In order to reduce rate of loss of workability generally water reducing agent / superplasticizer is used. Use of water reducing agent/ admixtures should be permitted after testing their suitability in proposed concrete mix.

Dose of water reducing agent/ super plasticizer should be decided on trials. Excessive dose may lead to segregation. In addition, there may be retardation also.

IS: 4926-2003 permits re-tempering i.e. addition of water / admixture in concrete at site to restore the workability, provided requirement for uniformity, as specified in IS: 4926-2003, are met with. Uniformity requirement specify variation limits for properties like air content, slump, compressive strength & density etc. At site, there is no way to judge whether specified uniformity requirement are met or not.

Moreover, addition of water affects the strength and durability characteristic of concrete. Thus, it is advised that re-tempering i.e. addition of water after mixing, should not be permitted at all either during transit or at the site of construction.

Ready Mix Concrete

2. Setting of concrete:

When the transit time of ready mix concrete is high, the initial setting of concrete may take place. To avoid setting of concrete, retarding admixtures can be used to prolong the setting of concrete. While permitting use of retarder, it should be ensured that the suitability and dose of retarder is decided after conducting necessary trials. It may be noted that generally retarding effect of retarder is smaller at higher temperature and sometimes few retarders seem to be in-effective at extremely high temperature. Thus, it is desirable to keep the temperature of concrete as low as possible.

In addition, it is also important that a dose of retarding admixture is tested by conducting trials. Large quantity of admixture/ retarder may delay the setting of concrete adversely or may prevent the setting of concrete totally. Some ti me dosing of admixture is done in stages to ensure desired workability. In such cases, admixture should be mixed at delivery site only. Addition of admixture should not be permitted during transit.

3. Time period for delivery of concrete:

In order to control loss of workability and setting of concrete, the concrete should be delivered completely to the site of work within one and half hours (when the atmospheric temperature is above 200C) and within two hours ( when the atmospheric temperature is at or below 200C)of adding the mixing water to the dry mix of cement and aggregate or adding the cement to the aggregate whichever is earlier.

Adequacy of the time period, required for delivery of concrete, should be checked. In case, location of site of construction is such that this time period is inadequate, delivery time period should be increased and specified clearly duly keeping some margin for hurdles in the way (i.e. level crossing/ check post/heavy traffic congestion locations etc).

Checking suitability of admixture:

As explained earlier, generally admixture like water reducing agents/ retarders are used in Ready Mixed Concrete for retention of workability and to avoid setting of concrete. IS: 9103 “Specification for admixtures for Concrete” may be referred to judge the suitability of admixtures.

According to it, the concrete mix should be prepared both with and without admixture using the same raw materials as proposed to be used for the work. The later being treated as the reference or controlled concrete mix.