The Strength Of Cement Concrete Is Due To Which Bond?

The Strength Of Cement Concrete Is Due To Which Bond
What is in This Stuff? The importance of concrete in modern society cannot be overestimated. 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, CaCO 3, 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. 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.

  1. This is called calcination,
  2. The third stage is called clinkering.
  3. During this stage, the calcium silicates are formed.
  4. The final stage is the cooling stage.
  5. The marble-sized pieces produced by the kiln are referred to as clinker,
  6. 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.

  1. The water causes the hardening of concrete through a process called hydration.
  2. Hydration is a chemical reaction in which the major compounds in cement form chemical bonds with water molecules and become hydrates or hydration products.
  3. Details of the hydration process are explored in the next section.
  4. 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 ceramic spheres perlite lightweight concrete which can be sawed or nailed, also for its insulating properties
lightweight expanded clay shale or slate crushed brick used primarily for making lightweight concrete for structures, also used for its insulating properties.
normal weight crushed limestone sand river gravel crushed recycled concrete used for normal concrete projects
heavyweight steel or iron shot steel or iron pellets used for making high density 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.

  1. Building of the molds and casting can occur on the work-site which reduces costs.
  2. Concrete is a non-combustible material which makes it fire-safe and able withstand high temperatures.
  3. It is resistant to wind, water, rodents, and insects.
  4. Hence, concrete is often used for storm shelters.
  5. 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.

Cement Compound Weight Percentage Chemical Formula
Tricalcium silicate 50 % Ca 3 SiO 5 or 3CaO, SiO 2
Dicalcium silicate 25 % Ca 2 SiO 4 or 2CaO, SiO 2
Tricalcium aluminate 10 % Ca 3 Al 2 O 6 or 3CaO, Al 2 O 3
Tetracalcium aluminoferrite 10 % Ca 4 Al 2 Fe 2 O 10 or 4CaO, Al 2 O 3, Fe 2 O 3
Gypsum 5 % CaSO 4,2H 2 O

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 Ca 3 SiO 5 + 7 H 2 O -> 3 CaO,2SiO 2,4H 2 O + 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 Ca 2 SiO 4 + 5 H 2 O-> 3 CaO,2SiO 2,4H 2 O + 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.

  1. The dormancy period can last from one to three hours.
  2. During this period, the concrete is in a plastic state which allows the concrete to be transported and placed without any major difficulty.
  3. This is particularly important for the construction trade who must transport concrete to the job site.
  4. It is at the end of this stage that initial setting begins.
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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. 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.

  1. Concrete’s strength may also be affected by the addition of admixtures.
  2. Admixtures are substances other than the key ingredients or reinforcements which are added during the mixing process.
  3. Some admixtures add fluidity to concrete while requiring less water to be used.
  4. 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.

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.

What are the factors that affect the strength of concrete?

(1) The Influence of Cement Strength Grades and Water-cement Ratio – Cement strength grades and water-cement ratio are the main factors impacting concrete strength. The chemically combined water needed in cement hydration generally account for 23% of the mass of cement. Figure 5.10, The Relationship between Concrete Strength and Water-cement Ratio It is proved that the smaller the water-cement ratio is, the higher the strength of cement will be, the higher the cohesive power will be, and the strength of concrete will be, under the same condition.

A large number of tests have proved that: at the age of 28d, the relationship between the concrete strength ( f cu,0 ), the actual strength of cement ( f ce ) and the water-cement ratio ( W / C ) is in line with the following formula: (5.3) f cu, 0 = α a ⋅ f ce C / W − α b In the formula: α a and α b are the regression coefficients.

If crushed stones are used, α a = 0.46 and α b = 0.07; if gravels are used, α a = 0.48 and α b = 0.33. In formula (5.3), if the actual strength of cement can be obtained, it can be calculated by the following formula: (5.4) f ce = γ c ⋅ f ce, g In the formula: f ce.g is the strength grade of cement (MPa); γ c is the safe coefficient of cement strength grade which should be determined by the actual statistics of various regions.

What is the strength of a base Bond in concrete?

Kinds of Strength in Concrete: –

  • Strength may be classified as follows:
  • 1. Compressive strength
  • 2. Tensile strength
  • 3. Shear strength, and

4. Bond strength.

  1. 1. Compressive Strength:
  2. For structural design the compressive strength is taken as the criterion of quality of concrete and the working stresses are prescribed as per codes in terms of percentages of the compressive strength as determined by standard tests.
  3. Compressive Tests:
  4. To determine the compressive strength of concrete following three types of specimens can be used:
  5. i. Cubes
  6. ii. Cylinders
  7. i. Cube Tests:

Generally specimens are cast in steel or cast iron moulds of 150 mm dimensions, which should confirm to cubical shape. The dimensions and planeness should be within the limits of tolerance. The mould should have rigid connection with base. The rigid connection with base is essential when the compaction is effected by means of vibration.

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This reduces the leakage of mortar. The cube is filled in three layers and compacted well either by vibration or standard tamping rod as per IS 516-1964. After compaction the top surface is made flush with edges of mould and the top surface finished by means of trowel. The finished surface is left undisturbed for 24 hours at a temperature of 66°F to 70°F and relative humidity not less than 90%.

After 24 hours, the mould is stripped and the specimen is stored in water for further curing. As far as possible the curing temperature should be maintained at 66°F to 70°F, usually these specimens are cured upto 28 days. The test should be carried out as per IS 516-1964.

  1. Ii. Cylinder Test: The standard cylinder is 15 cms in diameter and 30 cms height and is cast in a mould generally made of cast-iron or steel, preferably with a clamped base.
  2. Cylinder speci­mens are made as cubes specimens, but are com­pacted in three layers by a 16 mm diameter rod having one end of bullet shape.

The top surface of cylinder finished with a float is not smooth enough for testing and requires further prepa­ration. To overcome this difficulty capping of cylinders is done by cement paste or some other suitable material. Cylinders are used for the determination of compressive strength of con­crete in United States, France, Canada, Australia and New Zealand, while cubes are used in U.K.

Germany, India and Europe etc. The concrete strength is affected by the shape and size of the specimens, but high strength concretes is less affected than the low strength. Concrete Fig.14.3 shows the influence of height/diameter ratio on the strength of cylinder for different strength of concrete as suggested by Murdok and Kesler.

Failure of Compression Specimens : Compression test develops a more complex system of stresses. The compression load develops the lateral expansion in the test specimen (cube or cylinder) due to the Poisson’s ratio effect. The steel platens do not undergo the same lateral expansion as that concrete goes.

  1. Thus the steel restrains the expansion tendency of the concrete in the lateral direction.
  2. This restraint induces a tangential force between the end surfaces of the concrete specimen and the adjacent steel platens of the testing machine.
  3. It has been observed that lateral strain developed in the steel plates is only 0.4 times of the lateral strain developed in the concrete.

Thus the platens restrain the lateral expansion of the concrete in the part of the specimen near its ends. The degree of restraint exercised depends upon the friction actually develo­ped. In case the friction is eliminated by the application of any suitable greasing material as grease, graphite, or paraffin wax to the bearing surfaces, the specimen shows a greater lateral expansion, and eventually gets split along its full length.

Under normal conditions of the test, the elements within the specimen are subjected to shearing stresses as compressive stresses. The magnitude of shear stress decreases and the lateral expansion increases with the distance from the platens. Thus due to this restraint a cone of height of √3/2 d remain relatively undamaged in the specimen tested, where d is the lateral dimension of the specimen.

However if the length of the specimen is longer than 1.7 d, a part of it will be free from the restraining effect of the platens. Thus the specimens with lengths less than 1.5 d show considerably higher strength than those with greater lengths as shown in Fig.14.4.

Fig.14.4 shows the general pattern of the influence of the height diameter ratio on the compressive strength of cylinder. For values of H/D ratio smaller than 1.5, the measured strength increases rapidly due to non-restraining effects of the platens of the testing machine. For H/D between 1.5 to 4.0 strength variation is very little, and for H/D between 1.5 and 2.5 the variation of strength is within 5% of H/D ratio 2.0.

For H/D ratio above 5, strength falls rapidly. Hence the choice of H/D ratio of 2 is suitable. Comparison of Cube and Cylinder Strengths : Experimental results have shown that there is no simple relation between the cylinder and cube strength of the same concrete.

The ratio of cylinder/cube strength depends on the level of strength of concrete and is higher for high strength concrete. However for simplicity IS 516-1964 has suggested this ratio as 0.80. Table 14.1 below shows that this ratio varies from 0.77 to 0.96 in an irregular manner. The results are based on Evan’s work.

For 1000 kg/cm 2 strength concrete this ratio becomes 1.0.2. Tensile Strength: Concrete being a brittle material is not expected to resist direct tensile forces. However tension is of importance with regard to cracking, which is a tensile failure. Most of the cracking is due to the restraint of contraction induced by drying shrinkage or lowering of temperature.

  1. The tensile strength of concrete varies from 7% to 11% of the compressive strength but on average it is taken as 10% of compressive strength.
  2. Further it has been observed that higher the compressive strength, lower the relative tensile strength.
  3. The maximum tensile strength of concrete has been found of the order of 42.0 kg/cm 2,

Some researchers have observed that the type of coarse aggregate has a greater relative effect on tensile strength than on compressive strength. Generally for concrete quality control, tensile test is never made. However to have an idea of tensile strength an indirect method known as splitting test is applied.3.

Shear Strength: Shear is the action of two equal and opposite parallel forces applied in planes a short distance apart. Shear stress cannot exist without accompanying tensile and compressive stresses. Pure shear can be applied only through torsion of a cylindrical specimen in which case the stresses are equal in primary shear.

Secondary tension (maximum at 45° to shear) and secondary compression (maximum at 45° to shear, per­pendicular to tension). As the concrete is weaker in tension than in shear, failure in tension invariably occurs in diagonal tension. Direct determination of shear is very difficult.

Hence researchers have assumed the shear strength of concrete about 12% of the compressive strength.I.S.456-1978 has suggested the values of shear as follows: 4. Bond Strength: It can be defined as the resistance to slipping of the steel reinforcing bars which are embedded in concrete. This resistance is provided by the friction and adhesion between the concrete and steel friction between concrete and the lugs of deformed bars.

It is also affected by the shrinkage of concrete relative to the steel. Bond involves not only the properties of concrete, but also mechanical properties of steel and its position in the concrete member. In general bond strength is approximately proportional to the compressive strength of concrete upto about 200 kg/cm 2,

  • For higher strengths of concrete, the increase in bond strength becomes progressively smaller.
  • In the initial stages of failure (slip) the bond strength depends on the magnitude and uniformity of lateral pressure that exists or may be developed between steel and surrounding concrete.
  • Bond strength varies considerably with the type of cement, admixtures and water cement ratio i.e., on quality of paste.

It is not affected by air entrainment. Further it has been observed that bond strength increases with delayed vibration. It is higher for dry concrete than for wet concrete. Its value reduces at high temperatures. At 200°C to 300°C (400°F to 570°F) bond strength has been found 50% of the bond strength at room temperature.

Bond strength is also reduced by alternations of wetting and drying, freezing and thawing etc. Its value usually is determined by pull out test. The bond strength for deformed bars may be taken 40% more than ordinary bars of the same diameter. Bond strength is also a function of specific surface of gel.

Cement having higher percentage of C 2 S will give higher surface of gel, giving higher bond strength. On the other hand cement having higher per­centage of C 3 S or concrete cured at higher temperature gives smaller value of specific surface of gel, resul­ting in lower bond strength.

  • It has been observed that concrete cured at high pressure steam produces gel of about 1/20th specific surface of the gel surface produced at normal curing temperature.
  • Thus the bond strength of high pressure steam cured concrete is lower.
  • The values of bond strength are shown in Table 14.4.
  • As suggested by IS 456-2000.

All values are in N/mm 2 :

What do you mean by strength of concrete?

Strength of Concrete: Nature, Kinds and Factors | Concrete Technology The strength can be defined as the ability to resist force. With-regard to concrete for structural pur­poses it can be defined as the unit force required to cause rupture. Strength is a good index of most of the other properties of practical importance.

What is the maximum tensile strength of concrete?

Kinds of Strength in Concrete: –

  • Strength may be classified as follows:
  • 1. Compressive strength
  • 2. Tensile strength
  • 3. Shear strength, and

4. Bond strength.

  1. 1. Compressive Strength:
  2. For structural design the compressive strength is taken as the criterion of quality of concrete and the working stresses are prescribed as per codes in terms of percentages of the compressive strength as determined by standard tests.
  3. Compressive Tests:
  4. To determine the compressive strength of concrete following three types of specimens can be used:
  5. i. Cubes
  6. ii. Cylinders
  7. i. Cube Tests:
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Generally specimens are cast in steel or cast iron moulds of 150 mm dimensions, which should confirm to cubical shape. The dimensions and planeness should be within the limits of tolerance. The mould should have rigid connection with base. The rigid connection with base is essential when the compaction is effected by means of vibration.

  • This reduces the leakage of mortar.
  • The cube is filled in three layers and compacted well either by vibration or standard tamping rod as per IS 516-1964.
  • After compaction the top surface is made flush with edges of mould and the top surface finished by means of trowel.
  • The finished surface is left undisturbed for 24 hours at a temperature of 66°F to 70°F and relative humidity not less than 90%.

After 24 hours, the mould is stripped and the specimen is stored in water for further curing. As far as possible the curing temperature should be maintained at 66°F to 70°F, usually these specimens are cured upto 28 days. The test should be carried out as per IS 516-1964.

  1. Ii. Cylinder Test: The standard cylinder is 15 cms in diameter and 30 cms height and is cast in a mould generally made of cast-iron or steel, preferably with a clamped base.
  2. Cylinder speci­mens are made as cubes specimens, but are com­pacted in three layers by a 16 mm diameter rod having one end of bullet shape.

The top surface of cylinder finished with a float is not smooth enough for testing and requires further prepa­ration. To overcome this difficulty capping of cylinders is done by cement paste or some other suitable material. Cylinders are used for the determination of compressive strength of con­crete in United States, France, Canada, Australia and New Zealand, while cubes are used in U.K.

Germany, India and Europe etc. The concrete strength is affected by the shape and size of the specimens, but high strength concretes is less affected than the low strength. Concrete Fig.14.3 shows the influence of height/diameter ratio on the strength of cylinder for different strength of concrete as suggested by Murdok and Kesler.

Failure of Compression Specimens : Compression test develops a more complex system of stresses. The compression load develops the lateral expansion in the test specimen (cube or cylinder) due to the Poisson’s ratio effect. The steel platens do not undergo the same lateral expansion as that concrete goes.

Thus the steel restrains the expansion tendency of the concrete in the lateral direction. This restraint induces a tangential force between the end surfaces of the concrete specimen and the adjacent steel platens of the testing machine. It has been observed that lateral strain developed in the steel plates is only 0.4 times of the lateral strain developed in the concrete.

Thus the platens restrain the lateral expansion of the concrete in the part of the specimen near its ends. The degree of restraint exercised depends upon the friction actually develo­ped. In case the friction is eliminated by the application of any suitable greasing material as grease, graphite, or paraffin wax to the bearing surfaces, the specimen shows a greater lateral expansion, and eventually gets split along its full length.

Under normal conditions of the test, the elements within the specimen are subjected to shearing stresses as compressive stresses. The magnitude of shear stress decreases and the lateral expansion increases with the distance from the platens. Thus due to this restraint a cone of height of √3/2 d remain relatively undamaged in the specimen tested, where d is the lateral dimension of the specimen.

However if the length of the specimen is longer than 1.7 d, a part of it will be free from the restraining effect of the platens. Thus the specimens with lengths less than 1.5 d show considerably higher strength than those with greater lengths as shown in Fig.14.4.

  • Fig.14.4 shows the general pattern of the influence of the height diameter ratio on the compressive strength of cylinder.
  • For values of H/D ratio smaller than 1.5, the measured strength increases rapidly due to non-restraining effects of the platens of the testing machine.
  • For H/D between 1.5 to 4.0 strength variation is very little, and for H/D between 1.5 and 2.5 the variation of strength is within 5% of H/D ratio 2.0.

For H/D ratio above 5, strength falls rapidly. Hence the choice of H/D ratio of 2 is suitable. Comparison of Cube and Cylinder Strengths : Experimental results have shown that there is no simple relation between the cylinder and cube strength of the same concrete.

  1. The ratio of cylinder/cube strength depends on the level of strength of concrete and is higher for high strength concrete.
  2. However for simplicity IS 516-1964 has suggested this ratio as 0.80.
  3. Table 14.1 below shows that this ratio varies from 0.77 to 0.96 in an irregular manner.
  4. The results are based on Evan’s work.

For 1000 kg/cm 2 strength concrete this ratio becomes 1.0.2. Tensile Strength: Concrete being a brittle material is not expected to resist direct tensile forces. However tension is of importance with regard to cracking, which is a tensile failure. Most of the cracking is due to the restraint of contraction induced by drying shrinkage or lowering of temperature.

The tensile strength of concrete varies from 7% to 11% of the compressive strength but on average it is taken as 10% of compressive strength. Further it has been observed that higher the compressive strength, lower the relative tensile strength. The maximum tensile strength of concrete has been found of the order of 42.0 kg/cm 2,

Some researchers have observed that the type of coarse aggregate has a greater relative effect on tensile strength than on compressive strength. Generally for concrete quality control, tensile test is never made. However to have an idea of tensile strength an indirect method known as splitting test is applied.3.

Shear Strength: Shear is the action of two equal and opposite parallel forces applied in planes a short distance apart. Shear stress cannot exist without accompanying tensile and compressive stresses. Pure shear can be applied only through torsion of a cylindrical specimen in which case the stresses are equal in primary shear.

Secondary tension (maximum at 45° to shear) and secondary compression (maximum at 45° to shear, per­pendicular to tension). As the concrete is weaker in tension than in shear, failure in tension invariably occurs in diagonal tension. Direct determination of shear is very difficult.

Hence researchers have assumed the shear strength of concrete about 12% of the compressive strength.I.S.456-1978 has suggested the values of shear as follows: 4. Bond Strength: It can be defined as the resistance to slipping of the steel reinforcing bars which are embedded in concrete. This resistance is provided by the friction and adhesion between the concrete and steel friction between concrete and the lugs of deformed bars.

It is also affected by the shrinkage of concrete relative to the steel. Bond involves not only the properties of concrete, but also mechanical properties of steel and its position in the concrete member. In general bond strength is approximately proportional to the compressive strength of concrete upto about 200 kg/cm 2,

  1. For higher strengths of concrete, the increase in bond strength becomes progressively smaller.
  2. In the initial stages of failure (slip) the bond strength depends on the magnitude and uniformity of lateral pressure that exists or may be developed between steel and surrounding concrete.
  3. Bond strength varies considerably with the type of cement, admixtures and water cement ratio i.e., on quality of paste.

It is not affected by air entrainment. Further it has been observed that bond strength increases with delayed vibration. It is higher for dry concrete than for wet concrete. Its value reduces at high temperatures. At 200°C to 300°C (400°F to 570°F) bond strength has been found 50% of the bond strength at room temperature.

  1. Bond strength is also reduced by alternations of wetting and drying, freezing and thawing etc.
  2. Its value usually is determined by pull out test.
  3. The bond strength for deformed bars may be taken 40% more than ordinary bars of the same diameter.
  4. Bond strength is also a function of specific surface of gel.

Cement having higher percentage of C 2 S will give higher surface of gel, giving higher bond strength. On the other hand cement having higher per­centage of C 3 S or concrete cured at higher temperature gives smaller value of specific surface of gel, resul­ting in lower bond strength.

It has been observed that concrete cured at high pressure steam produces gel of about 1/20th specific surface of the gel surface produced at normal curing temperature. Thus the bond strength of high pressure steam cured concrete is lower. The values of bond strength are shown in Table 14.4. as suggested by IS 456-2000.

All values are in N/mm 2 :