Civil Engineering Reference
In-Depth Information
Cement content
Minimum 350 kg/m
3
2
Sand content
Not more than twice the cement content
3
Total aggregate
Not more than six times the cement content
4
Coarse aggregate
For 20 mm max. size - ideally not more than 20% to
pass a 10 mm sieve
5
Consistence
Not critical, but appropriate for good placement and
compaction around the steel
6
Water/cement ratio
Normally 0.5 or less
And, of course, the steel reinforcement should be provided with
appropriate cover for the exposure conditions. These seem simple enough,
but it is remarkable how often these simple rules are not followed and the
concrete is compromised from day one.
1.3 The role of non-destructive testing
1.3.1 Tests at the time of construction
NDT can be used initially at the time of construction to help to ensure that
the structure is correctly built with the right materials and the right cover.
While cube tests are being conducted, it is entirely possible to carry out some
Schmidt hammer tests (
Figure 1.2)
on the side of the cube prior to loading
and support these with some ultrasonic pulse velocity (UPV) measurements.
In this way, a set of reference data for strength against Schmidt hammer and
UPV can be built up. If there is any question regarding the quality of concrete
in a site component, or maybe just randomly on site, these techniques can
then be used to confirm the strength and quality of the concrete, as placed.
Far too often, if there is a dispute regarding cube tests or cylinder tests, the
engineer will call for core tests to be carried out. The relationship between
core tests and cube tests is very complex, depending on the type of member,
its curing, the type of concrete, orientation and a range of other factors.
Core testing often results in posing more questions than it answers. It is
possible using BS EN 13791 (BSI, 2007) and the recently published BS
6089 (BSI, 2010) which offers complementary guidance to BS EN 13791,
to estimate the in-situ strength of the concrete in a component. This can
be compared with the design strength and a decision can then be made on
adequacy of the concrete. In the author's view, attempting to correlate the
core and original cube tests, however, is so fraught with difficulty that it is
not recommended.
Table 1.2,
reproduced from a Concrete Society Technical
Report (Concrete Society, 2004), illustrates the problem. The data shows the
variability between core and cube strength for the same concrete cast into
different types of members, with different cement types, at different ages.
