Civil Engineering Reference
In-Depth Information
large quantities of dust are present. The volume proportions are found by the
method of point counting using a mechanical stage. The amount of coarse
aggregate can also be assessed by this method if a distinction can be made
between coarse and fine aggregate. The results obtained usually represent
the sample reasonably, but may not represent the concrete.
The amount of individual rock types present in the aggregate as a
whole are assessed and the saturated density of the sample is measured by
the method of immersion in water using vacuum impregnation to ensure
saturation. From this information and the volume proportions, the weight
fractions of aggregate, cement and water can be calculated.
Water/cement ratio
The hydration processes of cement paste vary significantly with the original
water/cement ratio. Concretes with a low water/cement ratio tend to leave
substantial quantities of unhydrated cement clinker and to develop only
limited amounts of coarsely crystalline calcium hydroxide. In particular,
the extent to which calcium hydroxide is separated into layers on aggregate
surfaces and occurs in voids and on void surfaces varies with the original
water/cement ratio. The number and proportion of unhydrated cement
clinker particles varies inversely with the original water/cement ratio.
Comparison with standard concretes made with known water/cement ratios
visually and by measurement allows the water/cement ratio of the cement
paste to be assessed directly. The standard error attached to the estimation of
water/cement ratio by this means is considered to be approximately ±0.03.
A Concrete Society Technical Report has recently been published giving
full details of what can be achieved using this technique (Concrete Society,
2010).
1.6 Schmidt hammer (rebound hammer)
The rebound principle for concrete testing for surface hardness is widely
accepted. The most popular equipment, the Schmidt rebound hammer, has
been in use world-wide for many years. Recommendations for the use of the
rebound method are given in BS EN 12504-2 (BSI, 2001) and ASTM C805
(ASTM, 2008).
Rebound test equipment and operation
The Swiss engineer Ernst Schmidt first developed a practicable rebound test
hammer in the late 1940s and modern versions are based on this. A spring-
controlled hammer mass slides on a plunger within a tubular housing. The
plunger retracts against a spring when pressed against the concrete surface
and this spring is automatically released when fully tensioned, causing the
hammer mass to impact against the concrete through the plunger. When the
spring-controlled mass rebounds, it takes with it a rider which slides along
