1. Tests, carried out mainly in a laboratory, to assist in deciding whether particular sources of material are suitable for concrete and conform to the specification requirements and, if so, in what proportions they should be combined to produce concrete of the required properties.
In the case of site-mixed concrete, these will usually consist of tests on the aggregate followed by trial mixes to establish the proportions for the various concretes to be used. For ready-mixed concrete, suppliers normally provide mix design certificates for the different concrete classes specified along with information about the aggregates and their properties.
In the case of site-mixed concrete, these will usually consist of tests on the aggregate followed by trial mixes to establish the proportions for the various concretes to be used. For ready-mixed concrete, suppliers normally provide mix design certificates for the different concrete classes specified along with information about the aggregates and their properties.
2. Routine on-site tests for both control and conformity.
Aggregates are tested for grading and moisture content in the case of site mixing, and both site- and ready-mixed concrete are tested for consistence and strength. Site tests are reasonably simple and, although staff do not need special skills, they must know the Standard requirements for those tests which they have to perform. Meaningful results can be obtained only if tests are carried out strictly in accordance with the relevant British Standard method.
This section concentrates on the site tests, though a few references are made to laboratory tests that, on occasions, may be used in the field.
Detailed descriptions of the methods of sampling and testing are given in a number of British Standards that are listed under References/further reading.
Sampling materials
The object of sampling is to produce a truly representative quantity of the consignment being sampled and of sufficient quantity for the tests required. Aggregate and concrete are heterogeneous materials, so great care is needed in sampling to ensure that the sample is truly representative if reliable test results are to be obtained. Materials being delivered to site in relatively small lots are best sampled during delivery to stockpiles.
Cement
The requirements for sampling cement are given in BS EN 196. The manufacturer supplies test reports on a regular basis, which should be studied by the concrete producer. Cement testing is rarely required on site.
Aggregates
Details of sampling aggregates are given in BS 812 : Part 102 and BS EN 932-1 .
The bulk sample of each type or size of aggregate to be tested should be obtained by collecting increments (scoopsful) to provide the quantity required for all the tests to be made. The minimum number of increments should not be less than those shown in Table.
Sampling may have to be done in a wide variety of conditions and it is not possible to describe in detail the procedure for obtaining increments in all circumstances. When sampling from heaps, the increments should be taken from different places in the stockpile, removing at each position the top 150 mm before digging in the scoop. For material being loaded or unloaded from a vehicle, or being discharged from a conveyor belt, the increments should be taken at fairly regular intervals distributed during the movement of the quantity being sampled. It is virtually impossible to obtain a representative sample from lorries, before or after discharge, which have been layer-loaded. When sampling from a stockpile, representative samples should be obtained by first digging holes of various depths from the top.
Table : Minimum number of sampling increments for aggregates.
Nominal
size of aggregate
D ma x
|
Minimum number
of sampling increments
|
Minimum
sample size for normal density aggregate
|
|
Large scoop
|
Small scoop
|
||
20 mm and larger
|
20
|
-
|
50 kg
|
5 mm to
20 mm
|
10
|
-
|
25 kg
|
5 mm and
smaller
|
10
half scoops
|
10
|
10 kg
|
The bulk sample usually has to be reduced to a smaller quantity, depending on the amounts required for the particular tests. A sample divider (riffle box) is the most convenient method. This is designed so that material poured in the top is divided approximately equally and diverted to two sides; material to one side of the box is discarded and the remainder tested or divided to a smaller sample (Figure 34). Coarse aggregates may be divided while damp, but sand should be surface-dry.
Concrete
Correct sampling of concrete is essential to obtain representative test results of a batch of concrete. Sampling is fully described in BS 1881: Part 101 and BS EN 12350-1, which also give the quantities required for the different tests. Whenever possible, the sampling should be done when the concrete is moving in a stream, such as when it flows down the discharge chute of a mixer or is being conveyed on a belt. Concrete may be sampled from a stationary lorry or heap, but this method is less satisfactory. Concrete cannot be sampled satisfactorily from a discharging tipper lorry or dumper.
A sample consists of a number of standard scoopsful taken from a batch; Table 19 gives the required number of scoopsful for some of the more usual tests. Figure 36 shows a suitable scoop that will provide about 5 kg of normal-weight concrete. As a general rule, the sample size should be about 1½ times the quantity required for testing.
Table : Quantities of concrete required.
Test or specimen
|
Number of standard scoopsful
|
Slump
|
4
|
Compacting factor
|
6
|
Degree of compactability
|
12
|
Air content
|
4
|
100 mm cube (per pair of cubes)
|
4
|
150 mm cube (per pair of cubes)
|
4
|
NOTE
Take sufficient scoopsful for tests to be made on different
samples
|
|
The batch should be nominally divided into a number of parts equal to the required number of scoopsful. For samples taken from a moving stream, such as a batch-mixer or ready-mixed concrete truck-mixer, the spoonsful should be taken at equally spaced intervals; avoiding the very first and very last parts of the discharge. Thus, when four scoopsful are needed to make up the required test sample, they should be taken about the time when one-fifth, two-fifths, three-fifths and four-fifths have been discharged, the scoop being passed through the whole width and thickness of the stream in a single movement.
When sampling from lorries or heaps, the spoonsful should be distributed through the depth of concrete as well as over the exposed surface.
Scoop dimensions
Dimension
|
Small scoop
Use for sampling aggregates only
|
Large scoop
Use for sampling aggregates and concrete
|
Length
|
200 mm
|
250 mm
|
Diameter
|
100 mm
|
125 mm
|
Where the point of mixing and the point of placing are some distance apart, there is the choice of taking samples at either place. Sampling and testing at the mixer has the advantage of enabling adjustments to the concrete to be made more quickly, but consistence tests can be more easily related to the placing conditions if done at the point of placing. On some jobs it may be useful to carry out a few tests at both places so as to ascertain, for example, the change in consistence during transport in hot weather or when long delays, of half an hour or so, occur between mixing and placing.
In the case of ready-mixed concrete an alternative method of sampling is permitted for the slump test only. For this, six standard scoopfuls should be collected in a bucket or other suitable container after about 0.3 m3 has been allowed to discharge, so that the load can be tested before the main discharge takes place.
For all tests the sample, consisting of a number of scoopfuls in one or more buckets, will require thorough mixing. This should be done by emptying it from the container onto a non-absorbent surface, preferably a 900 mm square metal tray, and shoveling it to form a cone, which should be turned over to form a new cone three times. When forming the cones each shovelful should be placed on the apex of the cone so that the portions that slide down the sides are distributed as evenly as possible. BS 1881:
Part 101 requires each sample to be accompanied by a certificate from the person responsible for taking the sample stating that the sampling was done in accordance with British Standard. Appendix 2 shows a suitable certificate.