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ACI Ryan Chairman Muwafaq A. Abu-Zaid Bijan Ahmadi J. Brandt Terence M. Browne Joseph G. Cabrera James N. Cornell, II D. Gene Daniel Richard D. Gaynor John G. Gendrich G. Terry Harris, Sr. Barry L. Houseal Frank A. Kozeliski Mark E. Leeman Kenneth B. Moore Dan Ravina John M. Scanlon Victor H.

Smith George V. Teodoru Habib M. Zein Al-Abidien Concrete mixed, transported, and placed under conditions of high ambient temperature, low humidity, solar radiation, or wind, requires an understanding of the effects these environmental factors have on concrete properties and construction operations. Measures can be taken to eliminate or minimize undesirable effects of these environmental factors.

Experience in hot weather with the types of construction involved will reduce the potential for serious problems. This committee report defines hot weather, lists possible potential problems, and presents practices intended to minimize them. Among these practices are such important measures as selecting materials and proportions, precooling ingredients, special batching, length of haul, consideration of concrete temperature as placed, facilities for handling concrete at the site, and during the early curing period, placing, and curing techniques, and appropriate testing and inspecting procedures in hot weather conditions.

A selected bibliography is included. These revisions involve an editorial revision of the document. The revisions focus in particular on the effects of hot weather on concrete properties, and the use of midrange water-reducing admixtures and extended set-control admixtures in hot weather.

Keywords: air entrainment; cooling; curing; evaporation; high temperature; hot weather construction; plastic shrinkage; production methods; retempering; slump tests; water content. All rights reserved including rights of reproduction and use in any form or by any means, including the making of copies by any photo process, or by electronic or mechanical device, printed, whitten, or oral, or recording for sound or visual reproduction or for use in any knowledge or retrieval system or device, unless permission in writing is obtained from the copyright proprietors.

ACI Committee Reports, Guides, Standard Practices, and Commentaries are intended for guidance in planning, designing, executing, and inspecting construction. This document is intended for the use of individuals who are competent to evaluate the significance and limitations of its content and recommendations and who will accept responsibility for the application of the material it contains. The American Concrete Institute disclaims any and all responsibility for the stated principles.

The Institute shall not be liable for any loss or damage arising therefrom. Reference to this document shall not be made in contract documents. These problems can adversely affect the properties and serviceability of the concrete. Most of these problems relate to the increased rate of cement hydration at higher temperature and increased evaporation rate of moisture from the freshly mixed concrete.

The rate of cement hydration is dependent on concrete temperature, cement composition and fineness, and admixtures used. This report will identify problems created by hot weather concreting and describe practices that will alleviate these potential adverse effects. These practices include suggested preparations and procedures for use in general types of hot weather construction, such as pavements, bridges, and buildings.

Temperature, volume changes, and cracking problems associated with mass concrete are treated more thoroughly in ACI Concrete durability is a general term that is difficult to quantify, but it is perceived to mean resistance of the concrete to weathering ACI Generally, if concrete strengths are satisfactory and curing practices are sufficient to avoid undesirable drying of surfaces, durability of hot weather concrete will not differ greatly from similar concrete placed at normal temperatures.

The presence of a desirable air-void system is needed if the concrete is going to be exposed to freezing cycles. Trial batches should be made at temperatures anticipated in the work and mixed following one of the procedures described in Section 2. The concrete supplier and contractor are generally responsible for determining concrete proportions to produce the required quality of concrete unless specified otherwise.

If the cylinders are allowed to dry at early ages, strengths will be reduced even further Cebeci Therefore, proper fabrication, curing, and testing of the test specimens during hot weather is critical, and steps should be taken to ensure that the specified procedures are followed.

High ambient temperature;? High concrete temperature;? Low relative humidity;? Wind speed; and? Solar radiation. The potential problems of hot weather concreting may occur at any time of the year in warm tropical or arid climates, and generally occur during the summer season in other climates.

Early cracking due to thermal shrinkage is generally more severe in the spring and fall. This is because the temperature differential for each 24 h period is greater during these times of the year. Precautionary measures required on a windy, sunny day will be more strict than those required on a calm, humid day, even if air temperatures are identical. Increased water demand;? Increased rate of slump loss and corresponding tendency to add water at the job site;?

Increased rate of setting, resulting in greater difficulty with handling, compacting, and finishing, and a greater risk of cold joints;? Increased tendency for plastic-shrinkage cracking; and? Increased difficulty in controlling entrained air content. The following list of practices and measures to reduce or avoid the potential problems of hot weather concreting are discussed in detail in Chapters 2, 3, and 4:? Select concrete materials and proportions with satisfactory records in hot weather conditions;?

Cool the concrete;? Use a concrete consistency that permits rapid placement and effective consolidation;? Minimize the time to transport, place, consolidate, and finish the concrete;? Plan the job to avoid adverse exposure of the concrete to the environment; schedule placing operations during times of the day or night when weather conditions are favorable;?

Protect the concrete from moisture loss during placing and curing periods; and? Schedule a preplacement conference to discuss the requirements of hot weather concreting. Harmful effects are minimized by control procedures outlined in this report.

Strength, impermeability, dimensional stability, and resistance of the concrete to weathering, wear, and chemical attack all depend on the following factors: selection and proper control of materials and mixture proportioning; initial concrete temperature; wind speed; solar radiation; ambient temperature; and humidity condition during the placing and curing period. The data in Fig.

Some researchers conclude that a relatively more uniform microstructure of the hydrated cement paste can account for higher strength of concrete mixtures cast and cured at lower temperatures Mehta The longer the delay between casting the cylinders and placing into standard moist storage, the greater the strength reduction.

The data illustrate that inadequate curing in combination with high placement temperatures impairs the hydration process and reduces strength. The tests were made on plain concrete without admixtures or pozzolans that might have improved its performance at elevated temperatures. Use of cements with increased rate of hydration;? Use of high-compressive-strength concrete, which requires higher cement contents;?

Design of thin concrete sections with correspondingly greater percentages of steel, which complicate placing and consolidation of concrete;? Economic necessity to continue work in extremely hot weather; and? Use of shrinkage-compensating cement. Good judgment is necessary to select the most appropriate compromise of quality, economy, and practicability.

The procedures selected will depend on: type of construction; characteristics of the materials being used; and experience of the local industry in dealing with high ambient temperature, high concrete temperatures, low relative humidity, wind speed, and solar radiation.

The most serious difficulties occur when personnel placing the concrete lack experience in constructing under hot weather conditions or in doing the particular type of construction. Last-minute improvisations are rarely successful. Early preventive measures should be applied with the emphasis on materials evaluation, advanced planning and purchasing, and coordination of all phases of work. Planning in advance for hot weather involves detailed procedures for mixing, placing, protection, curing, temperature monitoring, and testing of concrete.

Precautions to avoid plastic-shrinkage cracking are important. The potential for thermal cracking, either from overall volume changes or from internal restraint, should be anticipated. Methods to control cracking include: proper use of joints, increased amounts of reinforcing steel or fibers, limits on concrete temperature, reduced cement content, low-heat-of-hydration cement, increased form-stripping time, and selection and dosage of appropriate chemical and mineral admixtures.

Fig 2. It occurs in exposed concrete, primarily in flatwork, but also in beams and footings, and may develop in other climates when the surface of freshly cast concrete dries and subsequently shrinks. Surface drying is initiated whenever the evaporation rate is greater than the rate at which water rises to the surface of recently placed concrete by bleeding. High concrete temperatures, high wind speed, and low humidity, alone or in combination, cause rapid evaporation of surface water.

The rate of bleeding, on the other hand, depends on concrete mixture ingredients and proportions, on the depth of the member being cast, and on the type of consolidation and finishing.

Because surface drying is initiated when evaporation rate exceeds bleeding rate, the probability of plastic-shrinkage cracking therefore increases whenever the environmental conditions increase evaporation, or when the concrete has a reduced bleeding rate. For example, concrete mixtures incorporating fly ash, silica fume, or fine cements frequently have a low to negligible bleeding rate, making such mixtures highly sensitive to surface drying and plastic shrinkage, even under moderately evaporative conditions ACI R.

Table 2. The nomograph in Fig.


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ACI Ryan Chairman Muwafaq A. Abu-Zaid Bijan Ahmadi J. Brandt Terence M. Browne Joseph G.



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