Drying shrinkage occurs when water begins evaporating from the exposed surface and the moisture differential along the depth of the slab causes strain which induces tensile stresses.
The driving force for drying shrinkage is the evaporation of water from the hydrated cement paste which is exposed to air with a relative humidity lower than that within the capillary pores and the hydrated paste. This driving force depends on the size and shape of the pores and their continuity. It also depends on the relative humidity of the ambient air. The loss of water causes a decrease in volume and consequently causes drying shrinkage.
The effects of drying shrinkage are quite complicated. Their cases and preventive measures have often given rise to conflicting views and opinions on account of the non-availability of test data. In many structures drying shrinkage cracks are unavoidable and therefore the concrete requires properly spaced predetermined joints.
Water in Hydrated Cement Paste
Water exists in hydrated cement paste in different states. This classification is based on the degree of difficulty or ease with which water from hydrated cement paste can be removed. The different states in which exists in hydrated cement paste are as follows:
Capillary Water: The water present in capillary pores larger than 0.05 microns is generally free water, free from the influence of attractive forces exerted by solid surfaces. Removal of free water does not cause any volume change and therefore will not cause shrinkage.
However, water present in the capillary pores smaller than 0.05 micron is held by capillary tension and therefore its removal can cause shrinkage.
Absorbed Water: Water close to the solid surface is physically held by hydrogen bonding under the influence of attractive forces. Water is physically adsorbed onto the solid surface of hydrated cement paste. The bonding energy of water molecules decreases with increasing distance from the solid surface. A major part of this water can be lost because of the drying of hydrated cement paste. This loss of absorbed water is mainly accountable for the shrinkage which eventually results in the cracking of concrete.
Interlayer Water: The hydrated cement paste water layers are present between the layers of calcium-silicate hydrate structure (C-S-H). This water is strongly held by hydrogen bonding. The water is removed by hydrated cement paste only when strong drying takes place. This causes the C-S-H structure to shrink considerably due to the loss of water between its layers.
Chemically Combined Water: This water is an integral part of the hydrated product formed due to the chemical reaction between water and cement. This water is not lost on drying and therefore in no way is responsible for shrinkage.
Dimensional Stability Due to Drop in Relative Humidity
The saturated hydrated cement paste is not dimensionally stable. At 100% relative humidity there is no loss of water and hence no dimensional change occurs. As soon as relative humidity falls below 100% the free water held in large capillary pores begins to escape in the atmosphere. Since the free water has no physical or chemical bonding its loss would not be accompanied by shrinkage.
Further drop in relative humidity can cause loss of absorbed water or interlayer water. The loss of water through smaller capillary pores and loss of water which is physically bonded with the solid surfaces or between the solid layers needs a stronger driving force. Since water in small capillaries exerts hydrostatic tension its removal causes induction of compressive stress on the solid walls of the capillary pores or layers resulting in contraction of the mass or shrinkage.
Causes of Drying Shrinkage in Concrete
Both drying shrinkage and creep originate from the same source i.e. the C-S-H paste. It is seen from the above that loss of physically absorbed water from C-S-H results in shrinkage strain. When hydrated cement paste is subjected to sustained stress the C-S-H will lose a large amount of physically absorbed water depending on the magnitude and duration of the applied stress and the paste will show a creep strain.
The removal of absorbed water is the main cause of both drying shrinkage and creep strains in concrete. The difference is that drying shrinkage is mainly due to differential relative humidity between the concrete and it is surrounding while creep is mainly due to sustained applied stress.
The factors affecting drying shrinkage are as follows:
Material and Mix Proportions
The concrete mixes with high cement content undergo rapid hydration and have a high volume of hydrated cement paste. Therefore, a reduction in the volume of the hydrated cement paste system is also largely due to the drying of water. This results in a greater amount of drying shrinkage and cracking. Besides, for concrete mixes with a low water-cement ratio, water loss from the surface should be prevented. Wet curing needs to be carried out at a very early age. Curing using a membrane (curing compound) for low W/C ration mixes is not advisable. Wet curing is a must in the initial period. Length of curing will depend on the type of structure and circumstances under which it is concreted. Drying shrinkage causes tensile stresses which if exceed the tensile strength of the concrete will develop cracks.
Rich concrete mixes with low water to cement ratio (W/C) show greater shrinkage. Concrete with the lowest water to cement ratio has the highest one-year shrinkage regardless of initial moist curing time. While the concrete with the highest water to cement ratio has the lowest one-year shrinkage even with the least curing.
This is because concrete with high W/C and therefore low cement content will have the least amount of hydrated cement paste (C-S-H) which is mainly responsible for drying shrinkage. Besides high W/C ratio would also mean a greater amount of aggregate in the concrete mix which results in greater resistance to drying shrinkage.
It must be noted that the influence of cement and water content in concrete on drying shrinkage is not direct. It is more dependent on cement paste volume which undergoes moisture-dependent deformations causing shrinkage. However, for given cement content if there are an increase in W/C, the drying shrinkage increases.
Modulus of Elasticity of the Aggregate
Studies have indicated that the most important property of aggregate which influences drying shrinkage is the modulus of elasticity of the aggregate. Aggregates with high elastic modulus show lower shrinkage than aggregates with low elastic modulus. The porosity of aggregates also influences shrinkage. If the porosity of aggregates is higher drying shrinkage is also high.
Fineness and Composition of Portland Cement
The fineness and composition of Portland Cement may affect the rate of hydration or rate of gain of strength but not the volume and the characteristics of the hydrated paste. Studies have shown that normal changes in fineness or composition which tend to influence drying shrinkage behavior on small specimens of cement paste or mortar have a negligible effect on concrete. It is therefore important to note that cement, independent of its fineness or grade/strength, has negligible influence or no influence on drying shrinkage. The quantity of cement required in concrete is less if high strength cement is used instead of cement of lower strength for identical concrete strength and durability requirements. The use of high-strength cement results in a lesser quantity of hydrated cement paste and therefore chances of drying shrinkage are reduced.
Granulated Slag and Pozzolanas
These mineral admixtures tend to increase the volume of fine pores in cement hydration products. Since drying shrinkage directly depends on the water held by small pores in the range of 0.003 to 0.02 microns, concretes containing such admixtures which are capable of pore refinement usually show higher shrinkage.
Water Reducing Admixtures
Water-reducing admixtures cause better dispersion of cement particles in water. This also leads to pore refinement and hence shows higher shrinkage. Generally, water-reducing admixtures at low dosages are higher than 2% by weight of cement then there could be a considerable influence on the shrinkage of concrete.
The loss of water from concrete reduces with age because the hydration process continues and free water within its capillary pores is consumed with time. It is therefore seen that drying shrinkage to increases rapidly with time initially. At a later stage, there is a very gradual increase.
Long-term drying shrinkage has been studied for several years. Troxell and others studied drying shrinkage for 20 years for a wide range of concrete mix proportions, aggregate types, and environmental and loading conditions. Their findings are tabulated below.
|Period (Days)||Percentage of 20 years Drying Shrinkage|
|14||20 to 25|
|90||50 to 60|
|365||75 to 80|
However, the percentage of drying shrinkage at different periods will also greatly depend on the dimensions of the structure, their shape, condition of the exposed surface, and the relative humidity in the environment.
Atmospheric humidity has a great influence on the long-term drying shrinkage of concrete as it has a direct impact on the relative rate of flow of moisture. When environmental conditions are very humid then the moisture flow slows down considerably. When the environmental conditions are dry the moisture flow increases causing rapid drying shrinkage. The international recommendations for the Design and Construction of Concrete Structures, CEB/FIP, 1970 have published graphs which show drying shrinkage in microstrain versus relative humidity in percentage.
The microstrains at different relative humidity are given below:
|Relative Humidity (%)||Dry Shrinkage (Microstrain)|
Geometry of Concrete Member
The rate of water loss is proportional to the length of the path traveled by the water from the interior of the concrete to the atmosphere. Ultimate drying shrinkage will depend on this length which is generally expressed as effective or theoretical thickness. This thickness is defined as the area of the section divided by half the perimeter in contact with the atmosphere.
It must be noted that some structures show faster drying than others by virtue of their shape and location. Floor slabs dry much slower than slabs exposed on both sides or beams exposed on four sides.
The curling of concrete slabs is a consequence of drying shrinkage. The slab top surface shrinks due to drying but the slab bottom does not dry easily or takes a longer time. This shrinkage causes the top of the slab to become shorter than the bottom and results in curling upwards. The corners curl more than the sides curl because at a corner the curling is a function of the shrinkage along both the sides nearby it. This shrinkage causes the slab edges to be lifted from the sub-base and the weight of the concrete near the edges causes an uplifting force at the slab center. Curling stresses cause cracking because of loss of subgrade support under the curled portion.
Restraint of drying shrinkage is yet another cause of cracking of the concrete surface. Slab on grade rests on a sub-base and in contact with it forms a mechanical bond. When the slab tries to shorten or curl up the sub-base opposes the movement which is directly proportional to the frictional resistance between the sub-base and the slab concrete. This resistance induces stress in concrete. If this stress exceeds the stress carrying capacity of concrete, cracking of slab corners can occur.
The preventive/remedial measures needed to reduce or prevent drying shrinkage and as a consequence cracking, require to be carefully examined. It is frequently observed that whenever efforts are made to improve one property of concrete it affects other properties of concrete.
For improving the concrete durability and performance, concrete requires pore refinement or must have a large volume of hydrated paste. This is done by increasing the cement content and reducing the water to cement ratio. On one hand, concrete’s permeability is considerably reduced due to the above, and on the other hand tendency for drying shrinkage cracks to occur increases. The large volume of hydrated cement paste can increase drying shrinkage in high-performance concrete. As the volume of the cement paste is large, loss of water from within can cause a considerable reduction in the volume of the concrete mass. This makes the concrete with high cement content and/or low water to cement ratio more prone to drying shrinkage cracking.
Some remedial measures required to be taken for reducing drying shrinkage are listed below.
Reduce Cement Content
This should be done by using as little quantity of cement as possible without sacrificing strength or durability requirements. Contrary to the general belief high strength cement is used can result in a reduction of drying shrinkage due to a reduction in cement quantity or cement hydrated paste.
Reduce Water Content
This should be done by using less quantity of water in the mix without sacrificing the workability. This can be acquired by utilizing appropriate admixtures compatible with cement & other materials being utilized in the concrete mix.
If it isn’t possible to reduce cement content & water content beyond a certain quantity then curing has to be improved. Curing has to be continuous and for a longer duration than seven days especially when the exposed surfaces are large and moisture gradient between the opposite faces is likely to occur as in the case of concrete roads, pavement slabs & slabs on grade.
In rich mixes, rapid hydration of cement can take place and as the cement hydrated paste is large moisture loss from within the hydrated paste can cause excessive drying shrinkage and crack. Curing has to be initiated as early as possible and should be continuous throughout the curing period to prevent any moisture loss of concrete.
Quality of Aggregates
Aggregates that have high elastic modulus & low porosity are preferred.
Mineral and Chemical Additives
Mineral admixtures like flyash, slag, or micro silica result in the formation of fine pores in cement hydration products this, in turn, increases the chances of drying shrinkage. This is also observed in the case of chemical admixtures which disperse the cement particles and cause pore refinement.
Improvement in curing will greatly help whenever such additives are used to improve the durability of concrete in an aggressive environment.
In slabs on grade contraction joints are required to be provided to prevent drying shrinkage cracks occurring at random causing unsightly conditions and also resulting in concrete disintegration around the cracked surface.
If a continuous strip method of construction is used, contraction joints are induced by forming a groove in the plastic concrete or by sawing in the hardened concrete within 74 hrs. of finishing the slab.
The spacing of the joints is of utmost importance and is generally dependent on the thickness of the slab. Usually, contraction joints spacing is specified to be 2 to 3 times the slab thickness or 12 to 18 feet for 6 inches slabs. This rule-of-thumb may be too lenient. However, experience has indicated that if the joints are spaced 4.5 m, regardless of slab depth, cracking will be minimal. In road pavements with 30 cm thickness, the joint spacing is generally restricted to 4.5 m. If concrete is expected to shrink more than normal, the joints should be closer/nearer. If concrete is expected to shrink lesser than normal, the joints should be farther apart.