Production of High strength concrete
Five Division of High Strength Concretes
1. A minimum crushing strength of from 35 to 50 Mpa at 28 days for concrete subjected to normal to medium to high stresses.
2. A minimum strength of 35 to 70 Mpa at 28 days for particularly highly stressed concrete.
3. A minimum strength of 30 Mpa at 3 or 4 days for concrete that is to be post tensioned at an early age or precast and pre-tensioned when several days’ curing can be tolerated.
4. A minimum strength of 30 Mpa at 12 to 24 hours for post-tensioned concrete when earlier application of stress is required, or precast and pre-tensioned when the moulds are used once per day.
5. A minimum strength of 30 Mpa at 4 hours or less for precast a nd pre-tensioned concrete when moulds are used twice a day.
Important points in designing a mix concretes for high strength.
1. Don’t consider/count the lowest single test as the minimum strength for this may be because of errors in making the test. Permit a small perscentage of the test results to fall below the specified nominal strength of the test results to fall below the specified nominal minimum strength.
2. Make the specimens in groups of three from the same mix sample in order to reduce the risks of errors in making and in testing.
3. It is easier to work with a single graded coarse aggregate , but it may be necessary to use more than one sand, in order to maintain uniformity of grading from batch to batch, and to avoid segregation in handling prior to mixing.
4. A sand with an appreciably higher bulk density than the coarse is more satisfactory.
5. The maximum particle size of the sand should be less than the void size of the coarse aggregates so that the whole sand volume may be absorbed in the coarse aggregate without bulking.
6. Cement and water-cement ratio should me the minimum that will give the desired workabilityand strength; the result will be minimum shrinkage, low permeability and high durability.
7. The use of a water-reducing admixture is usually beneficial, but it is most important that calcium chloride admixtures shall not be used in prestressed concrete because of the resulting corrosion of the tendons.