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RE: Hot Weather Concreting

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The ACI 305.1 is as follows:

 

3.2.1 Limit the maximum allowable fresh concrete

temperature to 35 °C (95 °F), unless otherwise specified, or

unless a higher allowable temperature is accepted by Architect/

Engineer, based upon past field experience or preconstruction

testing using a concrete mixture similar to one known to have

been successfully used at a higher concrete temperature.

 

That leaves the Architect/Engineer the latitude to consider other means.  The potential problems listed in ACI 305 are as follows:

 

The amount of the water required to produce a given

slump increases with the time. For constant mixing

time, the amount of water required to produce a given

slump also increases with the temperature, as shown in

Fig. 2.2.1(a) and 2.2.1(b);

• Increased water content will create a decrease in

strength and durability, if the quantity of cementitious

material is not increased proportionately;

• Slump loss will be evident earlier after initial mixing

and at a more rapid rate, and may cause difficulties with

handling and placing operations;

• In an arid climate, plastic-shrinkage cracks are more

probable;

• In sections of large dimensions, there will be an

increased rate of hydration and heat evolution that will

increase differences in temperature between the interior

and the exterior concrete. This may cause thermal cracking

(ACI 207.1R);

• Early curing is critical and lack of it increasingly detrimental

as temperatures rise.

 
Each one of the above potential problems can be mitigated by some other means.  The use of a high range water reducer such as a polycarboxilate type can mitigate the first 2 items.  Fogging can mitigate the 3rd.  The fourth can be mitigated by insulation to minimize the thermal gradient.  The 5th can be mitigated by controled curing methods. 
 
The ACI 305 and 305.1 were developed from years of experience in various climates.  Admixtures and construction practices can help mitigate some of the potential problems.  You are free to work with your local supplier if you choose to deviate, but I would strongly urge a test placement of any concrete in which there is a deviation from what is tried and true. 

Regards,
Harold Sprague



From: tiger(--nospam--at)palaunet.com
To: seaint(--nospam--at)seaint.org
Subject: Hot Weather Concreting
Date: Thu, 28 Feb 2008 09:30:27 +0900

All,

I am hoping that someone on the listserver knows of literature that discusses the benefits of using crushed ice for concrete batching when sustained ambient temperatures for both placement and curing are in the range of 95 to 100 degrees F.

Let’s say I am placing concrete for two 8 inch precast walls (NOT mass concrete) in ambient temps of 95 to 100 deg F.

I use one mix design, but in one batch I replace some of the water with crushed ice (no change in W/C ratio) resulting in the temp of the concrete at discharge being less than 90 deg F.  For the second batch I do not use ice and so the concrete at discharge is 95 deg F or a bit above.  Both wall panels are shaded and moisture cured in exactly the same fashion at the same ambient temperature of 95 to 100 deg F for 28 days.

My question is, will the 28 day strength of the two panels differ significantly?

 

Based on literature I have been able to dig up, it would seem that there are two major problem areas with regard to placing concrete at these temps.  First would be the placement itself, as the hot weather would tend to reduce workability and decrease set time.  It seems to me that the addition of ice would help in this area by lowering the concrete temp at discharge.

Second problem area is overall decrease in ultimate concrete strength.  Since the temp of the iced concrete would rise and match that of the uniced concrete within a few hours and stay at that level through almost all of the curing time, it seems to me that there would likely NOT be a significant benefit by adding ice in terms of 28 day concrete strength.

 

I searched the listserver archives and didn’t come up with anything on this topic.  I am trying to determine if it is necessary to use ice at all, since other methods such as the addition of plasticizers to ensure workability could be used to address the first problem area.  If, however, there is a noticeable effect on 28-day strength than keeping the ice requirement would be preferable.

 

Thanks

 

 

Terangue *Tiger* Gillham, PE
GK2, Inc.

tiger(--nospam--at)palaunet.com <
mailto:tiger(--nospam--at)palaunet.com>

 



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