Chapter 3
Early Age Concrete
Early age concrete is defined as concrete from the time of final set to the time before the concrete achieves its characteristic strength. Usually the ages before 28 days are considered early ages. The problems of early age concrete, which are described in this chapter are:
-temperature cracking
-autogenous shrinkage.
3.1 Temperature Cracking
Temperature cracking is one of the important problems for pouring big volume concrete structures especially if the placing duration of the concrete cannot be made slowly. The work of pouring large volume of concrete is usually referred to as mass concrete work.
In massive concrete structures such as dams and mat foundations, temperature gradients occur inside the concrete due to heat of hydration. Consequently thermal stresses are developed. Since concrete possesses a low thermal conductivity, the produced heat is accumulated in the concrete. The accumulated hydration heat with temperature gradient can induce cracks especially at early age of concrete.
Cracks are formed at the early age state because heat is mostly generated and accumulated in the mass concrete causing maximum temperature at the early age state. Another factor is that strength of the concrete at the early age state is still low.
Thermal cracking reduced quality and durability of concrete and effects the serviceability of concrete structures is long term. Therefore, if is necessary to prevent the early-age thermal cracking to ensure that concrete structure can achieve its design service life.
3.1.1 Mechanisms
1) Cracking due to self-restraint within the concrete mass
The reactions between compounds in cement and water, or called hydration reactions, are exothermic reactions. The reactions generate heat and the rate of heat generation is rapid at a few hours after mixing the water with cement until a few days. The rate of heat generation slows down after a few days. As concrete has low thermal conductivity especially at the early ages, the generated heat slowly transfers to the outside environment and then accumulates in the concrete causing high temperature inside the concrete mass.
The temperature gradient will be more serious when rate of concrete placing is faster, the concrete section is larger and thicker, wind is stronger and relative humidity is lower.
The temperature difference within the concrete mass will create differential volume change which results in self-equilibrating stresses within the mass of concrete.
If the developed tensile stress due to the self-restraint becomes higher than the tensile strength, cracks will developed at the location of the highest temperature ingredient which is usually on or near the exposed surfaces.
The self-restraint cracking usually happens is the mass concrete during the temperature-rising period. The cracking patterns are usually random like map cracking. Fig.3.1 shows an example of the cracks observed on the surface of a concrete footing about a few days after casting. The cracks are thought to be temperature cracks, but not plastic shrinkage cracks, because curing of the footing had been done properly.