As you are probably aware, the current weather forecast continues to be exceptionally hot, dry and windy this summer. In fact, there are reports that 2014/2015 will experience some of the worst conditions on record.
These conditions lead to defects in the concrete, and pavements are most at risk because they have a large surface area to volume ratio. The surface will bear the brunt of poor weather conditions, and most defects will affect the surface as a result.
In anticipation of these conditions, Vic Mix has issued a "Hot Weather Warning" notice to its customers both in November and late December 2014. The notice provides some useful tips when pouring concrete in hot and highly evaporative conditions. Read the Notice by clicking here.
Retarders - Good or Bad?
A consequence of Summer conditions is faster setting of concrete. This is a result of heat driving faster chemical hydration of the cement and dewatering of the concrete due to high rates of evaporation.
Naturally, it makes sense that Customers look for solutions that enable the concrete to behave more favourably and "Retarding Admixtures" seem a good idea. After all, Accelerating Admixtures work well in cold conditions to speed up the set time of concrete, it seems logical that Retarding Admixtures will have the direct opposite effect in hot conditions. For the most part this is true and Retarders can work well, however there are some complications!
The biggest problem in summer, is dehydration near the surface of concrete!
Pavements are most venerable because they expose so much surface area to the environment and the greater the surface area, the greater the potential loss of water which can lead to Plastic Shrinkage Cracks. Click here for more information. Without protection of the concrete surface to prevent excessive evaporation, the environment will take water from the dormant bleeding concrete through evaporation. Once bleed capillaries are disrupted by hydration and the concrete transitions to a gel phase, water loss actually slows. Hence, under these circumstances, Retarders will delay set, extend the dormant period, and extend the time when the concrete is most venerable to high water loss. The consequence of loosing something is a reduction in length (SHRINKAGE). This leads directly to cracking. See the diagram below.
Another common experience with the use of Retarders (especially in conditions leading to rapid drying of the surface) is wavy or spongy feeling under foot. Caution is required with the use of set retarders, as the surface may appear ready for finishing, but the concrete below may still be plastic from the retarder. This can then affect the uniformity of the surface finish. For more information on Hot Weather Concreting click here.
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