Accelerated curing generates big returns.
By Mel Marshall, P. Eng.
Mel Marshall is owner of Mel C. Marshall Industrial Consultants Inc. and has more than 40 years of experience in the precast concrete industry.
Don’t let anyone tell you that concrete doesn’t need to be cured. Regardless of how well the concrete is consolidated and how high the density, concrete doesn’t exist if it isn’t cured. Curing is a process by which a gel is formed to “glue” all of the ingredients together.
Curing is more than simply hardening of the concrete. Fully cured concrete results from a chemical reaction (known as hydration) between the water and cementitious ingredients of a concrete mix. During this hydration process, calcium silicate hydrate (CSH) gel forms. This is the glue that binds all of the aggregates together to provide a strong, watertight and durable concrete product.
A moist, warm environment is necessary to successfully cure your concrete products, although accelerated curing hastens the hydration process so that concrete strengths can be reached much more quickly, rather than waiting a full 28 days. Autoclave, dry heat and live steam systems have all been utilized for accelerated heat curing. Since no precasters use autoclave furnaces (even most masonry block producers have abandoned this costly method), dry heat and live steam are the methods of choice.
Keep in mind that an increase of 18 F (10 C) in curing temperature doubles the rate of hydration. The higher the curing temperature, the more accelerated the hydration process and the more rapid the strength gain. In other words, the higher the curing temperature, the shorter the curing time.
Dry heat systems can be effective as long as the environment is humid enough to ensure moisture doesn’t evaporate from the product while curing. Dry heat with low humidity can crack the concrete. It is absolutely essential that the product doesn’t lose any of the mix water that is so necessary to ensure hydration of all the cementitious particles.
Mix designs are based on specific water/cementitious (w/cm) ratios – the lower the w/cm ratio, the stronger, more watertight and durable the product. But in order to achieve the mix design strength, all the cementitious material needs to be hydrated to form all of the glue that is required to coat the surface of all of the aggregates. If moisture evaporates from the product, there won’t be enough left to hydrate all of the cementitious particles. As a result, the cured product will include unhydrated particles of cement, so the concrete strength will be lower than the designed strength.
Blower heaters are not recommended, because the dry air flowing over the product will cause evaporation of mix water from it. The use of heat blankets, electric heat and radiant heaters are examples of effective sources of dry heat as long as misting systems are utilized to provide a humid environment.
An advantage of using live steam to accelerate curing is that steam supplies both the heat and elevated curing temperature so necessary for successful acceleration. When the product enters the curing cell, its curing temperature is lower than that of the cell. As a result, the steam condenses on the product, which prevents the evaporation of moisture from the product and transfers heat to it. As the product reaches the temperature of the curing cell, the hydration process rapidly accelerates.
Low-pressure steam boilers or direct-fired steam generators can provide live steam. When sizing your system for a boiler, you need to understand that the boiler rating system is based on converting water at 212 F (100 C) to steam at an operating pressure of 0 psi. In fact, however, the makeup water is likely 50 F to 60 F (10 C to 16 C) and the operating pressure will be 10 psi to 15 psi. Since steam generators are direct-fired, they are close to 100 percent thermal efficient.
Conventional live steam curing systems utilize curing cells (kilns) that are either concrete structures or curing canopies made from canvas, Polyweave, etc. Live steam is commonly supplied to the cells through holes in distribution piping or steam hoses attached to steam header lines. Live steam should be used properly to be effective and to improve product quality.
In order to monitor what is happening in the curing cells, it is a good idea to place temperature recorders in them. These recorders, either mechanical or electronic, have the ability to accurately record the curing cycle, showing times and temperatures. These enable the user to determine whether or not the cell temperatures are increasing too rapidly and if the temperatures are too low or too high.
A typical curing cycle includes:
- Preset Time (presteaming) – the precast product should preset for a period of time (preferably two hours) to allow the product temperature to increase prior to steaming at a higher temperature.
- Ramp Time (temperature rise) – this is the length of time required to increase the curing cell temperature from the preset temperature to the predetermined target temperature. The ramping temperature should be between 20 F (11 C) per hour and 40 F (22 C) per hour. Ramping at more than 40 F (22 C) per hour can “thermal shock” the product, which can result in cracking, while a minimum of 20 F (11C) per hour is required to set off the rapid acceleration process.
- Holding Time – once the curing cell reaches the predetermined target temperature, this temperature is maintained for the required length of time necessary to reach the desired concrete strength.
- Soak Time (cooling) – after the temperature has been held for the necessary length of time, the steam is shut off and the product is allowed to cool, prior to removing from the curing cell.
All of the above time periods can be easily recorded and monitored, as explained above. There will be a time lag between the temperature change of the curing cell and that of the product. This temperature delay, which will vary depending on the product weight and configuration, can be determined by casting temperature sensors in the product. By monitoring both the product and cell temperatures, you will be able to determine how long it takes for the product to reach the temperature of the curing cell.
Precast manufacturers will want to prevent temperature excursions within the product. This refers to product temperatures that exceed 160 F (71 C), which is the maximum recommended product temperature permitted in the United States. Curing product at too high a temperature or placing product in outdoor storage under the hot summer sun can result in harmful temperature excursions.
Many precasters cure by simply placing steam hoses under curing hoods, with no temperature control and no monitoring of the curing cycle under the hood. Because there is no control on the amount of steam flow to the hood, the curing temperature can vary greatly, depending on the weight of concrete being cured within the hood. At many plants, the weight to be cured can vary substantially from one day to another, but the amount of steam going into the hood remains the same. It is important to understand that the amount of steam (energy) required to cure precast products varies with the weight of concrete to be cured. Supplying too much steam can harm the product, while too little steam may not be sufficient to trigger rapid hydration.
Some producers waste energy (which is very costly today) and reduce the effectiveness of their curing hoods/tents through flueing of energy through holes in the tent fabric. If there are holes in the sides or top of your curing hoods, you are likely flueing energy and wasting money. When the hood is filled with steam, the steam will escape through one or more holes by setting up what resembles an invisible flue pipe. Once a flow has been established through a particular hole, the energy will rapidly escape. Energy may flue through one hole one day but through a different opening the next. Take a few minutes to immediately seal the holes with tape or replace with new material. Flueing will also occur in conventional kiln structures, through leaks in the block walls and through poorly sealed doors.
A simple way to determine if a curing cell is flueing energy is to hold a lit cigarette at the bottom of the curing cell. Smoke flowing into the cell indicates that cold air is being drawn in to replace hot air that is escaping through holes in the hood or leaks in conventional kiln structures. Also, the bottom of the concrete product will be “greener” than the top because of the cool air passing by it.
If the cigarette smoke blows away from the bottom of the curing cell, you can be assured that the steam has flowed around the entire product as desired. Also, the floor of the cell will be damp, because the warmer steam will condense on the cooler concrete floor.
Live steam curing can be a very effective and efficient method for accelerating hydration, but a good understanding of the process is recommended. Proper curing is essential to improving product strength, impermeability and durability. Effectively utilizing an accelerated curing system will enable you to achieve product quality quickly.