Understanding some of the most common concrete volume changes.
By Kayla Hanson, P.E.
Since concrete is typically restrained by foundations, reinforcement or connecting elements, volume changes can cause significant stresses. Concrete volume changes begin immediately as cement hydrates and continues throughout the life of the product. Understanding the nature of these changes and the effects of curing, temperature changes, applied loads, and organic and chemical factors helps eliminate detrimental or irreversible volume changes.
Chemical shrinkage, a result of cement hydration, is the reduction in absolute volume of cement paste. The total volume of constituent paste materials before mixing and hydration is greater than the volume after hydration. The process begins when cement grains come in contact with water and continues at a decreasing rate beyond initial set. Some forms of shrinkage are evidenced by subsidence and autogenous shrinkage prior to initial set.
The settlement of solids relative to liquids, air voids or water rising to the surface as aggregates settles causes subsidence, or settlement shrinkage. Subsidence is considered minor when concrete is appropriately consolidated and bleeding is kept to a minimum. The use of air entrainment and appropriate quantities of fine materials, in addition to a low water-cementitious material ratio, can reduce subsidence and resultant cracking.
The severity of autogenous, or internal, shrinkage is related to the presence of external curing water. The additional moisture applied through moist curing methods helps replenish the liquid lost to evaporation and during hydration. When moist curing methods aren’t used, hydration reactions are only able to consume pore water. This dehydrates the cement paste and intensifies the severity of volume loss and the extent of autogenous shrinkage.
Air voids are also a result from cement hydration. Voids begin accumulating at the beginning of hydration, progress through initial set and continue beyond final set. The reduction in relative volume, combined with the development of void space, makes up the total change in the concrete’s absolute volume caused by chemical shrinkage.
Another form of volume reduction in fresh concrete is plastic shrinkage. This is typically evidenced by rough-edged tears or cracks in the surface of concrete. Plastic shrinkage is caused when surface water evaporates faster than bleed water can travel to exposed surfaces. Like autogenous shrinkage, moist curing methods reduce plastic shrinkage.
Drying shrinkage, another common cause of concrete cracking, can cause a significant amount of stress within concrete. The stress intensifies as the level of restraint increases. As it cures and loses moisture to hydration and evaporation, concrete transfers stress to reinforcement and embedded objects. Unless special curing procedures are applied, the innermost portion of concrete retains unreacted mix water longer than the surrounding concrete. Interior concrete applies additional stress to the setting, drying concrete, which can result in cracks as it reduces in volume. Monitoring and adjusting curing procedures and moisture levels as needed can help control concrete volume reduction caused by drying. The most significant factor is the amount of water used per unit volume of fresh concrete.
Moisture lost through chemical shrinkage can be replenished by external water sources, referred to as swelling. Cement paste and concrete are able to absorb external water in an attempt to equalize concentrations on either side of the concrete surface. In this process, some of the absorbed water is held in capillaries and pores while another portion is consumed through hydration, which results in additional crystal growth. Removal of the external water source enables autogenous and drying shrinkage, which reverses swelling and the temporary increase in absolute volume caused by water absorption.
Applied loads can also cause an increase in the relative volume of concrete. Although there is an initial, immediate deformation when the load is applied, concrete continues to deform at a decreasing rate for the duration of the loading. This deformation is called creep. The curing method used before application of the load can affect the magnitude of creep. Although steam curing has been shown to reduce concrete creep, this method has a smaller impact on reducing drying shrinkage.
Thermal expansion and contraction
Some of the most influential factors in thermal expansion and contraction of concrete are aggregate characteristics and gradation, water-cementitious material ratio, relative humidity and fluctuations in temperature.
Cement hydration produces a significant amount of heat, which dissipates throughout thin concrete sections but is retained in larger elements. The increase in internal temperature causes minor, temporary expansion that counteracts chemical shrinkage. Additionally, the ambient temperatures concrete is exposed to play a role in its volume fluctuations.
As temperatures decrease, particularly below freezing, concrete contracts. Low temperatures have less of an impact on dry concrete than moist or wet concrete. The types of aggregate present and the concrete’s water-cementitious material ratio has a significant impact on the volume change that occurs at sub-freezing temperatures. Cracking at low temperatures is more likely when high levels of restraint such as rigid reinforcement, embedded objects and secondary pours exist, or when freezing water expands.
After reaching final set, high temperatures are generally less of a concern than low temperatures. Ambient temperatures exceeding 200 F that last for at least a few hours can cause irreversible damage to the concrete and its constituent materials. High temperatures cause the cement paste to dehydrate and shrink and the aggregate to expand. Due to varying coefficients of thermal expansion, aggregate volume increases tend to exceed the magnitude of paste shrinkage, resulting in an overall expansion.
Concrete is designed with the expectation that it will expand and contract in service due to ambient temperatures. Implementing a low water-cementitious material ratio; use of appropriate placement, consolidation, finishing and curing techniques; and use of aggregates ideal for certain environments can help mitigate the effects of temperature fluctuations.
Controlling volume changes
Concrete volume changes are inevitable and result from a wide range of factors. Understanding how and why concrete volume changes occur is the first step to ensuring that concrete products do not suffer any detrimental effects. Using this understanding, the next step is to implement proper design, manufacturing and curing techniques to control the level of volume change that occurs. If you have further questions about volume changes, the National Precast Concrete Association technical department can help. Visit precast.org to learn more or to contact NPCA staff.
Kayla Hanson, P.E. is a technical services engineer with NPCA.