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Tech Brief: Hot Weather Concrete

SCP Tech Brief: Hot Weather Concrete

Placing concrete in hot weather poses one of the greatest challenges in construction. This is because concrete gains the majority of its properties during the temperature-dependent cement hydration reaction. Temperature of the concrete, surrounding air temperatures, and form temperatures can all play a substantial role in the quality of the final concrete product. Large volumes of concrete are placed during the hottest months of the year, therefore managing and mitigating threats associated with placing concrete in hot weather is crucial in today’s fast-tracked construction industry. Hot weather is defined 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F or higher, and an evaporation rate that exceeds 1kg/m2/h or as revised by the Architect/Engineer [1].

Understanding the threats that hot weather poses and how to mitigate those threats in an efficient and effective manner can determine the success of a project.

Some of the main effects of hot weather to concrete are:

  • Workability – slump or workability decreases as temperature increases [Fig 1]
  • Set time – set time decreases as temperature increases [Fig 1]
  • Compressive strength – A high early compressive strength followed by lower long-term compressive strength gains
  • Plastic shrinkage cracking – cracks that occur due to rapid evaporation of bleed water.

Planning during hot weather concrete placements is recommended in ACI 305 “Hot Weather Concreting”. Planning can consist of preplacement conferences, use of admixture and evaporation products, adjusting concrete mix proportions, and others. All of these should be focused on reducing and controlling temperatures and evaporation rates. Wind, relative humidity, ambient temperature, and concrete temperature are the largest factors when considering evaporation rate. The nomograph below has been used throughout the industry for decades and still prevails as the most widely used tool for calculating evaporation rates [Fig. 2].

There are many different ways to mitigate risks that come with hot weather concreting. A combination of measures taken at the ready-mix plant and the jobsite can allow for concrete to be placed at high temperatures. Acceptable production methods to reduce the temperature of concrete at include: shading aggregate stockpiles, sprinkling water on coarse aggregate stockpiles; using chilled water for concrete production; substituting chipped or shaved iced for portions of the mixing water; and cooling concrete materials with liquid nitrogen [1]. Acceptable jobsite measures include timely placement and finishing, control of bleed-water evaporation, curing, and protection. These jobsite measures are all focused on reducing evaporation rates when conditions are threatening. Methods to reduce evaporation rates on site include erecting wind shades and sun shades, using evaporation retarders, using proper curing methods, and others.

The initial curing of concrete strength specimens is often overlooked in hot weather. Because the specimens are small quantities of concrete compared to actual cast in place members, cylinders and beams can be even more severely affected by high temperatures. ASTM C31 Casting of Concrete Strength Specimens in the Field requires that strength samples

be stored at temperatures between 60 and 80 degrees F for normal strength concrete, and between 68 and 78 degrees F for concrete with a specified compressive strength of 6000 psi and higher. Additionally, the specimens should be shielded from direct sunlight and protected from moisture loss [2].

Another consideration is thermal shrinkage of the concrete structure due to rapid drops in concrete temperature of 22C (40F) in the first twenty-four hours. Thermal shrinkage can be reduced by using chilled concrete, using cool pipes in the concrete structure, using supplemental cementitious materials (SCM), using mist water, shading the slab, or using curing methods [3].


[1] Specification for Hot Weather Concreting. ACI 305.1-06. Retrieved December 18, 2018, from http://www.arquitectosrp.com/archivo/download/ACI 305.1-06 Specification for Hot Weather Cncreting.pdf

[2] ASTM C31 / C31M-18b, Standard Practice for Making and Curing Concrete Test Specimens in the Field, ASTM International, West Conshohocken, PA, 2018, www.astm.org

[3] Association, N. R. (2009). CIP 42 – Thermal Cracking of Concrete. Retrieved January 17, 2019, from https://www.nrmca.org/aboutconcrete/cips/42p.pdf

[Fig 1] CIP 12 – Hot Weather Concreting. Concrete in Practice – What, Why, How?Retrieved December 18, 2018, (2000) from https://www.nrmca.org/aboutconcrete/cips/12p.pdf.

[Fig 2] Specification for Hot Weather Concreting. ACI 305.1-06. Retrieved December 18, 2018, from http://www.arquitectosrp.com/archivo/download/ACI 305.1-06 Specification for Hot Weather Cncreting.pdf

 

Tech Brief: Cold Weather Concrete

SCP Tech Brief: Cold Weather Concrete

Portland cement concrete relies on chemical reactions between cementitious materials and water, known collectively as hydration, to harden and develop strength. Like most chemical reactions, the rate of early-age hydration is significantly temperature-dependent. Generally, as concrete temperature rises, the time for hydration to occur shortens and vice-versa. The American Concrete Institute produces two key documents that discuss concrete operations in cold weather: ACI 306.1-90 Standard Specification for Cold Weather Concreting and ACI 306R-16 Guide to Cold Weather Concreting. The information provided in both the specification and guide should be considered any time concrete will be placed during cold weather, defined by the Guide as “…when the air temperature has fallen to, or is expected to fall below, 40° F (4° C) during the protection period.” For most concrete mixes, this protection period lasts until the concrete has achieved at least 500 psi.¹

Generally, protective measures include providing sufficient concrete temperatures as mixed, not placing concrete on frozen base material or subgrade, and not allowing concrete to freeze before reaching 500 psi, generally at least 48 hours from time of placement. Heated aggregates and/or hot mixing water are often used by ready mixed concrete companies to achieve required minimum as-delivered concrete temperatures. Heated enclosures and curing blankets are often used to maintain concrete above freezing temperatures in the field once placed. Care should always be taken to vent the exhaust of hydrocarbon-burning heaters outdoors if they are used due to the risk of death of workers and carbonation of the concrete surface.

The damage from early-age freezing of concrete is often readily identifiable upon visual inspection due to the formation of ice “lenses” in the concrete surface. Concrete that has frozen during its early age (during the protection period) often will never achieve required compressive strength or durability characteristics.

Concrete hydration is an exothermic reaction. However, the rate at which the heat is lost to the atmosphere can overcome concrete’s ability to replenish the heat in the absence of insulating materials. ACI 306R gives recommendations on minimum temperatures to resist damage from early-age freezing based on cement content, concrete member thickness, and insulation R-value employed.

One obvious result of cold weather is that concrete takes longer to reach initial set. Generally, chemical reactions double in time required for every 10 degrees C variation. So, for concrete without accelerator and with normally encountered cementitious contents that sets in 6 hours at 70 degrees F (21 C) the following is a general guide for what to expect:

(*) Concrete below 50 F (10 C) may have extended set times beyond what is expected.

Cold Weather and Spray-Lock Concrete Protection Products

Various techniques are used by concrete companies to offset the effects of low concrete temperature, including heating aggregates, heating water, and the use of chemical accelerators. Spray-Lock Concrete Protection (SCP) products should be kept from freezing during storage and should not be applied to concrete with temperatures less than 40° F (4.4° C). When chemical accelerators are used in the concrete, care should be taken when using SCP products due to early setting of the product on the surface after application. If this occurs, delay SCP product application until the product penetrates without reacting on the surface. Otherwise, SCP products can be expected to perform normally as long as cold weather concreting procedures are in place.

Conclusion

Quality concrete construction is achievable in cold weather, but takes careful preparation, planning, and execution. Generally, producers and contractors that operate in cold weather often are accustomed to the requirements of cold weather construction. Problems typically occur more often in warmer regions that rarely have cold weather for extended periods where cold weather concreting procedures are rarely used.


¹ ACI 306R-16 Guide to Cold Weather Concreting. American Concrete Institute, Farmington Hills, MI.