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Ready mix concrete is generally produced in large quantity and is transported to distant places for placement in structural elements. Sometimes the distance can be in many kilometers or miles. So, the effect of this transit times can reduce the quality of concrete. Following are the various effects of transit times on Ready Mix Concrete:
1. Loss of workability:
The concrete should always be laid in position without loss of time to avoid setting and stiffening of concrete to reduce its workability. When the transit time is high, it will have effect on workability of concrete. This happens due to hydration reaction taking place when cement mixes with water, evaporation of mixed water in concrete and due to absorption of water by aggregates.
While the workability of concrete depends on many factors such as the constituent material, mixed proportion, ambient temperature, humidity and method of transport etc., the reduction in workability may lead to difficulty in placement of concrete. Thus, while planning for use of Ready Mixed Concrete, the aspect of loss of workability in the likely transit time involved should be taken into account.
By measuring slump or compacting factor of concrete immediately after mixing at RMC plant and after transit time, loss of workability may be estimated.
How to Prevent Reduction of Workability of Ready Mix Concrete:
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.
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.