Concrete buildings continue to increase in complexity, size, and required compressive strength. As a result, engineers, contractors, and ready-mix suppliers are faced with new challenges. Among these is considering whether they are dealing with mass concrete and if so what thermal issues might come into play.
Traditionally an issue only in heavy civil applications, more commercial building teams are recognizing that foundations, transfer beams, and walls greater than 4 feet thick and concrete mixes with more than 700 pounds per cubic yard (pcy) of cementitious material could require temperature control measures. In today’s concrete buildings, it is not uncommon to have a 6-foot-thick transfer beam, with 8,000 psi compressive strength concrete that requires 900 pounds of cementitious material. Foundations for tall buildings tend to cause the most concern.
In these situations, high temperatures can cause thermal cracking within hours or years later, leading to delayed ettringite formation (DEF), which can cause highly accelerated deterioration of the concrete. ACI 301-16, Specifications for Structural Concrete, is the current version of the standard specification adopted into most building codes; the Mandatory and the Optional Requirements Checklists in ACI 301 are not actually part of the Specification, but rather are provided to assist the specifier. The temperature limits of ACI 301 are generally the default in most project specifications.
How much heat?
In ACI 301-16, Section 8.1.3, the default maximum temperature in concrete after placement is 160° F. To determine if a mix will gain enough heat to be of concern, Mass Concrete for Buildings and Bridges, Portland Cement Association, offers an equation for estimating adiabatic temperature gain on concrete greater than 4 feet thick in the least dimension.
ΔT = 0.14 (pounds cement + 0.5 pounds Class F fly ash + 0.6 pounds Class C flyash+0.8 pounds slag + 0.95 pounds metakaolin)
For example, estimated temperature gain on a mix with 900 pcy total cementitious content with 30% Class F fly ash replacement (270 pcy) would be:
ΔT = 0.14 (630 pcy cement + 0.5 x 270 pcy Class F fly ash) = 107° F
To meet the 160° F maximum, the starting temperature would need to be at or below 53° F. Without any temperature controls and less cement replacement that concrete could exceed the boiling point of water and easily violate the ACI standard increasing the probability of DEF. Testing should be done to verify the actual heat gain. Cooling a mix by 20 to 40° F will be extremely expensive, potentially more than doubling the cost per cubic yard. In the interest of the project, alternative approaches should be considered.
One technical challenge is to measure the in-place temperature of the concrete. Several manufacturers have solutions with probes, readers, and wireless transmitters. On the Proton Therapy project in Washington D.C, Baker Concrete Construction chose the Intellirock system by Flir to measure temperature. Probes at the core measure the maximum temperature and probes near the surface tell us the differential. Insulating blankets are used to keep the surface temperature from cooling to the ambient air temperature.
In Washington D.C. in the fall, this would create a potential 70° F or greater temperature difference between the core and surface, which would probably cause thermal cracking. Blankets keep the surface warmer and thereby keep the differential within an acceptable range. The goal is to have enough insulation over the mass concrete and forms to slowly cool the concrete but not so slowly that the cooling and monitoring process takes longer than five to seven days. Insulating durations of up to three weeks are common in mass concrete so the pours have to be staggered or the delay has to be taken into account in the overall project schedule.
Let’s look at each of the thermal requirements and how to deal with them.
The maximum temperature of concrete is seldom measured on commercial buildings. It is assumed to be low enough to not be of concern. The Optional Requirements Checklist in ACI 301-10, Section 8.1.2 states that “Concrete that contains supplementary cementitious materials may have a reduced risk of DEF and may justify internal temperatures above 158° F.” Thereby, ACI opens the door to higher maximum temperatures by using fly ash and slag, although this statement was deleted from ACI 301-16.
In heavy civil construction, the Florida Department of Transportation (FDOT) sets the maximum temperature of concrete at 180° F. This comes with the requirement that any mix considered mass concrete (expected to have a core temperature greater than 165° F) must have 35% to 50% fly ash or 50% to 70% slag. FDOT is very strict about having a specialty engineer develop the mass concrete plan and having it approved by FDOT.
In extreme cases, a contractor’s bid might need to include substantial costs to precool high strength concrete—as much as $50 to $75/cy if using liquid nitrogen. If the concrete delivery rate is greater than 60 cubic yards/ hour (more than one concrete pump) then the contractor might need to place cooling elements with water pipes to stay below the maximum temperature of 160° F. This would cost $70 to $100/cy. These costs would decrease as the allowable maximum temperature is increased.
The default allowable differential temperature between the core and the surface is 35° F. But since concrete temperature is not typically monitored in commercial buildings, the differential is seldom known.
The ACI 301-16 Optional Checklist, 8.1.3 states that, “A higher temperature difference limit may be acceptable depending on concrete properties, placement dimensions, and reinforcement configuration.” A mix can be evaluated “through numerical simulations and comparing calculated thermally-induced tensile stresses with the developing tensile strength at the surface of the concrete placement.”
Other mass concrete plans used by contractors, prepared by specialty engineers, have allowed max differentials of 50° F to 60° F, based upon the aggregate and considering the strength of the concrete. As strength increases, so does tensile resistance and allowable differential. The New York DOT, for example, has allowed the differential to increase based on the age of the concrete: 30° F from 0 to 24 hours, 40° F for 24 to 48 hours, and 50° F after 48 hours.
As the differential increases the insulating duration decreases and the forms can be removed faster. The old adage “time is money” applies. A contractor would have to either work faster or have more formwork with a tight concrete differential like 35° F.
Maximum delivery temperature
This is monitored on building projects because the testing lab usually takes the delivery temperature along with slump while making cylinders. The challenge is that many locations have high daily ambient temperatures in the summer and possibly high overnight low temperatures. Without night delivery, the concrete delivery temperature will usually be the daily maximum temperature +/-5° F. Therefore, from May to October the concrete will, on average, be above 95° F from late morning to evening, impacting schedule and adding cost to precool. Delivery temperature does not generally indicate the long-term strength or durability but can be an indicator of the potential maximum temperature.
ACI 301-05 had a maximum delivery temperature of 90° F and this still remains in many specifications. ACI 301-10 modified the maximum delivery temperature to 95°F, still the current default. The 301-16 Optional Checklist, 126.96.36.199(b) states “If concrete delivered in hot weather with a temperature higher than 95° F has been used successfully in given climates or situations, the higher temperature may be specified in place of the 95° F limit.” Numerous locations in the U.S. have a history of delivering concrete above the 95° F threshold.
The default duration of monitoring is until the core is within 35° F of the average outside air temperature. The basis for increasing this is similar to the reason for allowing a higher maximum differential temperature. As the concrete ages, its resistance to internal temperature differences increases. FDOT for example allows all monitoring and temperature control to stop when concrete is within 50° F of the average daily temperature.
Other mass concrete plans cease monitoring after the core temperature is decreasing and within 60°F of the average daily temperature. Any relief on standard differential will decrease the contractor’s cost with little risk of damage, due to mature concrete.
The financial impact of the most conservative interpretation of ACI 301 can be dramatic. As with any engineering cost versus value evaluation, the owner should be informed that any relief from the default standards will come with some increased but hard-to-quantify risk. There are numerous specialty engineers that can provide a mass concrete plan when required by the contract documents or requested by a prudent concrete contractor.
Michael Hernandez is the Engineering / BIM & Formwork Asset Manager SE Region, for Baker Concrete Construction.