Name: managing heat in concrete placement
Uploaded: 2024-11-07T14:48:43.000Z
Duration: 1 min 20 s
Channel: JoVE

In dry and hot conditions, placing fresh concrete at high temperatures accelerates the setting and leads to rapid evaporation of water from the concrete, which can cause cracking.
Preferably, the concrete&#39;s temperature in hot weather conditions should not exceed 60 degrees Fahrenheit.
This is achieved by reducing the cement content, using Sulfate-free aggregates with less moisture absorption, incorporating set-retarding admixture, adding ice to the mixing water, and ensuring its complete melting before the concrete mixing is completed.
Additionally, on hot weather sites, precautions include shading the aggregate stockpiles from direct sunlight, spraying them with water to facilitate cooling by evaporation, burying the water pipes, and painting concrete handling equipment and tanks white to reflect sunlight.
Formwork is sprayed with water before the concrete is placed, and the placement is done during the cooler evening hours.
After placement, the concrete may be covered with white-colored plastic sheeting to protect it from the sun and rapid evaporation.
Lastly, in dry weather conditions, wetting the concrete and allowing evaporation of water facilitates good curing and cooling.

hot weather concreting

Hot Weather Concreting

Concreting at elevated temperatures accelerates the hydration process, leading to quicker setting but potentially reducing the long-term strength of the concrete structure. Additionally, low air humidity fosters rapid moisture loss from the concrete, resulting in reduced workability, pronounced plastic shrinkage, and a higher likelihood of crazing.
Mitigating the heat increase in concrete can be economically achieved by shading aggregate stockpiles to prevent heating from solar radiation, burying water pipes, and painting exposed pipes and tanks white to reflect heat. Lowering the cement content in the mix can reduce the total heat released from hydration; in this case, the decrease in workability due to less cement content can be compensated by selecting aggregates with low absorption rates and minimal impurities. Adjusting workability and setting time is also possible through the application of set-retarding admixture. Incorporating ice into the mixing water helps to control the temperature. It is crucial, however, to ensure the ice is completely dissolved during the mixing process.
It is advisable to place the concrete within&#160; 45 to 60 minutes after mixing on dampened formworks and during the cooler evening hours. Post-placement, it is crucial to ensure the&#160; evaporation rate does not exceed 0.25 kg/m2&#160;per hour on the concrete surface. To protect the concrete from excessive daytime heat, it should be covered. During dry conditions, wetting the concrete can provide dual benefits of cooling and promoting effective curing.

In dry and hot conditions, placing fresh concrete at high temperatures accelerates the setting and leads to rapid evaporation of water from the concrete, ...

Education

JoVE Core

Civil Engineering

Unit 1

Elasticity, Creep, and Shrinkage in Concrete

elasticity-creep-and-shrinkage-in-concrete

KEY TERMS AND CONCEPTS

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 heat before mixing. Aggregate stockpiles can be shaded so they do not warm up in direct sunlight. Water pipes can be buried, and exposed pipes and tanks can be painted white to reflect heat.

The mix itself can also be adjusted. Using less cement lowers the total heat released during hydration. If that reduces workability, aggregates with low absorption and few impurities can help. Set-retarding admixtures can also slow setting and improve working time. Ice can be added to the mixing water to control temperature, but it must fully melt during mixing.

Timing and placement matter as well. Concrete should be placed within 45 to 60 minutes after mixing. Dampened formworks and cooler evening hours are better choices. After placement, the evaporation rate should stay below 0.25 kg/m2 per hour. The surface should be covered to protect it from daytime heat, and wetting the concrete during dry weather can cool it and support curing.

Managing Heat in Concrete Placement

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...

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 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 hot weather affects concrete setting, strength, and curing, and how shade, ice, admixtures, and timing help control heat loss.

Managing Heat in Concrete Placement | 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

Managing Heat in Concrete Placement

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