By Kayla Hanson, P.E.
Although the calendar tells us summer has not officially started yet, the days are getting sunnier, temperatures are rising, flowers are in full bloom and cold drinks beckon. And, we aren’t the only ones feeling the early onset of summer – concrete is feeling it too.
High ambient temperatures, low humidity, wind or any combination of the three can negatively impact raw concrete materials and sabotage your usual mixing, transporting, placing, finishing and curing practices. ACI 305, “Guide to Hot Weather Concreting,” outlines best practices to help minimize harmful effects on fresh and hardened concrete as a result of environmental factors in warmer months.
According to ACI 305, “If the initial 24-hour curing is at 100 degrees F, the 28-day compressive strength of the test specimens may be 10-to-15% lower than if cured at the required ASTM C31 curing temperature.”
What counts as “hot” weather?
An 80 degrees F day during the month of May might seem like a heatwave in Wisconsin or a surprisingly comfortable day in Texas, so it’s important to clearly define hot weather in your plant-specific quality control manual and know when to take the proper precautions.
ACI 305, which is used both in precast and cast-in-place applications, defines hot weather as “job-site conditions that accelerate the rate of moisture loss or rate of cement hydration of freshly mixed concrete, including an ambient temperature of 80 degrees F or higher, and an evaporation rate that exceeds 1.8 lb./yd.2 per hour, or as revised by the architect or engineer.”
The term “job site” as applied to precast operations covers batching, mixing, placing and curing areas.
It’s important to remember that hot weather concreting is not limited to high temperatures. Windy conditions, direct exposure to sunlight and low humidity can also create hot weather concreting conditions. Anything that will raise concrete temperature and dry out the surface may create short- or long-term issues.
Clues to watch for in fresh and hardened concrete
The most commonly occurring fresh and hardened concrete issues associated with hot weather concreting are outlined here, along with possible causes and tips for troubleshooting.
A Sticky mix with low workability: Fresh concrete should have a consistent texture and flowability. Watch for mixes that look lumpy or dry in certain areas, feel sticky or flow less smoothly than expected during placing. The concrete may also adhere to the finishing equipment or trowel when finishing the unformed surfaces.
Possible causes: Many factors could be at play here, however; with respect to hot weather concreting considerations, the most likely cause is insufficient mix water.
Solutions to consider: First, check your batch plant printout from the batch in question to determine how much total water was added as well as the total weight of cementitious materials (portland cement, blended cement, and supplementary cementitious materials like fly ash, slag or silica fume). If the w/c is lower than expected, the water meter likely needs immediate calibration. If the w/c is higher than expected and out of tolerance, the mix must be discarded. If the w/c is within the target range, check to see if the coarse and fine aggregate moisture contents were tested. In the warmer months, and especially on windy days, aggregates will be batched into the mixer in moisture conditions shockingly close to an oven dry condition. Be sure the aggregate moisture contents are checked at the necessary frequency and the moisture adjustments are being made correctly for each batch. Also investigate the mix proportions to ensure they are ideal for the given conditions and are designed to produce the desired fresh concrete properties. Contact your admixture, cement and supplementary cementitious materials suppliers for specific recommendations. Your material suppliers, particularly chemical and mineral admixture suppliers, will likely have specific requirements for batching sequences and mixing durations after each raw material addition. Lastly, if the fresh concrete was dispensed from the mixer and appeared flowing and workable as usual and the first realization the mix was sticky came when the unformed surfaces were being finished, the unformed surfaces may simply need to be finished sooner in the summer when the mix is capable of curing faster.
High fresh concrete temperatures: The temperature of fresh concrete at the time of placing should never exceed 90 degrees F. If the concrete temperature exceeds 90 degrees F, there is a significant risk that concrete strength and durability may be compromised.
Possible causes: The combination of hot raw materials batched into a mix, as well as exposure to hot surfaces such as chutes, reinforcing and forms, may significantly raise fresh concrete temperature.
Solutions to consider: If the fresh concrete temperature exceeds 90 degrees F or is out of tolerance, consider implementing methods for reducing the temperature of different components of the mix. Wetting or watering aggregate stockpiles with a sprinkler system can help cool the aggregates slightly. Consider using chilled water in the mix. Water can be cooled to as low as 33 degrees F. Also consider adding ice as a partial mix water replacement. The ice should not account for more than 75% of the total mix water and all the ice should completely melt during mixing. Ice should be in the form of chips, shavings or small pieces rather than cubes, blocks or large chunks. Try to keep the forms and reinforcing out of direct sunlight prior to casting. If the fresh concrete temperature is already approaching its upper limit when it is placed into a form that has been sitting in the sun all morning, the heat absorbed by the form will increase the fresh concrete temperature and could impact curing.
Plastic shrinkage cracks: Plastic shrinkage cracks manifest as thin, shallow, spidery cracks on unformed product surfaces within the first 24 hours after casting. They may also appear as long, thin cracks running nearly the full unformed length or width of a larger, flat precast concrete product.
Possible causes: Plastic shrinkage cracks occur when moisture near the surface of freshly placed concrete evaporates faster than the replenishing ability of rising bleed water. The expedited evaporation while concrete is in the plastic, or non-hardened, state causes shrinkage and contraction at the concrete surface and results in shallow surface cracks. The cracks can be caused by high ambient temperatures, wind, low humidity, lack of protection from those elements or any combination thereof. Typically, long, thin plastic shrinkage cracks on larger unformed surfaces of flat products are the result of wind, and often run perpendicular to the direction of the wind. Additionally, according to ACI, “concrete mixtures that incorporate fly ash, silica fume, or fine cements frequently have a low-to-negligible bleeding rate, making such mixtures highly sensitive to surface drying and plastic shrinkage, even under moderately evaporative conditions.”
Solutions to consider: Protect products from the hot, drying environment. Cover the unformed surfaces with tarps or plastic sheeting that are preferably light in color so they retain less heat. Cover the products as soon as the unformed surfaces are finished. Also consider applying moist pieces of burlap on the unformed surfaces before covering the products with tarps or plastic sheeting. A curing compound may also be used to help prevent surface moisture evaporation in extreme cases where other preventive measures may not be as effective.
Reinforcement shadowing: Reinforcement shadowing appears as dark gray, almost black concrete on the formed product surfaces that seem to show exactly where the steel reinforcement is placed.
Possible causes: Although this could be the result of not having the required 1/2 inch of concrete cover over the reinforcement, another likely cause could be the reinforcement was warm when the fresh concrete was poured around it. The warm, or sometimes hot, reinforcement can cause the fresh concrete surrounding it to cure more quickly, sometimes resulting in a dark shadow-like outline which shows where the reinforcement is located.
Solutions to consider: Store reinforcement indoors, if possible, or protect it from the sun’s rays with light-colored tarps. Try to ensure the reinforcement temperature is similar to the fresh concrete at the time of placement.
Low one-day compressive strength breaks: When performing 24-hour, one-day or stripping-strength compressive strength cylinder tests, the average of the results from two cylinders from the same batch, cured in the same manner and tested at the same age show a statistical outlier that is unexpectedly low. The results may not be out of tolerance, but they are noticeably lower than usual for the particular mix design.
Possible causes: In hot temperatures, precasters may use a mix design that has been modified with a set-retarding admixture, a low-early- strength cement or a slightly higher amount of supplementary cementitious material, or any combination thereof. All raw materials must be carefully measured, dosed and weighed during batching. Set-retarding admixtures are beneficial in warm temperatures to help regulate the cement hydration reactions, prevent accelerated setting and ensure the fresh concrete will remain workable for an appropriate amount of time. Use of SCMs as a partial replacement for portland cement can result in slightly slower compressive strength development when compared to a mix that uses only portland cement.
Solutions to consider: Check the batch plant printout for the batch in question to determine which admixtures, cement type and SCMs were used, and verify that the batched quantities fell within the tolerances set by the mix qualification documentation. If the quantities were within tolerance, check the ambient curing temperature record from the day prior when the cylinders were cast. Was the temperature as high as anticipated? If not, especially if the temperature was quite a bit lower than expected, using the hot weather retarding mix design that day may have been unnecessary. Also, check with your admixture or SCM supplier to determine if the mix proportions should be adjusted. If the quantities on the batch plant printout are out of tolerance, the batching equipment most likely needs to be calibrated.
High one-day compressive strength breaks: When performing 24-hour, one-day or stripping-strength cylinder breaks, the average of the results from two cylinders from the same batch, cured in the same manner and tested at the same age shows a statistical outlier that is surprisingly high. Although this may not seem like an issue, it is worth investigating to determine the cause as there could be long-term issues or cost-savings hiding behind those flattering test results.
Possible causes: Higher ambient temperatures or higher fresh concrete temperatures expedite cement hydration reactions. When the reactions occur at a faster rate, the concrete gains strength at a faster rate. If the concrete gains strength at too much of an elevated rate, it could result in hardened concrete issues like lower overall long-term strengths and risks to durability.
Solutions to consider: Check the batch plant printout from the batch in question to verify the type of raw materials and the quantities of each raw material were as intended and within tolerance. Review other fresh concrete testing data from the previous day when the batch was mixed. If any other test results seem out of the ordinary, what could have caused the variation? Was the winter mix design with high-early-strength cement accidentally used? Was more cement batched into the mixer than usual even if the total amount was still within tolerance? Did any other raw material quantities show variations or raise questions? Were they all appropriate for their intended use? For example, a high-early-strength cement or a set-accelerator are probably not necessary in most hot weather precast applications. Although high strength results may indicate you’re making a strong mix, strength results that are out of the ordinary in either direction deserve investigation – some variation in the material or process caused it. It’s a great opportunity to identify an oversight, catch scales that need to be calibrated or tighten up QC practices.
Prepare for warmer weather
With any variation in either fresh or hardened concrete properties, it’s imperative to investigate the factors at play and identify – and resolve – the cause of the variation in order to put a practice in place to help prevent it from happening again.
As you’re investigating, be sure the raw materials and the mix proportions you’re using as we head into the warmer months are appropriate for their intended use and are designed to provide the desired fresh and hardened concrete properties. Now is also a good time to review ACI 305 in its entirety, brush up on hot weather concreting best practices, ensure the procedures outlined in your plant-specific QC manual accurately reflect your operations, and be able to identify signs of issues in fresh and hardened concrete as a result of the warmer weather.
Kayla Hanson, P.E. is NPCA’s director of technical services.