Shrinkage

Three main types of shrinkage have been held responsible for cracking; plastic shrinkage, hydration shrinkage and drying shrinkage.

Plastic Shrinkage

This occurs while the concrete is still plastic and before it has attained significant strength. Much of the early cracking of concrete results from this early drying. It occurs almost entirely on horizontal surfaces exposed to the atmosphere.

The cause of plastic shrinkage cracks is the rapid evaporation of water from the surface of the concrete. When the concrete is placed and vibrated, the solid ingredients settle and produce a layer of water at the surface. Under hot, dry and windy conditions, the surface water evaporates rapidly and the top surface of the concrete is placed in tension. Concrete at its early stage of setting has a negligible tensile strength and cracks are formed as a result to relieve the tension.

Hydration Shrinkage

Hydration proceeds after a cement is set. A hydrated cement gel which is responsible for the strength of the concrete is formed as a result of this hydration reaction. The hydrated gel has a smaller volume than the sum of the cement and the water from which it is formed. If the water lost by hydration is not replaced, shrinkage occurs. Shrinkage from this cause may be up to 10-4.; three quarter of this shrinkage takes place in the first three months after mixing. For instance: a concrete with modulus of elasticity at this stage of 30,000 N/mm2 would have a tensile stress due to shrinkage alone of 3 N/mm2. Since the ultimate tensile strength of the concrete was only 2.9 N/mm2, cracks appeared.

Drying Shrinkage

Drying shrinkage is defined as the contracting of a hardened concrete mixture due to the loss of capillary water. This shrinkage causes an increase in tensile stress, which may lead to cracking, internal warping, and external deflection, before the concrete is subjected to any kind of loading. All portland cement concrete undergoes drying shrinkage or hydral volume change as the concrete ages. The hydral volume change in concrete is very important to the engineer in the design of a structure. Drying shrinkage can occur in slabs, beams, columns, bearing walls, prestressed members, tanks, and foundations.

Drying shrinkage is dependent upon several factors. The shrinkage potential of a particular concrete is influenced by the amount of mixing, the elapsed time after the addition of water, temperature fluctuation, slumping, placement, and curing.

Other factors affecting drying shrinkage include:

• cement composition
• cement content
• quantity and quality of paste
• characteristics and amounts of admixtures used
• mineral composition
• maximum size aggregate
• mixture proportions
• size and shape of the concrete mass
• amount and distribution of reinforcing steel
• curing conditions
• humidity of surrounding air during the drying period
• length of the drying period

Shrinkage Cracking

Figure 1: Typical cases of internally loaded and time dependent strains caused by shrinkage

The elements that experiences shrinkage-induced cracks are floors, slabs, and pavements. Typical cases of dry shrinkage can be seen in figure 1. Both cases depend on the balance of time dependent stresses and the material properties. The amount and rate of shrinkage depends on the type of concrete and the surrounding temperatures. In Figure 1-a, the single crack has a crack width proportional to the dry shrinkage strain. In Figure 1-b, the concrete cracks are in a distribution patter due to the effect of the frictional bond between the sub-base. The most severe initial cracks, such as the one seen in Figure 1-a, are randomly located throughout the concrete, and filters to the surface from low external stress. These cracks are separate of each other and called primary cracks. However, under high external stress, additional cracks form between the primary cracks. These are secondary cracks. Furthermore, when the axial stresses increase and no more primary cracks form, the secondary cracks begin to widen. The contribution of drying shrinkage to concrete cracking can be controlled by calculating a proper mix design, proportioning the concrete member to minimize differential shrinking stresses, optimizing curing procedures, and proper use and application of joints.

Dry Shrinkage Warping

Shrinkage warping results in a time dependent curvature on a reinforced concrete section and a time dependent deflection of a reinforced concrete flexural member. The curvature is caused by the shrinkage internal force (D T). When the internal force cause bottom cracking, the section properties of concrete are reduced, allowing an increase in curvature. Therefore, the curvature due to drying shrinkage is greater on cracked sections than uncracked sections. The shrinkage stresses and strains on an uncracked section are shown in Figure 2.

Figure 2: Shrinkage stresses and strains on an uncracked section

Figure 3: Shrinkage stresses and strains on a fully cracked section

As the concrete shrinks, the tensile stress gradually increases, creating curvature and gradual warping of the beam. The amount of curvature depends on the size of the concrete member and the magnitude of the cracking.

Curing Influence on Shrinkage

Drying Shrinkage can take place over long periods of time. Some shrinkage has even been documented after twenty-eight years. See Figure 4.

Figure 4: Percentage of shrinkage versus time

This long-term shrinkage of concrete may be related to carbonation. The rate of drying shrinkage decreases quickly with time—
14-34 percent of the 20-year shrinkage occurs in 2 weeks
40-80 percent of the 20-year shrinkage occurs in 3 months
66-85 percent of the 20-year shrinkage occurs in 1 year.
(A.M. Neville, 382).

The greater the amount of hydrated cement means a smaller volume of unhydrated cement. This leads to a greater drying shrinkage over long curing periods. However, the cement paste becomes stronger with age and can absorb a fraction of its shrinkage without cracking. Drying shrinkage is also affected by the relative humidity of the medium surrounding the concrete. See Figure 5.

Figure 5: Shrinkage versus time for different relative humidities

The same figure shows that swelling is 6 times smaller than shrinkage in air of relative humidity of 70 percent, or eight times smaller than shrinkage in air at 50 percent (A.M. Neville, 383). Therefore, concrete placed in dry unsaturated air shrinks, but it also swells in water at 100 percent relative humidity.

Composition

The compositional makeup of concrete contributes directly to the drying shrinkage of concrete. Loss of moisture in the hydrated cement paste results in shrinkage. Different compositions and fineness of cements have variable effects on the shrinkage of cement paste. Difference in shrinkage is reduced significantly due to the adjustment of the amount of gypsum added to the different cement compositions. The size of aggregate is not as important, but has an indirect influence on the water content of concrete. Shrinkage decreases with the volumetric increase of aggregate concentration causing a linear relationship between free shrinkage and crack width. High density aggregates and high modulus of elasticity of aggregates will decrease the compressibility and increase the shrinkage of concrete. The use of admixtures may alter the hydration reaction, which results directly in a high increase of drying shrinkage.

Moisture

The concrete properties influence on drying shrinkage depends on the ratio of water to cementitious materials content, aggregate content, and total water content. The total water content is the most important of these. The relationship between the amount of water content of fresh concrete and the drying shrinkage is linear. Increase of the water content by one percent will approximately increase the drying shrinkage by three percent. Constant water to cementitious materials ratio coincides with changes in the amount of aggregate used.

Dry Environment

The amount of drying shrinkage depends on the environmental conditions; relative humidity, temperature, and air circulation. Concrete subjected to a dry atmosphere will, in most cases, have a greater drying shrinkage than if subjected to an alternative wetting and drying cycle. Lower temperatures generally produce a decrease in drying shrinkage because of higher humidity and slower evaporation.

References

(1) The California Producers Committee an Volume Change and Affiliated Technical Organizations, Drying Shrinkage of Concrete, California Producers Committee, Oakland California, March 1966.

(2) Neville, Adam M., Properties of Concrete, 3rd edition,
Pitman Publishing Inc.,Massachusetts, 1981.

(3) Gilbert, R. I., Time Effects in Concrete Structures, Elsevier Science Publishing
Company Inc., New York, 1988.

(4) Ropke, John C., Concrete Problems: Causes and Cures, McGraw-Hill Inc.,
New York, 1982

(5) American Concrete Institute Publication SP-30, Cracking, Deflection, and
Ultimate Load of Concrete Slab System, American Concrete Institute,
Michigan, 1971

(6) American Concrete Institute Publication SP-20, Causes, Mechanism, and Control
Cracking in Concrete, American Concrete Institute, Michigan, 1971