Although most concrete nowadays is delivered ready-mixed, there may be occasions when it is more economic and practicable for the concrete to be batched and mixed on site.
There are many different types and sizes of batching plants and mixers; the following general recommendations apply to all mixer set-ups, and may also be relevant to ready-mixed concrete.
There are many different types and sizes of batching plants and mixers; the following general recommendations apply to all mixer set-ups, and may also be relevant to ready-mixed concrete.
The main objective is to produce every batch with the required consistence, strength and other specification requirements.
Storage of materials
Materials must be stored so that they are not harmed in any way during storage.
Cement must be kept dry either in silos or, if in bags, kept under cover and off the ground as described on page 9 under Delivery and storage of cement.
Aggregates should be handled and stored so as to avoid segregation and contamination by other aggregates, or by fuel, mud, etc. Each site will impose its own conditions and will have to be considered individually, but factors to be taken into account include:
- Access for delivery
- Adequate storage area available in relation to the quantity to be stored
- Drainage
- Avoidance of double handling
- Convenience in relation to subsequent use.
Further information on aggregate storage is given on page 1 3 under Storage of aggregates. Storage of water is not usually a problem if a normal mains supply is used. If water is taken from a stream, well or lake, some storage may be needed in addition to the tank provided on the batching and mixing plant. Water-heating facilities may be required if concreting is to continue during cold weather.
Special arrangements should be in place to prevent contamination of the site when cleaning down plant and equipment.
Concrete mixers
Concrete mixers are designated by a number representing the nominal batch capacity in litres and a letter indicating the type of mixer, as follows:
- Tilting drum, type T
- Non-tilting drum, type NT
- Reversing drum, type R
- Forced action, type P (commonly known as a pan mixer). Thus a 200 litre tilting drum mixer is designated as 200T.
A concrete mixer must be accurately levelled, and checked regularly to see that it stays so; inaccurate levelling results in poor mixing and increases mechanical wear as well as affecting weighing accuracy.
Mixers should not be overloaded beyond their rated capacities, otherwise spillage of materials will occur and the mixing will be less efficient, leading to lack of uniformity within the batches of concrete; mechanical wear will also be increased.
When bagged cement is to be used, the selection of the size of mixer should be related to the number of whole bags required for each batch.
Batching
For all but the smallest of jobs and for all strength classes of concrete over 20 N/mm2, all materials should be weigh-batched. Provided the weighing mechanisms are carefully maintained and regularly calibrated, reasonable accuracy should be achieved in the material proportioning.
There are many different types and sizes of batching plant and the choice for a particular job will usually depend on the amount of concrete to be produced, daily and in total.
For large quantities of concrete the aggregate weigh hoppers are likely to be fed from overhead storage bins and then discharged directly into the mixer, or onto a conveyor belt feeding the mixer. The cement from a silo with its own weigh hopper is usually fed directly into the mixer.
On smaller sites, where the output may be 20 - 50m3 a day, the materials are often weigh-batched into a loading hopper that is integral with the mixer (Figure 14). When in the lowered position the hopper rests on a load cell or hydraulic capsule connected to a weighing dial, which should be in a position such that it can easily be seen by both the mixer driver and the drag-line skip operator. The cement will be weighed in the cement silo dispenser and fed directly into the mixer or into the mixer hopper.
The method and order in which the materials are fed into the mixer can affect the uniformity of the concrete; this applies particularly to the water. Ideally, the cement, sand and coarse aggregate should be fed into the mixer simultaneously; this produces a more uniform concrete than when the materials are introduced one after another. Similarly, the water and admixtures should enter the mixer at the same time and over the same period as the other materials. This is not always possible, in which case it is advisable to start the flow of water a little in advance of the other ingredients. If all the water is added before or after the other ingredients, the batch of concrete is liable to vary in consistence from part to part.
Where the loading hopper turns upside down to discharge into the mixer, it is best if the coarse aggregate goes into the hopper first so that it pushes the sand and cement out in front of it and gives a clean discharge of the hopper. When the cement is fed into the hopper from the silo dispenser, it is best for it to be sandwiched between the coarse and fine aggregates. If bagged cement is used, the weights of sand and coarse aggregate should be adjusted to suit a whole number of bags. Attempting to judge half or quarter bags by splitting them leads 1 large errors and variability between batches.
On the rare occasions when volume batching of aggregates is unavoidable it should be done with buckets or gauge boxes, and on no account should batching by the 'shovelful' be permitted. Allowance should also be made for bulking of the sand; sand increases in volume, up to 20 - 30%, as the moisture content rise to about 5 - 6%. Further increases in moisture content result in a decrease in bulking until, when the sand is completely saturated, its volume is almost the same as it was in a dry condition. Unless tests are made, it is usual to assume an average value of 20% for the bulking of damp sand.
Operation of site mixers
It is the mixer operator's responsibility to ensure that the concrete is properly mixed, uniform throughout each batch and at the required consistence. He must see that the materials are being accurately batched and, when a loading hopper is in use, that they are loaded in the right order and that the hopper is uniformly loaded (non-uniform loading can lead to weighing inaccuracies).
Adding the right amount of water to each batch is the mixer driver's main responsibility, so that consistence is maintained from batch to batch. The free water content should be the same for each batch. However, as described on page 1 3 under Storage of aggregates, the moisture contents of the aggregates are likely to vary so that the actual amount of water to be added may also have to be varied in order to keep the free water in each batch the same. This is best dealt with by adding most of the water, estimated from the average moisture contents of the aggregates, but keeping a little back to add later if it is needed. A skilled mixer driver can tell by looking at the concrete in the mixer as it gets to the end of mixing whether enough water has been added to give it the right consistence. An ammeter or kilowatt meter connected to the mixer motor also give a good indication. Normally, the amount of water to be added from batch to batch will not vary much, only by about 5-1 0 litres/m3.
Mix proportions are usually based on the saturated surface-dry weights of aggregates. For weigh-batching purposes an allowance has to be added for the moisture contents.
Batch weights of aggregate need to be adjusted to allow for variations in their moisture contents in order to reduce variations in consistence and strength, and if aggregate deliveries can be seen to have widely different moisture contents and they are to be used immediately, batch weights may require adjustment; similarly, after rain has fallen on exposed stockpiles, adjustments may be necessary. If, on the other hand, the weather is settled and stockpiles and deliveries are known not to have widely varying moisture contents, such adjustments are not necessary because of their smallness in comparison with the total batch weights.
Thorough mixing of concrete is essential. Mixing times will vary according to the mix, the mixer and whether or not it is being filled to capacity. A uniform colour is usually the best guide to whether the mixing has been efficient. For rotating drums up to about 1 m3 capacity, the mixing time needs to be 1½ - 2 minutes after all the materials have been fed in. Very small mixers used on building sites and some large free-fall mixers require longer times. For pan mixers, because of the forced action, 30 - 45 seconds is usually enough. When the concrete is mixed it should be discharged in one operation before loading the next batch.
With a clean mixer - at the beginning of a day's concreting, for example - some of the finer material from the first batch will stick to the mixer sides and blades and the batch will be discharged harsh and stony, short of sand and cement. To make up for this loss of fine material, the amount of coarse aggregate in the first batch should be reduced by about half and the water addition reduced to maintain the required consistence.
The mixer drum must be thoroughly cleaned out after the end of concrete mixing for the day, and before long stoppages such as meal breaks, by filling the mixer with coarse aggregate and water and allowing it to rotate for about five minutes before emptying; this will remove any build-up of hardened mortar on the blades or sides of the mixer.
Weigh hoppers should be cleaned daily to prevent any build-up of material, especially if the cement is being put into the hopper, otherwise dial gauge readings may be inaccurate. There is also a risk of aggregate spillage building up around the weighing mechanism under the hopper, resulting in inaccurate weighing, and any such build-up should be removed daily.