Page 271 - Civil Engineering Project Management, Fourth Edition

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Site concreting and reinforcement
reason, the contractor has to take swift action to prevent concrete solidifying
in the pipeline. Compressed air is used to force the final concrete batch through
the line, followed by water to clean the pipes. Plasticizers are frequently used
in pumped concrete; these increase its workability without requiring increased
cement or water. There are a wide variety based on different chemicals; BS
5075:1982 gives their main characteristics, but they should not be permitted
by the resident engineer except to the extent allowed in the specification or
sanctioned by the engineer.
Concrete can also be blown through a delivery pipe using a blower or
compressed air. One batch at a time is blown through. The end of the delivery
pipe must be directed into the area to be concreted, not against formwork
which may be dislodged by the force of the ejected concrete. Proper warnings
must be given to personnel before each ‘shot’ because aggregate can rebound
and be dangerous, especially when blowing concrete into closed spaces such
as the soffit to a tunnel lining.
The skip method of placing concrete is widely used. Skips can be either
bottom-opening, or tip-over. In either case there can be a considerable bounce
and sway of the skip when the concrete is discharged. The work should
always be under the charge of an experienced ganger who keeps a continuous
watch over the safety of his men.

19.10 Construction and other joints

The resident engineer must agree with the contractor where construction
joints should be placed; but he should not require them to be placed in
impracticable positions and must allow for the manner in which formwork
must necessarily be erected. There are positions for construction joints which
are ‘traditional’ even though the position may not seem to be the most desir-
able from a structural point of view. For instance a construction joint usually
has to occur at the base of a wall even though it cantilevers from a base slab,
which is a point of maximum tensile stress in one face of the wall concrete.
This joint is best sited 150mm above the base slab so as to give a firm fixing
for the wall shutters and the best possibility of achieving a sound joint. In
water-retaining work it is important to keep the number of construction joints
to a minimum.
The bonding of one layer of concrete to a previous layer is usually accom-
plished by cleaning the surface of the old concrete with a high pressure water
jet, and placing a layer at least 2cm thick of mortar on the exposed surface
immediately before the new concrete is placed. Sometimes a proprietary
bonding mortar is used, especially when refilling cut-out portions of defective
concrete. Wire brushing of the old surface is not so effective as water jetting,
is laborious, and can seldom be properly done when reinforcement passes
through a joint. A problem frequently encountered is that of finding debris on
a construction joint at the bottom of erected formwork. Such debris must be