In the course of testing concrete slabs poured in cold weather, Dennis Purinton, owner of Purinton Builders in East Granby, Conn. and his team, which included five ACI 306 Committee members, were seeking answers to the following questions:
- How do cold grade and ambient temperature effect concrete slabs poured on grade?
- Does changing concrete temperature change its performance?
- Does cold concrete temperature affect the performance of accelerators?
- What effect do Supplemental Cementitious Materials (SCM’s) have on concrete slabs poured in cold weather?
“It was a great winter for field testing the performance of slabs on grade in cold temperatures,” says Dennis Purinton, owner of Purinton Builders in East Granby, Conn.
The team chose February 10, 2014 as the placement day and it was a beautiful, cold day in Suffield, Conn. Purinton says the morning started with single digit temperatures and a couple of inches of new snow from the night before. Meanwhile, the ambient temperature never got above the mid 20s.
The testing consisted of 15 different 10 foot by 10 foot by 5 inch panels that replicated 15 different field conditions. "We placed three loads of concrete, provided by Tilcon, Conn. Two loads were a straight Portland cement design," says Purinton. "The only variable was its batch temperature. The third load was a ternary blend of 45 percent SCM’s."
Two of the 15 panels were constructed in a temporary building that had cold subgrades. A Wacker Neuson indirect fired heater was utilized here.
The balance of the slabs were placed outside in the open air; some with cold sub-grades, some utilizing hydronic ground heaters to heat the bases, and one with two inches of XPS directly under the slab.
All 15 panels were monitored with Con-Cure radio remote maturity meters. “There were four probes in every panel,” he says. “One was placed two inches below grade, one at the interface of the concrete and base, and two in the slab. (See diagram 1) We also monitored surface temperatures with a hollowed out block of insulation and a digital thermometer." (See picture 1)
The time of concrete placement and the surface temperature of the concrete were recorded at every step in the finishing process. In the slabs utilizing higher temperature concrete, the surface temperature dropped as much as 12 degrees Fahrenheit during the finishing process. In the slabs utilizing the lower temperature concrete, the surface temperature dropped as much as 15 degrees Fahrenheit. Even with surface temperatures dropping, no finishing issues occurred.
An ACI certified lab technician was also on site to record concrete temperature, test for air content, and make cylinders to be cured at the lab and tested for compressive strength.
After the finishing process was complete, all panels were covered with poly and blankets. “In addition, we added hydronic heat piping on top of the poly and under the blankets in three of the panels, to simulate moist, heated curing environment,” Purinton says.
Coring the slabs
During the seven days after placement, the average ambient daily high temperature was 29 degrees Fahrenheit. The nights averaged 13-degrees Fahrenheit with a low of zero degrees. Two days after placement, a 12 inch snow storm walloped the area of placement.
On the eighth day, the maturity meters were removed for reading, along with the blankets and poly on all panels. Water was present under the plastic on all panels, including the edges and corners. The surface temperature of all slabs was still above 32 degrees Fahrenheit despite only a few hours of temperatures above 32 degrees Fahrenheit over the previous seven days.
“We cored all 15 slabs at eight days and again at 28 days,” he explains. “The cores were taken to the Connecticut-based, Materials Testing, Inc. and tested for compressive strength.”
The following are just a few of the interesting facts that were observed in the testing process:
- There was minimal drop in the concrete temperature where the interface of the concrete and the 33 degree Fahrenheit sub-grade met.
- The 33 degree Fahrenheit sub-grade temperature had little or no effect on the concrete two inches above sub-grade.
- The sub-grade absorbed heat from the initial hydration that was slowly consumed by the slab in the curing process over some number of days.
- Two inches of XPS directly under the concrete accelerated set and finish times and reduced the compressive strengths in the cores.
- A warm sub-grade will shorten finish time, but higher concrete temperature will reduce finish time even further.
- Two trucks of the same mix design were ordered. The only difference between the two was the concrete on the first truck was 88 degrees Fahrenheit, while the concrete on the second truck was 54 degrees Fahrenheit. The 88 degrees Fahrenheit concrete compressive strength in lab cured cylinders was over 20 percent less than the 54 degrees Fahrenheit concrete cylinders. The cores in the field panels showed a similar reduction in compressive strengths.
- Finish times for the 45 percent ternary blend mix design showed a 10 percent increase in comprehensive strength over a 53 degrees Fahrenheit Portland cement mix design, and over 30 percent higher than the 88 degrees Fahrenheit Portland cement mix design. All were cured under the same conditions.
- Non-chloride accelerators are temperature sensitive in their effectiveness on finish time.
Purinton says the amount of data collected from this cold weather slab testing was daunting, resulting in hundreds of possible performance comparison combinations. “We have just begun in the effort to draw conclusions to many of these comparisons.”
Ed. Note: None of this testing could have been carried out without the valuable help received from many companies and individuals. Thanks to Mike Barry; Gene Daniel; Euclid Chemical Company; Ron Kozilcowski; Lloyd Concrete; Bill Lyons; Materials Testing, Inc.; Mike Purinton; Smith Brothers Concrete; Tilcon, CT; Wacker Neuson; Con-Cure Corporation; JGW Concrete.