Mena Infrastructure – Q2 2010
ALIREZA BIPARVA explains how to optimise concrete performance in hot weather construction environments.
While many industries may flourish in warm climates, high temperatures and humidity create many challenges for the construction industry. Hot, dry weather can lead to excessive surface moisture evaporation during concrete curing. Wind is also the enemy in this environment as it exacerbates the accelerated evaporation process. These factors lead to shrinkage, cracking and finishing difficulties that in turn lead to reduced concrete strength in buildings and structures – problems that should be mitigated from the onset of and throughout construction. Kryton’s Krystol Internal Membrane (KIM) is a waterproofing admixture designed to specifically address many of these issues.
Although Kryton’s KIM’s primary function is to act as a concrete waterproofer, it ultimately accomplishes this function while enhancing the durability of the concrete, since there is a direct correlation between permeability and durability. By managing the heat of hydration, KIM can reduce shrinkage and cracking by up to 25 percent and assist in internal curing that ultimately leads to a more durable and waterproof concrete structure.
A loss of concrete consistency is another major concern when working in hot weather conditions. Hot weather impacts the setting time of concrete by accelerating the hydration and causing it to dry too quickly. In general, acceleration in the hydration process will increase slump loss, whereas retardation in the hydration process will decrease slump loss. An effective solution is to use an admixture to act as a set retarder to mitigate this process. In many cases the addition of Kryton’s KIM will greatly enhance performance in this area.
The workability and consistency of the concrete mix should be sufficient to place, consolidate and finish the concrete. However, concrete loses fluidity over time, known as slump loss. Slump loss is directly related to the reduction of the free water in fresh concrete. The free water in fresh concrete is reduced mainly due to hydration of the cement and by evaporation. Therefore, conditions that accelerate the rate of moisture loss and the rate of cement hydration, such as high ambient and concrete temperature, low relative humidity and high wind speed, can also accelerate the rate of slump loss.
All concrete has slump loss or it would never harden. In normal slow setting, the concrete gradually loses its entire slump and then proceeds to harden. However, what is important is an abnormally high rate of slump loss and there are several ways to manage this.
The most common solution is retempering. Retempering is defined as the “addition of water and remixing of concrete that has lost enough workability to become unplaceable or unstable”. However, while adding more water makes concrete workable for placing, it will reduce the strength of the cured concrete due to an increase in the water/cement ratio. In addition, excess water will cause an increase in shrinkage, resulting in more cracking and less durable concrete.
Keep the concrete cool. Cooling the mix by replacing ice for part of the mixing water will significantly lower the concrete temperature, decreasing slump loss. On site equipment – from the mixers to the conveyor belts -should be kept shaded and cool as much as possible to protect the concrete
A good example of how Kryton’s KIM has performed in this environment is the Emirates Call Center in Dubai. This project was running behind schedule and due to tight timelines, Kryton’s KIM was selected as the waterproofing system in place of membranes. Additionally, this project was being completed in August 2008 where temperatures were often in the mid to high 40°C range. The entire raft slab and vertical walls were cast with Kryton’s KIM dosed concrete, while the horizontal joints were waterproofed with the Krystol Waterstop System. This project was delivered on schedule in large part because of the efficiency of the hot weather concreting process assisted by Kryton’s KIM admixture.
“High temperatures and humidity create many challenges for the construction industry”
Alireza Biparva is Technical Services Rep/Concrete Specialist, Research & Development at Kryton International Inc. Biparva received his MASC. in civil engineering from the University of British Columbia specialising in cement-based materials, and Bachelor Degrees of Engineering from Shiraz University of Iran. He has published several resear papers in international journals and conferences.