Early-age cracking can be a significant problem in concrete. Volume changes in concrete will drive tensile stress development when they are restrained. Cracks can develop when the tensile stress exceeds the tensile strength, which is generally only 10 percent of the compressive strength. At early ages, this strength is still developing while stresses are generated by volume changes. Controlling the variables that affect volume change can minimize high stresses and cracking.
Mechanisms of Early-Age Volume Change
The volume of concrete begins to change shortly after it is cast. Early volume changes, within 24 hours, can influence tensile stress and crack formation in hardened concrete.
Chemical shrinkage occurs due to the reduction in absolute volume of solids and liquids in the hydrating paste. Chemical shrinkage continues to occur as long as cement hydrates. After initial set, the paste resists deformation, causing the formation of voids in the microstructure.
Autogenous shrinkage is the dimensional change of cement paste, mortar, or concrete caused by chemical shrinkage (Figure 1). When internal relative humidity is reduced below a given threshold (i.e., extra water is not available), self-desiccation of the paste occurs, resulting in a uniform reduction of volume.
Figure 1 – Chemical shrinkage and autogenous shrinkage volume changes of fresh concrete. Not to scale.
Creep is the time-dependent deformation of concrete under sustained load. During early age, concrete creep is generally as much as three to five times higher than for mature concrete. Early load application due to construction forces or prestressing operations can therefore have a significant impact on total deformation. Furthermore, the magnitude of creep in tension is greater than in compression, and early tensile creep can be relied upon as a stress relaxation mechanism. Creep is influenced by drying or self-desiccation at early age, and this synergy is often referred to as the Pickett Effect, after Gerald Pickett, a Portland Cement Association researcher who discovered the phenomena in the 1940s (Pickett 1947).
Concrete, mortar, and cement paste will sometimes swell when sealed or in the presence of external water. Swelling is generally caused by pore pressure, but can be accentuated by the formation of some expansive hydration products. The swelling is not significant, between 50-100 millionths at early ages; therefore, we will not be discussing swelling further.
As cement hydrates, the reaction provides a significant amount of heat. In large elements, this heat is trapped and can induce significant expansion. When thermal changes are superimposed upon autogenous shrinkage at early age, cracking can occur. In particular, differential thermal stress can occur due to rapid cooling of massive concrete elements.
Testing of Early-Age Volume Changes
Chemical shrinkage test
Volume change due to chemical shrinkage can be estimated from the hydrated cement phases and their crystal densities or can be determined by physical test. The physical test places a measured amount of lime-saturated water in an open pipit over a known amount of cement paste inside a closed container. The change in water level within the pipit indicates the change in volume due to chemical shrinkage (Figure 2).
Figure 2 – Test for chemical shrinkage of cement paste showing flask for cement paste and pipit for absorbed water measurement.
ASTM C157 - Modified for Early-Age Shrinkage
The standard drying shrinkage test for concrete can be modified to capture early age volume change by elimination of the curing period (usually 7-28 days) and beginning measurements as early as possible. Prisms may also be sealed after casting to provide an estimate of autogenous shrinkage. Surfaces should be sealed as quickly as possible to eliminate loss of moisture. It should be emphasized that autogenous shrinkage depends on temperature history (maturity) and will be different in a laboratory prism when compared to a larger concrete member in service under variable ambient temperatures. As with drying shrinkage measurements, the test result will not represent the actual shrinkage in the structure.
ASTM C1581 – Restrained Ring Shrinkage
The restrained ring shrinkage test consists of a concrete ring specimen six inches tall, .5 inches thick and 13 inches diameter that is cast surrounding an instrumented steel ring (Figure 3). The steel ring prevents the concrete from shrinking from the time that the concrete is first cast. Shrinkage stresses continue to grow as the concrete passes from initial set to final set and beyond. Tensile creep relaxation alleviates stress development and is considered beneficial at early age. The instrumented ring uses strain sensors to monitor the development of stress. If the shrinkage in the concrete is significant, the stresses will eventually cause cracking. The strain sensors provide an indicator of the cracking time, which is used to compare the cracking tendency between different concrete mixtures.