Notes on Physical Testing
A Modern Concrete Crushing Machine
(Photograph Courtesey of Qualitest)
The compressive strength of a concrete has
traditionally been used as a specification tool by engineers to specify
the quality of concrete delivered to a job. It has been used for many
years because it is a relatively simple property to measure, unlike
chemical analysis which requires a well equipped chemistry laboratory to
determine whether an adequate cement content has been used. On site, a
simple slump test will normally be carried out to show whether the
concrete complies with the specified workability.
The concrete will normally be specified as a
"designed mix", meaning that the engineer will have asked for a concrete
of a certain strength, either to ensure that he has adequate strength
for structural purposes, or, more usually, to ensure the overriding
requirement that the concrete has adequate durability. The first thing
to realise when measuring concrete strength is that concrete is not a
uniform material and, when placed, it is even less so. Even with good
quality control, a spread of results of 16 N/mm2 is likely
with a Grade 40 concrete. This means that to be sure of getting most
(95%) of his results above 40 N/mm2, the concrete supplier
will aim for a target mean strength of some 48 N/mm2.
Designated mixes give the specifier a simpler means
of specifying concrete for a particular job. For example, the specifier
wanting a concrete suitable for a domestic driveway would call for a
PAV1 mix. This automatically requires a minimum strength of 35 N/mm2,
a minimum cement content of 300 kg/m3 and a maximum
water/cement ratio of 0.6, with air entrainment. For heavier duty, a
PAV2 mix may be specified, and so on.
Recent experience suggests that concretes made with a
high replacement content of GGBS (50% typically) may not be suitable for
winter concreting, especially for heavy duty external paving.
These concretes have a tendency to bleed and to form a thick layer of
laitance on the surface, which causes extreme dusting. Similarly,
specifiers should be cautious about the use of polypropylene fibre
reinforcement in external paving subjected to freezing and thawing,
where air entrainment is not specified. Experience has shown that
it is the quality of the cement matrix and the aggregates which govern
freeze-thaw performance and that these fibres are not a substitute for
air-entrainment.
It must be appreciated that the results gained from
concrete core samples will not be directly comparable with those from
the original cube tests. Cube samples are fully compacted and have been
stored under ideal curing conditions for 28 days, prior to test. Core
samples, on the other hand, have been taken from insitu concrete, cured
in the structure, often inadequately, with perhaps less than perfect
compaction. There are also settlement effects, with results on cores
from the bottom of a wall or column differing by 15-30% from those of
cores taken near the top. Not surprisingly, there are large differences
in the strength measured on cores and on cubes made from the original
concrete, as supplied. Concrete Society Technical report No. 11 gives
excellent guidance to anyone wishing to find their way through the maze
of measuring strength in concrete. It provides guidance on planning an
investigation, interpreting the results and comparing the test values
with those from the original concrete as delivered. A further Concrete
Society technical report has studied the effect of a range of different
placement situations and also the use of ggbs and pfa cement replacement
materials. This report has shown a very wide variation in strength
correlation between cores and cubes for a wide range of different
concretes and types of construction. The end result is that
proving compliance or otherwise of concrete by core testing is virtually
impossible.
Measuring Concrete
Strength
Core samples are the most common form of sample for
this purpose, removed from the structure by diamond drilling. Typically
cores will be 100mm in diameter, and should ideally be at least three
times the maximum aggregate size in diameter.
The cores are usually visually described and
photographed, concentrating especially on compaction, distribution of
aggregate, presence of steel etc. and then trimmed to a length to
diameter ratio approaching 1:1. Various methods are available to ensure
accuracy of the ends of the core; grinding to a perpendicular flat
surface, capping with high alumina cement mortar and capping with hot
sulphur are all methods which are used.
The capped core is then crushed (after appropriate
curing for HAC capped specimens) in a calibrated compression testing
machine. The resulting failure load is converted firstly to a cylinder
strength and secondly to an equivalent insitu cube strength. This can
vary from some 70% to 150% or so of the original cube result, depending
on where the core was taken, the type of member and the age, before
allowing for any additional correction due to compaction effects.
