Name: preventing thermal cracks in large concrete
Uploaded: 2024-11-07T14:48:43.000Z
Duration: 1 min 22 s
Channel: JoVE

When large volumes of concrete are placed, if the temperature of the concrete on the surface and the interior region is different, thermal cracks may occur.&#160;
To mitigate thermal cracking, the concrete mix uses low-heat cement, pozzolans such as fly ash, cooled aggregates with angular particles, and water-reducing admixture.
The ingredients are then mixed using chilled water to produce fresh concrete at a temperature of about 45 degrees Fahrenheit.
Fresh concrete is then placed in formwork made of thermally insulating materials.
Covering the surfaces of the structure with plastic-coated quilts controls the heat loss.
Additionally, cooling pipes can be embedded into the structure to facilitate cooling post-concreting.
To ensure the structure is insulated, thermocouples are positioned at various locations to monitor the concrete&#39;s temperature.
The formwork and insulation are kept in place for up to two weeks for sections with thicknesses greater than twenty inches.&#160;&#160;
However, for externally restrained large concrete sections, construction should take place in lifts, and movement joints are to be provided at suitable locations to prevent cracking.

mass concreting

Mass Concreting

Mass concreting refers to the process of placing large volumes of concrete, such as in gravity dams. The heat generated during the cement hydration process and differential cooling rates within the concrete mass can lead to a temperature gradient, which can result in thermal cracks in the concrete mass.
To reduce the risk of such cracking, the concrete mix may incorporate low-heat cement and pozzolans to reduce the temperature rise. Pre-cooled angular aggregates and water-reducing admixtures further assist in managing heat generation. Concrete is mixed using chilled water, resulting in fresh concrete with an approximate temperature of 45 degrees Fahrenheit.
Fresh concrete is then placed in formworks made of thermal insulators and completely covered using plastic-covered quilts or soft boards to control heat losses. Furthermore, cooling conduits may be installed within the structure to provide continuous temperature management after placing the concrete. Temperature monitoring is achieved by placing thermocouples at strategic points within the concrete mass. This monitoring ensures that insulation is effective and maintained as needed. For sections over twenty inches thick, the formwork and insulation might be required to remain in place for at least two weeks.
For larger concrete sections that are externally restrained, the construction process is adapted to include the creation of layers or lifts and the strategic positioning of movement joints for accommodating the stress and strain that may occur due to temperature changes, thereby helping to avert the development of cracks.

When large volumes of concrete are placed, if the temperature of the concrete on the surface and the interior region is different, thermal cracks may ...

Education

JoVE Core

Civil Engineering

Unit 1

Elasticity, Creep, and Shrinkage in Concrete

elasticity-creep-and-shrinkage-in-concrete

KEY TERMS AND CONCEPTS

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 the temperature rise. Pre-cooled angular aggregates and water-reducing admixtures also help control heat generation. The fresh concrete is mixed with chilled water, and its temperature is brought to about 45 degrees Fahrenheit.

After mixing, the concrete is placed in formworks made of thermal insulators. It is then fully covered with plastic-covered quilts or soft boards to limit heat loss. Cooling conduits may also be built into the structure so temperature can be managed after the concrete is placed. Thermocouples are set at key points inside the concrete mass to monitor temperature and check that the insulation is working.

For sections thicker than twenty inches, the formwork and insulation may need to stay in place for at least two weeks. Larger externally restrained sections are also built in layers or lifts. Movement joints are placed in strategic locations so the structure can handle stress and strain from temperature changes and avoid cracks.

Preventing Thermal Cracks in Large Concrete

preventing-thermal-cracks-in-large-concrete

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...

shrinkage in concrete

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...

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...

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

Learn how mass concreting limits heat buildup, uses chilled water and insulation, and adds cooling conduits and joints to prevent thermal cracks.

Preventing Thermal Cracks in Large Concrete | 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

Preventing Thermal Cracks in Large Concrete

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