This primer of tried-and-true mix and manufacturing strategies for preventing freeze/thaw damage is a timely review as we head into another cold-weather production season.
By Phillip Cutler, P.E.
If you are a precaster who resides in an area of the country where the potential for freezing temperatures is a way of life in the winter, it is likely that you have to deal with an old concrete nemesis: freeze/thaw cycles. We’re all too familiar with its potential to harm precast or prestressed concrete structures.
What is the first thing that comes to mind when we think about freeze/thaw conditions? The obvious answer is our industry’s best offensive tool: ACI 306R, “Guide for Cold Weather Concreting,” with its recommended practical procedures for successful cold-weather concreting. And when ACI 306 pops into your mind, our next thought is: “Oh! Precast must never be exposed to multiple freeze/thaw cycles at a very early age (≤ 500 psi compressive strength) or during the beginning of the curing process.” Right? Well, whether or not ACI guidelines were your first thought, let’s review some fundamental freeze/thaw facts and strategies.
What the research says
According to industry research on concrete mixtures, the No.1 culprit is too much water when facing cold temperatures when there is a risk of freezing. In our industry, “too much water” can exist in two distinct ways: at the time of mixing, and saturation (by any means) of the structure during post placement. In either case, too much moisture can create a freeze/thaw problem if not properly addressed. Precasters today have many more preventive options than in the past, but recommended precautions and strategies should be reviewed so that appropriate steps are taken to counteract the potential adverse effects of freeze/thaw conditions.
Three freeze/thaw precautions
1. Don’t add too much water to your mix. Keep the w/c (water-to-cement) ratio as low as practical. The w/c ratio should be ≤ 0.40 for reinforced concrete exposed to deicing chemicals. Too much water can lead to excessive accumulation of bleed water, which is not desirable when conditions for freezing may take place – especially for early-age concrete. A low w/c ratio can be achieved easily with today’s technology by incorporating a water-reducing admixture or a high-range water-reducing admixture. The goal is to provide enough water for hydration to occur.
2. Remember that aggregates can add moisture. If your coarse aggregates are overly porous, they can retain moisture, and this water will tend to migrate out of the aggregate under freezing temperatures due to ice expansion. Generally speaking, it is usually not the aggregate that is the culprit. If aggregates are overly porous and exposed to freeze/thaw cycles, the typical proof will be exhibited by a pop-out on the surface of the structure. Although pop-outs can be caused by other conditions, moisture within the aggregate under freeze/thaw conditions would represent the major cause of this surface damage.
3. Understand how ice formation in concrete leads to deterioration. Ultimately, it is the pressure generated by the expansion of water as it freezes in the concrete matrix that can be detrimental. If excess matrix water is not abated, deterioration can result. As the water within the concrete begins to freeze, it expands and literally wants to push itself out of confined areas. If there is no room in the concrete matrix for this expansion and the ice pressure exceeds the localized tensile strength of the concrete, small microscopic cracks are generated. The cracks generated then become miniature pathways for external water intrusion into the concrete structure. Over time with multiple freeze/thaw cycles, the concrete exhibits visible deterioration under these exposure conditions.
What can you do to enhance freeze/thaw durability in precast and prestressed concrete?
• Review cold-weather concreting procedures. First and foremost, precast production facilities should have adequate cold-weather procedures in their plant-specific quality control manual. Cold-weather procedures should be reviewed periodically so that everyone is aware of proper concrete manufacturing practices for cold-weather exposure.
• Use air-entraining and water-reducing admixtures. These admixtures counter the potential detrimental effects of freeze/thaw on precast concrete. We have already cautioned to keep the w/c ratio as low as possible (≤ 0.40) and to consider the use of water-reducing agents. The use of air-entraining admixtures is the next best approach for durable cold-weather concrete.
Speaking of air-entraining admixtures …
Many precasters have likely heard the claim that air-entraining admixtures reduce precast concrete compressive strength. While this may be true under certain conditions and to some extent, any chemical or additive used in excess when proportioning a precast concrete mix design is not necessarily a good thing. Used in proper and reasonable proportions, air entrainment has been shown to provide excellent results for counteracting adverse freeze/thaw effects.
Air-entraining admixtures have been used successfully in appropriate amounts based on the desired slump range and/or nominal maximum aggregate size (see Table 6.3.3 from ACI 211.1, “Standard Practice for Selecting Proportions for Normal, Heavy-Weight, and Mass Concrete”). Proportioning guidelines for SCC (self-consolidating concrete) can be obtained from your admixture supplier and/or ACI 237, “Self-Consolidating Concrete.”
The theory behind entrained air (artificially induced with chemical agents as opposed to entrapped air that is caused by the mechanical mixing of constituents) is to provide a microscopic void system within the concrete that allows ice molecules adequate space to expand under freezing conditions. Without the presence of these air voids, the expansive movement generated by the water-to-ice formation has no place to go. Expansion from ice formation and the internal pressure generated create microscopic cracks that continue to grow over time. As the frequency of freeze/thaw cycles increases, concrete may exhibit potentially severe deterioration.
Available technologies and a knowledgeable staff
The answer to the threats from our freeze/thaw adversary is clear: a producer who takes advantage of current mix design technologies and maintains a knowledgeable production staff. Use of quality materials and proven production practices and processes in precast concrete manufacturing plants will result in a strong, durable product for cold-weather service.
Phillip Cutler, P.E., is NPCA’s director of Technical Services.
PCA Manual Design and Control of Concrete Mixtures, 15th edition
ACI 306 Cold Weather Concreting
ACI 201.2 Guide to Durable Concrete