Provided certain simple rules are followed good concrete can be achieved by methods varying from the ‘bucket and spade’ hand-labour method to use of the most sophisticated weigh-batching and mixing plant. The following shows the principal matters that should receive the resident engineer’s attention.
First, choose good aggregates. The best guide is to use well-known local aggregates that have been and are being used satisfactorily on other jobs elsewhere. A reputable supplier will be able to name many jobs where his aggregate has been used, and the resident engineer will not be over-cautious if he visits one or two of these where the concrete is exposed to view. When the aggregates are being delivered on the job (not just the first few loads, but the loads when the supply has really got going), random loads as delivered should be examined. Handfuls of aggregate should be taken up and examined in detail, looking for small balls of clay, soft spongy stones, flaky stones, pieces of brick, soft shale, crumbly bits of sandstone, and whether clay or dirt is left on the hands after returning the handful. If the engineer finds more than a few handfuls, he should request the contractor to bring this to the notice of the supplier. He need not reject the load out of hand, but it will do no harm to let the supplier know the aggregates are being watched. If a load contains numerous weak stones or several pieces of clay, it should be rejected.
Diagnosing whether an aggregate is likely to give rise to alkali-silica reaction (which can cause expansion and disruption of concrete in a few years in the presence of moisture) requires specialist knowledge. The most practical approach for the engineer is to ask the supplier if his aggregate has been tested for this; if not, structures built some years previously with the aggregate should be checked for signs of cracking due to alkali-silica reaction. Guidance and precautions are set out in certain publications (References 1 and 2), but if it is proposed to use an aggregate not used before, the site staff should refer the problem to the engineer.
Second, choose tested cement. The same principle applies to cement as with the choice of aggregates; find the supplier of cement to other jobs and request a recent test certificate. Troubles can start when imported cement has to be used or cement from a variety of suppliers. Overseas it is not unusual for a small contractor to buy his cement a few bags at a time from the local bazaar.
Testing such cement on site before any concrete is placed in an important part of a structure is essential. BS 12 provides methods for testing the compressive strengths of 1:3 mortar cubes or 1:2:4 concrete cubes but, if this is difficult to arrange, the flexural test mentioned in Section 19.3 can be applied on site.
Third, ensure reasonably graded aggregates. In delivery and stockpiling of coarse aggregate there is a tendency for the mix to segregate, the larger material remaining on top. Care has to be taken to ensure that certain batches are not made up from all the coarsest material and others from most of the fines.
Crushed rock often has a considerable amount of dust in it, although this does not normally present a problem one does not want a batch made up mostly from dust and fines taken from the bottom of a stockpile.
Fourth, use washed aggregates. Unwashed aggregates suitable for concreting are rare: they are usually comprised of crushed clean homogeneous rock.
Sometimes a river sand is supplied unwashed – it being assumed that the sand has already been ‘washed’ by the river. This should not be accepted as a fact, since a river also carries silts and clays. Sea-bed or beach sands must be washed in fresh water to remove the salt from them.
Fifth, achieve the right workability. Mechanical mixers are seldom at fault with regard to mixing, and hand-mixing can also be quite satisfactory; but it is the water content of a mix that requires the most vigilant attention. The site engineer should never let ‘slop’ be produced. Although the slump test and the compacting factor test are useful in defining the degree of stiffness of a mix, in practice judging the water content of a mix ‘by eye’ is both necessary and possible.
The right sort of mix should look stiff as it comes out of the mixer or when turned over by hand on mixing boards. It should stand as a ‘heap’ and not as a ‘pool’ of concrete. When a shovel is thrust into such a pile, the shovelcut should remain open for some minutes. Such a mix will look quite different after it is discharged and worked into some wet concrete already placed.
As soon as it is worked with shovels or vibrated, it will settle and appear to flow into and become part of the previously placed concrete.
The same characteristic makes it possible to judge the water content by noticing what happens if the freshly mixed concrete is carried in a dumper hopper to the point of discharge. The ‘heap’ of stiff concrete discharged from the mixer to the dumper hopper will appear to change to a pool of concrete as the dumper bumps its way round the usual site roads. When the dumper hopper is tipped, however, the concrete discharged should again appear stiff.
But if, in transport, the concrete slops as a semi-fluid over the side of the dumper hopper, this shows too much water has been added. Asimple density test on freshly mixed concrete (see Section 19.8) may assist in finding if the mix has too much water.
Sixth, ram the concrete well in place. Properly shovelled, rodded, or vibrated, the concrete should be seen to fill the corners of shuttering and to easily wrap around the reinforcing bars. When hand shovelling or rodding is adopted, it is scarcely possible to over-compact the concrete. But when mechanical vibrators are used the vibration should not be so prolonged as to produce a watery mix on the surface. Vibrators of the poker immersion type should be kept moving slowly in and out of the concrete. They should not be withdrawn quickly or they may leave an unfilled hole in the concrete; nor should they be left vibrating continuously in one location. Where vibrators are used, it is necessary for the contractor also to have available suitable hand rammers in case the vibrators break down in the middle of a pour.
Seventh, ensure the mix has sufficient cement in it. Normally contractors will use a little more cement than is theoretically necessary and this is helpful since batches of concrete vary. But if a contractor becomes too keen on cutting the cement to the bare minimum, a number of the cube crushing tests may fail to reach the required strength, and much delay may be caused by conducting the investigations required to seek out the cause.