Troubleshooting - Concrete Mix Design, Quality Control and Specification - page 271

is an aggravating factor. Therefore, if pumping problems are being
experienced, pumping more slowly and ensuring that one truck is not
emptied before a replacement arrives may assist.
8. The use of a priming slurry that is too wet is a common cause of
pump blockage. This is because the focus is on wetting the pipeline
not segregating the concrete that follows. The optimum slurry mix
should be cohesive with the similar composition as the concrete but
without the aggregates.
12.1.3 Unsatisfactory appearance
Unsatisfactory appearance may be due to inept placing, poor formwork,
or many other factors that are beyond the scope of this topic. However, it
is also often due to bleeding, the remedies for which were covered earlier.
If bleeding happens at all, it often results in a flow of water up the face
of formwork, leaving clearly visible signs. A slight formwork leak (just of
water) can cause an internal surface flow of water over an area of more
than a square metre and result in a large black stain, known as a hydra-
tion stain.
Presetting or plastic cracks—There are two kinds of presetting cracks
with diametrically opposite causes: settlement cracks and evaporation
cracks.
Plastic settlement cracks—Plastic settlement cracks result from settle-
ment of the concrete due to loss of bleedwater. In settling, the con-
crete “breaks its back” over anything resisting settlement in one
location and not another, for example, reinforcing bars, cast-in
plumbing, or sharp changes in depth of section. Measures to avoid
bleeding were dealt with earlier.
Plastic shrinkage or evaporation cracks—Plastic shrinkage or evap-
oration cracks result from evaporation of water from the surface
layer of concrete beyond the rate of bleed from the concrete to
replace the loss. If a concrete has very low bleeding (e.g., silica
fume concrete), it is susceptible to such cracks and measures
must be taken to avoid evaporation (e.g., use of an aliphatic
alcohol evaporation retardant such as Confilm, a sheet material
such as polythene, or a mist spray of water drifting across the
surface).
Thermal stresses—Another frequent cause of early-age cracking is
thermal stress. This can be reduced by substituting pozzolanic
material for cement in the mix design and reducing placement tem-
perature. However action other than mix change may be needed,
such as avoiding restraint to thermal shortening (in the case of long
slabs); reducing temperature differentials between the element and