It might seem like a tall order to combine a faster turnaround time for building products and components and a flexible manufacturing location (either on- or off-site), with an overall reduction in energy use and wastage. However, modern researchers have succeeded in doing just this.
A leader in this field has been Australia's CSIRO Sustainable Materials Engineering, whose scientists developed an innovative clean technology for curing concrete products. The technology, called LEAP (low energy accelerated processing), uses carefully controlled microwave energy to speed up concrete curing.
What is Cement Curing?
Curing of cement refers to stabilizing the moisture content and temperature of poured fresh concrete for a duration sufficient to develop the required characteristics. When Portland cement is properly mixed with sufficient water, a chemical reaction called hydration takes place. This converts the mix of calcium-, silica- and alumina-based minerals that make up Portland cement into a hydrated gel of calcium silicates. This process develops the strength of the cement continuously over time.
The reaction begins from the surface and proceeds inwards, with the heat generation being maximum up to 24 hours, and then decreasing gradually over a month.
The advantages of adequate curing are considerable, as it ensures the cement is durable, strong, watertight, resistant to abrasion attack and freeze-thaw cycles, as well as to deicer application. It also preserves the volume stability.
For most applications using precast concrete, reducing curing speed (that is, achieving the required concrete strength within the minimum time possible) is a major objective, as it frees molds for re-use, thus cutting down the time to final completion of the order. It also helps minimize the disruption of function in the case of required repairs to precast concrete structures like roads, which are in constant use.
Heating the concrete mix is one way of doing this, but conventional heating methods such as steam heating or autoclaving rely on the conduction of heat from the outside to the inside, requiring considerable time to equalize these temperatures. This is where LEAP comes into its own.
How Does Microwave Curing Work?
The basis of this microwave curing technology is the high penetration depth of microwave radiation, which allows the material to heat up from inside. The penetration is sufficient to ensure complete heating of materials of up to medium thickness.
Microwaves cause internal energy dissipation by causing excitation of molecular dipoles in electromagnetic fields. This leads to rapid and more uniform heating, thus causing high early strength to form within a short process duration.
Results of Using Microwave Curing
Many studies have shown that using microwave curing for 15 to 30 minutes results in better 3-day and 28-day compressive strength by about 40-45%, while the flexural strength increases by up to 15%. This progressive increase in strength with the aging of the cured concrete is due to the elimination of water from the fresh mix at the time of microwaving. This closes the capillary spaces within the concrete and causes densification.
Microwave curing has proved itself to be capable of curing precast concrete in six hours or less, in the absence of any other accelerants. This is almost three times as fast as any other method now in use, including the use of steam or other heating agents, which takes from 12-24 hours.
LEAP technology was intensively tested on both laboratory and pilot model scales, at CSIRO, using both solid and hollow-core concrete products. One significant finding is that actual microwave heating occurs for only about 10% of the total time taken for curing, for the energy consumption of 40 MJ per ton.
What are the Advantages of Microwave Curing of Concrete?
This technique presents multiple important advantages, namely, a shortened curing phase for pre-cast concrete, which speeds up final delivery, as well as a significant and rapid increase in strength, which is proportional to the age of the concrete.
The first application envisaged for this product was the rapid curing of long-bed pre-cast concrete structures like railway sleepers, or hollow-core wall panels and slabs, but it is now clear that it can be used for curing many more conventional products in the construction industry, like bricks and paving, roof tiles and piping, as well as utility poles and even, potentially, concrete roads.
A huge benefit of using microwave curing is that it is versatile enough to be used as a stand-alone or in combination with other curing technologies used for pre-cast concrete, like conventional steam chambers, water- or steam-heated casting beds, and autoclaves. Another advantage is the ability to heat the concrete from a cold start, obviating the need to pre-heat casting beds, which reduces overall energy costs.
The range of markets for this product thus ranges from low-cost housing to high-end, high-performance products, in developed countries and in the Third World, which is currently engaged in building large volumes of low- and medium-cost housing developments. These require precision parameters to reach quality standards, among which rapid-curing of the concrete takes a prominent place.
Sources and Further Reading
This article was updated on the 19th August, 2019.