What Is A Mass Concrete in Construction?

What Is A Mass Concrete in Construction?

What Is A Mass Concrete in Construction?

Mass concrete is a type of concrete that has large dimensions and requires special measures to prevent cracking due to the heat generated from the hydration of cement and the resulting volume changes.

The temperature within a mass concrete structure can vary greatly, leading to potential cracking, due to factors such as the size of the structure, the ambient temperature, the initial temperature of the concrete at the time of placement and curing, the type of cement used, and the cement content in the mix.

Mass concrete is typically used in the construction of large structures such as dams and bridge piers, but it is also being used more frequently in smaller projects like bridges and buildings due to the popularity of fast-track construction and the use of high-performance concretes with high cement content.

To ensure that mass concrete structures have a long service life, it is important to understand and address the causes of internal temperature increases and temperature-related cracking that can occur with mass concrete.

Examples of mass concrete structures include foundations, dams, and other concrete structures that are more than 3 feet (1 meter) wide or deep. In the past, mass concrete referred to concrete that was cast in place without reinforcement using shuttering and was commonly used in the UK between 1850 and 1900 for various structures such as walls, dams, reservoirs, retaining walls, and maritime structures.

At that time, the term was not formally defined and did not consider the effects of heat generation from cement hydration.

Material Uses Of Mass Concrete

Material uses of mass concrete include the use of Portland cement with low heat of hydration, aggregate, pozzolanas, mixing water, admixture, and coolants. To achieve the desired strength and economic efficiency, it is important to use the minimum amount of Portland cement.

Pozzolanas such as fly ash can help to reduce the heat of hydration, improve workability, and delay strength gain. The grading of fine and coarse aggregate should meet the requirements set by the IS.

For structural concrete, a 20-30 mm aggregate is suitable, while a 40 mm aggregate may be used for dams. The ratio of fine aggregate to total aggregate should be low. Mixing water should be kept to a minimum to achieve a low slump (0-50 mm).

Admixtures, such as water reducing and retarding admixtures, can be useful. In hot weather conditions, it is important to control the temperature of mass concrete dams to within a range of 5-20C.

This can be achieved by using finely chopped ice instead of water, keeping the aggregate damp and under shade, and spraying steel forms with cold water.

Properties Of Mass Concrete

Mass concrete is a type of concrete that is characterized by its large mass and high volume. It is used in construction projects where large amounts of concrete are needed, such as in dams and foundations. There are several properties of mass concrete that are important to consider in order to ensure the quality and effectiveness of the final product. These properties include workability, durability, watertightness and strength.

Workability refers to the ease with which concrete can be worked and compacted. In mass concrete projects, it is important to have an optimal level of workability in order to achieve full compaction. Uniformity of workability is also crucial in mass concrete projects, as the use of low cement content is often necessary for economic reasons.

Durability is the ability of concrete to withstand weathering, chemical attack, and abrasion. The durability of mass concrete is influenced by the mix design, placing techniques and curing methods used.

To improve the chemical resistance of mass concrete, it is important to carefully select the materials used in the mix. Weathering durability can be enhanced by adding small air bubbles to the concrete mix.

Watertightness is an important property of mass concrete, especially in hydraulic structures like dams. To achieve water tightness in mass concrete, it is necessary to use good quality aggregate, maintain a low water-cement ratio, and ensure proper consolidation and curing during the placement process.

Lean mixes, which are mixes with low cement content, are often used in mass concrete projects to improve watertightness.

Strength refers to the ability of concrete to resist loads and prevent failure. The strength of mass concrete is determined by factors such as the water-cement ratio, the quality of the aggregates used and the compaction and curing techniques employed.

In some cases, it may be economical to add pozzolana or use portland pozzolana cement in mass concrete mixes in order to increase the ultimate strength and watertightness of the concrete. Pozzolanas can also improve the plasticity and workability of mass concrete.

How To Control Temperature During The Process Of Mass Concrete

There are two ways to control the temperature of concrete: artificial cooling of the concrete to its final stable temperature, and controlling the temperature drop from maximum to final in order to avoid the build-up of thermal stresses.

The duration of initial and fine cooling, and the rate of temperature drop, depending on the diffusivity of the concrete. The diffusivity of concrete can be measured and used to schedule construction in order to control the temperature rise in the concrete mass.

Pre-cooling the concrete materials, limiting the amount of heat released, and immediate cooling after placement can all help to control the temperature of the concrete.

Pre-cooling the concrete materials can lower the maximum temperature, but will not significantly change the temperature rise due to cement hydration. Cooling the coarse and fine aggregate is the most effective, as these materials make up the majority of the concrete.

In some cases, crushed ice can be used in place of part of the mixed water to lower the batch temperature.

 

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