The Art Of Curing
- Written by CP Staff
Generally accepted as fundamental among optimum concrete curing conditions are temperatures between 85_ and 120_F, with relative humidity as high as practical and some form of air circulation. Cement content, a factor addressed later in the discussion, also plays a part in curing. For pavers, the optimum standard temperature would be 85_ to 105_F. Temperature variations can cause major problems, such as strength and color variation due to differences in the cure rates.
Optimum humidity should be maintained between 85 percent and 95 percent. Saturation of the air, occurring at 100 percent humidity, should be avoided because the result is condensation, which can negatively affect concrete surfaces and metal equipment. Some form of consistent air circulation is recommended, because hot air and steam naturally rise, and exceedingly hot humid dead spots develop without air movement. The negative result is that the chambers are hotter at the top while much cooler at the bottom. Consequently, uncontrolled faster cure rates occur at the top and much slower rates at the bottom of the chamber.
Over the years, various methods of heating curing chambers have been used, including direct steam injection, oil heated radiators and heat pump systems. A major problem with older systems was the escape of hot humid air into the main production area, where it would condense on the roof structure when the chambers were opened.
Another problem is related to steam, which produced at 212_F, is 100 percent saturated. It condenses as it cools and can rain within the chamber, causing drips on the product and corrosion on any uncoated steel in the chamber. Without good circulation and insulation, the heat energy in the steam can also be lost, making those systems inefficient and expensive to maintain. Some systems made use of hot air or steam circulation, and some did not. It is important to control humidity but, this was frequently not possible with the older systems. U.K. producer Brett Landscaping planned to solve these problems with its recent installation of a CDS Curing System.
With the CDS system, the air within the curing chambers is efficiently circulated to achieve consistent curing conditions. The temperature and the humidity levels are automatically monitored and controlled to tight tolerances. The air is well circulated and controlled which results in the benefits of improved energy efficiencies and lower operating costs. In fact, with this system, the humidity is maintained between the desired range of 85 to 95 percent without saturating the air. No condensation occurs because there are no cold bridges, and the insulation enclosing the chamber is designed correctly. It was this target of efficiency that Brett Landscaping was aiming to achieve when they planned to add a new block plant.
The curing system at Brett's Cliffe plant consists of a burner, air handling unit and insulated ductwork to equally distribute the warm humid air to all parts of the chamber. The system also includes motorized dampers, a sophisticated temperature and a humidity control system with a patented water atomization system, which effectively avoids uncontrolled vapor generation.
The Cliffe plant curing system was a further improvement of the CDS system outlined above by having the curing racks, finger car, elevator and the lowerator all totally enclosed. The well-insulated roof and the walls form part of the main building structure. The front wall of the chamber is manufactured from insulated cladding panels provided with access doors. The products enter and leave the curing chamber through openings fitted with air curtains that can be adjusted to allow for varying cube heights. The extra isolation that is provided by these curtains means that very little air escapes from the curing chamber into the production plant.
One significant advantage of having an enclosed curing chamber system is that fresh product is in the optimum curing environment within the shortest possible time after leaving the block making machine. This starts the curing process at the earliest possible moment and avoids the risk of surface evaporation that can cause efflorescence and a friable surface. The improvement in surface hardness can be seen when different products are compared. The pallets are also suitable for the higher temperatures in the chamber.
When operators examine the curing system control panel, they see a display of a temperature of 95_F and a relative humidity of 95 percent even though there is no condensation within the chamber. The racks, finger car, elevator and lowerator are all dry. The image of the mild-steel plate left inside the curing chamber for a year (photo, page 48) shows it to be rust free even though the center was left untreated while both ends were coated with a rust preventing coating.
There are other possible benefits from curing with controlled temperature, humidity and air circulation. Curing temperature and humidity are maintained constant Û summer and winter alike Û and there is no need to use additional cement to combat cold weather conditions.
For the manufacture of segmental pavers, landscape flags and segmental retaining wall or other decorative products, the key factors are better color consistency, lower energy consumption and high early strength at the lowest possible process costs. The consistent curing conditions maintained within the curing chamber provide color consistency throughout the whole chamber, top to bottom and side to side, giving brighter colors and greatly reduced efflorescence. Low energy consumption is achieved because the heat of hydration of the cement is more completely used. Very little energy is lost through the well-insulated walls and during warm weather conditions, with continuous operation, the burners may not actually fire at all because the heat that is needed is being generated by the hydrating cement.
The temperature in the chambers is continuously monitored and fuel is burned only when it is necessary to maintain the temperature. Elevated temperature curing (up to 120_F) is used to generate early strength. This makes it possible to reduce the cement content and cost, or to use less expensive cement substitutes such as furnace slag, etc., for further cost reduction.
For Brett Landscaping one of the most important benefits was higher product quality. The company favors bright and vibrant colors. The higher product quality experienced by Brett includes good color consistency with efflorescence virtually eliminated. The improved and reliable results achieved by the curing system have helped to ensure that their products are preferred by their customers.
Process cost savings has always been important to Brett, which has used furnace slag with the CDS system to provide significant pigment and cement savings. When furnace slag is used as a cement replacement material early strength can be a challenge. This is overcome by the elevated temperature curing provided by the CDS system. At the Cliffe plant, Brett staff has been able to compare the results achieved by two similar manufacturing plants: one operating with the benefits of the CDS system and the other plant with a chamber without any special curing system. Brett has been able to confirm that much more cementitious material could be used in the plant with the CDS system while still maintaining the required strength, frost and abrasion resistance specifications.
The controlled and consistent curing environment provided by the CDS system can make more cost-saving opportunities possible as a company like Brett develops its manufacturing and process procedures. An example is that when pavers from the CDS equipped plants are tumbled in line, the improvement achieved by the extra hardness and toughness of their surface is easily noted.
Based on the outcome and performance at the Cliffe plant, Brett has recently installed two more CDS systems.