Temperature changes can have wide, varied effects on concrete’s properties.
Summer has finally arrived, and with it comes warm weather, busy schedules, yard work, pool time and … goosebumps? On a hot summer day, the hair on your arms may raise on end. When our bodies experience a temperature difference that makes us feel cold, like exiting a pool or running through a sprinkler, we release adrenaline. Adrenaline causes tiny muscles under the skin to contract, resulting in the bumpy appearance we commonly call goosebumps. Just as temperature fluctuations can shock our bodies, they can shock concrete too.
Understanding temperature’s impact
Temperature is a complex thermodynamic property that is typically addressed as a simple, one-dimensional measurement in units of degrees Fahrenheit or Celsius. Temperature affects us in a subjective sense, and also affects material properties on an objective scale. Physical material properties including density, volume, conductivity, pressure, chemical reaction rates and even the speed of sound are all temperature-dependent. Precast concrete is no different, and quality precast relies heavily on appropriate curing temperatures.
Temperature differentials – the difference in temperature between two specified points – play a role throughout production. They exist among raw materials as well as between concrete and embedded items, concrete and forming equipment, concrete and ambient air, and even among different areas of a single concrete mass. Ketan Sompura, director of concrete technology at Sika Corp., said the greatest contributing factor for thermal shock and thermal cracking in concrete is the temperature differential between the surface and interior portions.
Precasters must also take temperature gradient into account. A product that is stripped from its form 24 hours after it is cast may feel cool to the touch, but a temperature gradient exists temporarily as the concrete temperature increases closer to the center of the mass.
Fast start, slow finish
The temperature of fresh concrete, or even “green” dry-cast concrete, depends on constituent materials, ambient temperature, mixing equipment, forming equipment and consolidation procedures. Controlling this is usually important as cement hydration reactions, setting time, compressive strength gain, workability and concrete volume are notably affected by temperature in addition to air entrainment and water demand.
In general, higher temperatures during the first few days of curing promote faster early strength gain, but can reduce 28-day strength. For example, with all other factors maintained constant at an optimal humidity of 80% and without special external treatments, early age cement hydration rates and concrete compressive strength steadily increase when comparing curing temperatures of 73, 90, 105 and 120 degrees F.
Concrete strength continues to increase throughout a product’s life as remaining cement hydrates. However, strength development rates slow approaching 28 days and taper near 36 days. How much the rates slow down depends on curing temperature, so as time goes on, things flip and concrete cured at lower temperatures will display higher compressive strength values than concrete cured at higher temperatures (PCA Figure 15-11).
However, if temperatures get too low, a different set of issues arise. Hydration rates slow as temperature decreases and hydration and compressive strength development discontinue below 32 F.
The low down on low temperatures
Concrete must reach 500 psi prior to exposure to freezing temperatures. This compressive strength value is considered the minimum acceptable point at which the hydrating cement paste is strong enough to resist stresses applied to it by expansive, freezing pore water. Concrete that freezes prior to reaching 500 psi may develop as little as 50% of its intended ultimate strength. The effects of early freezing are irreversible. ACI 306, “Guide to Cold Weather Concreting,” outlines precautions, procedures and considerations for manufacturing concrete in cold weather. Section 4.4.7 of the NPCA Quality Control Manual for Precast Concrete Plants states, “In cold weather the temperature of concrete at the time of placing shall not be less than 45 degrees F.”
Sompura recommends precast plants located in colder climates prevent thermal shock in concrete by doing the following:
- Control maximum internal temperature.
- Insulate the surface of concrete to ensure gradual cooling.
- Cover precast products to reduce temperature differential within concrete with different thicknesses.
“The concrete temperature should be high enough to prevent the concrete from freezing before reaching final set,” Sompura said. “This is the primary concern when concrete is placed in an open environment.”
Other available options include using set-accelerating admixtures, heated mix water, heated forms, high early strength cement or accelerated curing.
On the other end of the spectrum, special thermal considerations must also be made for hot weather concreting. Dry aggregate stockpiles can be misted with water to provide an appropriate moisture content prior to batching, iced or chilled mix water can be used instead of room-temperature water, and set-retarding admixtures or low heat hydration cement can be used. ACI 305, “Guide to Hot Weather Concreting,” outlines precautions, procedures and considerations for producing concrete in especially warm, dry weather. Section 4.4.6 of the NPCA Quality Control Manual for Precast Concrete Plants states, “In hot weather the temperature of concrete at the time of placing shall not exceed 90 degrees F.”
Manufacturing procedures that aren’t tailored to the curing environment can cause irreparable damage to curing concrete. Certain conditions like high concrete temperature, high ambient temperature, wind and low humidity can cause water near the concrete surface to evaporate faster than it can be replenished by rising bleed water. This process results in rapid drying shrinkage, tensile stresses and, in turn, plastic shrinkage cracks or crazing. Plastic shrinkage cracks, usually irregularly shaped and short in length, typically occur on unformed surfaces shortly after placing or finishing. Crazing appears as a more connected, yet irregular, network of very shallow cracks.
Sompura said precautions made by precast plants in warmer weather are somewhat similar to the points listed for plants located in colder climates. He recommends plants can do the following to prevent thermal shock:
- Minimize the temperature difference between the concrete form and concrete.
- Use supplementary cementitious materials like fly ash and, when possible, Type I cement to control the temperature of concrete.
- Spray water on aggregates to control the internal temperature of concrete and maintain an appropriate aggregate moisture content.
The right cure
Precast concrete manufacturers often employ accelerated curing methods, particularly in cold weather applications. Accelerated curing usually involves applying steam or heat and moisture to products enclosed in curtains or tarps for a set period of time and at a strictly regulated temperature.
To avoid disrupting hydration and early strength development, concrete must reach initial set prior to applying any form of accelerated curing. Attaining initial set, which is tested in accordance with ASTM C403, “Standard Test Method for Time of Setting of Concrete Mixtures by Penetration Resistance,” ensures curing concrete has developed sufficient strength to withstand the thermal stresses that result from applying steam and heat.
Clay Scott, assistant quality control manager of Forterra Pipe & Precast in Austin, Texas, explained that at his plant, steam curing is used to help maintain appropriate curing conditions for dry-cast pipe and box products during winter months and misting is used during summer months.
“As soon as one (curing zone) is filled up and production has finished their work, they lower the curtain,” Scott said. “It keeps the pipe moist and the curing temperatures up until the curtain is raised the next morning. Curing temperatures are monitored to make sure they don’t increase too fast.”
Curing conditions, especially when employing accelerating curing procedures, must be carefully regulated to maintain an ideal environment. No matter the curing method, the ambient curing temperature must remain below 150 F at all times to mitigate the potential for delayed ettringite formation condition. The ambient temperature must also not increase or decrease by more than 40 F per hour. Maintaining proper temperatures helps avoid shocking the concrete or interrupting hydration and strength development. Care must also be taken into consideration when using gas-fired heaters to cure products. To prevent severe concrete carbonation, the heat source must not be applied directly to exposed concrete surfaces.
Temperature’s lasting impact
In hardened concrete, the material property most notably affected by temperature is volume. When an object’s temperature increases, the atoms within it vibrate faster and cause the material to expand. Conversely, when an object’s temperature decreases, the atoms slow down and cause the material to contract. Think of how doors may open and close freely in winter, but stick during hot summer months. This is the same principle at work.
A material’s coefficient of thermal expansion (α) indicates how it will react to temperature change. Materials with comparable α exhibit similar temperature-induced length and volume changes, which keeps the thermal stresses between them to a minimum. In contrast, materials with dissimilar α display very different length and volume changes. When these changes occur in confined areas and cannot be accommodated, significant stresses develop, leading to cracking.
The α values of concrete and steel indicate that when exposed to a certain temperature change, a like-sized sample of each will display a similar change, but water will change 6 times as much as concrete.
In any scenario, concrete temperature and ambient temperature must be maintained at appropriate levels throughout mixing, casting, finishing and curing. Straying from the guidelines for any length of time can cause serious, irreversible damage to concrete that prevents strength development and dramatically reduces durability, permeability, freeze-thaw resistance, abrasion resistance, resilience and service life.
Kayla Hanson, P.E. is a technical services engineer with NPCA.