Concrete undergoes deformation due to the applied stress. But in addition to this, volume changes due to shrinkage, and temperature variations also take place. In practice these variations are of considerable importance as these movements partly or wholly are restrained and thus they introduce stress. These volume changes affect the long term strength and durability of concrete. Though shrinkage, swelling and thermal changes are categorised as independent of stress, but actually in practice it is not so simple.
The restraint caused to the volume changes may induce tensile stresses in the concrete, which may result in the development of cracks in the concrete as concrete is very week in tension. Cracks in concrete must be avoided as far as possible. If it is not possible to avoid cracks, steps should be taken to control and minimise them as they affect the durability and structural integrity. Cracks are also aesthetically undesirable.
One of the most objectionable defects of concrete is the presence of cracks, more specially in floors and pavements. Shrinkage is one of the most important factors which contribute to the development of cracks in the floor and pavements. It is very difficult to produce a concrete which does not shrink and crack, however the magnitude of shrinkage may be reduced. Shrinkage is an inherent-property of concrete. It needs the understanding of factors which influence the shrinkage of concrete.
Definition of Shrinkage:
Shrinkage may be defined as the volume charge in concrete due to loss of water or moisture due to evaporation or by hydration of cement or by carbonation. The reduction in volume i.e. volumetric strain is equal to 3 times the linear contraction. In practice the shrinkage is simply measured as linear strain. Its units are thus mm per mm, usually expressed in 10–6.
Differential Shrinkage of Concrete:
In addition to internal restraint to shrinkage by aggregate and reinforcement, some restraint also is induced from non-uniform shrinkage with in the concrete member itself. Moisture loss takes place at the surface of the concrete specimen, which is subject to differential shrinkage. This shrinkage is compensated by strain due to internal stresses. Tensile stresses occur near the surface and compressive stresses in the core. When drying takes place in an unsymmetrical manner, warping of concrete takes place.
The progress of shrinkage extends gradually from the drying surface into the interior of the concrete extremely slowly. Complete dryness at a depth of 75 mm was observed after one month but it took 10 years to reach a depth of 600 mm. Ross found the difference in shrinkage of a mortar slab at the surface and at a depth of 150 mm as 470 x 10–6 after 200 days.
If the modulus of elasticity of mortar is 21 GPa (3 x 106 PSI) the differential shrinkage would induce a stress of 10 MPa (1400 PSI). As the stress increases gradually it would be relieved by the creep, but even then surface cracking may occur. The increase in volume of aggregate would restrain the shrinkage considerably. Thus the advantage of using concrete rather the neat cement paste or mortar is clear.
Moisture Movement of Concrete:
We know that when concrete is allowed to dry in air at a lower humidity it shrinks and when it is kept in 100% relative humidity or placed in water, it swells. Just as drying shrinkage is an ever continuing process, similarly swelling is also a continuing process if placed in water or 100% humidity conditions. If a concrete specimen is allowed to dry in air at a given humidity and subsequently placed in water it will swell. It has been observed that during this process all initial drying shrinkage does not recover fully even after a prolonged storage in water.
For the usual range of concrete, the irreversible part of shrinkage is between 0.3 and 0.6 of the drying shrinkage, the lower value being more common. The absence of fully reversible behaviour may be due to the introduction of additional links with in the gel during the period of drying. During this period closer contact between the gel particles is established. In case, the cement paste has hydrated to a considerable degree, before drying, it will be less affected by the closer configuration of the gel in fully dry condition.
As drying shrinkage is due to the loss of adsorbed water around the gel particles, swelling is due to the adsorption of water by the cement gel.
The water molecules act against the cohesive force and push the gel particles away from each other, resulting in the swelling of concrete or paste. Further the ingress of water decreases the surface tension of the gel.
The property of swelling when placed in water or 100% humidity and shrinking when placed in dry conditions is known as moisture movement in concrete. Cement mortar mix, stored alternatively in water and then dried in air at 50% relative humidity. Fig. 16.14 shows the typical moisture movement of 1:1. Cement mortar mix, stored alternatively in water and then dried in air at 50% relative humidity.
The moisture movement in concrete induces compressive and tensile stresses alternately. These alternate compressive and tensile stresses may cause the fatigue in concrete which may reduce the durability of concrete. The typical values of moisture movement of concrete and cement mortar are shown in Table 16.2. Concrete dried at 50% relative humidity and immersed in water.
Thermal Movement of Concrete:
Cement concrete also has a positive coefficient of thermal expansion like other engineering materials.
The value of coefficient of thermal expansion for concrete depends on the following factors:
1. Composition of concrete.
2. Moisture condition of concrete at the time of temperature change.
1. Influence of Composition of Concrete:
The main constituents of concrete are cement paste and aggregate. The coefficients of thermal expansion for these two components are dissimilar. Thus the coefficient of thermal expansion of concrete is affected by these two values of thermal expansion and also the volumetric proportions of aggregate and cement paste and their elastic properties.
The role of aggregate in thermal movement is similar to that in creep and shrinkage i.e., the aggregate restraints the thermal movement of the cement paste, which has highest thermal coefficient of expansion. Coefficient of thermal expansion may be defined as the change in unit length per degree change in temperature.
αc. = coefficient of thermal expansion of concrete
αg = coefficient of thermal expansion of aggregate
αp = coefficient of thermal expansion of cement paste
g = volumetric content of aggregate
Kp/kg = stiffness ratio of cement paste to aggregate. This ratio is approximately equal to their moduli of elasticity ratio.
The value of coefficient of thermal expansion of cement paste varies from 11 x 10–6 to 20 x 10–6 per °C depending upon the moisture condition.
This dependence is due to the fact that thermal coefficient of expansion of cement consists of the following two components:
(a) Kinetic or True Thermal Coefficient:
This is caused by the molecular movement of the paste.
(b) Hygrothermal Expansion Coefficient:
This expansion coefficient occurs due to an increase in the internal relative humidity (water vapour pressure) as the temperature increases, resulting expansion of the cement paste. No hygrothermal expansion is possible when the paste is totally dry or when it is fully saturated as in both these situations no increase in water vapour pressure is possible. However hygrothermal expansion occurs at some intermediate moisture content as shown in Fig. 16.15.
However hygrothermal expansion occurs at some intermediate moisture content. It has been observed that for a green or young paste the maximum hygrothermal expansion takes place at 70% relative humidity. For an older paste the max hygrothermal expansion is smaller and occurs at a lower internal relative humidity. In concrete the hygrothermal expansion is also smaller.
The thermal coefficient of 1:6 concrete cured in air and at relative humidity of 64% and also for saturated concrete made with different types of aggregates is shown in Table 16.3.
The values of coefficient of thermal expansion of aggregates vary from 5.0 x 10–6 to 12 x 10–6 per °C. Lime stone and Gabbros have low values whereas gravel and quartzite have higher values of coefficient of thermal expansion.
The values of coefficient of thermal expansion near freezing temperature have been observed minimum. Further at temperatures lower than freezing point, the coefficient of thermal expansion is found higher again. Fig. 16.16 shows values of saturated concrete tested in saturated air. In another case, concrete slightly dried after a period of initial curing and then stored and tested at 90% relative humidity, showed no decrease in the coefficient of thermal expansion shown by dotted curve in Fig.16.16.
The coefficient of thermal expansion of concrete subjected to a temperature variation of less than 65°C. It has been observed that concrete subjected to higher temperatures shows somewhat different values of thermal expansion due to presumably less moisture content in the concrete. The importance of coefficient of thermal expansion is felt at higher temperatures when dealing with fire hazard. The values of coefficient of thermal expansion of concrete made with different types of aggregates at higher temperatures are shown in table 16.4.
Effect of Restraint and Cracking on Concrete:
In structural concrete the importance of shrinkage largely is related to cracking. We are concerned with the cracking tendency of the concrete as the initiation or start and absence of cracking depends not only on contraction but also its ability to expand, it strength and its degree of restraint to deformation that may cause cracking. The restraint in the form of reinforcing bars or gradient of stress increases its ability to expand (extensibility) as it allows concrete to develop strain well beyond the corresponding maximum stress.
The high extensibility of concrete is generally desirable as it permits concrete to withstand the greater volume changes. The Bureau of Reclamation carried out some thermal cycle tests on concrete at a constant strain and found that tension developed on cooling it.to the original temperature. The failing (tensile) stress was found lower in the concrete made with ordinary or modified cement than with low heat or Portland pozzolana cement which could withstand a greater temperature drop before failure. The crack pattern developed when stress is relieved by creep is shown in Fig. 16.17.
The crack pattern developed when stress is relieved by creep. The cracking can be avoided only if the stress induced by the free shrinkage strain reduced by creep is smaller than the tensile strength of concrete.
The effects of restraint and cracking may also be explained as follows:
As the stress and strain occur together, any restraint of movement, introduces a stress corresponding to the restrained strain, if this stress and the restrained strain are allowed to develop to such an extent that they exceed the strength or strain capacity of concrete, then creeks will develop.
Restraint can induce both compression and tension, but in the majority of the cases, only tension causes problem.
Restraints may be of the following two types:
1. External Restraint:
The external restraint exists when the movement in concrete member is fully or partly prevented by an external rigid or partly rigid adjacent member or foundations. The effect of external restraint can be explained by considering a section of a completely insulated concrete member whose ends are fully restrained This concrete section is subjected to a cycle of temperature i.e., the section is heated to a certain temperature and then cooled to its original temperature.
As the temperature rises and the concrete is prevented from expanding. In this condition the compressive stresses are developed in the concrete uniformly across the section. Usually these stresses are very small in comparison to the compressive strength of concrete. More over these stresses partly are relieved by creep at early ages. When the temperature falls and concrete cools, it is also prevented from contracting, at first any residual compressive stress is recovered and on further cooling tensile stress is developed.
If these temperature changes take place slowly, the stress partly would be relieved by creep. In case concrete is more mature at this stage, the magnitude of creep would be smaller and the tensile stress will become much larger to reach the present tensile strength of the concrete, resulting in cracking across the section. If sufficient reinforcement is present in the concrete in such a condition, cracks will still develop in the concrete section, but they will be well evenly distributed and narrower in width while in case of un reinforced concretes cracks will be few but wider in width.
2. Internal Restraint:
This type of restraint exists when temperature and moisture gradients exist within the concrete section. A combination of external and internal restraint is also possible.
The internal restraint is an un-insulated concrete mass as a dam, in which heat develops due to the hydration of cement. The heat is dissipated from the surface of the concrete resulting in a temperature gradient across the section. As no relative movement of any part of the concrete is possible, there is a restrained thermal strain. This thermal strain induces a stress.
Creeps contribute to thermal cracking with large, but slow temperature cycles due to the reduced effectiveness of stress relaxation by creep with time. However in other cases creep is beneficial in preventing cracking. For example if a thin concrete member is externally restrained, so that contraction due to shrinkage is prevented, the induced uniform elastic tensile stress is relieved by creep. The point where the tensile stress is relieved by creep, cracks will develop.
In case of thicker sections, with no external restraint, but having a moisture gradient, the shrinkage of the surface layer is restrained by the core of the section. In this case the compressive stress is developed inside the concrete member and tensile stress exists on its surface. Creep again relieves the stresses, but if tensile stress exceeds the current strength of the concrete, the shrinkage cracking will take place.
Types of Cracking in Concrete:
The intrinsic concrete cracks may be grouped as follows:
1. Plastic cracks
2. Early age thermal cracks
1. Plastic Cracks:
These cracks develop before the concrete has hardened i.e., between 1 to 8 hours of placement of concrete. As the evaporation of water takes place from the surface of the concrete, it contracts resulting inducement of tensile stresses in concrete causing cracks. These cracks are in the form of plastic shrinkage cracks and plastic settlement cracks. The plastic shrinkage cracks can be prevented by restricting the rate of evaporation of water from the surface to less than 0.5 kg/hour/m2.
2. Plastic Settlement Cracks:
These cracks develop due to the uneven settlement of concrete on bleeding due to the presence of obstructions. These obstructions may be in the form of reinforcing bars or un equal depth of concrete placed morolithically. The plastic settlement cracks may be reduced by using air entrainment admixtures as the air entrainment reduces bleeding of concrete. These cracks may also be eliminated by revibrating the concrete at a suitable time. The cause and location of different type of cracks is shown in Table 16.5.