Concrete Curing Control
In the case of uncertainty about the procedure and duration of water curing, thermal curing and membrane curing, the following items should be checked:
- Several samples should be provided and stored under the on-site conditions in terms of curing exactly like the pieces of the structure. At the same time, a sample of the same concrete must be prepared and stored under the laboratory curing conditions.
- The results of these two types of samples that were cured under different conditions are obtained after 28 days (i.e. the characteristic strength age). The curing is acceptable if one of the following conditions is met
- The cured compressive strength under the on-site conditions is the cured compressive strength of the samples in the laboratory and the characteristic strength of concrete compression.
- If none of these conditions are met, the curing procedure (method and duration) is not acceptable and steps must be taken to improve it. It should be noted that a %15 decrease of the strength on the site is acceptable.
- The concrete strength control in the desired times under the real on-site curing conditions (awareness sample):
- To check the quality of concrete at certain times the following should be considered:
- A sample of the concrete (including at least 2 specimens of the same days of age) should be provided and stored in the actual on-site curing conditions and its strength should be determined at the desired age. In this case, there is no need to compare it with the samples in the laboratory.
- If the strength has reached the desired level, the samples can be demolded or the pre-made pieces can be moved, or the type and duration of the accelerated curing can be determined.
- Controlling concrete cores of the hardened concrete pieces of the structure
To check the items including the low-strength concrete, the procedure of concrete placing, compaction and curing, the constructed structures and their safety, and the damaged structures, the following should be considered.
- If a weak area has been identified or its probability is considered, of weakness is given there, the site documents should be checked and concrete hammer test (by Schmidt hammer) or ultrasonic concrete test should be done followed by at least three concrete cores of the weak spots should be provided. Suspicious areas or the damaged spots must be specified by the supervisor or consulting engineer.
- It is better to do the core drilling in the free of rebar spots, i.e. in the spots that cause not major damages. Core tests from the pieces with less height (thickness) are not possible. In this case, the rebar locators are used.
- Both ends of the provided concrete core should be cut and capped. It is recommended to cut by 2 to 3 cm even if the outer surface is smooth so that the probable cracks caused by shrinkage do not affect the current strength.
- The cored specimens are tested in dry or saturated conditions. The operating conditions are usually taken into account in testing and the moisture conditions of the cores are specified. Dry cores should be kept for 7 days at 16°c to 27°c at and the relative humidity less than % 60. If the saturation conditions are considered, the cores should be immersed in water (or lime water) for at least 40 hours. Obviously, if saturation is necessary, capping is done after the cores are taken out of the water.
- The results of the compressive strength test of the cores should be converted to the standard cylindrical strength. Two types of conversion coefficients are used in this regard. Conversion coefficients of the core strength due to lack of height-to-diameter ratio, are obtained from the relevant tables. The minimum diameter of the core is usually 100 mm, so the height of the core cannot be less than 100 mm. If the diameter of the core is 100 mm, it should be divided by 1.02 to obtain the strength of a diameter of 150 mm.
