Civil Engineering Reference
In-Depth Information
involved, proper curing is often omitted and the result can be premature
drying out, excessive shrinkage or delamination.
13.8 Quality assurance
Introduction
Sprayed concrete is more dependent on those who place it than is conventional
concrete; a trained nozzleman is therefore imperative (
Figure 13.7).
This
also makes it difficult to separate quality control from quality assurance.
The nozzleman controls the water in the dry process and, as a result the
in
situ
mix proportions will be different from the batched proportions. If the
requirement is that it should adhere to the substrate, it is the nozzleman and his
assistant whose actions will control whether or not it does. The nozzleman's
method of working may create variations in the quality of succeeding layers
and therefore density (or perhaps even homogeneity), as well as strength,
needs to be checked. Cores are used for this purpose and these can also help
to check the thickness of the cover to the reinforcement.
Prequalification of nozzlemen
Each nozzleman should demonstrate his skill prior to commencement of
spraying and this is usually done by spraying into test panels, usually about
750 × 750 × 100 mm thick, set at the orientation of the proposed works
(vertical, sloping, horizontal or overhead). The test panels should replicate
the type of concrete, i.e. reinforced (bar, fabric or fibres) or unreinforced.
Cores are subsequently taken to ensure that strength, density, freeze/
thaw resistance, chemical resistance and filling behind reinforcement are all
acceptable.
13.9 Quality control
Quality control must be continuous throughout the works, from drawings
through to finished product and must be carried out by someone with
experience of this type of work. The following should be included:
Review
The method of placing the concrete must be considered whilst the type, size
and location of the repairs are being determined. Reinforcement is a main
consideration - bar sizes, locations and staggering of overlaps, methods of
(rigid) support are all important. Large bars, laps and small gaps behind
exposed bars can all cause shadowing; that is, where the sprayed concrete
behind the bar does not get fully compacted.
