Concrete Pour at High Temperature

[AminSaalaar]

Hi

Is it harmful, if freshly poured concrete is subjected to 40 to 50 degrees celcius during its curing period?

Does anybody think I should be using retarder in the mix?

Thanks guys

 

[Ron]

Yes, it can have detrimental effects. The three most common effects are lack of consolidation because it stiffens too quickly, cracking due to increased drying shrinkage, and the initial strength will be higher than normal but the ultimate strength will be lower than normal.

 

[civilperson]

No, if the rise in temperature is moderate, say five or six degrees an hour from the plastic state with 30 degree maximum starting temperature then the 50 degree temperature will only accelerate the cure. This is standard proceedure for precast plants using boilers to steam heat the product and forms at a slow rate.

 

[cvg]

yes, but these are cured in an autoclave at relative humidity of nearly 100%. For high temperature curing outside of the boiler, with sun, wind and dry air, not as good for your concrete. ACI has special procedures that should be followed.

ACI 305R Hot Weather Concreting

 

[civilperson]

No autoclaves are found in precast factories that I have seen. Merely canvas tops and steam pipes below using membrane cure on exposed concrete that slows moisture loss.

 

[cvg]

irregardless, steam curing was not proposed by the original post

 

[Ron]

ACI publications, including 305R, give the detrimental effects. 50C is 122F...that's high for placing concrete, considering the autogenous heat of hydration, and the likelihood that the mix water temperature will be closer to 80F or less, that's a fairly significant differential and potential heat gain.

Assuming it is ready-mix concrete, then that temperature range greatly exceeds the typical limits of ASTM C94.

We have had to go to nitrogen cooling and other means (ice replacing mix water) in temperatures that were not nearly as high as 50C, just to keep the concrete properties within reason.

 

[AminSaalaar]

Just to clarify

We are replacing a 12-inch top slab with a 10-inch slab (19' x 13')for a steam manhole on existing walls. Live Steam piping within manhole at time of pour. Currently, temperature inside manhole with the existing top slab is about 50 degrees that is why I am assuming the same temperature for the pour and curing period.

By the way, is it correct to assume that concrete gains 2 MPa compressive strength per day after it's poured? i.e. 20 MPa after 10 days........

Thanks

 

[cvg]

We limit concrete pours to less than 100F during the pour for thin slabs, however curing can continue after the pour with temperatures up to 120F.

I assume no steam is released, so humidity is relatively low? can you place insulation over the steam lines prior to pouring the concrete? Can you ventilate the space through the manhole opening? You might also be concerned with the temperature differential between the inside and the outside during the curing period. What is the ambient temperature outside of the vault? Is it much cooler than 50?

 

[AminSaalaar]

Yes, we will be re-insulating all piping before the pour considering it is high pressure steam line. good insulation may reduce the hot air in the manhole. I guess there will be some ventilation through the opening too. outside temperature could be between 50F to 70F. But if you're saying temperature limits could be between 100F and 120F then it should be fine, shouldn't it?

 

[cvg]

I would add additional temporary insulation to reduce the temperature in the vault during the pour. But you are saying the temperature inside could be as high as 122F and outside it will be as low as 50F? That is a 72F degree differential in temperature. Will that cause uneven curing, shrinkage and cracking on your slab?

 

[kslee1000]

Only one person mentioned "humidity" in this whole episode, which, it is to me, holes the key.

Mass concrete requires cooling for placement in the normal temperature range, because of the heat generated through hydration is much higher than the ambient temperature. Without cooling, the crust sets much faster than the core, resulting in random shrinkage cracks, and affecting durability of the concrete.

In cold region, the mixing water and aggregates need to be heated, and the finished surface needs to be covered to avoid freezing.

In hot region, fast vaporization is the culprit, especially on a dry, breezy day (On the other hand, humid, airless day helps fresh concrete mix to hold on to the surface water). Here again, with proper surface protection to avoid escaping of surface water, the high temperature itself hardly has any effect on concrete curing. However, the temperature differential between day and night colud be crucial, which shall be take into account when selecting curing method.

I don't think any chemical additive would do better than a good planning on curing/surface protection technique.

 

[concretemasonry]

There is a long history of curing at higher temperatures using atmospheric pressure and stems to accelerate the strength gain under controlled conditions very often using controls that are not possible on the job site.

The autoclave is the ultimate curing method,but is limited to certain size and shape elements and is not possible on a job site. - Absolute control of the moisture and temperature.

For a construction site, do not be mislead by the air temperature since the controlling factor is the temperature of the concrete, assuming the mix is an ordinary mix using ASTM spec cement and admixtures and not the highly research materials, cements and admixtures designed for site work.

Uber accelerated conditions. the strength gain can be measured in psi per hour and not per day. Your conditions are not normal and it appears you are trying to use a conventional fot that application, so you should use the traditional limitations and guidelines. Concrete cylinders will give no meaningful information.