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.
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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