Cement undergoes both initial expansion and subsequent, more pronounced shrinkage. While the chemical reaction that hardens the material causes a brief volume increase, the overall, long-term behavior is dominated by contraction. This dynamic interplay of expansion and shrinkage is a fundamental property of cement-based materials and is a major consideration in construction engineering. Understanding this volume change, known as “volume instability,” is crucial for predicting the long-term performance and durability of structures.
Defining Cement, Concrete, and Volume Instability
Cement is a fine, powdery binder that, when mixed with water, forms a paste that hardens over time. Concrete, the final product used in construction, is a composite material made by mixing this cement paste with aggregates, such as sand and gravel. The cement paste acts as the glue, binding the aggregates together to create the hard, stone-like structure.
Volume instability refers to the inherent tendency of cement paste and concrete to change volume over time, either by expanding or contracting. This instability is driven by chemical reactions within the paste and the physical loss or gain of moisture. Because the cement paste reacts and holds the material together, the amount of paste in the mix directly influences the severity of volume changes.
The Initial Expansion Caused by Hydration
The initial volume increase results from hydration, the chemical process that begins immediately upon mixing cement with water. Hydration is an exothermic reaction where cement compounds react with water to form new, stable solid products, primarily Calcium Silicate Hydrate (C-S-H) gel and calcium hydroxide.
These newly formed solid hydration products occupy a greater physical volume than the original cement powder and water, leading to a slight expansion of the fresh material. This is often referred to as autogenous expansion, a volume change that occurs internally without moisture exchange. The thermal energy released during the exothermic process also causes the concrete to heat up, resulting in temporary thermal expansion that contributes to the early-age volume increase.
The Dominant Effect of Drying Shrinkage
While there is an initial expansion, the long-term volume change is dominated by contraction, known as drying shrinkage. This shrinkage occurs as excess water, beyond what is needed for hydration, evaporates from the hardened concrete matrix over weeks and months. Concrete mixtures contain more water than chemically required to ensure the material is workable enough to be placed and finished.
As this excess water leaves the material, particularly from the fine capillary pores, surface tension forces develop within the remaining water. This capillary tension creates an internal pressure that pulls the walls of the pores closer together, physically contracting the cement paste. This volume reduction can continue for years and is the main reason concrete cracks when its movement is restrained by foundations, reinforcement, or adjacent structures. Shrinkage strain values are generally less than 0.05%, but this small movement generates high tensile stresses that the material is weak against.
Managing Volume Change in Construction
Engineers and contractors use several methods to mitigate the stresses caused by volume change and prevent random cracking. The most effective strategy is proper curing, which involves keeping the newly placed concrete moist for an extended period, often seven days or more. Wet curing slows the rate of water loss, allowing the concrete to gain strength before significant internal stresses build up from drying shrinkage.
The concrete mix design is also carefully controlled to reduce the potential for shrinkage. This is achieved by minimizing the total water content and maximizing the volume of coarse aggregate. Coarse aggregate is dimensionally stable and restrains the shrinking cement paste.
In addition to material changes, physical design elements like control joints and expansion joints are installed to manage anticipated movement. Control joints are saw-cut grooves or pre-formed breaks that create a weakened plane, forcing the inevitable shrinkage cracks to occur in a neat, predetermined location.