added to water to determine solubility, the students may conclude that cement is insoluble because its rate of dissolution is relatively slow. However, a quick check of pH will demonstrate that something is happening.
• Note: Vermiculite is a compound.
Answers to Questions:
1 a. The sample that has the highest mass dissolved per volume of water is the most
soluble.
b. Answers will vary.
2. This is dependent upon the samples used. However, mixtures are usually obvious from appearance except solutions.
3. color, odor, size, magnetism, solubility
4. Size of particles and solubility of substances used to make it. The physical properties help determine the purity of concrete's ingredients which greatly affect the produced concrete's characteristics.
Experiment 2
How Dense Is It?
An Introduction to Concrete Density and Aggregates
Objective: The objective of this experiment is to determine the density of a concrete sample and to learn the effect of various types of aggregates on concrete's density.
Scientific Principles:
Density is the physical property of matter that measures the quantity of a substance per unit of space. Density is recorded in units of grams per cubic centimeter (g/cm3) for solids, grams per milliliter (g/mL) for liquids, and grams per liter (g/L) for gases. Density is a way of determining how compact one substance is compared to another. Density is also the property that enables one object to be buoyant or another to sink. The object that is less dense will float in a more dense substance. The density of a typical concrete is 2.3 g/cm3.
Time: 45-50 minutes
Materials and Supplies:
cured 1/2” x 2” cylinders of concrete samples (each consisting of a different aggregate)
sample concrete recipes
|
water |
25 mL |
15 mL |
25 mL |
20 mL |
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cement |
50 g |
50 g |
50 g |
50 g |
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aggregate |
150 g sand |
35 g pea gravel |
4 g dry vermiculite |
none |
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100 ml graduated cylinder
water
balance
General Safety Guidelines:
• To avoid cracking the graduated cylinder, gently slide the concrete cylinder down the side.
• Wear goggles.
• Concrete can be caustic. Wash hands after direct contact.
Procedure:
1. Estimate which sample will have the highest and which will have the lowest density.
Record your ideas on the data chart.
2. Mass each cured cylinder on the balance. Record in data table.
3. Half-fill a graduated cylinder with water. Record the initial volume of water.
4. Gently slide the concrete cylinder into the graduated cylinder so as to not splash out any
water or break the glass bottom.
5. Record the final volume of water with the concrete cylinder in it.
Subtract the final volume from the initial volume of water to obtain the volume of the cylinder.
6. Remove the concrete cylinder from the water.
7. Repeat steps 1-6 for all other cylinders.
8. Calculate the density of each cylinder by dividing cylinder's mass by its volume.
Data and Analysis:
Estimates
highest density cylinder_________________
lowest density cylinder _________________
Data Table
Questions
1. a. What type of cylinder was the least dense?
b. What type of cylinder was the most dense?
2. Give a use for the cylinder in question 1a.
3. Give a use for the cylinder in question 1b.
4. Were your estimates correct? Did any of the results surprise you? Why?
5. How do your values compare to the typical density of concrete (2.3 g/cm3) ?
Notes for Teacher:
Many science courses already have a density experiment as part of the course. The use of concrete cylinders will add a new touch to these experiments. It is assumed the concrete cylinders were previously prepared by the teacher.
This experiment could be performed as an investigation in which students prepare concrete mixtures of different ingredient proportions to study their effect on density. These cylinders of varying composition could later be used in the Stress and Strain experiment. This could also be done with different kinds of aggregates. Aggregate densities could be determined, as well as the density of hardened paste. Lightweight aggregates are usually available from a ready mix company. Horticultural vermiculite or perlite can be used. The density of aggregates could be measured and correlated with the density of concrete. (A good project for advanced students.) When measuring the density of aggregates, it is advisable to measure the density of saturated aggregates (soak in water for at least one hour or overnight). Otherwise the aggregates will absorb water during the displacement measurement and give erroneous results. Remember, aggregates in concrete become fully saturated.
This could be turned into a materials science competition in which the students must make a cylinder with the greatest or least density.
Expected Results:
The four cylinder types listed from most dense to least dense:
gravel, sand, paste, vermiculite
Answers to Questions:
1. a. vermiculite
b. gravel
2. Any objects that float. Answers will vary.
3. Answers will vary. Roads, bridges, and underwater structures.
4. Answers will vary.
5. Sand or gravel should be closest to this value.
Experiment 3
Hot and Cold pHun!
Cement Hydration and pH Evolution during Curing
Objective: Students will calculate the amount of water that reacts during hydration when cement becomes concrete under differing curing temperatures and observe the pH change that occurs in the curing process.
Scientific Principles:
Water is the reactant that makes concrete hard. The degree of hydration, or maturity, of the concrete determines the porosity of the concrete. Porosity is the amount of empty space in the concrete. Low porosity concrete has high strength and lasts for a long time.
During the hydration of the cement, the compound calcium hydroxide, Ca(OH)2, is produced. Calcium hydroxide is a basic compound (alkaline). Bases are caustic (eats skin tissue), feel slippery on your skin (like soap), and have a bitter taste (don’t taste it!). Calcium hydroxide is a mild base, but can irritate sensitive skin, so be careful!
Bases also affect indicators (chemicals that change color). There are many different indicators. Universal indicator is a mixture of different indicators so that colors can be achieved all along the pH scale. The pH scale is a numerical range used to determine the acidity or alkalinity of a substance. The scale ranges from 1 to 14. pH’s from 1 to 6 are acidic. A pH of 7 is neutral. Values ranging from 8 to 14 are basic.
The environmental conditions, such as temperature and humidity, under which the concrete is cured can also affect the concrete’s processing and properties. Cooler surroundings results in concrete hydrating at a slower rate.
Time: 100 - 120 minutes (2 1/2 class periods)
Materials and Supplies for Groups of 2-3:
Fresh cement
mixing container
stir stick
balance
oven
refrigerator
labels
pH paper
3 glass or plastic petri dishes (see Teacher's Notes)
plastic wrap or Ziploc bags
gloves
graduated cylinder
releasing agent (PAM cooking spray, cooking oil, aluminum foil, plastic wrap)
General Safety Guidelines:
• Prolonged exposure may cause severe chemical burns.
Wash exposed skin after contact.
• Gloves should be worn to protect the skin, especially if your skin is sensitive.
• All heating must be done in a oven only.
Procedure:
Day 1:
1. Prepare a batch of fresh paste using 100 g of cement and 40 mL of water.
2 a. Place a releasing agent (PAM cooking spray, aluminum foil, plastic wrap, cooking oil) in all three petri dishes. Be sure to cover the entire inside of the dish.
b. Mass the three petri dishes and record in the data table.
3. a. Fill each petri dish 1/4 full with the paste and level it off.
b. Mass each dish separately and record in the data table.
c. Wrap two dishes in plastic wrap or place in Ziploc bags. Set one aside at room temperature. Set the second in a refrigerator.
d. Put the third dish into a sellable container with water. Be sure the water level is well below the edge of the dish. Tightly seal the container and place it in an oven set at 40˚ C.
4. Using pH paper, test the pH of tap water and any leftover fresh concrete paste. Record your values on the data page.
5. Allow forms to cure 24 hours.
Day 2:
1. Remove the concrete samples from their controlled environments.
2. Mass each dish separately and record in the data table.
3. Determine the mass of original water by using the equation found in the data and calculations section.
4. Determine the mass of the original cement by using the equation found in the data and calculations section.
5. Place each sample in an oven set at 100 -110 ˚ C. Allow the samples to remain in the oven for 24 hours.
Day 3:
1. Remove the samples from the oven and allow to cool.
2. Weigh each petri dish and then determine the mass of only the cement paste.
3. Determine the mass of water driven off in the oven by using the equations in the data and calculations section.
4. Calculate the mass of water combined during hydration using the equations in the data and calculations section.
5. Calculate the percent of water reacted using the equations in the data and calculations section.
Data and Analysis:
Calculation Hints:
Data:
pH of mixed cement _________________
pH of tap water ____________________
Day 1 |
ROOM |
REFRIGERATOR |
OVEN |
mass of petri dish |
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mass of concrete and dish |
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Day 2 |
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mass of concrete and dish |
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mass of sample |
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mass of original water |
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mass of original cement |
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Day 3 |
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mass of dish and dry sample |
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mass of dry sample |
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mass of water driven off |
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mass of water combined in reaction |
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% of water reacted (hydration) |
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Questions:
1. Which curing environment yielded the greatest percent of water reacted?
2. a. What is the relationship between curing environment and amount of hydration?
b. Do your results support your answer?
3. Which concrete sample would you hypothesize to have the greatest strength?
Explain your reasoning.
4. What proof do you have that hydration is a chemical change?
5. Suppose you came across a container filled with a substance that looked like concrete.
When you tested it with pH paper, the paper turned red. Was the substance concrete?
Explain your reasoning.
Notes for Teacher:
• Students may use an universal batch or make their own mixtures to study water to cement ratio effect on pH and hydration.
• If plastic petri dishes are used, place a few sheets of paper on the oven rack before setting the dishes in the oven. Then they shouldn't melt. If plastic dishes are used, a releasing agent is unnecessary since the dish is disposable.
• Students may investigate the effect of admixtures. Calcium chloride can be used to accelerate setting (2% CaCl2•2H2O by weight of cement). Sugar can be used to retard setting (0.1% by weight of cement). Commercial accelerators and retarders can be obtained from your local ready mix company.
• An alternative experiment is to vary the amounts of water available for hydration and determine the results. Samples are all kept at room temperature. One is covered with wet paper towels and plastic sheeting, one with plastic sheeting only, and one is covered with only a wet paper towel. The test specimens would then be treated as described in Day 2.
• Students could perform the same procedure on cylinders of different ages to prove that hydration continues over a long period of time.
Expected Results:
Hydration of the sample in the oven is accelerated by a warm environment. Therefore, the sample in the oven will use more water in its hydration products. So it will have the highest water reacted and the cooler environment in the refrigerator will have the least.
Answers to Questions:
1. Oven
2. a. Colder environment results in less hydration occurring, more free water is available.
b. Answers will vary
3. Oven sample, because more hydration could have occurred.
4. The pH paper changed color and new products were formed, because water was retained, not evaporated.
5. No. pH paper changes to blue, indicating a base is produced during the formation of concrete. A change to red indicates an acidic change, not basic.
Experiment 4
The Fleet Afloat!
A Design Project
Objective: Students will design a concrete barge which will float and carry a maximum cargo. In this experiment the students will be learning to work with the component materials used to make concrete and building and testing a concrete structure for physical properties.
Scientific Principles:
Density is a measure of the compactness of a material. It is a measure of how much matter is squeezed into a given space. Density is the amount of matter per unit of volume.
Whether an object will sink or float in water depends on its density. An object will float if it is less dense than water. An object will sink if it is more dense than water. If an object has a density equal to that of water, it will neither sink nor float. The density of water is 1.00 g/cm3. The apparent density of an object can be changed by either changing the mass of the object, the shape of the object, or both. For a given mass of concrete, the apparent density can be altered by changing the volume it occupies (i.e. volume displaced when placed in water). Concrete can be made to float if it is shaped like a boat. A boat-shaped or hollow object will displace a volume of water greater than the actual volume of solid material in the object. The object is said to be "buoyant" when it floats due to low density. By spreading out the concrete used to make the boat over a larger volume, the apparent density of the boat becomes less than that of water. Hence the boat floats!
Time: 90 min. (1 class period for making the barges and 1 class period for testing)
Materials and Supplies:
The following materials are needed for each group of 2-3 students:
mold to make barge
cement
water
various aggregates
sand
pea gravel
vermiculite
small Styrofoam balls
perlite
crushed corn cobs
2-3 pairs of gloves for concrete handling
newspapers
spreading utensil for spreading concrete in barges
boat or tray to demonstrate buoyancy with cargo
1-2 pounds of washers, weights or lead shot
General Safety Guidelines:
• It is easiest to form the canoe by hand, but cement is caustic. Wear plastic gloves.
• Wash your hands well after use.
Design Competition Parameters:
1. The barge or canoe must be constructed from materials (cement and various aggregates) provided by the instructor. The form used to create the barge or canoe is to be provided by the students.
2. The proportions of the ingredients used in the cement mixture are to be chosen by the students using the following as a guide, not a recipe:
3 parts aggregate: 2 parts cement : 1 part water
100 grams of cement is sufficient for one Dairy Queen boat.
3. Once constructed, the barge or canoe should be cured for at least 24 hours.
Design Testing Procedures:
1. The mass of each empty barge or canoe will be measured. The lighter the ship the more points it receives. Any vessel less than 150 grams earns the maximum points value.
2. Each barge or canoe will be placed in the water. The empty barge or canoe must float for one minute before the cargo will be added. A cargo will be added in increments until the point when the ship begins to take on water. Each ship must float for 1 minute prior to the addition of each additional cargo increment. For the canoe, whether the canoe leans or floats level will be noted.
3. The mass of the cargo will be measured. For every 50 grams the ship holds, it will receive one point.
4. A ratio of mass of cargo will be calculated to determine each barge’s class rank.
mass of ship
5. Barges which fail to float without any cargo will be assigned a predetermined point
value.
6. Points will be awarded to each barge based on its class rank.
7. The cost of construction will be calculated for each boat which successfully floats for 1-2 minutes to determine its class rank. (optional)
8. The amount of leakage will be determined and point values will be given for the different levels of leakage.
Other Procedures:
1. Suppose you and a friend are in a canoe with a load of rocks. If you throw half of the rocks overboard, will the water level of the pond go up, down, or stay the same? Make note of your prediction.
2. In a container of water, float a boat loaded with a cargo of washers or other uniform weight. Make sure the water's height is greater than the combined height of the washers and the boat.
3. Mark the water level on the container and on the side of the boat.
4. Put the cargo of washers in the water. Mark the new water levels on the container and the side of the boat.
Questions
1. What happens to the water level on the side of the boat when the cargo is moved from the boat to the water?
2. If you see a boat riding high in the water, which is it more likely to be carrying: lead
bricks, glass marbles, or Styrofoam cups?
3. What happens to the water level on the side of the container when the cargo is moved from the boat to the water? Was your prediction correct?
4. Describe the shape, dimensions, and composition of your barge. Draw an illustration.
5. What was the limiting factor in your barge design?
b. How would you change your barge’s design to improve this ratio?
7. How could you change the density of your barge/canoe without changing its mass?
Notes for Teacher:
• Several criteria are listed so that the teacher may choose the guidelines upon which to rank the barges/canoes.
• This experiment emphasizes the physical geometry of the barge and therefore may be most suitable for a physics course when discussing principles of density and buoyancy.
• The cargo could be lead shot, washers, weights, or any other material of uniform mass. The one minute waiting period prior to the addition of more cargo may need to be shortened depending on class period length and the number of barges being tested. However, a waiting period is necessary to provide time for water to enter the barge.
• The form materials could be provided or the students could be directed to bring in their own forms, allowing their imaginations to wander. Limits might need to be set in terms of dimension, weight, or amount of concrete/cement available. Possibilities for the barge forms are travel soap dish or a travel toothbrush holder or even an old tennis shoe. Dairy Queen banana split boats, foam insulation, plastic weighing dishes, old butter/margarine containers are suitable for the canoe.
• Foam meat trays or plywood boards make good bases for the barges/canoes to be set on and are very helpful when moving them from the building site to the curing site (if different). The ships should be covered with wet rags or plastic while curing to help retard evaporation and improve hydration.
• A wash tub, 5 gallon bucket, or a small child’s swimming pool could be used for the testing waters. Do not use ponds or swimming pools. Cement is caustic to living things. A video recording of the testing could also be made. The video could be set out in the hallway and replayed during parent conferences for the parents to view while waiting to speak with you - great public relations.
• This activity could be tied into a holiday such as Labor Day, Columbus Day or Washington’s Birthday observance. Decorations could be added to the barges and a competition held to determine the most aesthetically pleasing barge. The art classes might be asked to judge the barges.
• Note: Pre-assign dollar amounts to quantities of materials in order to calculate the cost of the boat. (optional) This option allows discussion of economy and efficiency.
Answers to Questions:
1. Will go down. i.e., boat rides high in the water
2. Styrofoam cups
3. The level goes down!
Removing the cargo lowers the water level by a volume, V1, equal to the cargo's mass over the apparent density of the boat and cargo. The water volume, V2, raises from the cargo put into the water by its mass over its density.
Simple math yields: V2-V1=-0.57*Cargo Mass
Note that the negative sign means down.
4. Answers vary based on students design.
5. Possible answers: heavy mass; low surface area; poor construction; setting time inadequate.
6. a. Answers vary.
b. Possible answers: change aggregates; change concrete component proportions; different shaped form.
7. By changing its volume.
Experiment 5
Stress and Strain
An Introduction to Stress and Strain
Objective: Students will design and make a concrete cylinder and beam that can withstand the greatest applied load.
Scientific Principles:
Solids have rigidity because the bonding forces between the atoms resist changes. When an external force is applied to a solid, the interatomic forces allow for deformations to occur. When the distorting external force is removed, the forces between the atoms bring them back to their original positions and the solid, as a whole, back to its original shape. This property is called elasticity. All solid materials are elastic to some extent, even concrete. The amount of elastic deformation that can occur in a material is dependent on the type of stress applied.
Two common types of stress are compressive and tensile. Compressive forces exist when you push together a material. Walking on concrete is one such example. Tensile stresses describe a material when it is pulled upon. The cable that supports the load of a crane is a good example of tensile stresses.
While in elastic regions, compressive and tensile stresses do no damage to the material. A material will return to its original dimensions once the stress has been removed if it is within this region. If the stress becomes too great in concrete, microscopic cracks begin to form inside the concrete. Once cracks begin to form, the concrete will not return exactly to its original dimensions and thus behaves inelastically. These changes are very small and not easy to measure without expensive equipment. As the load increases, these microscopic cracks come together so as to form one large macroscopic crack, and the concrete fractures or fails. Concrete has very good compressive properties but very poor tensile properties unless steel reinforcements are added.
Time: 90 min. (1 class period for making the beams & cylinders and 1 period for testing)
Materials and Supplies:
The following materials are needed for each group of 2-3 students:
cement
various kinds of aggregate
mixing containers
stirring utensils
forms for making beams and cylinders (enough for each student to work individually)
LOW-TECH COMPRESSION TESTER
two 8' long 2x4's
one 2' long 2x4
large gate hinge
sheet metal -cut into two 4 "squares
2 copper pipe caps to fit over concrete cylinders
2 rubber stoppers or washers to fit inside pipe caps
MEDIUM TECH COMPRESSION TESTER
2 C-clamps ---6" span or larger
2 copper pipe caps to fit over concrete cylinders
PVC pipe slightly longer than concrete cylinders and slightly larger in diameter
2 rubber stoppers or washers to fit inside pipe caps
HIGH TECH COMPRESSION TESTER
hydraulic jack with pressure gauge
2 copper pipe caps to fit over concrete cylinders
2 rubber stoppers or washers to fit inside copper caps
Design Competition Parameters:
1. All cylinders and beams will be made using uniform molds which will be provided.
2. The concrete used to make each cylinder and beam must be made using the raw materials provided.
3. Suggested recipe for concrete is 3 parts aggregate, 2 parts cement, and 1 part water. Any combination of aggregate or none at all may be used. The exact amounts of each component to be used is up to you!
4. The beams must span at least 30 centimeters but may be longer.
Beam Testing Procedure:
1. Support the beam between two tables of the same height.
2. Twist the ends of a wire together so that it forms a circle around a bucket handle. Put this loop around the middle of the beam.
3. Add weight to the bucket in uniform increments. Wait 30 seconds between each addition of weight.
4. Continue to add weight until the beam breaks.
Cylinder Testing Procedure:
1. Place the cylinder in the testing apparatus.
2. Apply force until the concrete is smashed.
Questions and Analysis:
1. If a horizontal concrete beam is supported at one end and a downwards force is applied to the other end, where would the microscopic cracks tend to be formed? The top layer, the bottom layer, or the middle layer?
2. If a concrete beam is supported but not held at both ends and a load is applied in the middle, which layer would be under compression? The top layer, the bottom layer, or the middle layer?
3. In the construction of a bridge, would concrete be better suited for the columns
supporting the bridge or for the bridge decking? Explain your answer.
4. Write the formula needed to calculate the
a. tensile strength of a beam
b. compression strength of a cylinder
5. Calculate the strength each of your structures exhibited in the competitions.
6 a. Write the equation to calculate the force exerted on a cylinder.
b. Calculate the force that was exerted on your cylinder.
7. What changes would you make to improve your beam design?
8. What changes would you make to improve your cylinder design?
Notes for Teacher:
We used a 24” truss spacer as our beam forms. These can be found at a hardware store. Our truss spacers cost 69 cents each and are reusable. A toothpaste box reinforced with tape can be used as a form. The plastic cases that toothbrushes come in could also be used. However, obtaining a classroom set of these might prove to be a challenge. The beam testing could also be set up as a cantilever style with only one end of the beam fixed and the load applied to the free end.
Free weights can be added to the bucket until breakage occurs. If free weights are not available, water can be used. Be careful of water spillage.
Cylindrical forms can be plastic or paper tubes, pipe insulation, or any other small diameter tubing. The concrete needs to be easily removable from the forms. Pipe insulation from a hardware store works well. This foam material is easy to work with, reusable, and inexpensive. The bottom of the forms can be taped, corked, or set on glass plates. The goal is to achieve smooth top and bottom surfaces which are necessary when performing the compression test. If the surface is not smooth, the force will be applied unequally. We recommend the cylinders be no more than 1/2” in diameter. Cylinders with a larger diameter will require a greater force to fail. A good rule of thumb is the height of the cylinder should be at least twice the diameter of the cylinder. However, if the cylinder is too tall it will tend to buckle.
The aggregate selection could be limited to just sand or could include gravel. The strength of the specimen can be adjusted by changing the water to cement ratio of the concrete to suit the capacity of your test set up. The length of curing can also be varied.
The amount of force needed to produce failure in a beam depends on the beam’s dimensions. The theoretical maximum tensile strength of a beam can be calculated by:
The amount of force needed to produce failure in a cylinder depends on the cylinder’s cross-sectional area. The theoretical maximum compressive strength of a cylinder can be calculated by:
Some common values needed for failure are: Beam - 20MN/m2 (3000 psi)
Cylinder - 35 MN/m2 (5000 psi)
The experiment could be used to show the development of strength over time. The students could make several cylinders and beams from each of their concrete batches and the compression strength could be tested over a long period of time such as 1 day, 3 days, 7 days, and 28 days.
The principle of reinforced concrete could be demonstrated by placing a strip of hardware cloth (galvanized steel screen) or chicken wire into the beam in the tension zone.
A cost factor could be incorporated into this lab to make it a true engineering project. The competition could involve assigning points based on most strength for lowest cost.
We offer three testing apparatus designs for measuring the compression strength of the cylinders. Each of these designs is pictured and described on the following pages.
Low Tech Compression Testing Apparatus - The Cement Cracker
Two 8 feet long 2 x 4’s were hinged together using a large gate hinge. The hinge plates will need to be bent so as to fit to the 2 x 4 resulting in a device similar to a nut cracker. A separate 2 x 4 which is about two feet long should be nailed to the bottom side of one of the 8’ long boards to provide stability. Otherwise, the unit tends to roll to the side. To provide additional strength to the 8’ long boards, we attached two small pieces of sheet metal at the location where the cylinders would be seated for testing, about 2 or 3 inches from the hinge. To insure uniform pressure acting on the cylinders, copper pipe caps were placed over the ends of the concrete cylinders. Before placing the copper caps on the cylinders, rubber inserts were placed inside the copper pipe caps to cushion the ends of the cylinders. Place the copper capped concrete cylinders on the sheet metal pads and apply a force on the end of the 8’ long boards. By calculating the mechanical advantage of the lever and knowing the applied force, the force exerted on the cylinders can be determined.
Sample calculation: Assuming the cylinder is placed 3” from the hinge on an 8’ (96”) long board, the mechanical advantage of the lever is 32
If a 100 pound student stands at the end of the 8’ long board, the force exerted on the cylinder is 3200 pounds. If this is attempted, ensure the safety of the student.
Medium Tech Testing Apparatus - The C-Clamp
Rubber inserts are first placed inside copper pipe caps to provide a cushion for the concrete cylinders. The copper pipe capped cylinder is placed inside a piece of 1” diameter PVC pipe (clear or white) which has been cut long enough to completely cover the concrete cylinder. The PVC pipe is used as a safety means to prevent pieces of concrete from flying outwards when the cylinder breaks. It also provides stability to the test cylinder. Any other piece of pipe that has an inside diameter greater than that of the copper pipe caps could be used. The PVC covered cylinder is placed in the throat of a large (6” or greater) C-clamp. Tighten the threaded shaft onto the copper caps on the cylinder. This entire set up is then clamped to a table top using a second C-clamp. A socket which fits a torque wrench is fitted over the handle of the C-clamp. This socket will need to have slots cut into it so that it snugly fits the handle of the C-clamp. The handle of the C-clamp can be tightened further down onto the concrete cylinder. The torque value can be recorded from the torque wrench. Because the cylinders break rather quickly, we suggest that a video camera is used to record the readings on the torque wrench and played back to obtain more accurate readings. By calculating the mechanical advantage of the C-clamp screw and knowing the torque applied, the force exerted on the cylinder can be determined.
Sample Calculation: The mechanical advantage of a screw can be calculated by:
The mechanical advantage of the screw multiplied by the torque reading equals the maximum force applied to the cylinder, neglecting friction.
High Tech Testing Apparatus - Hydraulic Jack
A hydraulic jack with a pressure gauge provides the easiest method for determining the force exerted on the concrete cylinders. Brodhead-Garrett Co. ( 223 S. Illinois Ave., Mansfield, OH 44901) sells a hydraulic jack with a pressure gauge and platform for about $100. (Dan Phillip 1-800-949-8324)
Place rubber inserts inside copper pipe caps to provide a cushion for the concrete cylinders. Cap the concrete cylinder and place in the platform of the jack. Begin applying pressure while observing the pressure gauge. Allow time between pumps for the cylinders to fail - a minute or two. Safety glasses are a must as pieces of the cylinder fly outward!! A safety shield around the sample would also be a good idea. A video camera may be used to record readings on the pressure gauge and replayed to obtain more accurate readings.
To calculate the compressive strength that is applied to the cylinder, use the following equation.
Expected Results:
For 1/2" cylinders, the cement paste broke at 2000+ psi, gravel at 2000 psi, and vermiculite at 400 psi. The beams' maximum load will vary depending on the form used. Loads as heavy as 100 kilograms have been observed!
Answers to Questions:
1. Top layer, because it is in tension and concrete is week in tension.
2. The top layer will be in compression. The bottom will be in tension.
3. The columns of bridges are concrete while the decking is usually steel.
The columns are only under compression while the decking is under tension.
4. a. See equation #1
b. See equation #2
5. Answers will vary, units will either be psi or N/meter2
6. a. See equations #3 and #4 for low tech apparatus and equation #5 for medium and high tech apparatus.
b. Answers will vary, units should be newtons or pounds
7. and 8. answers might include: use different aggregate, vary the amounts of components used in the cement, use a different form and change the dimensions.
Experiment 6
Make and Take
A Take-home Project
Objective: Students will develop an understanding of concrete as a construction material. A concrete product will be designed and constructed.
Scientific Principles:
Concrete is everywhere. Take a moment and think about all the concrete encounters you have had in the last 24 hours. All of these applications of concrete are created from a material that is a mixture formed from the chemical reaction of cement and water (hydration) with added aggregate. It important to distinguish between cement and concrete as they are not the same. Cement is used to make concrete.
Water is a key reactant. Water and cement initially form a cement paste that begins to react through the chemical process of hydration. In the hydration of cement, chemical changes occur slowly, eventually creating new crystalline products, heat evolution, and other measurable signs. The hydration process hardens (sets) the paste which binds the aggregate particles together.
The aggregate is the solid particles that are bound together by the cement paste to create the synthetic rock known as concrete. Aggregates can be both fine, such as sand, and/or coarse, such as gravel. The relative amounts of each type and the sizes of each type of aggregate determines the physical properties of the concrete.
Time: 1 Class period for each object made. (45 min.)
Materials and Supplies for groups of 2-3:
cement
pea gravel
water
2 flower pots or suitable plastic containers
wire mesh
mixing bowl or wheelbarrow
finishing tools
damp sand
1" greased dowel rod
Procedure:
1. Prepare enough concrete to fill the space between your two containers when they are stacked with a 1" separation.
2. Coat the inside of the bottom container and the outside of the top container with plastic wrap.
3. Place a 1" lubricated dowel (future drain hole) that is 1" long, in the bottom center of the first mold for a round based flower pot or four dowels if you are making a rectangular flower pot. This will serve as a spacer now and a drain in the hardened pot.
4. Surround the dowels with concrete mix, place the top mold on the mix being sure to allow for about 1" wall thickness. Add concrete mix between the forms until the bottom form is filled. Tamp concrete to remove air pockets. Allow to harden for 15- 20 minutes. If decorations are desired on the outside of the pot, remove outer mold and add glass beads, colored stones, etc. Allow to harden 24 hours.
5. Remove molds and place in water for three days to harden. Be cautious of this step if your pot is decorated.
Notes for Teacher:
• Use larger sized aggregates to create a nice appearance.
• Use as a money making project.
• Many different shapes can be made and may be a great way to inspire some creativity.
CONCRETE NAME________________________
QUIZ 1
1. Distinguish between cement and concrete.
2. Name at least three items you have encountered today which are concrete.
3. What are the major ingredients for concrete, and what role do they play?
4. What is meant by "workable?" Why is it important for concrete to be workable?
5. Give an example of an aggregate. What is the practical use for this aggregate in making concrete?
CONCRETE NAME _____________________________
QUIZ 2
1. What can be used to slow the hardening of concrete (give example?) Why would slowing this process be desirable?
2. What can be used to speed the hardening of concrete (give example?) Why would speeding up this process be desirable?
3. Suppose you were in charge of building a skyscraper. What would be your choice for aggregate and why?
CONCRETE NAME______________________________
QUIZ 3
1. What will happen to concrete if it dries out too quickly?
2. Suppose you were to be the chief designer in charge of building a concrete ship to carry people overseas. What aggregate might you choose to put in your concrete and why?
3. Explain what the dormancy period of fresh concrete is. How do contractors make use of the dormancy period?
4. Explain how can you measure the consistency of freshly mixed concrete?
CONCRETE NAME________________________
QUIZ 4
1. Briefly discuss the importance of a proper water to cement ratio.
2. Explain the purpose of a superplasticizer in making concrete.
3. Why should gloves be worn when mixing concrete? Be specific.
4. Water is important in making concrete, however, it can also be harmful to concrete. Explain this statement.
CONCRETE NAME KEY
QUIZ 1
1. Distinguish between cement and concrete.
Cement is a component of concrete. Cement and water make the "glue" which holds concrete together.
2. Name at least three items you have encountered today which are concrete.
Answers will vary.
3. What are the major ingredients for concrete, and what role do they play?
cement- reacts with water to form "glue"
water- reacts with cement, the amount also determines strength
aggregate- makes concrete stronger, more durable, and less costly
4. What is meant by "workable?" Why is it important for concrete to be workable?
Cement which is workable is able to be poured into forms without difficulty. A slump test is used to measure workability.
5. Give an example of an aggregate. What is the practical use for this aggregate in making concrete?
gravel, sand, vermiculite, perlite,
Aggregate makes the concrete stronger and cheaper.
CONCRETE NAME__KEY________________________
QUIZ 2
1. What can be used to slow the hardening of concrete (give example?) Why would slowing this process be desirable?
Sugar can be added to the concrete to retard hardening. For example, if the concrete needs to be transported a long distance, then a retarding admixture would be desired.
2. What can be used to speed the hardening of concrete (give example?) Why would speeding up this process be desirable?
Calcium chloride solution can be added to speed the hardening of concrete. For example, in cold weather it is desirable to speed up the hardening process and produced higher heat of hydration.
3. Suppose you were in charge of building a skyscraper. What would be your choice for aggregate and why?
The aggregate would depend upon how the concrete is to be used in the building. Lightweight aggregates like shale are used for insulating properties. However, normal weight aggregate would be required for strength. Availability and economy of aggregate are important, too.
CONCRETE NAME__KEY________________________
QUIZ 3
1. What will happen to concrete if it dries out too quickly?
Concrete will most likely crack due to drying shrinkage. The hydration reaction which strengthens concrete will be halted from lack of water resulting in weaker concrete.
2. Suppose you were to be the chief designer in charge of building a concrete ship to carry people overseas. What aggregate might you choose to put in your concrete and why?
A lightweight aggregate would be desirable for building a ship needing to float. However, the boat would be dangerous because of poor tensile properties of concrete. It would have to be reinforced to be safe.
3. Explain what the dormancy period of fresh concrete is. How do contractors make use of the dormancy period?
The dormancy period of fresh concrete is the period during which the concrete is in a plastic state and the reaction is very, very slow. This state lasts from 1 to 3 hours and allows contractors to transport concrete to the job site and consolidate it before it hardens. After the dormancy period, the hydration reaction accelerates, and the concrete sets and becomes hard.
4. Explain how can you measure the consistency of freshly mixed concrete?
A slump test can be performed on freshly mixed concrete to determine its consistency. This is done by pouring it into an inverted cup with the bottom cut out. Once the cup is removed, the concrete is observed. It is desirable that the concrete stay 50-75% of its original height for good workability.
CONCRETE NAME__KEY__________________
QUIZ 4
1. Briefly discuss the importance of a proper water to cement ratio.
The water to cement ratio determines the strength of concrete. The less water that is used to obtain a workable concrete, the more strength the resulting hardened concrete will have. However, remember that workability is lost if water to cement ratio is too low.
2. Explain the purpose of a superplasticizer in making concrete.
A superplasticizer is an admixture which is used to make concrete more workable with the use of less water. Using a superplasticizer will result in a stronger concrete because less water is used.
3. Why should gloves be worn when mixing concrete? Be specific.
Gloves should be worn while mixing concrete because one of the products of the hydration reaction is calcium hydroxide, a base. In fact, upon mixing concrete, the pH rises to 12 which means the solution is strongly basic. This can burn, irritate, and dry out the skin.
4. Water is important in making concrete, however, it can also be harmful to concrete. Explain this statement.
Water transports harmful substances that lead to concrete degradation. Water is the central issue in freeze-thaw damage of concrete.
Glossary
Accelerators: Admixtures that decrease the setting time by increasing the rate of hydration.
Admixture: A material other than water, aggregates, or cement that is used as an ingredient of concrete or mortar to control setting and early hardening, workability, or to provide additional cementing properties.
Aggregate: Inert solid bodies such as crushed rock, sand, gravel.
Binder: Hardened cement paste.
Bleed: To have water seep to the surface of the cement paste due to settling.
Calcination: Decomposition due to the loss of bound water and carbon dioxide.
Cement: Finely powdered mixtures of inorganic compounds which when combined with water hardens with hydration.
Cement paste: Cement plus water. When the mass has reacted with water and developed strength it is called hardened cement paste.
Clay: Type of soil consisting of very fine particles.
Clinker: The material that emerges from the cement kiln after burning. It is in the form of dark, porous nodules which are ground with a small amount of gypsum to give cement.
Compression: Forces acting inwardly on a body.
Concrete: A hard compact building material formed when a mixture of cement, sand, gravel, and water undergoes hydration.
Cure: To keep concrete moist during initial hardening.
Deformation: The process of changing the dimensions of a structure by applying a force.
Dormancy period: Time period that concrete retains it workability.
Elasticity: The ability of a material to return to its original shape after being stretched.
Forms: Holders in which concrete is placed to harden.
Gypsum: Calcium sulfate dihydrate, CaSO4.2H2O added to cement to regulate setting.
Hydration: The reaction of cement with water to form a chemical compound.
Kiln: High temperature oven.
Limestone: Mineral rock of calcium carbonate.
Mortar: Cement paste mixed with sand.
Pozzolan cement: Volcanic rock powdered and used in making hydraulic cement.
Porosity: The amount of empty space in concrete.
Portland cement: A cement consisting predominantly of calcium silicates which reacts with water to form a hard mass.
Retardants: Admixtures that increase the setting time by slowing down hydration.
Set: Transformation of cement paste or concrete from a fluid-like consistency to a stiff mass.
Slump test: Test used to determine workability.
Tension: The stress resulting from elongation.
Workability: How easily fresh concrete can be placed and consolidated in forms.
Source: http://matse1.matse.illinois.edu/concrete/concrete.doc
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