On occasion during periods of hot summer weather, a testing company may report a low compressive strength test result and subsequent tests carried out on cores removed from the concrete appear to confirm the initial low test results. Often, further testing reveals that the concrete was satisfactory at the time of delivery and that, with the exception of surface layer, the cast-in-place concrete relating to these test failures was also satisfactory. In the meantime delays have occurred and money spent on unnecessary testing. Assuming the concrete is delivered to the site with the potential to achieve the specified minimum compressive strength then how can the situation described above be avoided?
It is normal procedure on many sites for concrete cylinders to be stored in some convenient location during the first day following manufacture, with consideration rarely being given to the ambient air temperature except during cold weather when the cylinders may be stored in an insulated and thermostatically controlled curing box. Often the tops of the cylinders are left unsealed allowing the surface layer of concrete to dry out.
Over the years we have found that non-representative low cylinder test results during periods of hot weather may result from one, or a combination, of the following condition occurring on site.
- Cylinders curing during the first day (or weekend) at high temperatures;
- The concrete forming the tops of the cylinders drying out.
It is important during hot weather to control the curing temperature of cylinders while they are on site during the first day following manufacturer, otherwise low compressive strength test in the laboratory at 7 and 28 days. The best way of achieving this is by storing the cylinders such that they are almost completely immersed in a water bath. On building sites where CCIG is providing testing services, we arrange for a water bath to be provided. Such a bath usually consists of water contained in a sawn-off end of a 45 gallon drum.
The water bath should be located in an area of permanent shade, and the concrete forming the tops of the cylinders should be prevented from drying out by placing plastic caps or polyethylene bags on top. The water in the drum does not require heating or cooling. In our experience, it will settle down to an almost ideal temperature of around 20 Celsius provided only one set (3 cylinders) is placed in a single tank.
We have found by comparison testing that a curing tank of the type described above will make a difference to the type described above will make a difference to the 28 day compressive strength of as much as 10 MPa, particularly after a long weekend with the cylinders having been left on site.
We have also taken two cylinders from a laboratory which had reported that they had failed by some 8 Mpa below the specified strength, cut off the top 11/2 inches from each of the cylinders and on retesting the same cylinders found that they attained a compressive strength well over that specified.
Sometimes an incorrect low strength cylinder test result in confirmed by equally incorrect results of tests carried out on cores removed from the related concrete normally forming a floor slab. The low core test result may well be due to the presence of a weak layer of concrete at the slab surface due to the concrete drying out.
Removal of the top layer of concrete before testing will often result in a satisfactory compressive strength being achieved by the concrete forming the rest of the core. In our experience, the removal of the surface layer has the potential to as much as double the compressive strength of the core. The solution to low core test results may be as simple as wet curing the top of the slab until the surface layer of concrete achieves a satisfactory strength.
The procedures to follow in avoiding misleading compressive strength test results due to hot weather temperatures effects are therefore simple to:
- Ensure both that all concrete test specimens are stored in water from the time of manufacturer to the time they leave the site and that the tops of the cylinders are sealed by a plastic cap or polyethylene bag to prevent the surface layer of concrete from drying out.
- When testing cores removed from the structure, take into account the fact that the surface layer of concrete may have a lower compressive strength than the rest of the concrete due to premature drying out of the surface concrete.
- It is, of course, essential in the event the strength gain of the surface layer of concrete has been affected by premature drying out of the concrete that the structural engineer review the acceptability of this condition and that additional curing of the concrete be carried out necessary.
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