The curing or setting of concrete is an often misunderstood process. As concrete is wet when it is mixed and poured, many believe curing is a drying process that strengthens and sets the mixture in place. However, maintaining adequate moisture levels during the curing process is essential, with premature drying often leading to unwanted cracking.
The curing of concrete is a hydration process which takes time. By controlling temperature and humidity levels, you can help to create the ideal environment and optimize the hardness and strength of concrete.
Optimal conditions for concrete curing
As well as leading to cracking, improperly cured concrete is often weaker, less resistant to abrasion and can suffer from scaling. Concrete exposed to the elements can be particularly susceptible to these problems. Drying winds can reduce moisture levels, and fluctuating temperatures affect optimal curing conditions.
The strength of concrete increases over time, and it can continue to strengthen over decades. Around 90% of the strength, however, is typically achieved during the first four weeks, with the first three days of hardening being most critical. It is therefore essential to ensure the best curing conditions for concrete during this early phase, and there are several different curing systems to choose from.
But which system is best for you? Below we will describe some of the different curing systems available and their respective areas of use.
Curing of concrete specimens
Laboratories often require the curing of individual concrete specimens. Specific standards are set by the ASTM and AASHTO, along with recommendations for accepted curing methods. Curing tanks and moisture rooms are two such methods, each with their specific requirements. A comparison of the two will help you to decide which is most suitable to your situation.
Curing tanks are one of the simplest and least expensive solutions for the controlled curing of concrete specimens. Water storage tanks such as those used for livestock feeding can effectively hold several smaller concrete specimens. This makes them perfect for shorter-term or temporary applications, or where space is limited.
Curing tanks are filled with calcium hydroxide-saturated water, which maintains moisture levels around the specimens. The calcium hydroxide prevents leaching of this chemical into the water. Additional equipment is then used to maintain optimal temperature levels in the water – monitoring, circulating and heating it as necessary. This ideal temperature is 23.0±2.0°C (73.4±3.5°F). They can even be interconnected to increase their capacity and the efficiency of the temperature regulation system.
Curing tanks are low-maintenance and easy-to-assemble yet provide all the conditions necessary for optimal curing of concrete specimens. Spillages from the tanks can occur, however, and the chemicals in the water prevent the specimens from being handled without protective gloves. The large surface area of water exposed can also lead to humidification of the rooms in which they are housed.
A more advanced solution for the curing of concrete specimens is a moisture room; a room made from moisture resistant materials in which temperature and humidity levels can be maintained. These can be pre-fabricated units or can be constructed in situ to the required size. This allows for the simultaneous curing of larger and greater quantities of specimens.
According to specifications humidity levels in a moisture room must be kept at or above 95% relative humidity. There are various systems for creating the inside atmosphere in a moisture room. One way is to use fogging humidifiers in combination with temperature regulation equipment to maintain levels. As with curing tanks, this temperature is 23.0±2.0°C (73.4±3.5°F).
A more advanced system involves atomizing spray heads which release temperature-controlled water into the room, providing both heat and humidity at the same time. Both systems are likely to incorporate an externally mounted control panel which regulates the temperature and humidity in the room. This allows for easy monitoring of the curing specimens.
Moisture rooms allow for easier and more effective storage of specimens. By using racks or shelving the specimens can be maximally exposed to the humid air. Organization and handling are also much easier than in curing tanks. Moisture rooms, therefore, allow for more effective curing of greater quantities of specimens but can be more expensive to build and maintain.
Which curing system you choose ultimately depends on your circumstances. If only a few specimens are to be cured, or you only require a temporary installation, then curing tanks will likely provide the most cost-effective solution without sacrificing on quality. When larger or a great number of individual specimens are to be cured, it may be necessary to take advantage of a moisture room’s greater capacity. For longer-term applications, the opportunities for more effective organization and easier handling can also make moisture rooms an excellent choice.