Thermal Effects of Mass Concrete Pour
Thermal Effects of Mass Concrete Pour
(OP)
We are performing materials testing on a 4-ft thick concrete slab pour with 1500 cy. The specifications are loose and the contractor is going to do one continuous pour. The maximum specified placement temp is 95 deg and there are no specific requirements for cooling. The pour will start in early evening and go all night. Night time temp is 75 deg F and daytime temp is 100 deg F. No chance to delay project until cooler weather. We didn't design this but are concerned the heat of hydration may cause problems. I am a geotech with no experience with thick concrete pours. Any comments on whether there may be problems with this thick of a pour and ways to address this? Thanks.





RE: Thermal Effects of Mass Concrete Pour
Temperatures can be monitored using thermocouple devices for this purpose.
Dik
RE: Thermal Effects of Mass Concrete Pour
ACI has guidelines for hot weather placement and for mass concrete placement.
RE: Thermal Effects of Mass Concrete Pour
RE: Thermal Effects of Mass Concrete Pour
Four feet is a thickness that can cause problems in the climate noted by mesaus.
RE: Thermal Effects of Mass Concrete Pour
RE: Thermal Effects of Mass Concrete Pour
RE: Thermal Effects of Mass Concrete Pour
There are two temperatures you need to worry about. The first is the maximum internal temperature measured near the geometric center of the placement. If it rises higher 160 F, there is risk of delayed ettringite formation, significantly reducing the service life of the structure. I've seen agenciess specify a maximum temperature as low as 145 F.
The other is the temperature differential between the center of mass and the outer faces. If the temperature differential gets too high, there is significant risk of thermal cracking. Some agencies specify a differential as low as 30 F, but I think the standard is 40 F. As the concrete gains strength, this number becomes less of an issue. I would place more thermocouples 2" below the top surface near the center.
I typically place a minimum of two thermocouples per location, several feet apart to provide redundancy in the equipment, and far enough apart so that if they blast an area with the pump hose and it takes out the thermocouple, it shouldn't take them both out at the same time.
Greg
RE: Thermal Effects of Mass Concrete Pour
RE: Thermal Effects of Mass Concrete Pour
If the differential is getting too great, the slab may need to blanketed. Either way it should probably be blanketed or at least covered in some manor for curing and temperature control.
Starting with a cool initial temp is key, and 4-feet thick shouldn't be too much of a problem with too much heat as long as the initial mix design is prepared with mass concrete in mind.
Greg
RE: Thermal Effects of Mass Concrete Pour
RE: Thermal Effects of Mass Concrete Pour
Regarding the aggregate, it may be possible to spray it with water to drop the temperature a bit and also saturate the surface. This can have an impact on the water in the mix design.
Dik
RE: Thermal Effects of Mass Concrete Pour
RE: Thermal Effects of Mass Concrete Pour
RE: Thermal Effects of Mass Concrete Pour
Your call.
RE: Thermal Effects of Mass Concrete Pour
Dik
RE: Thermal Effects of Mass Concrete Pour
RE: Thermal Effects of Mass Concrete Pour
RE: Thermal Effects of Mass Concrete Pour
The trucks need to be shaded during delivery and in general the aim has to be to get the concrete into the forms as cool as possible.
As you state the main problem is differential temperature differences so althouh ponding would be ok for curing try not to have flowing water which will cool the surface as the core temperature is still rising.
RE: Thermal Effects of Mass Concrete Pour
RESULTS. The pour was friday night/sat morning. Took about 10 hours. Concrete placement temp = 90 - 95 deg. Started at 9:00 pm - 85 deg air temp. Finished at 7:30 am - 80 deg air temp. afternoon air temp = 101 deg. Two thermocouples worked and 2 did not. One was in the center of the slab. End of pour = 130 deg. 12 hrs was max temp at 152 deg. 24 hrs = 151 deg. 48 hrs = 140 deg.
A second thermocouple was 1 ft from side edge and in the middle vertically. Temp was 10 to 20 deg less than the other thermocouple.
So based on all the comments and our results, I would recommend the specs have explicit requirements. The bid docs have to have specifics for the contractor. I don't like just referenceing ACI as they are thick documents and usually very general. I would say 40 deg differential in concrete. Place 8 thermocouplers - 4 near edge and 4 in center. Assume half will not work. Maximum temp = 145 deg. As always, local specs/codes take precidence. Hot weather vs. cold weather temp must be taken into account, but the differential should handle that.
Strategies to help achieve the above would be retarder in mix, cool the mix before placement, or place sequentially in checkerboard squares. Other comments?
RE: Thermal Effects of Mass Concrete Pour
MASS CONCRETE
CONCRETE POURS HAVING A TOTAL VOLUME IN EXCESS OF 5 CU-M AND A MINIMUM DIMENSION IN EXCESS OF 3 FT SHALL BE CONSIDERED AS MASS CONCRETE
CONCRETE WORK SHALL COMPLY WITH THE RECOMMENDATIONS OF ACI COMMITTEE 207
CEMENT TYPE FOR MASS CONCRETE SHALL BE TYPE LH UNLESS APPROVED BY THE [CONSULTANT | ENGINEER]
MAXIMUM AGGREGATE SIZE SHALL BE 2" UNLESS APPROVED BY THE [CONSULTANT | ENGINEER]
PROVIDE PAIRS OF THERMOCOUPLES IN THE CENTRE OF MASS OF THE CONCRETE POUR
PROVIDE SURFACE THERMOCOUPLE PAIRS AT THE TOP SURFACE OF THE CONCRETE POUR DIRECTLY ABOVE THE THERMOCOUPLE PAIR AT THE CENTRE OF MASS, NEAR THE CENTRE OF TWO ADJACENT VERTICAL FACES AND AT ONE CORNER (5 SURFACE PAIRS TOTAL)
SURFACE THERMOCOUPLE PAIRS SHALL BE PLACED 2" FROM THE OUTSIDE SURFACE AND AT MID DEPTH OF THE CONCRETE POUR
THERMOCOUPLE PAIRS SHALL BE SEPARATED BY A DISTANCE OF APPROX 4'
MAXIMUM TEMPERATURE AT THE CENTRE OF MASS SHALL NOT EXCEED 50%%DC
MAXIMUM TEMPERATURE DIFFERENTIAL BETWEEN THE CENTRE OF MASS AND THE CONCRETE SURFACE SHALL NOT EXCEED 25%%DC
CONTRACTOR SHALL PROVIDE A PROCEDURE OUTLINING PROPOSED METHOD FOR MAINTAINING TEMPERATURE REQUIREMENTS FOR REVIEW BY THE [CONSULTANT | ENGINEER]
Dik