The mission of ACI Technical Committee 306 – Cold Weather Concreting is to develop and report information on cold weather concreting, in low temperature events.
The American Concrete Institute (ACI, www.concrete.org) web session entitled the Effect of Frozen Rebar on Surrounding Concrete During Cold Weather Placement addresses monitoring for cold weather concreting and cold rebar during low temperature events, as part of the ACI 306 Cold Weather Session in March 2014 in Reno, Nevada. The mission of ACI Technical Committee 300 Design and Construction, 306 – Cold Weather Concreting is to “develop and report information on cold weather concreting.”
As part of a research project on cold weather concreting and cold rebar during low temperature events, speaker Ronald L. Kozikowski of North Starr Concrete Consulting conducted a scientific study to address the following two questions:
- Do actual measurements in the field match FEM (finite element method) models?
- Is heating steel to 32ºF necessary?
Variables in the research project included various rebar sizes from #3 rebar to #18 rebar and a steel volume ranging from 1% to 5%, with a concrete temperature of 55ºF and a steel temperature at -5ºF. Kozikowski and his team instrumented the rebar with thermistors in three areas in each test concrete mold, first at the center of the rebar, second at the surface of the rebar, and third at varying distances from the rebar outward in each of the molds, at ¼, ½, 1 and 2 diameter spacing, based on the respective rebar diameter.
Next, the team analyzed the “freezing plateaus” by temperature in degrees Fahrenheit (ºF) and by time in hours, charting water, paste, mortar and concrete phase changes from liquid water to liquid-solid water and ice to solid ice. The team used the same volume of each material, water, paste, mortar and concrete, for each test. Lower percentages of water in each material caused shorter freezing plateaus, with water at about 14 hours in freezing plateau and with concrete and mortar at about 2-3 hours in freezing plateau.
In summary, Kozikowski published a series of tables of temperature and time data for reinforcing steel (rebar) at various percentages of steel concentration. The tables include initial steel temperature (ºF, ºC), initial concrete temperature (ºF, ºC), time for bar surface to heat to 32ºF or 0ºC (minutes), time for bar-concrete thermal equilibrium (minutes), and bar-concrete equilibrium temperature (ºF, ºC). Conclusions highlighted no observed temperature plateaus, and that the measured equilibrium values matched the calculated values.
(Source: American Concrete Institute (ACI, www.concrete.org) Effect of Frozen Rebar on Surrounding Concrete During Cold Weather Placement, Ronald L. Kozikowski, North Starr Concrete Consulting)
More in the next blog about weather planning, Weather Controls™ and weather risk management in the construction industry and built environment…