Page 9 - Concrete-Technology-Interacted_Book_Prof-MIK_Neat

2/4 Plastic and thermal cracking

tensile forces and influences the crack pattern. Other forms of cracks from chemical
actions (ASR, freeze–thaw, reinforcement corrosion) also occur.

The presence of cracks can influence the behaviour and durability of a concrete member.
They can reduce the shear capacity of a section or provide a path by which moisture,
oxygen, carbon dioxide, chlorides etc. can penetrate into the concrete surrounding the
reinforcement which in time may result in reinforcement corrosion. These aspects are
covered in more detail in later chapters. Cracks and crack patterns have different
characteristics depending on the underlying cause.

Different types of crack occur at different times in the life of a concrete element (see
Table 2.1). So as well as a recognition of a crack pattern, a knowledge of the time of the
first appearance of cracks is helpful in diagnosing the underlying cause.

Table 2.1 Typical times for appearance of defects (from Concrete Society Technical
Report 54)

Type of defect Typical time of appearance

Plastic settlement cracks Ten minutes to three hours
Plastic shrinkage cracks Thirty minutes to six hours
Crazing One to seven days – sometimes much longer
Early thermal contraction cracks One day to two or three weeks
Long-term drying shrinkage cracks Several weeks or months

The Concrete Society (1992) provides information on the most common forms of
‘intrinsic’ cracks in concrete. Figure 2.1 (taken from the Technical Report) illustrates
most of the types of crack that are likely to be experienced in the lifetime of a concrete
structure.

Type of cracking

I A Plastic settlement A, B, C
A
J Plastic shrinkage D, E, F

BI Early thermal contraction G, H

E Long-term drying shrinkage I

K B Crazing J, K
C B
L Corrosion of reinforcement L, M
Tkoipckoefr M
Shear F Alkali–silica reaction N
cracks ‘Bad’, i.e.
Cracks at ineffective, Tension bending cracks
kicker joints N Plus rust stains
G joint

H D

H
I

Figure 2.1 Examples of intrinsic cracks in hypothetical structure (from Concrete Society, 1992).

The following sections are chiefly concerned with early-age movements but also
discuss the longer-term effects of drying shrinkage.

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