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Lafarge Readymix aims to be right first time, every time, delivering the correct product at the correct time to its customers. Nevertheless, it is important that the customers check delivery tickets carefully before any readymix concrete is discharged. The few minutes spent doing it properly could avoid a time-consuming, costly problem later. The procedure is particularly relevant on a large site where there can be several concerting gangs expecting deliveries of concrete with different specifications.
Check that the delivery note shows the correct quantity and grade of concrete being delivered, together with essential details of the concrete specification required for the particular job. Another important piece of information is the time of batching the load. This will indicate whether the transport time was normal in relation to the distance traveled or whether there has been an excessive delay between dispatch from the batch plant to site.
Establishing a time limit on the acceptance of concrete as workable and suitable for placement is not straightforward. The rate of change in the properties of concrete depends on the mix constituents, including any special additives, the ambient temperature and general weather conditions. At what stage the concrete becomes unusable also depends on the nature of the job. As a result, the acceptance time period could be less than half an hour or more than six hours and the decision on acceptability is based more on experience than on an objective test.
In an attempt to overcome the reliance on operator judgment, some contract specifications lay down a maximum permitted time between batching and discharging. After which the concrete is deemed to be unsuitable. Unfortunately this can often create another problem.
During a pour, a load may be rejected due to the expiry of the arbitrary time limit but, in the time required to organize a replacement load, the pour site may experience a cold joint problem. In this case, the quality of the rejected load may still have been workable and the better solution could have been to accept the load and continue the pour without interruption. Conversely, the situation can arise in hot weather where a contactor finds that a concrete mix has stiffened unacceptably before the specified time period has lapsed.
The delivered mix may still be within the original tolerance limits, but the concrete is either too stiff or too wet and the originally specified workability is no longer valid. In such instances, Lafarge Readymix should be notified and the arrangements made to supply a concrete mix with workability suitable for the new conditions.
Control through workability
The workability of a mix is generally considered to be the key quality control indicator for production control purposes at the readymix batching plant and also for the on-site placement. Only when workability is satisfactory are the other properties of the concrete also likely to be correct. Each mix is designed to have a specific workability, which impacts on all the main characteristics of a concrete: with the control of workability being largely based on the free water/cement ratio, this influences final strength and durability, the quality of the surface finish and the ease with which the concrete is transported and compacted.
It is therefore essential that the correct level of workability is specified and maintained within the relevant tolerance. SANS method 861 specifies that the acceptable slump is within a tolerance of 25mm or 30% of the target slump.
When monitoring concrete workability as the primary quality control indicator, it is important to realize that the repeatability of the test results are very dependant on using a methodical testing procedure. Results obtained by two different people on the same concrete sample can differ significantly, especially if the correct test procedures are not rigorously followed.