Mass concrete is defined as heavy volume concrete work with large dimensions and boundary conditions that are prone to elevated temperatures due to a higher rate of the heat of hydration.
The high temperature in concrete results in high thermal stresses, cracking, and reduced long-term strength gain. Hence, the main factor distinguishing mass concrete from normal concrete is its thermal behavior.
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Mass concrete is a term associated with the large in-situ concrete poured structures like dams, bridge piers, foundations of a large building, and large concrete placements (minimum 1 m deep).
This article explains the important features and composition of mass concrete as per the American Concrete Institute (ACI).
Contents:
Characteristics of Mass Concrete
The American Concrete Institute (ACI 207.1R) defines mass concrete as “any volume of structural concrete in which a combination of dimensions of the member being cast, the boundary conditions, the characteristics of the concrete mixture, and the ambient conditions can lead to undesirable thermal stresses, cracking, deleterious chemical reactions, or reduction in the long-term strength as a result of elevated concrete temperature due to heat of hydration.”
The hydration reaction in mass concrete releases enormous heat. The absence of a proper mode to dissipate this heat would result in a higher temperature difference between the interior and exterior surface of the mass structure, which further leads to thermal stresses and cracks.
Thermal cracks provide pathways for air and water to reach the reinforcing steel, leading to corrosion. Hence, thermal cracks create a durability issue.
Mass concreting is always designed and performed by emphasizing the thermal behavior, which if ignored affects the structural integrity and monolithic action of the structure. Thermal cracks observed in various mass concrete structures are listed below:
| Type of Mass Concrete Structure | Form of Thermal Crack |
| Large Foundation | Random map cracks |
| Walls | Series of vertical cracks, wider at the base |
| Beams | Uniformly spaced cracks perpendicular to the longest dimension of the beam |
At temperatures greater than 160 oF, unstable hydration products are produced, called delayed ettringite formation (DEF). DEF expands with the concrete with time, which results in significant cracking.
Temperature Control in Mass Concrete
The main factors that can influence concrete temperature are:
- Cement composition and fineness
- Aggregate content and its coefficient of thermal expansion
- The geometry of section
- The temperature of materials
- The temperature during placement
Methods to pre-cool concrete materials to achieve lower maximum temperature for mass concrete include:
- The use of crushed ice with water during summers to reduce the temperature of mass concrete mix by 10 oF.
- The fine and coarse aggregates can be pre-cooled in moist conditions using vacuum saturation and liquid nitrogen injection (common in the United States).
- Post-cooling is sometimes conducted to keep the maximum temperature of hardened concrete below 75 oF.
As per ACI 301,
- After placing, the maximum temperature in mass concrete must not exceed 160 oF to prevent DEF.
- The temperature difference between the center and surface of placement must not be greater than 35 oF.
Composition of Mass Concrete-ASTM Standards
The characteristics of cement, pozzolans, admixtures, aggregates, and water used for mass concrete construction are as follows:
1. Cement in Mass Concrete
The types of cement that can be used for mass concrete construction are given in ACI 207.2R and 207.4R. Portland cement (ASTM C150), blended cement (ASTM C595), and hydraulic cement (ASTM C 1157) are some of the cements used for mass concrete construction.
Portland cement can be used with pozzolans or any other type of cement for mass concrete. During the process, the batching is performed separately at the mixing plant. Limiting the cement content can reduce the temperature and help achieve economy.
2. Pozzolans in Mass Concrete
Pozzolans in mass concrete reduce the internal heat generation, improve workability, lessen the possibility of alkali-aggregate reaction, and provide a better economy. However, the properties of pozzolans vary. Hence, they should be tested combined with the project cement to measure its contribution to mass concrete.
The pozzolan classification is given in ASTM C 618.
3. Ground-Granulated Iron Blast Furnace (GGBS) in Mass Concrete
GGBS or slag cement is used separately with Portland cement in mass concrete. The standard requirements of slag cement used in concrete are given in ASTM C 989.
The incorporation of slags reduces the rate of heat generation as they decrease the rate of hydration. They also reduce permeability, expansion of reactive aggregates, and improve workability.
4. Chemical Admixtures in Mass Concrete
The important chemical admixtures used for mass concrete are air-entraining, water-reducing, and set-reducing. The detailed features of these mixtures are given in ACI 212.3R and ASTM C494.
Chemical admixtures help lower the heat of evolution during the hardening of mass concrete. In addition to this, they help increase strength, durability, abrasion, and erosion resistance.
5. Aggregates in Mass Concrete
The detailed information on the fine and coarse aggregates used in construction is defined in ASTM C125 and ACI 221R.
The grading of fine aggregates influences the workability of mass concrete. A good grading of sand for mass concrete is given in Table-2, as per the U.S. Bureau of Reclamation.
Coarse aggregate used for mass concrete must be hard, dense, durable, and uncoated particles.
6. Water in Mass Concrete
The water used for mass concrete mixing must be free of materials that significantly affect the hydration reaction of Portland cement. The limits of chloride content for water used for various construction purposes are mentioned in ACI 201.2R.
More details on mass concrete are provided in ACI 301-20, “Specification for Concrete Construction’, and ACI 207.1R-05, “Guide to Mass Concrete.”
FAQs
The American Concrete Institute (ACI 207.1R) defines mass concrete as “any volume of structural concrete in which a combination of dimensions of the member being cast, the boundary conditions, the characteristics of the concrete mixture, and the ambient conditions can lead to undesirable thermal stresses, cracking, deleterious chemical reactions, or reduction in the long-term strength as a result of elevated concrete temperature due to heat of hydration.”
As per ACI 301,
1. After placing, the maximum temperature in mass concrete must not exceed 160 oF to prevent DEF.
2. The temperature difference between the center and surface of placement must not be greater than 35 oF.
The types of cement that can be used for mass concrete construction are given in ACI 207.2R and 207.4R. Portland cement (ASTM C150), blended cement (ASTM C595), and hydraulic cement (ASTM C 1157) are some of the cements used for mass concrete construction.
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