Question:
What is match curing and can I use it to determine specified release strengths and design strengths?
Answer:
Match curing is a system in which a standard concrete cylinder–usually 4×8 in. (102×203 mm)–is cured at the same temperature as that measured in a concrete member. The system includes a temperature sensor in the member, a controller, a special insulated cylinder mold with a built-in heating system, and a temperature sensor in the mold.
A reference sensor is located in the member to obtain the temperature of the freshly placed concrete. The reference sensor and the sensor from the cylinder mold are connected to the controller. The controller continuously compares the reference temperature with the temperature of the cylinder mold. When the reference sensor temperature exceeds the cylinder temperature, the controller activates the heater on the cylinder until the cylinder temperature and reference temperature are equal. One controller can be used with several molds. The controller can be replaced with a personal computer that can also record
temperature versus time.
As illustrated in the article in the previous article, curing temperature can have a significant effect on measured concrete compressive strength at release and a lesser effect on later-age strengths. This effect is more significant with HPC because the higher amounts of cementitious materials produce more heat of hydration and higher temperatures.
Whether or not the match-curing technique can be used to determine specified strengths will depend on the specifications. Several state DOTs now allow its use to determine release strengths but still use member curing or moist curing for design strengths . Some states use the technique for HPC bridges only. Other states are experimenting with the technique. As research data indicate, a match-cured cylinder produces a compressive strength that more closely matches the strength of the concrete in the member than the strength measured using other curing methods. This is particularly true at early ages. If your specifications do not currently permit the use of match curing and you are producing high strength HPC, it is time to consider a change to your specifications.
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