Keywords: plastic shrinkage cracking, bleeding, evaporation of water, mix proportion, heat of hydration, surface temperature of concrete.

Plastic shrinkage cracking is normally noticed on flat concrete surfaces, such as slabs and pavements, when they are subjected to a very rapid loss of moisture caused by a number of factors, including air and concrete temperatures, relative humidity, and wind velocity, at the surface of the concrete. Combined, these factors can cause high rates of surface evaporation in either hot or cold weather. The rapid evaporation of surface water leads not only to a decreased degree of hydration in the surface, but also to shrinkage and accompanying shrinkage stresses in the freshly placed concrete. Regardless of the ultimate strength of the concrete, if sufficient tensile strength to overcome tensile stress due to the restraint provided by the concrete below the drying surface layer is not developed within the first few hours after placing, the occurrence of plastic shrinkage cracking is inevitable.

The drying that initiates this cracking begins at the precise moment when the rate of evaporation of water from the surface of the concrete exceeds the rate at which bleed water is supplied to the surface. Therefore, the bleeding phenomenon is an important factor that must also be taken into account when predicting the plastic shrinkage cracking of concrete. Therefore, an accurate prediction of cracking requires the consideration of changes in the concrete mixture, such as the water to cement ratio, unit water content, and fine aggregate to total aggregate ratio, because bleeding seems to depend largely on the properties of the concrete.

Until now, most research on plastic shrinkage cracking has been concentrated on experimental studies. Analytical studies on plastic shrinkage cracking are very rare because of the uncertainties and complex mechanism of the crack occurrence. ACI 305 merely assumes a value for the critical rate of evaporation, and it suggests that precautions against plastic shrinkage cracking are necessary if the rate of evaporation exceeds the critical value. However, since the bleeding characteristics of concrete are influenced by many factors, such as concrete mix and member size, the application of a single critical rate of evaporation to all concrete members without consideration of mix proportion and member size is apt to give an inaccurate prediction of plastic shrinkage cracking. Accordingly, this paper proposes an analytical model that can predict the occurrence of plastic shrinkage cracking on slabs by evaluating the balance between bleed water and bleed water evaporation.

First, based on correlation studies between experimental data and the numerical results obtained by using the analytical model for the bleeding of cement paste proposed by Tan et al. [1], it is verified that the analytical model can be used directly to calculate the bleed water of concrete without any modifications. Next, we propose a numerical model for the evaporation of bleed water that considers the variation of concrete temperature caused by hydration heat, and the validity of the model is verified by comparing the numerical results with the experimental data obtained by Berhane [2]. Finally, these two numerical models are used to analyze the bleeding of the concrete and to calculate the bleed water evaporation. The time at which the concrete surface begins to dry, which is obtained by the two analytical models, is compared with the time at which plastic shrinkage cracking occurs in the experiment of Almusallam et al. [3,4]. This correlation study allows us to conclude that the proposed analytical model can be effectively used to predict the occurrence of plastic shrinkage cracking.

1

Tan, T.S., Wee, T.H., Tan, S.A., Tam, C.T., Lee, S.L, "A Consolidation Model for Bleeding of Cement Paste", Advances in Cement Research, 1(1), 18-26, 1987.

2

Berhane, Z., "Evaporation of Water from Fresh Mortar and Concrete at Different Environmental Conditions", ACI Journal, 81(6), 560-565, 1984.

3

Almusallam, A.A., Maslehuddin, M., Abdul-Waris, M., Khan, M.M., "Effect of Mix Proportions on Plastic Shrinkage Cracking of Concrete in Hot Environments", Construction and Building Materials, 12(6/7), 353-358, 1998. doi:10.1016/S0950-0618(98)00019-1

4

Almusallam, A.A., "Effect of Environmental Conditions on the Properties of Fresh and Hardened Concrete", Cement and Concrete Composites, 23(4/5), 353-361, 2001. doi:10.1016/S0958-9465(01)00007-5

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