Civil Engineering Project Management (4th Edition)

252 Civil Engineering Project Management
removed before the mortar layer and new concrete is placed. Usually it is the
job of the resident engineer’s inspector to inspect formwork and the cleanliness
of construction joints before permission is given to the contractor to start
concreting. If the contractor runs ‘Quality Assurance’ one of his staff should
act as inspector of formwork, but this does not relieve the resident engineer of
his need to inspect on behalf of the engineer.
In liquid-retaining structures resilient plastic waterstops are usually provided
at contraction joints. Fixing half their width in the stop-end shuttering
to a narrow reinforced concrete wall often leaves a congested space for the
concrete which must therefore be most carefully vibrated in place to ensure
that the waterstop is bedded in sound concrete. If the concrete face of the joint
is to be bitumen painted before the next wall section is built, bitumen must
not get on the waterstop.
Floor joint grooves need cleaning out by water jetting, then surface drying
as much as possible with an air blower before the priming compound supplied
by the manufacturer of the joint filler is applied to the groove faces. It is
essential that this primer is not omitted, and the filler must be pushed down
to the bottom of the groove. Joint grooves are normally filled after the concrete
has been allowed to dry out for 2 or 3 weeks when most shrinkage on drying
should have taken place (see Section 19.11).
Leaks from liquid retaining concrete structures are most likely to occur
from opening up of wall joints due to wall movement, especially at the corners
of rectangular tanks; and puncturing of the floor joint filler under liquid
pressure where the filler has not been solidly filled to the base of the groove.
19.11 Concrete finish problems
The skill required by carpenters to make and erect formwork for concrete is
seldom fully appreciated. The formwork must remain ‘true to line and level’
despite substantial loading from the wet concrete. Column and wall faces
have to be strictly vertical, and beam soffits strictly level, or any departure
will be easily visible by eye. Formwork for concrete which is to remain
exposed to view has to be planned and built as carefully as if it were a permanent
feature of the building. Many methods have been tried to make the
appearance of exposed concrete attractive: but any of them can be ruined by
honeycombing, a bad construction joint, or by subsequent weathering revealing
that one pour of concrete has not been identical with adjacent pours, or
that the amount of vibration used in compacting one panel has been different
from that used in others. If concrete has to remain exposed to public view,
then the resident engineer should endeavour to agree with the contractor
what is the most suitable method for achieving the finish required if the specification
or drawings do not give exact guidance on the matter. The problem
is that if, through lack of detailed attention, a ‘mishap’ on the exposed surface
is revealed when the formwork is struck, it is virtually impossible to rectify it.
Sometimes rendering the whole surface is the only acceptable remedy.