Concrete Curing Temperature Makes a Difference

How Warm Does it Have To Be To Pour Concrete?

Whether the conditions are hot or freezing, the ideal concrete curing temperature should be maintained at about 55°F to achieve the optimum concrete strength.

Curing the Hoover Dam

At its completion in 1935, the Hoover Dam was the largest dam in the world and a marvel of labor and engineering.  The first pour began on June 6, 1933. Rather than being a single block of concrete, they built the dam as a series of individual columns. The trapezoidal columns rose in five foot lifts. This method allowed the tremendous heat produced by the curing concrete to dissipate. If the dam were built in a single continuous pour, the concrete would have gotten so hot that it would have taken 125 years for the concrete to cool to ambient temperatures. The resulting stresses would have caused the dam to crack and crumble away (The Story).  

The heat and dryness of Nevada posed additional complex problems with the pour and concrete curing temperature.  When the concrete was first poured, river water circulated through cooling coils of 1″ thin-walled steel pipes. Once the concrete had received a first initial cooling, chilled water from a refrigeration plant on the lower cofferdam circulated through the coils to finish the cooling (The Story).

Concrete Curing is an Art

We live in a world where faster always seems better; however, concrete that cures too quickly or under hot concrete curing conditions can actually result in weak or unstable concrete.  If concrete is cured in cooler ambient  temperatures (32°F to 50°F) with moisture continually present, strength gain will be slow but the concrete will eventually reach a high strength. Concrete should not be allowed to get hotter than 90°F or to dry out during the curing period.

Best Concrete Curing Temperature

By “best” we mean “most thoroughly,” not the fastest.  High temperatures mean faster curing, but fast curing equates to weaker strength in the end.  The following study by Paul Klieger in the Portland Cement Association Research Bulletin 103 illustrates this concept.

concrete cure time chart with concrete curing temperature

Concrete Cure Time Chart with Temperature

At an age of 1 day the 120°F concrete was strongest and the 25°F concrete was weakest. By 7 days the high-temperature cured concretes had no more strength than the 73°F concrete or even less. By the age of 28 days the high-temperature concretes were weaker than the 73°F concrete. From 28 days to 1 year the 55°F concrete was considerably stronger than the 73°F concrete. All of this suggests that, provided there is continuous curing, concrete cured at about 55°F for the first 28 days ultimately reaches the highest strength (Concrete).

Hot Weather Concrete Temperature Limits

Hot weather concreting doesn’t simply involve temperature.  High ambient temperatures, winds, and relative humidity all play a role in “hot weather.”  Under hot heather conditions, the primary curing issue is having the top of the slab of concrete dry much faster than the bottom. As concrete dries it shrinks. This means that the top will be shrinking while the bottom is not. This creates internal problems with the concrete that will result in a damaged slab. The top and the bottom of the pour need to cure at the same rate (Placing).

Concrete Curing Temperature Solutions

Is it too HOT?

Powerblanket ICE is portable cooling equipment that will keep newly poured concrete safe from heat.  Portable, insulated, and efficient, Powerblanket ICE effectively regulates the temperature of concrete under both regular and hot conditions.

The Powerblanket ICE Circulation Blanket is combined with either a cooler or chiller to achieve optimum results. The Circulation Blanket draws heat to the blanket in order to cool the concrete.

  • Use Powerblanket’s patented heat-spreading technology in reverse–the cooling blanket draws heat away and lowers the temperature of the concrete.
  • Blanket cover and insulation are the same as the robust system used in Powerblanket heating products
  • Portable
  • Control the curing speed of newly poured concrete even in hot conditions

Is it too COLD?

Powerblanket concrete curing blankets provide a manageable way to cure concrete effectively and confidently in the cold weather months. Even in warm weather, Powerblanket curing blankets increase production by rapidly curing with consistent, even heat.

  •   Cure concrete 2.8 times faster than conventional, insulated blankets
  •   Produce cold weather concreting strength of up to 3,925 psi in 72 hours
  •   Maintain moisture throughout hydrating process
  •   Easily installed and removed
  •   Prevent a freeze cycle
  •   Thaw ground and frost from job site prior to pour
  •   Reduce downtime & increase profitability
  •   Maintain ACI compliance for cold weather concreting

 

Concrete Curing Temperature

Works Cited

Concrete Construction Staff.  “Best Curing Temperatures”. Concrete Construction Magazine. 16 May 2017. http://www.concreteconstruction.net/how-to/best-curing-temperatures_o

“Placing Concrete in hot or cold weather”. Sakrete Blog. 16 May 2017. http://www.sakrete.com/media-center/blog-detail.cfm/bp_alias/Placing-Concrete-in-hot-or-cold-weather

“The Story of Hoover Dam – Essays”. Bureau of Reclamation. 16 May 2017. https://www.usbr.gov/lc/hooverdam/history/essays/concrete.html

 

Hot Weather Concreting Made Easier

Hot Weather ConcretingHot weather concreting

Hot weather concreting problems are most often encountered in the summer; however, any condition that increases curing rates and strips the concrete of moisture is considered hot weather concreting.

Pouring Concrete in Hot Weather

With temperatures rising and summer approaching, attentions shift from protecting concrete from the cold to concreting in hot conditions.  When pouring concrete in hot weather, special procedures should be followed for proper mixing, placing, finishing, and curing.  High ambient temperatures, high concrete temperatures, low relative humidity, and/or high winds impair the quality of freshly mixed and recently placed concrete (Hot).  Pouring concrete in hot weather affects laboratory test results, showing that higher temperatures affect the compressive strength gain of hardened concrete. Even though concrete poured in hot weather will produce higher early strength but as time goes by, the ultimate strength will be lower than expected (Rodriguez).

Water Loss

Hot weather concreting causes increased setting rates and rapid water loss.  Shrinkage and cracking are usually associated with hot windy weather.   The major side effect of faster curing and water loss is decreased overall concrete strength.  It is critical to prevent moisture from evaporating from the concrete surface. The evaporation rate removes surface water necessary for hydration , and thermal cracking may result from rapid changes in temperature, ie. pouring concrete on a hot day followed by a cool night (CIP 12). Proper mix design can compensate for these conditions, and in combination with protective measures to prevent rapid evaporation, quality concrete can be poured in hot temperatures (Rodriquez).

Adding water can increase concrete workability. However, adding water beyond the amount required by the approved mix design increases the water to cement ratio. This, in turn, can result in decreased compressive strength and an increased chance of cracking (Hot).

Recommendations for Hot Weather Concreting

Preparation is key and there are several basic precautions that can reduce the damaging effects of hot weather on concrete:

  • Use mix designs that are less susceptible to the effects of hot weather. The use of low-heat-of-hydration cement and certain admixtures (such as hydration retarding and/or water-reducing admixtures) are two standard approaches.
  • Keep concrete as cool as reasonably possible. ACI 305R does not state a maximum “as-placed” or “as-delivered” concrete temperature, but 90° F is commonly used. Substituting chilled water or shaved ice for a portion of the required mix water can help.
  • Limit the amount of time between loading the concrete at the plant and placement/finishing at the site.
  • Limit water addition at the job site, except to adjust slump upon arrival (when permitted by mix design).
  • Avoid or limit hydration accelerating admixture use.
  • Schedule large concrete pours in the early morning or evening when temperatures are cooler and have the manpower available to complete the job as quickly as possible.
  • Use temporary wind screens and water misting nozzles to reduce surface moisture loss (Hot).
  • Consider using a concrete cooling blanket to both retain moisture and protect the concrete’s surface from high ambient temperatures.

Powerblanket ICE Concrete Cooling

Powerblanket ICE® is portable cooling equipment that will keep newly poured concrete safe from heat.  Portable, insulated, and efficient, Powerblanket ICE effectively regulates the temperature of concrete under both regular and hot conditions.

The Powerblanket Ice Circulation Blanket is combined with either a cooler or chiller to achieve optimum results. The Circulation Blanket draws heat to the blanket while also cooling the concrete.

  • Blankets use Powerblanket’s patented heat-spreading technology in reverse–the cooling blanket draws heat away from the drum and lowers the temperature of the concrete.
  • Blanket cover and insulation are the same as the robust system used in the Powerblanket heating products
  • Powerblanket Ice industrial cooling systems are portable (120VAC required)
  • Control the curing speed of newly poured concrete even in hot conditions

Hot Weather Concreting

 

Works Cited

“CIP 12 Hot Weather Concreting”. NRMCA. 5 May 2017. www.nrmca.org/aboutconcrete/cips/12pr.pdf

“Hot Weather Concrete”. Engineering Consulting Services.  5 May 2017. www.ecslimited.com/blog/hot-weather-concrete

Rodriguez, Juan. “Pouring Concrete in Hot Weather: Tips and Tricks”. The Balance.  5 May 2017. www.thebalance.com/pouring-concrete-in-hot-weather-845030