Mixing of Concrete
The mixing operation consists of rotation or stirring, the objective being to coat the surface the all aggregate particles with cement paste, and to blind all the ingredients of the concrete into a uniform mass; this uniformity must not be disturbed by the process of discharging from the mixer.
Batch mixer
The usual type of mixer is a batch mixer, which means that one batch of concrete is mixed and discharged before any more materials are put into the mixer. There are four types of batch mixer.
Tilting drum mixer:
A tilting drum mixer is one whose drum in which mixing take place is tilted for discharging. The drum is conical or bowl shaped with internal vanes, and the discharge is rapid and unsegregated so that these mixers are suitable for mixes of low workability and for those containing large size aggregate.
Non tilting drum mixer:
A non tilting drum is one in which the axis of the mixer is always horizontal, and discharge take place by inserting a chute into the drum or by reversing the direction or rotation of drum. Because of slow rate of discharge, some segregation may occur.
Pan type mixer:
A pan type mixer is a forced–action mixer, as distinct from drum mixer which relies on the free fall of the concrete inside the drum. The pan mixer consist of a circular pan rotating about its axis with one or two stars paddles rotating about vertical axis of pan.
Dual drum mixer:
A dual drum is sometimes used in highway construction. Here there are two drums in series, concrete being mixed part of the time in one and then transferred to the other for the remainder of the mixing time before discharging.
Continuous mixers:
These are fed automatically by a continuous weigh-batching system.
Charging the mixer:
There are no general rules on the order of feeding the ingredients into the mixer as this depend on the properties of the mixer and mix. Usually a small quantity of water is fed first, followed by all the solids materials. If possible greater part of the water should also be fed during the same time, the remainder being added after the solids. However, when using very dry mixes in drum mixers it is necessary to feed the coarse aggregate just after the small initial water feed in order to ensure that the aggregate surface is sufficiently wetted.
Uniformity of Mixing
In any mixer, it is essential that a sufficient interchange of materials occurs between parts of the chamber, so that a uniform concrete is produced. The efficiency of the mixer can be measured by the variability of the samples from the mix. ASTM prescribes samples to be taken from about points 1/6 and 5/6 of the discharge of the batch and the difference in the properties of the two samples should not exceed any of the following:
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Density of concrete 1 lb/ft³
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Air content 1%
-
Slump 1″ when average is less than 4″
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1.5″ when average is less than 4 to 6″
-
% of aggregate retained on 4 No. sieve 6%
- Compressive strength 7 day, 3 cylinders 7.5%
Mixing time:
It is important to know the minimum mixing time necessary to produce a concrete of uniform composition, and of reliable strength. The mixing time or period should be measured from time all the cementing materials and aggregates are in mixer drum till taking out the concrete. Mixing time depends on the type and size of mixer, on the speed of rotation, and on the quality of blending of ingredients during charging of the mixer. Generally, a mixing time of less than 1 to 1.25 minutes produces appreciable non-uniformity in composition and a significant lower strength; mixing beyond 2 minutes causes no significant improvement in these properties.
Table: Recommended minimum mixing times
|
Capacity of mixer (yd³) |
Mixing time (Minutes) |
|
Up to 1 |
1 |
|
2 |
1.25 |
|
3 |
1.5 |
|
4 |
1.75 |
|
5 |
2 |
|
6 |
2.25 |
|
10 |
3.25 |
Prolong mixing:
If mixing take place over a long period, evaporation of water from the mix can occur, with a consequent decrease in workability and an increase in strength. A secondary effect is that of grinding of the aggregate, particularly if soft; the grading thus becomes finer and the workability lower. In case of air entrained concrete, prolong mixing reduces the air content.
Ready mixed concrete:
If instead of being batched and mixed on site, concrete is delivered for placing from a central plant. It is referred to as ready-mixed or pre-mixed concrete. This type of concrete is used extensively abroad as it offers numerous advantages in comparison with other methods of manufacture:
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Close quality control of batching which reduces the variability of the desired properties of hardened concrete.
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Use on congested sites or in highway construction where there is little space for a mixing plant and aggregate stockpiles;
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Use of agitator trucks to ensure care in transportation of concrete, thus prevention segregation and maintaining workability
-
Convenience when small quantities of concrete or intermittent placing is required.
There are two categories of ready-mixed concrete: central-mixed and transit mixed or truck mixed. In the first category, mixing is done in a central plant and then Concrete is transported in an agitator truck. In the second category, the materials are batched at a central plant but are mixed in a truck.