The History of Concrete: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.

Hoover Dam (4,360,000 cubic yards of concrete)

What is in Concrete? Concrete is a composite material which is made up of a filler and a binder. Typical concrete is a mixture of fine aggregate (sand), coarse aggregate (rock), cement, and water.

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Under no circumstances should the word "cement" be used to refer to the composite product "concrete".

Portland cement, so named for its color similarity with limestone near Portland England, is composed primarily of four complex compounds: tricalcium silicate, dicalcium silicate, tricalcium aluminate, and tetracalcium aluminoferrite.

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, typically drinkable, 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 in order to keep 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. The final properties of the concrete will depend on the cement characteristics, the type and amount of aggregate, the water-cement ratio, and the completeness of the reaction (subject to time, humidity, and temperature).

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.

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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.

The compressive strength of concrete is usually at least ten times its tensile strength, and five to six times its flexural strength. The principal factors governing compressive strength are given below:

Water-cement ratio is by far the most important factor. The age of the cured concrete is also important. Concrete gradually builds

strength after mixing due to the chemical interaction between the cement and the water. It is normally tested for its 28 day strength, but the strength of the concrete may continue to increase for a year after mixing.

Character of the cement, curing conditions, moisture, and temperature. The greater the period of moist storage (100% humidity) and the higher the temperature, the greater the strength at any given age.

Air entrainment, the introduction of very small air voids into the concrete mix, serves to greatly increase the final product's resistance to cracking from freezing-thawing cycles. Most outdoor structures today employ this technique.

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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 palestine to form a natural deposit of cement compounds.

3000 BCEgyptians

Used mud mixed with straw to bind dried bricks. They also used gypsum 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 BCGreeks, Crete & Cyprus

Used lime mortars which were much harder than later Roman mortars.

300 BCBabylonians & As Syrians

Used bitumen to bind stones and bricks.

300 BC - 476 AD Romans

Used pozzolana cement from Pozzuoli, Italy near Mt. Vesuvius to build the 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 Middle Ages

The quality of cementing materials deteriorated. The use of burning lime 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.

1780Bry 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

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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.

1796James 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.

1810Edgar 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.

1828I. 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.

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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.

1880J. 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.

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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.

Concrete - MaterialsTable 1. Classes of Aggregates

class examples of aggregates used uses

ultra-lightweight

vermiculiteceramic spheresperlite

lightweight concrete which can be sawed or nailed, also for its insulating properties

lightweightexpanded clayshale or slatecrushed brick

used primarily for making lightweight concrete for structures, also used for its insulating properties.

normal weight

crushed limestonesandriver gravelcrushed recycled concrete

used for normal concrete projects

heavyweight steel or iron shotsteel or iron pellets

used for making high density concrete for shielding against nuclear radiation

Examples of classes of concrete aggregate are shown in Table 1. 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.

Hydration of Portland Cement

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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.

Table 2. Composition of portland cement with chemical composition and weight percent.

Cement Compound Weight Percentage Chemical FormulaTricalcium silicate 50 % Ca3SiO5 or 3CaO.SiO2

Dicalcium silicate 25 % Ca2SiO4 or 2CaO.SiO2

Tricalcium aluminate 10 % Ca3Al2O6 or 3CaO .Al2O3

Tetracalcium aluminoferrite 10 % Ca4Al2Fe10 or 4CaO.Al2O3.Fe2O3

Gypsum 5 % CaSO4.2H2O

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

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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 1. Schematic illustration of the pores in calcium silicate through different stages of hydration.

The diagrams shown in Figure 1 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

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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.

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 2. Rate of heat evolution during the hydration of portland cement

As shown in Figure 2, 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.

Strength of Concrete

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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. The relationship between the water/cement ratio and porosity is illustrated in Figure 3.

Figure 3. 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 Figure 4.

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Figure 4. 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, as shown in Table 3.

Table 3. A table of admixtures and their functions.

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TYPE FUNCTION

AIR ENTRAININGimproves 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)

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.

Mix Design ProcedureIn order to determine the proper amounts of water, cement, coarse aggregate, and fine aggregate that is to be used for a concrete mix, factors such as strength requirements, the type of structure, the environmental exposure of the structure, and the workability must be considered. This is demonstrated in the procedure outlined below.

Concrete Mix Design Procedure

The structural and environmental design requirements of the project must be met. (Information related to these requirements is presented in Tables 1 and 2 and Figure 1 below and in the ENGR 314 Laboratory Manual. Take the specified safety factor, if any,

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into account by increasing the minimum strength value by the specified amount, when determining the necessary compressive strength. If a water-to-cement ratio between the values given in the table is desired, linear interpolation should be used to determine the appropriate amount of each constituent material required for the mix. For example, if a .43 water-to-cement ratio is desired, interpolate between the data for the .40 and .45 water-to-cement ratios. The weight of each component required to prepare the desired volume of concrete must be determined. The weight of each component in pounds that is required to produce 1 cubic yard of concrete is shown in Table 2. If a volume other than 1 cubic yard of concrete is required, the appropriate conversion factor must be used with the data from Table 2 to calculate the required amount of each constituent. If the sand is known to absorb water, be sure to take this into account and increase the amount of water used by an appropriate amount so that the appropriate amount of water will be available for the hydration reaction.

Concrete Mix Design Example

A retaining wall is to be built in Jackson, Mississippi. Pouring conditions call for a 4 inch slump, and Type I Portland cement with a sack weight of 94 lb/cu ft is to be used. Sand available in the area may be used, but it has been dried so that it will absorb 0.25% of its weight in water. Coarse aggregate with a maximum size of 3/4 inch is available in the area and will be used for the project. The 28 day strength requirement for this project is 5000 psi. Initially, a trial mix of 2 cubic yards is to be produced. Determine the amounts of water, cement, fine aggregate, and coarse aggregate that are needed for this trial mix.

Based on Table 1, the maximum water-to-cement ratio that should be used for the retaining wall in an area with mild temperatures should be selected on the basis of strength and workability requirements, but minimum cement content should not be less than 470 lb. per cubic yard. Based on Figure 1, the maximum water-to-cement ratio that will meet the compressive strength requirements of 5750 psi (5000 psi + 15%) is 0.50. Table 2 is used to determine the amount of each component required. Since the sand will absorb 0.25% of its weight in water, this amount of additional water must be added to the mix in order to have the necessary amount of water available for the hydration reaction. Note that Table 2 gives the amount of each component that is needed for 1 cubic yard of concrete. The amounts must be adjusted according to the amount of concrete needed. For example, 2 cubic yards are needed in this example, so the amounts in Table 2 must be multiplied by 2.

Table 1

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ACI Recommended Maximum Permissible Water-Cement Ratios

For Different Types of Structures and Degrees of Exposure.

Type of Structure Exposure ConditionSevere Temperatures

Freeze and Thawing

(Air-entrained only)

Mild Temperatures

Rainy, or arid

Air

Fresh

Water

Salt

Water Air

Fresh

Water

Salt

Water1. This section such as reinforced piles and pipe

0.49 0.44 0.40 0.53 0.49 0.40

2. Bridge decks 0.44 0.44 0.40 0.49 0.49 0.443. Very thin section such as curbs, architectural and all sections with less than 1 inch of concrete over reinforcement

0.49 - - 0.53 0.49 -

4. Moderate sections such as retaining wall, pier, beam

0.53 0.49 0.44 * 0.53 0.44

5. Concrete slab laid on ground 0.53 - - * - -6. Pavement 0.49 - - 0.53 - -

* Water-cement ratio should be selected on basis of strength and workability requirements, but minimum cement content should not be less than 470 lb. per cubic yard.

See Recommended Practice for Selecting Proportions for Concrete (ACI 613-54) for more details.

Table 2

Suggested Trial Mixes for Non-air-entrained Concrete

of Medium Consistency ( 3-4 inch slump).

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Water-Cement

Ratio lb/lb

Max Size

Aggregate

Air Content

Entrapped %

Water

lb/cu yd

of concrete

Cement

lb/cu yd

of concrete

Sand

% of

total

Sand

lb/cu yd

of concrete

Rock

lb/cu yd

of concrete

0.40 3/8

1/2

3/4

1

1 1/2

3

2.5

2

1.5

1

385

365

340

325

300

965

915

850

815

750

54

47

39

36

33

1350

1220

1080

1020

1000

1150

1400

1680

1830

19900.45 3/8

1/2

3/4

1

1 1/2

3

2.5

2

1.5

1

385

365

340

325

300

855

810

755

720

665

56

48

41

38

35

1440

1300

1160

1100

1080

1150

1400

1680

1830

1990

0.50 3/8

1/2

3/4

1

1 1/2

3

2.5

2

1.5

1

385

365

340

325

300

770

730

680

650

600

57

49

42

39

38

1570

1430

1270

1210

1180

1150

1400

1680

1830

19900.55 3/8

1/2

3/4

1

1 1/2

3

2.5

2

1.5

1

385

365

340

325

300

700

665

620

590

545

58

51

43

40

37

1570

1430

1270

1210

1180

1150

1400

1680

1830

1990

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Figure 1. Water-to-cement Ratio vs. Compressive Strength (psi) Data.

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