Mass Concrete and Associated Temperature Limits & Potential Damages

  1.     What is Mass Concrete? Mass concrete - It is distinguished from other types of concrete owing to its thermal behavior - which requi...

 


1.    What is Mass Concrete?

Mass concrete - It is distinguished from other types of concrete owing to its thermal behavior- which requires contemplating the actions to control the massive heat of hydration and associated temperature rise due to the large/mass concrete quantum.

According to ACI 116R, Mass Concrete is defined as “Any volume of concrete with dimensions large enough to require that measures to be taken to cope with generation of heat from the hydration of the cement and attendant volume change to minimize cracking”

2.    Where is Mass Concrete Used?

Mass concrete is usually associated with a large quantity of cast-in-situ concrete structures such as dams, high-rise buildings foundations, Turbine foundations, Boiler foundations, Storage tanks, silo foundations, etc.

3.    My Practical experiences of Mass Concrete Foundations

I have been involved in the construction of various mass concrete foundations such as Gas turbine & Generator Foundations, Steam Turbine & Generator Foundations, HRSG/Boiler Foundations, and HSD Oil storage tanks foundations.

An idea about the dimensions of these foundations is:

Gas Turbine + Steam Turbine + Generator Foundation: 54.3m x 8.21 m x 2.50 m

Gas Turbine + Generator Foundation: 41.14m x 11.6m x 2.80m

Steam Turbine + Generator Foundation: 37.7m x 15m x 2.5m

HRSG/Boiler Foundation: 31 m x 16 m x 1.2 m

4.    Permissible Temperature Limits for Mass Concrete

As we have already discussed the major attribute associated with mass concrete is to deal with the heat of hydration to ensure that in any case, safe/permissible temperature limits are not exceeded.

What are the safe/permissible temperature limits? ACI 301 provides the answer to this question.

“ACI 301 Section-08, Clause 8.1.2” states that

Unless otherwise specified, the following criteria shall apply for mass concrete placement:

§  The maximum temperature in concrete after placement shall not exceed 158 0F (700C).

§  The maximum temperature difference between the center and surface of placement shall not exceed 350F (~19oC).

5.    Why Do These Temperature Limits Apply?

The maximum temperature limit of 1580F (700C) exits to prevent Delayed Ettringite Formation (DEF), while the minimum temperature difference limit of 350F (~19oC) exits to prevent thermal cracking.

These two (02) cases are discussed in detail in the following:                                                    

    5.1  Delayed Ettringite Formation (DEF)

Ettringite is normally one of the earliest products formed during cement hydration. However, when the early-age concrete core temperature (inside mass concrete foundation) exceeds 158 0F, the formation of Ettringite is prevented or destroyed due to high heat. Thus, the formation of Ettringite occurs in the later stage when the temperature comes down to the safe limits, thus called Delayed Ettringite Formation.

The term “Delayed Ettringite Formation” (DEF) is commonly used to refer to the potentially deleterious reformation of Ettringite in already hydrated concrete after the destruction of primary Ettringite by high temperature.

As the formation of Ettringite is associated with the positive volume change, therefore, formation or reformation of Ettringite in already hardened concrete [DEF] can lead to extensive damage to the structural concrete

The common form of concrete deterioration due to DEF results in cracking thus damaging the structural concrete and affecting the structure's durability. DEF can also increase the risk of secondary forms of deterioration such as freeze/thaw attack or reinforcement corrosion.

    5.2. Temperature Difference Limit

The temperature difference is between the center/hottest portion and the surface of the mass concrete foundation.

The temperature difference limit of 35 0F, is to minimize the potential of thermal cracking within the concrete structure.

The increasing temperature difference between the center and surface of the mass concrete raft creates tensile stresses in the concrete. When the tensile stresses exceed the tensile strength of the concrete, cracking occurs. The depth and severity of cracking depend primarily on the magnitude of the temperature differential.

Summary

Mass concrete is distinguished from other concrete types based on its thermal behavior. The large concrete quantity; a defining tagline of mass concrete, demands measures to cater the heat of hydration and to keep the associated temperature rise within the specified limits to prevent structural distresses.

The temperature limits for mass concrete as provided in ACI 301- Section 08 are stated below:

·    The maximum temperature within the core of the mass concrete raft should not exceed 158 0F (700C) to curb the Delayed Ettringite Formation (DEF), which otherwise could cause cracking in the structural concrete.

·     The maximum temperature difference between the center and surface of placement should not exceed 350F (~19oC) to prevent thermal cracking due to differential temperatures between the two surfaces.


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Umair Ashraf is a highly skilled Civil Engineer with extensive experience in successfully handling complex and large-scale construction projects. His goal is to share his expertise and experience to support and empower others through his writing ultimately making a meaningful contribution to the construction industry.
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CIVIL ENGINEERING TRIBUNE: Mass Concrete and Associated Temperature Limits & Potential Damages
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