Elasticity, Creep, and Shrinkage in Concrete

Concrete shows a non-linear stress-strain response when it is loaded in uniaxial compression or tension at moderate to high stress levels. As the load is removed, the stress-strain path does not return along the same line. This creates a hysteresis loop and shows that concrete is not perfectly elastic.

The modulus of elasticity is used to describe stiffness. In many materials, it is taken from the slope of the linear part of the stress-strain curve. Concrete does not have a simple linear...

Sonic testing of concrete beams is used to measure the dynamic modulus of elasticity. This value describes how a concrete specimen deforms under impact or other dynamic loads. It is usually higher than the static modulus of elasticity, which is measured under slow, steady loading.

A common test uses a concrete beam that is either 6 x 6 x 30 inches or 4 x 4 x 20 inches. The beam is clamped at its center. Vibrations start at one end with an electromagnetic exciter unit powered by a...

Concrete creep is the time-dependent increase in strain under a sustained load. It is different from shrinkage, swelling, and thermal expansion, which are other time-based changes in concrete. When concrete stays under constant stress, its shape can slowly change over time.

The main cause of creep is the loss of physically adsorbed water from calcium silicate hydrate in the hydrated cement paste. Concrete also shows a non-linear stress-strain relationship, which adds to this behavior.

Concrete creep depends on the materials in the mix, the load applied, and the curing environment. In normal-weight aggregate concrete, the hardened cement paste is the main part that creeps. The aggregates are stiffer than the cement paste, so they resist stress-induced deformation more effectively.

Aggregate properties can change how much a concrete member creeps. The stiffness of the aggregates is described by their modulus of elasticity. When aggregates make up a larger volume of the...

Creep in concrete is the slow deformation that happens when a load stays on a structure for a long time. It can change how beams, columns, bridges, and other concrete members behave over time. Engineers must account for it when they design safe and durable structures.

In reinforced concrete beams, creep increases deflection, which means the beam bends more than expected. That extra bending can widen cracks and may require added design measures. In slender columns under eccentric loads, creep...

Concrete shrinkage is the loss of volume that happens when water leaves the mix or when cement reacts and carbonates. This contraction creates volumetric strain in the material. In practice, engineers often measure shrinkage as linear strain, which is about one-third of the volumetric strain.

Fresh concrete can also shrink while it is still plastic. This plastic shrinkage can reduce the volume by about 1% of the concrete’s absolute volume. It usually happens when water evaporates from the...

Concrete shrinkage happens when hardened concrete loses water to air with a relative humidity below 100 percent. Free water in the capillaries evaporates first. Then water that was adsorbed, or held on the calcium silicate hydrates, moves into these empty spaces and evaporates too.

As more water leaves, the concrete volume decreases. This is called drying shrinkage. Some of this change can be reversed. If the concrete later meets moisture, it can take in water again and expand slightly.

Not...

Concrete carbonation begins when atmospheric carbon dioxide enters the pores of concrete. If moisture is present, the gas forms carbonic acid. That acid reacts with calcium hydroxide in hydrated cement and produces calcium carbonate.

This reaction causes carbonation shrinkage, which means the concrete loses some volume. The calcium carbonate can also fill pores, so permeability drops slightly. At the same time, the water released during the reaction can help any unreacted cement continue...

Non-structural concrete cracks can form during placement, curing, and drying. The main groups are plastic cracks, early-age thermal cracks, and drying shrinkage cracks. Another surface pattern called crazing can also appear when finishing or curing is not done well.

Plastic cracks form while the concrete is still fresh. Plastic shrinkage cracks usually appear within hours after pouring. The top of the concrete dries faster than the lower part, which creates tensile stress. The soft concrete...

Large concrete pours can build up heat and crack as they cool. This is a key challenge in mass concreting, such as the placement of concrete in gravity dams. Heat from cement hydration, which is the chemical reaction that hardens cement, can create a temperature difference between the inside and outside of the concrete mass. That temperature gradient may lead to thermal cracks.

To lower that risk, the concrete mix is adjusted before placement. Low-heat cement and pozzolans are used to reduce...

Hot weather changes how concrete sets and cures. High temperatures speed up hydration, which makes the mix set faster. That faster set can lower the long-term strength of the finished structure.

Low air humidity creates another problem. Water leaves the fresh concrete more quickly, so workability drops. This can also increase plastic shrinkage, which is shrinkage before the concrete hardens, and raise the chance of crazing, a network of fine surface cracks.

Several simple steps can reduce...

Fresh concrete can be damaged if it freezes before it sets. When the water in the mix turns to ice, it expands. That expansion disrupts setting, slows the chemical reactions needed for hardening, and can leave more pores in the finished concrete. Those extra pores weaken the final structure.

If the concrete gains enough strength before freezing, the damage may be reduced. Cold-weather protection focuses on keeping the concrete warm during and after placement. Mixing with warm water and heated...