Mixing and Testing Concrete
Topic 2 – Mixing and Testing Concrete
Concrete’s finished quality is determined by the water/cement ratio (W/C). If mixing by hand the proportions of cement to aggregate are 1:5. The amount of water needed to achieve the correct W/C ratio is added to the mix as well. The lower the ratio the more durable the finish but the less workable the concrete. The amount of aggregate you put in a mix has little to do with the strength of concrete. As you add more aggregate to the mixer, the mix becomes dryer and harder to place, but the strength does not change (provided there is enough cement paste to fill the spaces between the aggregates).
The advantages of ready mixed concrete are:
- Uniform product
- Convenience
- Economy
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Water should not be added to a ready-mix order as it will reduce the quality of the mix, increase wait time, and take longer to finish. In hot weather, dampening the subgrade will slow down evaporation without affecting the workability of the mix.
A slump test is used to measure the consistency of the mix and should be done to ensure the proper slump has been delivered.
Concrete should be tested to ensure certain design properties are in the mix. The concrete is sampled at different intervals during placement. The samples are taken from a wheelbarrow instead of the chute or forms and stored in test cylinders. Tests include:
- Air entrainment tests
- Compressive strength tests
- Wear resistance test
- Permeability tests.
Concrete strength is based on a curing time of 28 days. Sample tests are taken during placement, stored and then tested to see if the mixture has the desired strength at different stages in the curing time.
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For additional information review Principles and Practices of Commercial “Concrete Work.” (P 233)
Review Instruction Sheet (IS) on Brightspace:
CONC 100/122, p. 17 – 39:
- IS 3.2 Water-Cement Ratio and Mixing a Trial Batch
- IS 3.3 Ready-Mixed Concrete
- IS 3.4 Concrete Mixing Using a Mixer
- IS 3.5 Terminology Used in Concrete Construction
- IS 1.2 Testing Concrete