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Heading: 1.1.1.1.2Cement Temperature
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What is the maximum cement temperature at delivery that should be accepted?

At this temperature what effects can I expect in the concrete (not including, hotter concrete) and what should be done to counter-act these effects.

Thanks

Peter


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Peter Schwenger


The Portland Cement Association did an exhaustive study on this and the net result was that higher cement temperatures caused no meaningful problems other than to increase the concrete temperature. In the hot summer months, hot materials regardless whether they are cement, stone, sand or water will elevate concrete temperatures where it may become difficult to meet concrete temperature ceilings called for in project specifications. This may mean that a producer will need to sprinkle stockpiles to keep aggregate temperatures down. He may need to add ice as a partial replacement of some of his mix water. The most important factor is not the cement temperature but the concrete temperature. High concrete temperatures lead to lower strengths, slump loss, adding water on jobsites, increased shrinkage, higher potential for cracking, etc. Some contractors pour early morning to avoid high ambient temperatures in the PM.


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Fred Croen


Concrete temp is the determining factor, and cement hydration, in and of itself, produces chemical heat. Higher starting cement temps only exacerbate this action. I have had personal experience with high temp cement. In thick section concrete, the seven day breaks were in excess of 5,000 psi for 5.5 bag, natural aggregate, 2” slump concrete, with cement temps in the 180f + range. The 28 day breaks did not gain more than 10%. The solution was finding cooler cement. Hot concrete can cause hydration cracks as the concrete cools, and other problems.

I have also witnessed the effect of flash setting of concrete in the truck mixer. The appearance of the concrete is normal, but the 7, 14, 28, and 56 day breaks all failed. This was determined to be caused by flash setting due to excessively hot cement. This is particularly dangerous as results are not known for at least 7 days, and often 28 days.

There are many other factors that affect concrete temp, but they are too numerous to list here. The simple solution is a specification that calls for the maximum temp of the cement at the time of delivery to 105 F. That at least eliminates one of the factors contributing to higher concrete temps.

Bill Mangum


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Bill mangum Mangum


I would say that in the hot summer months, when cement demand is high, it may not be possible for suppliers to maintain a maximum cement temperature at time of shipping below 105F. That could be difficult even if the cement supplier has a million dollar cooler installed at their facility. I think the normal expected range would be up to 160F. No strength gain from 7 days to 28 days is a function of the chemistry of the cement C2S vs C3S and of the initial curing temperatures of the cylinders being used to test the concrete. They do make field curing chambers that will maintain 73 (+-)4F and 100% relative humidity. We use these in the field when situations warrant.

Fred Croen
Keystone Cement
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Fred Croen


To reiterate on a couple of the points already put forward…

In past years we often had to deal with concerns from concrete producers regarding the supposed affect on concrete temperature of "hot cement". It was not uncommon to receive comments from plant operators that a load of "hot cement" that had been delivered to a plant had to be the cause of slump retention problems.

Concrete producers were not aware that generally all local deliveries of cement are not only at a high temperature, about 75°C (165°F) or so, but also are at similar temperatures. And that is only for deliveries direct from a cement plant. Cement delivered from secondary dispatch depots after several handling operations would be at a lower temperature.

Important concrete literature and explanations written by Neville and many others state that a 9°C change in cement temperature will affect concrete temperature by about 1°C; in imperial units 9°F change for 1°F.
So even if the cement temperature rose from 50°C to 70°C the concrete temperature would only increase by 2°C.
By far the biggest contributor to the temperature of a concrete mix is the heat energy in the aggregate. Depending upon the actual mix design aggregate occupies 65 to 70% of the total concrete volume. Cement occupies about 8 to 15%.

Now that when concrete plant operators are aware of these points very few comments are raised regarding “hot cement”.

So if a lower concrete temperature is required, say to comply with a project specification or to maintain workability, the first item to look at would be to lower the temperature of the aggregates. Shading or wetting the aggregate stockpiles somehow. Wetting aggregate and then allowing the water to evaporate uses up heat energy.

1 deg unit of Centigrade = 1.8 deg units of Fahrenheit.
75°C = (75 x 1.8) + 32 = 167°F.

Paul Kidd

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Paul Kidd
 
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