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Re: Hot Weather Concreting

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You've gotton some good recommendations already.  My experience in
Phoenix is that using ice in the mix is worth it.  You need to get the
mix placed and consolidated quickly in hot weather.  The ice helps a
lot giving more time for the batch to be placed and worked.  Spraying
the rebar down that is exposed to the sun also is recommended.

Once your concrete exceeds 90 to 95 degrees, the placement is
questionable and with questionable placement and consolidation, the 28
day strength starts to get effected.  If you get a hot batch and it is
placed quickly and cured properly, you usually make the design
strength.

We use ice all the time in Phoenix.  The key is get the batch placed
quickly, consolidate quickly and start the curing as soon as possible.

Paul.
Phoenix, AZ

On 2/28/08, Harold Sprague <spraguehope(--nospam--at)hotmail.com> wrote:
>
> 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@palaunet.comTo: seaint@seaint.orgSubject: Hot Weather
> ConcretingDate: 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, PEGK2, Inc.tiger(--nospam--at)palaunet.com
> <mailto:tiger(--nospam--at)palaunet.com>
>
> _________________________________________________________________
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