Why Concrete Temperature Testing Is Important During Extreme Weather

The heat produced by concrete during curing is called heat of hydration. This exothermic reaction occurs when water and cement react. The amount of heat produced during the reaction is largely related to the composition (Table 1) and fineness of the cement.

Concrete Maturity e-book cover

Monitoring the temperature of your concrete pour after placement according to ASTM guidelines is one of the most important steps in the construction of a concrete structure. That is why ensuring optimal curing conditions for your element is critical, especially during extreme weather conditions. The hydration process can be drastically impacted if freshly placed concrete is exposed to temperatures that are too high or too low, compromising the strength development of a mix design. Furthermore, if the temperature differentials are too high, thermal cracking can occur. By closely monitoring temperature variances in your concrete element during curing you will ensure that the strength, quality, and durability of your structure is acceptable.

The heat produced by concrete during curing is called heat of hydration. This exothermic reaction occurs when water and cement react. The amount of heat produced during the reaction is largely related to the composition (Table 1) and fineness of the cement.

Want to learn more about what concrete curing is? Read this blog

The Five Phases of Heat Evolution in Concrete

Heat evolution in concrete is a very complex and extensively researched topic. To simplify this process, heat evolution over time can be separated into five distinct phases. The heat profile can change depending on the type of cement. Typical hydration for Type I cement is graphically represented in the figure below.

Table 1: Portland cement composition

Portland Cement Phases

Abbreviation (Chemical Formula)

Dicalcium silicateC2S
Tricalcium silicateC3S
Tricalcium aluminateC3A
Tetracalcium aluminoferriteC4AF
Calcium sulphate *CaSO4, CaSO4·2H2O (gypsum), CaSO4·½H2O

*Calcium sulphate only represent  ̴10% of the cement mass. The other four phases are the principal compounds of the Portland cement and their individual mass fraction changes based on the type of cement.

concrete temperature

 

Figure 1: Heat of hydration for Type 1 cement

Phase I: Pre-Induction

Shortly after water comes into contact with cement, there is a sharp increase in temperature, which happens very quickly (within a couple minutes). During this period, the primary reactive phases of the concrete are the aluminate phases (C3A and C4AF). The aluminate and ferrite phases react with the calcium and sulphate ions to produce ettringite, which precipitates on the surface of the cement particles. During this phase, at a lesser extent, the silicate phases (mainly C3S) will also react in very small fractions compared to their total volume and form a very thin layer of calcium-silicate-hydrate (C-S-H).

Phase II: Dormant Period

This phase is also known as the induction phase. During this period, the rate of hydration significantly slows down. This is believed to be due to the precipitation of the aforementioned compounds on the surface of the cement particles, which leads to a diffusion barrier between these particles and water. Nevertheless, there is significant debate regarding the physical and chemical properties behind the occurrence of this stage and the methods to predict it.

During this period fresh concrete is transported and placed. At this time, the concrete has not yet hardened and is still workable (plastic and fluid). The length of the dormant period has been shown to vary depending on multiple factors (cement type, admixtures, w/cm, etc.). The end of the dormant period is typically characterized by the initial set.

Phase III and IV: Strength Gain

In this phase, the concrete starts to harden and gain strength. The heat generated during this phase can last for multiple hours and is caused mostly by the reaction of the calcium silicates (mainly C3S and to a lesser extent C2S). The reaction of the calcium silicate creates “second-stage” calcium silicate hydrate (C-S-H), which is the main reaction product that provides strength to the cement paste. Depending on the type of cement, it is also possible to observe a third, lower heat peak from the renewed activity of C3A.

Phase V: Steady State

At this point, the temperature of the concrete stabilizes with the ambient temperature. The hydration process will significantly slow down but will not completely stop. Hydration can continue for months, years, or even decades provided there is sufficient water and free silicates to hydrate. However, strength gain will be minimal during this period.

What is a Thermal Control Plan?

A thermal control plan allows contractors to monitor the temperature of early-age concrete in order to ensure that the maximal temperature gradient is not reached so that it cures properly. These temperature monitoring procedures are specific to the project and concrete mix being used. They define the concrete temperature limits as well as the maximum concrete temperature differential between the core and surface in order to prevent concrete cracking and other quality issues. They also state how often concrete temperature should be measured.

Learn more about thermal control plans here!

Why Monitor Concrete Temperature?

In Phase II, the temperature of concrete can be measured as it is poured. Temperature is typically measured to make sure the concrete is in compliance with certain specifications that define an allowable temperature range. Typical specifications require the temperature of the concrete during placement to be within a range of 50°F to 90°F (10°C to 32°C). However, different specified limits are provided depending on the element size and ambient conditions (ACI 301, 207). The temperature the concrete exhibits during placement affects the temperature of concrete during the next hydration phase.

Monitoring the temperature of the concrete during phase III and IV is a quality control component that is regularly performed. The main reason behind this measurement is to ensure the concrete does not reach temperatures that are too high or too low. This allows proper strength development and improvement of the durability of the concrete. Another reason for monitoring concrete curing temperature during this phase is to evaluate the in-place strength, where the rate of hydration is the principal behind the maturity method (ASTM C1074).

A Complete Guide to the Maturity Method

Cold Weather Concreting

If the ambient temperature is too low, the hydration of the cement will significantly slow down or completely stop until the temperature increases again. In other words, there will be a significant reduction or an end to the strength development. If the concrete temperature freezes before reaching a certain strength (3.5 MPa/500 psi), the concrete will have a reduced overall strength. This will also cause cracking as the concrete does not have sufficient strength to resist the expansion of water due to the formation of ice.

To ensure proper strength development and avoid cracking of the concrete, the general guidelines suggest that concrete temperature must be maintained higher than a certain temperature for a specific amount of time (40°F) >5°C)) for 48 hours) (ACI 306).

Read More About Monitoring Your Concrete’s Curing Temperatures

Hot Weather Concreting

Generally, a concrete temperature is limited to 160°F (70°C) during hydration. If the temperature of the concrete during hydration is too high, it will cause the concrete to have high early strength development but consequently gain less strength in the later stage, resulting in lower durability of the structure overall. Furthermore, it has been observed that such temperatures interfere with the formation of ettringite in the initial stage and subsequently its formation in the later stages is promoted; which causes an expansive reaction and subsequent cracking.

Additionally, high temperature issues are of concern, especially in mass concrete pours, where the core temperature can be very high due to the mass effect, while the surface temperature is lower. This causes a temperature gradient between the surface and the core, if this differential in temperature is too large it will cause thermal cracking.

Learn how Giatec 360 can help you analyze concrete temperature gradients!

Mitigating Improper Concrete Hydration Temperatures

Multiple methods currently exist to mitigate the adverse effects of improper hydration temperatures. Two approaches, or a combination of both, can be taken to control the temperature during the dormant and strength-gain phases of the hydration process. One approach is to control the surrounding elements or mix constituents’ temperature. The second approach is to optimize the mix design.

Concrete Temperature Control During Mixing and Curing

In cold weather, the temperature of the concrete can be controlled by providing appropriate curing conditions to maintain insulation and strength gain, such as the use of heating systems. High curing temperatures can also be controlled when mass pouring with cooling pipes.

The temperature when the concrete is placed can be somewhat controlled by using cold water for the mix, cooling down aggregates using ice, or pouring at night when temperatures are naturally lower.

Concrete Temperature Controlling During Mix Design

An effective approach to controlling heat generation during cement hydration is to have a mix design that is suited to the application and the ambient conditions. Here are some things to consider:

  • Selecting the appropriate cement type changes the heat of hydration generated. Compared to Type I cement, Type III generates more heat while Type II generates moderate heat, and Type IV generates less than the others;
  • Adjusting the fineness of the cement. A finer cement will generate more heat;
  • Using supplementary cementitious materials (SCMs) is also an effective means of reducing the heat generated during hydration. Replacing a portion of the cement with, for instance, slag or fly ash, reduces the amount of reactive material in the early stages. In turn, this reduces the amount of heat generated and delays concrete strength gain; and
  • Adding other types of admixtures such as retarders and accelerators. However, these mixtures will not typically affect heat generation; rather they will be used to control the length of the dormant period.

 

Learn More About Optimizing Your Concrete Mix Design

Keep in mind that appropriate curing is crucial to ensuring that the concrete has enough moisture to hydrate properly. Overall, the general contractor, engineer, and ready-mix supplier need to be in good communication with each other. This is to ensure the structure is stable and safe so that the project is successful. Testing concrete temperatures regularly to avoid extremes during mixing, pouring, and curing, as well as having a plan in place in case temperatures do drop or exceed recommended limits is essential in any extreme weather.

**Editor’s Note: This post was originally published On March 23, 2018 and has been updated for accuracy and comprehensiveness.

11 Responses

  1. It was interesting to read that if the ambient temperature is too low, the hydration of the cement will significantly slow down or stop. Getting this professionally tested would probably be important if the concrete is used in any type of structural application. Making sure that the cement sets up correctly, even in hot or cold temperatures probably makes a difference when it comes to the integrity of it.

  2. Thanks for going over some things that can help control the temperature of concrete. It’s kind of cool that you can monitor it and check to see if it’s following certain specifications. I’m kind of interested to learn more about the tools that are used to accomplish this and if it’s something that a dealer can use.

    1. Hi Taylor,

      Thanks for your comment. We’ll have someone from our team reach out to you to discuss the applications of SmartRock2!

  3. Well first of all thanks for the useful article, and for the subsequent discussion. It is indeed important to consider the age at which the elevated curing temperature should be applied to achieve the desired performance of concrete mixes. However this should be performed consciously, as despite the fact that increasing the curing temperature might increase the mechanical strength of the concrete mixes, many evidences have shown that increasing the curing temperature at the hardening stage might significantly alter the pore size distribution of the pore structure. This has been attributed to the rapid formation of the C-S-H gel which in a way or another forms wider pores in the elevated curing temperature.This of coarse might have a significant impact on the future durability performance of the mixes if this increase in the pore sizes affected both the disconnected and the capillary porosity of the mixes. Therefore the decision to whether using elevated curing temperature or not should consider both the required future mechanical performance of the concrete mix, as well as the required future durability performance of the concrete member.

    1. Hussam,

      Thanks for contributing to the discussion! We will be focusing on the influence of temperature on porosity, pore-size distribution and overall durability in a future post. Stay posted!

  4. Dear Sara….

    100% agree with you.

    So we need new blog show that the effects of temperature on concrete in cases :
    1. fresh zone ( plastic) and
    2.harden zone…
    And how the smartrock sensor will be useful in both case

    Again as usually you give us very useful technical information…. And to be honest with you, am the one of very interested in your articles, keep on … .
    Many thanks….

  5. First of all thanks for great article… But i have little bit points i want to discuss with you..

    Nobody has doubt that the Temperature itself has good effect on concrete by accrelate the hydration of concrete …that why the most of precast producers use steam curing in high temperature for fasting harden products..

    So.. When the temperature has negative effect on concrete??? …

    Answer is… The adverse effect of temperature will be happen in the cases :

    1- if apply heat in the zone of delay time period ( time between casting and initial setting of concrete) in phase one which cause the DEF due to gypsum get soluble In C3A. .

    2- apply temperature in early age of concrete may be cause loss of water inside by evaporation which reduce the rate of hydration. So must of engineers Not recommend to exceed 32C

    3- apply temperature in the rang more than 80 C which cause DEF due to thermal tensile crack between outer pore and inner pore particle’s…

    So the key words to get best quality of concrete are :
    1- select the ideal time to apply heat ( high temperature need longer delay time) and
    2_ retention the water of mix inside concrete by cooling the water of mixing and cover by plastic sheet after casting to increase hydration of cement..
    3- monitoring the temperature in the early age of concrete.

    On the other hand in the cold weather place the theory is same but in adverse case…. Engineers worried about freezing the water of mix inside concrete which will cause the expansion crack ….. So that why should heat water and aggregate…

    1. Hi Radad,
      Thank you very much for your comment and for adding some clarification about some of the points mentioned in the blog.
      As you mentioned one of the main problems with high early concrete temperature is the delayed ettringite formation (DEF), which causes expansion and cracking, and reduces the durability and the later age strength of the concrete.
      High concrete temperature can also contribute to:
      – Reducing slump, which is usually compensated by adding more water to the mix.
      – Reducing the setting time, which affects the placement and the finishing.
      – The loss of water through rapid evaporation, which can cause plastic shrinkage of fresh concrete.
      – Drying shrinkage and thermal cracking of the harden concrete.
      In addition to high concrete temperature, hot weather concreting is greatly affected by the ambient condition such as low humidity, wind, and high environment temperatures. It is always important to have a good plan, monitor the concrete temperature at early age, and provide adequate curing in terms of temperature and moisture.

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