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Plastic settlement cracks in concrete are formed during initial setting of concrete. Mechanism,
appearance and prevention of plastic settlement cracks are discussed below.
Mechanism of Plastic Settlement Cracks in Concrete
If the settlement of solids in the concrete can freely take place without hindrance there will be
a reduction in depth and volume of the cast concrete but no cracking. However, any restraint
to this movement, e.g. reinforcement, can result in plastic settlement cracks.
Where the solids continue to settle in comparison to those which are prevented from further
downward movement, the concrete will ‘break its back’ and a tear appears in the surface as it
is forced into tension. Cracks may develop at regular spacing reflecting the reinforcement
layout.
Cracks in concrete often occur in conjunction with voids under the bars as shown in Figure 1.
Figure 1(a) shows initiation and Figure 1(b) the condition after a few hours. These crescentshaped voids may initially be filled with bleed water. The region of bond between the bar and
concrete is thus reduced.
Fig. 1: Formation of plastic settlement crack (initial and final)
Appearance of Plastic Settlement Cracks in Concrete
Visual appearance of Plastic Settlement Cracks
The most common restraint in slabs is from the reinforcement. The cracks occur on the top
surface and usually follow the line of the uppermost bars, giving a series of parallel cracks;
there may also be shorter cracks at right angles over the bars running in the opposite
direction.
Plastic settlement cracks are typically 1 mm wide and usually run from the surface to the bars
(see Figures 2 and 3). The settlement may also result in visible undulations on the concrete
surface, with the high points over the top reinforcing bars.
Fig. 2: General Plan View of Cracks
Fig. 3: Section showing undulations
In some cases where the bars in the top layer of reinforcement are close together, the whole
surface layer of the concrete may be ‘suspended’ on the reinforcement while the concrete
below settles. This can lead to a horizontal discontinuity beneath the bars, resulting in a loss
of bond and with time delamination of concrete cover that protects the reinforcing steel
against corrosion.
Unlike cracks in hardened concrete, due to overloading for instance, these cracks form at a
very early age and pass through the cement paste and do not pass through aggregate particle
pieces. The path is therefore more tortuous. This form of crack can be potentially serious as it
passes longitudinal with the reinforcement and extends to the steel, negating the resistance to
corrosion provided by the concrete.
Fine cracks can occur in relatively narrow formed surfaces such as columns. The concrete
may arch between the containing form faces. Settlement below the restrained concrete results
in a crack being formed, generally coinciding with the links.
It is sometimes possible for plastic settlement cracks to form on a vertical face where
reinforcement has restricted the free flow of concrete within the formwork. In such cases it is
possible that the cracks are formed between the lines of the reinforcement.
The concrete can also be supported by the formwork face. This causes restraint to the
concrete between connected members and is especially evident where changes in section
cause differential settlement, the concrete in the deeper section settling more than the
shallower section resulting in a crack.
This is noticeable in the transition between a flared column head (mushroom) and the plain
column, and in trough and waffle slabs where more settlement takes place in the web than the
comparatively thin flange (see Figures 4 and 5). The cracks may pass through the flange and
appear similar to shrinkage cracks.
Fig. 4: (a) Arching near the top of column, cracking coincides with links. (b) Cracks at
change of section.
Fig. 5: Cracks at change of section in trough and waffle slabs.
It can also occur at other locations, such as under spacer blocks. Cracks at mushroom heads
of columns are generally horizontal. They are also typically 1 mm wide and can cross the full
section.
If the sub-base or other material against which the concrete is placed has a high absorbency
(dry soil, permanent forms) the settlement can be exaggerated, again the cracking following
the reinforcement layout.
Prevention of Plastic Settlement Cracks in Concrete
The restraints that cause plastic settlement cracking are inherent in the construction and
generally cannot be avoided. Abrupt changes in section depth could be avoided at the design
detailing stage but the main reduction of risk is through mix design and suitable cohesion of
the concrete to reduce bleeding.
In simple terms prevention of plastic settlement cracks in concrete can be achieved by
increasing the sand content. However, there is a limit to this at which the bleeding will
increase. Very clean (marine-dredged) sand tend to assist water movement, so blending with
a ‘dirtier’ sand with a higher fines (<150 micron) content can be beneficial.
A tendency to bleed largely depends on the properties of the cement. Fineness is a controlling
factor, possibly because the finer particles hydrate more quickly thereby reducing the rate of
sedimentation. Rich mixes are less prone than lean mixes.
Pozzolanic additions may also help as they reduce water content and add to the fines. High
GGBS contents should be avoided as slower setting times allow for the bleeding to continue
longer. Air entrainment admixtures can be used, the entrained air stabilizing the matrix and
reducing the water movement. A similar effect is claimed for polypropylene fibres within
concrete (British Board of Agreement, 1995).
Remedial Measures for Plastic Settlement Cracks
Plastic settlement cracks rarely pass through the full section, except in cases similar to trough
and waffle slabs, mainly because they stop at the reinforcement that causes the restraint.
Structural integrity is therefore not compromised, however the cracks need to be sealed
especially on slabs, to reduce the risk of reinforcement corrosion.
Where plastic settlement cracking is apparent in the newly placed concrete, the most effective
way of eliminating their occurrence is to re-vibrate the concrete after the cracks have formed
but before initial set. Assessing the most appropriate time is the responsibility of the
operative. This will vary depending on the mix characteristics (cement type) and ambient
conditions. The concrete must be capable of being re-fluidized by a poker vibrator.
In general, timely and proper re-vibration can serve only to improve the situation, if not
completely restore the bond beneath the bar. Tamping the surface may close the surface but is
unlikely to remove the voids beneath the bar. If noticed early enough, and the cracks remain
clean and not filled with detritus, cement can be brushed into the openings and allowed to set.
This however does not resolve any bond reduction caused by voids beneath the
reinforcement.
Treating cracks in older hardened concrete will depend on the service conditions, i.e.
exposure class, and the severity of the cracking. They can be treated with resin injection
although in some instances full-depth breaking out and reinstatement may be necessary.