Short-term construction economy should not be obtained at the expense of long-term durability. If an owner decides that it is less cost effective to wait for milder temperatures before beginning concrete construction, then the minor short-term costs of protecting against cold weather should be strictly adhered to. Neglect of protection against early freezing can cause instant ruin or permanently weakened concrete. Therefore, if cold weather concrete is performed, sufficient protection from low temperatures and proper curing are essential.
The first step in any concrete construction project is to have the concrete supplier, contractor, and owner (or architect/engineer) meet at a pre-pour conference. At the pre-pour meeting, clearly define how cold weather methods will be used to protect the freshly placed concrete, and ensure that the performance specifications of the project are met. A plan should be in place, and all necessary equipment should be on site before concrete is placed, NOT after concrete is down and temperature conditions approach the freezing point.
Since it has been decided that it is beneficial to go ahead with concrete construction during cold weather conditions, it is strongly recommended that contractors use an accelerator admixture as well as hot water when placing orders. The combination best allows for concrete to set up in a reasonable time, as well as keeping the internal temperature elevated.
When the internal temperature of concrete gets to 32 degrees F, the water in the mix will begin to freeze. Water in a solid (frozen) state expands in volume, and the effect when that happens in concrete is to create large voids, which severely weakens the compressive strength of concrete. It can also lead to increased cracking problems when the concrete eventually sets up as the ambient temperature rises. Even if the air temperature is not approaching 32, the ground temperature may be as much as 5-10 degrees lower than the air temp based on soil type, and time of day. Thus, hot water helps protect the concrete from freezing by helping to keep the concrete temperature above the ambient conditions, even if the air temperature is in the low 40s.
Accelerator is added to concrete to speed up hydration (the process by which cement and water react to harden). Depending on temperature, set times can be reduced by several hours, or even cut in half. There are two main points to remember when deciding to work with accelerator: 1) it is the concrete temperature, and not the ambient temperature that dictates the effectiveness of accelerator and 2) accelerator tends to rapidly lose effectiveness under a concrete temp of 50 degrees F. Thus, adding hot water in tandem to accelerator admixture really produces a winning combination that protects concrete, as well as decreases labor costs by reducing set and finishing times.
Ex. A great project to highlight here is one slab on grade we did in December of 2014 for American River AG. 7/11 poured three 1,000 cubic yard slabs, on consecutive nights, at an ambient temperature of 30-35 degrees F, and a ground temp of around 27 degrees F. The contractor wisely chose to use accelerator and hot water after the first night, as the final finishing operations were not completed until approx. 20 hours after the first load was discharged. The last two pours had the hot water/accelerator in the mix, the concrete temperature stayed above 50 degrees F all night, and the final finishing operations were completed approx. 12 hours after the first load was poured. By choosing these great cold weather admixtures, the concrete was better protected, covers were not needed to protect the concrete from freezing while waiting for the sun to come up, and best of all 8 hours of labor was saved. The cost/benefit is clear when using accelerator and hot water when placing and finishing concrete in cold weather conditions.
In areas that are exposed to cycles of freezing and thawing (generally in consistent snow regions), concrete should be properly air-entrained to prevent cracking and the weakening of concrete over time. In snow regions, the melt creates liquid water which can enter into existing concrete, and when temperatures drop, can freeze inside of the concrete. As mentioned above, frozen water expands, thus causing voids and increased internal pressure inside of concrete. This process weakens concrete over time. Air-entrainment allows for increased air voids inside of concrete, which creates areas water can move into within concrete. During a freezing cycle, these voids allow the water to freeze and expand, without weakening the existing structure.
The last topic to discuss in preparing for cold weather concrete placement is slump. Generally low slump concrete (4” or less) should be used where applicable. Using lower slump concrete minimizes bleed water, and decreases set time. During cold weather, bleed water may remain on the surface for an extended period of time. This interferes with and impedes finishing operations, as well as increasing the probability of plastic shrinkage and cracking. If bleed water persists on the surface, it should be skimmed off prior to troweling with a rope or hose. Working excess bleed water into the surface will result in a lowered strength surface, which can lead to dusting, cracking, and/or delamination. Thus to reduce labor costs as well as protecting the concrete, it is best to use lower slump concrete during cold weather.