Procedures for implementing improved practice for mix proportions in concrete

Procedures for implementing improved practice for mix proportions.

Enforce the specifications for mix proportions in a gradual and orderly way so that work is not slowed down. Follow the sequence given below.

• Keep all equipment and tools in a clean and serviceable condition so that good workmanship will result.

• Ensure correct amounts of materials, with improved control even on volume batching.• Allow for bulking of moist sand ( in case of volume batching ) at least on a rough basis at the beginning . Move over to more exact determination later.• Supply clean and serviceable containers for measuring water to be added to concrete mix. Allow an experienced mixer operator to adjust the amount of water, depending on his judgment. In the next stage, use slump test as a check on workability and as a help to the mixer operator. In the third stage, give a rough indication of the correction required for surface moisture carried by aggregates as a further help in substantiating the amount of water to be added. Use slump test as a guide in the fourth stage, and in important works, carry out laboratory test to find out the moisture content on aggregate. Do suitable correction for mixing water.

• Introduce the simple tests for finding clay and organic matter, in sand . With these tests, it will be possible to arrange for washing the aggregate whenever required. If the source proves to be good enough, washing can be stopped and the two tests can be made, say once a week, as a check.

• Introduce the grading requirements of aggregates gradually, in the following order:

(i) Remove excess fractions of aggregates by screening whenever the aggregate appears to be oversized or containing too much fine materials. It is not necessary to remove all fines but the amount should be within specifications and present uniformly throughout.

(ii) At the second stage, change the relative proportions of sand and coarse aggregate ( keeping the total weight constant) whenever changes in grading of sand leads to change in workability. For example, reduce the sand content if the mix is consistently over sanded. Increase the coarse aggregate content correspondingly.

(iii) At the third stage, do proper sampling of aggregates and sieve analysis, to detect changes in grading. If a change in grading is shown by consecutive sieve test (say three days), a change in the mix proportion can be made.

• If weigh batching is followed , the above methods can be adopted, except that correction for bulking of moist sand will not be necessary.