Name: concrete shrinkage and crack risk
Uploaded: 2024-11-07T14:48:42.000Z
Duration: 1 min 27 s
Channel: JoVE

The loss of water or chemical reactions reduces the concrete&#39;s volume, a process known as shrinkage.&#160;
Practically, shrinkage in concrete is measured as linear strain even though shrinkage contributes to volumetric strain, which is three times the linear strain.
In its plastic state, concrete loses water due to evaporation from its surface or water absorption by the dry underlying layers, leading to a volume contraction in the concrete, known as plastic shrinkage.
The higher the rate of evaporation, the more plastic shrinkage. This evaporation rate is influenced by the concrete&#39;s temperature, the surrounding air&#39;s temperature, relative humidity, and wind speed.
Moreover, concrete mixes with higher cement content exhibit more plastic shrinkage than those with lower cement content.
As the cement hydrates, the internal pores within the cement paste begin to dry out, developing capillary tension within these pores, resulting in compressive forces on the hydration products.
As a result, a uniform reduction in the concrete&#39;s volume occurs, known as autogenous shrinkage.
Lastly, high-performance concrete exhibits greater autogenous shrinkage compared to normal concrete due to its low water-to-cement ratio.

shrinkage in concrete

Shrinkage in Concrete

Shrinkage in concrete is primarily due to water loss from evaporation, hydration of cement, or carbonation, leading to a reduction in volume. The volumetric contraction results in volumetric strain in concrete. However, in practice, shrinkage is measured as linear strain, which is one-third of the volumetric strain.
When concrete is still in its plastic state, it can undergo a decrease in volume by about 1% of its absolute volume. This decrease is known as plastic shrinkage. It arises either from water evaporating off the concrete&#39;s surface or being absorbed by the concrete beneath. The American Concrete Institute specifies that evaporation rates exceeding 0.25 kg/h/m&#178; should be avoided to prevent plastic cracking. Furthermore, concrete&#39;s cement content is directly proportional to the degree of plastic shrinkage experienced; mixes with greater aggregate content exhibit less plastic shrinkage.
Even without moisture transfer from the environment, concrete can experience autogenous shrinkage, particularly in high-performance variants, although it is minimal in ordinary concrete. This is due to the internal compressive stresses exerted on hydration products due to the internal loss of water as the cement hydrates, and not due to external drying.

The loss of water or chemical reactions reduces the concrete&#39;s volume, a process known as shrinkage.&#160;
Practically, shrinkage in concrete is ...

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JoVE Core

Civil Engineering

Unit 1

Elasticity, Creep, and Shrinkage in Concrete

elasticity-creep-and-shrinkage-in-concrete

KEY TERMS AND CONCEPTS

concrete shrinkage and crack risk

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 surface or is drawn into the concrete below.

The American Concrete Institute says evaporation rates above 0.25 kg/h/m² should be avoided to reduce plastic cracking. Cement content also affects this behavior. Mixes with more aggregate tend to show less plastic shrinkage.

Concrete can also undergo autogenous shrinkage even when there is no moisture transfer from the outside environment. This effect is especially important in high-performance concrete, but it is small in ordinary concrete. It comes from internal compressive stress on hydration products as the cement uses up its own water during hydration, not from external drying.

Concrete Shrinkage and Crack Risk

elasticity in concrete

Concrete Stress-Strain Modulus Methods

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...

dynamic modulus of elasticity of concrete

dynamic-modulus-of-elasticity-of-concrete

Sonic Testing of Concrete Beams

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...

creep in concrete

Concrete Creep Under Constant Load

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.

factors affecting creep

Concrete Creep: How Material Choices Matter

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...

effects of creep

Creep in Concrete Structures

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...

drying shrinkage

Concrete Shrinkage as Water Evaporates

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...

carbonation shrinkage

Concrete Carbonation and Steel Corrosion

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...

types of non-structural cracks in concrete

types-of-non-structural-cracks-in-concrete

Concrete Cracks from Shrinkage and Heat

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...

mass concreting

Preventing Thermal Cracks in Large 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 concreting

Managing Heat in Concrete Placement

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...

cold weather concreting

Preventing Freeze Damage in Fresh Concrete

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...

key terms and concepts

Explore how water loss, hydration, and carbonation cause concrete shrinkage, including plastic shrinkage, autogenous shrinkage, and crack risk in fresh mixes.

Concrete Shrinkage and Crack Risk | JoVE K12

JoVE Core Civil Engineering offers a foundational exploration of essential construction materials such as bricks, stones, steel, wood, and concrete, along with fluid mechanics topics like flow over immersed bodies, open channel flow, and dimensional modeling. It also covers key surveying concepts, including leveling, traverse adjustments, curve design, topographic mapping, and geospatial technologies (GPS & GIS). Science-in-action videos showcase laboratory research on materials, fluids, and surveying practices in real-world engineering systems.

civil engineering

Concrete Shrinkage and Crack Risk

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