Concrete can arrive on your job site in two ways, either centrally mixed or truck mixed. While the former is mixed completely in a stationary mixer and then transferred to another piece of equipment for delivery, the latter is a process by which previously proportioned concrete materials from a batch plant are charged into a ready-mix truck for mixing and delivery to the construction site.

The water-cement ratio plays a key role in the workability of the concrete. The maximum allowance provided in the specifications should never be exceeded. Longer haul distances and warmer weather make it very difficult to control consistency. Various options are available to overcome these situations. For example, you can control the concrete temperature with hot or cold water, withhold some of the mixing water until you arrive on the job site, use a retarder to slow down the reaction, or utilize a superplasticizer to increase slump while maintaining a low water-cement ratio.

The table, shown below, highlights some other general principles that are recommended when mixing concrete:
 

1. Uniform concrete will be produced if you feed the cement, sand, and coarse aggregate into the mixer simultaneously. Filling the hopper in horizontal layers of gravel, cement and sand, and possibly with another layer of a different gravel on top will provide the best conditions. You will also avoid the formation of a hardened layer of mortar at the bottom of the hopper, if you place gravel at the bottom of the hopper. Do not pile-up any one size of material at the throat of the hopper.

2. Admixtures, which help to achieve certain properties in concrete or ensure quality in adverse weather conditions, should be added accordingly. For example, minerals should be directly combined with the cement. If it is water soluble, it should be dissolved in the mixing water. If more than one admixture is needed, they should be batched separately in order to avoid adverse interactions between the two.

3. It is desirable to add up to ten-percent of the mixing water before the aggregates are incorporated into the mixer. The next eighty-percent of the mixing water should be uniformly added during the addition of solid ingredients. The final ten-percent of mixing water should be added after all of the aggregates have been charged.

4. Mix the concrete until the concrete is of uniform color and consistency.

5. Do not load the mixer beyond its rated capacity.

6. Properly set the axis of rotation of the mixer drum in the horizontal position, unless using the tilting-drum type.

7. The whole batch being mixed should be discharged at the same time. For example, it is not advisable to discharge in small separate quantities. This can lead to an uneven distribution of stone and sand. This is known as honeycombing as shown in Figure 1.

8. The speed of the mixer should be run at the manufactured rating.

9. Ten percent of cement and sand should be added to the first batch since the first batch of concrete mixed is left on the blades and around the drum. This will avoid difficulties in placing due to a shortage of fines.

10. Regular cleaning of the mixer should take place in order to prevent concrete from building up.

11. Replace any worn or bent blades since they will reduce the efficiency of the mixer.

12. Rub grease or oil over the mixer. This will reduce the adherence of cement.

Source: Murdock, L.J. Concrete: Materials and Practice

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