Q: We're placing massive concrete walls using forms that have expanded polystyrene form liners. The wall sections are 25 feet high, 32 feet long and 12 to 24 inches thick.
The concrete is made with 600 pounds of Type III cement per cubic yard. Typically we pour the concrete at 1 p.m. and start stripping forms at 8 a.m. the next day. The job started last January, and we had no problems until April, when the form liner started melting and sticking to the concrete, making form stripping difficult.
The expanded polystyrene has a melting point of about 160º to 180ºF. Does the concrete generate enough heat during hydration to melt the liner?
A.: It's possible. In massive structures, the following equation can be used to estimate temperature rise when heat neither enters nor leaves the concrete:
T = CH/S
T = Temperature rise of the concrete in degrees Fahrenheit due to heat generation of cement when no external heat enters the concrete and no heat escapes
C = Proportion of cement in the concrete, by weight
H = Heat generation due to hydration of cement, in British thermal units per pound
S = Specific heat of concrete, in Btu per pound per degree Fahrenheit. Assuming that your concrete weighs 4,000 pounds per cubic yard
C = 600/4,000 = 0.15. From the graph below, heat of hydration of the Type III cement at one day is 140 Btu per pound. It would be a little less since you're starting to strip forms in less than 24 hours after concrete placement, but we'll use 140.
Specific heat of concrete ranges from 0.20 to 0.28; we'll use the lower-end value, 0.20. Thus, T = 0.15(140)/0.20 = 105°F temperature rise. This assumes that because of the expanded polystyrene form liner, no heat was lost at the formed surface. If the concrete was placed at a temperature of 70º F, maximum temperature would be 105 70 = 175°F, which is within the 160º to 180º F melting point range you gave.To test your hypothesis, embed a thermocouple near the formed surface before pouring the wall, and then measure the concrete temperature up to the time the forms are stripped.
Steven Kosmatka and William Panarese, Design and Control of Concrete Mixtures, 13th ed., Portland Cement Association, Skokie, Ill., 1994.