Q.: I am required to monitor the differences between interior and exterior concrete temperatures on a mass concrete placement. How many thermometers, or thermocouples, are needed to get a reliable measure of the internal temperatures?
A.: We asked Matt Miltenberger, senior project engineer with Master Builders Inc., Cleveland, who suggested that you install as many thermocouples as possible since they can easily be damaged during concrete placement.
The heat profile over a cross section is parabolic, so Miltenberger suggests closer spacing of the thermocouples near the form faces because the temperature-change rate of the structural section is greatest near the surfaces but diminishes toward the center. If the cross section is symmetrical, space the thermocouples symmetrically. As shown in the drawing for a rectangular section, place one row of thermocouples at midheight, with the first thermocouple 1 inch from the form, the next one about 6 to 12 inches from the form, and the rest spaced about 24 inches apart until you reach the middle of the section. Repeat the spacing as you approach the opposite side of the section, and repeat this array in a line perpendicular to the first set and at the center of the cross section. Also add a thermocouple near the base -- although not as much heat is lost there -- and one near the top surface, where heat loss may be the most rapid.
Miltenberger says you should measure the temperature before and immediately after concrete placement, and then every hour after the temperature at the core starts to rise. You can use an automatic data logger or a voltmeter that reads the thermocouples manually. He suggests that if you have only a few channels available on a data logger, you use at least five thermocouples -- one at the center of the placement and four along the surfaces.