Precast Cracking and Temperature
How to reduce cracking? What is the correlation with temperature? Is it possible to cure faster if the temperature is increased to over 70 C (158 F)? What other alternatives are available for Precasters?
The temperature handling for concrete is a science but it does not need to be over complicated. Some would argue more so for precast concrete because of the range of environmental conditions that a casting can experience between the plant, yard, site and final installation.
Proper temperature handling during the early stages of the product life cycle is key to ensuring durable, high quality precast and it is therefore a major consideration when planning a new production process, plant, or specialized product. It can be a subject of immense concern for precasters who are charged with conforming to strict specifications, particularly (but not limited to) public works contracts for example.The Basics – Simplified
During curing, a chemical reaction, known as hydration, occurs between the water and cementitious material in the concrete mix. The reaction produces CSH (Calcium Silcate Hydrate) gel in the paste fraction of the concrete, which binds the aggregates together, similar to glue. The reaction is also exothermic and releases a significant amount of heat, known as “heat of hydration”. Moreover, for complete hydration to occur, the concrete must be kept sufficiently moist and the heat of hydration maintained for an extended period of time to actively promote proper curing to meet or exceed the desired result for strength and durability.
Alternatives for Precasters
Three major curing methods are used to cure concrete:
- Moist curing, where excess water is applied to the surface of the casting after setting via ponding, sprinkling and wet coverings such as wet burlap or even sand.
- Enclosed curing, where the loss of moisture is prevented after setting by covering the casting with polythene sheeting, leaving the formwork in place, or by applying curing compounds to the surface that trap the moisture within the casting.
- Accelerated active curing through the use of steam, hot air, radiant heat or direct heating of the forms. The use of curing accelerators is also common practice.
Accelerated curing is the most common method used in precast production, and it is carried out under precise, controlled conditions in the plant or yard. The objective of an effective and efficient accelerated active curing procedure enables the producer to increase production capacity and efficiency, and reduce the amount of cement in the mix.It usually takes place in a special curing chamber, tarped enclosure or in the forms. In order to be effective, an appropriate curing cycle must be followed. Many systems are available commercially to control and monitor curing operations.
Typically, an accelerated curing cycle consists of 4 phases: Preset, Ramping, Holding and Cooling.
In general, if the holding temperature is higher, the desired concrete strength is achieved faster. The goal of most precasters is to achieve an acceptable strength for stripping – usually close to the 28 day strength – within 16 hours of casting or less, thus allowing a second cast in 24 hours. However most precasters know that there is a danger in pushing curing temperatures too high – Delayed Ettringite Formation (DEF).
The American Concrete Institute (ACI) defines DEF as a form of sulfite attack whereby curing at too high of a temperature (over 70 degrees C) stops the natural formation of ettringite during the early hydration process. Later, if the concrete is exposed to a wet environment, the ettringite crystals slowly form in the concrete, leading to internal pressures from the expanding crystals and destructive cracking. This was a serious problem in the 1990’s when cement suppliers increased the fineness and sulphate content of their cement under an increased demand for accelerated strength development, in conjunction with producers using higher curing temperatures. Today, it is widely acknowledged that the use of supplemental cementitious materials (Fly Ash, Silica Fume, Blast Furnace Slag) and respecting a maximum curing temperature of 70 degrees is the best practice to avoid DEF.
The use of low-pressure steam curing has been accepted as a standard method of accelerated curing throughout the world. However, some authorities require precasters to carry out secondary “post curing” operations involving wet curing for up to 7 days after accelerated curing is complete. This has been particularly for suppliers of precast girders for transportation projects, where DEF has been a problem in the past. Needless to say there is significant operational cost associated with secondary curing and there has been much debate as to the effectiveness of such measures.
In Canada in 2013/14, the CPCI commissioned a series of studies which compared the effects of different curing regimes (air curing after 16 hours accelerated curing, air curing after 72 hours moist curing and air curing after 168 hours moist curing) on the compressive strength and rapid chloride penetration (RCP) properties of samples prepared by 9 different plants. The concrete samples included supplementary cementitious materials within normally specified ranges. After evaluating the results, Researchers concluded that accelerated curing for 16 hours followed by air drying produces concretes that are statistically the same (within a 95% confidence range) as concrete that has been wet cured for an extended period, and that a maximum curing temperature of 70 degree C is acceptable to prevent DEF.
This research has proved to be valuable in allowing precasters to be confident that accelerated curing cycles are effective when maintained within the specified temperature and humidity limits, resulting in savings for the producer and more sustainable products that use less cement than what is necessary.