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References are included that provide supplementary data on the effects of curing temperature on concrete strength. If items found in these documents All rights reserved including rights of reproduction and use in any form or by any are desired to be part of the Project Documents they should means, including the making of copies by any photo process, or by any electronic or be phrased in mandatory language and incorporated into the mechanical device.
Under such 7. Rapid Chapter 10 - References, p. Concrete placed tended serviceability of the structure. Concrete struc- during cold weather will develop these qualities only if tures are intended for a useful life of many years. The it is properly produced, placed, and protected. The attainment of satisfactory strength for day, stan- necessary degree of protection increases as the ambient dard-cured cylinders is irrelevant if the structure has temperature decreases.
Cold weather, as ble corrosion of reinforcement see Section 9. Short- defined in this report, usually starts during fall and term construction economy should not be obtained at usually continues until spring. The practices and procedures de- scribed in this report stem from the following princi- specification checklist. When no external water is available, the 1. They do nor necessarily represent common metric ranges or sizes. For practical applica- No further protection is necessary unless a cer- tion.
The user should adjust them to conform with local practice. See Chapters 5 mum values should be made well before freezing tem- and 6. Necessary equipment 1.
See Chapter 8. See During periods not defined as cold weather, such as Chapter 9. The user should keep in mind the pri- placement. Concrete protected in this manner will be mary intent of these recommendations and should use safe from damage by freezing at an early age. If the discretion in deciding what is adequate for each partic- concrete is air entrained and properly cured, the ulti- ular circumstance.
Protection from freezing during the first 24 1. Protection and curing should continue excessive, considering what is required and the result- long enough - and at a temperature sufficiently above ing benefits. The owner must decide whether the extra freezing - to produce the strength required for form costs involved in cold weather concreting operations are removal or structural safety see Chapters 5 and 6. Neglect of protection against 2. Therefore, if cold time of placement should not be lower than the values weather concreting is performed, adequate protection given in Chapter 3.
In action, to prevent freezing at from low temperatures and proper curing are essential. Experience indicates that freshly mixed light- occur see Section 7.
For this, expendable thermis- weight concrete loses heat more slowly than freshly tors or thermocouples cast in the concrete may be used. Lighter weight insulat- ing concretes lose heat even more slowly.
However, when exposed to freezing temperatures, such concretes 2. Heated enclosures must be strong enough to be The temperature of concrete at the time of place- windproof and weatherproof.
Otherwise, proper tem- ment should always be near the minimum temperatures peratures at corners, edges, and in thin sections may given in Chapter 3, Table 3. Placement temperatures not be maintained despite high energy consumption. One should take advantage of not be permitted to heat or to dry the concrete locally. Concrete that is placed at low sulting from the use of salamanders or other combus- temperatures [40 to 55 F 5 to 13 C ] is protected tion heaters that exhaust flue gases into an enclosed against freezing and receives long-time curing, thus de- area, may be damaged by carbonation of the concrete.
It is, therefore, less subject to ther- crazing depending on the concentration of carbon mal cracking than similar concrete placed at higher dioxide, the concrete temperature, and the relative hu- temperatures. Placement at higher temperatures may midity see Section 7. Carbon monoxide, which can expedite finishing in cold weather, but it will impair result from partial combustion, and high levels of car- long-term concrete properties. In addition, strict fire prevention measures should be enforced.
Fire can destroy the protective enclosures as 2. Concrete can be damaged The actual temperature at the concrete surface deter- by fire at any age. However, at a very early age addi- mines the effectiveness of protection, regardless of air tional damage can occur by subsequent freezing of the temperature. Therefore, it is desirable to monitor and concrete before new protective enclosures are provided. Temperature record- ing and monitoring must consider the following: 2. The water-cement ratio should 2.
Temperatures of concrete sive strength of psi 24 MPa. Therefore, new and the outdoor air should be recorded at regular time sidewalks and other flatwork exposed to melting snow intervals but not less than twice per hr period. The during daytime and freezing during nighttime should be record should include temperatures at several points air entrained and protected from freezing until a within the enclosure and on the concrete surface, cor- strength of at least psi 24 MPa has been at- ners, and edges.
There should be a sufficient number of tained. Temperature measuring de- vices embedded in the concrete surface are ideal, but 2. During cold weather, bleed water 2. Data re- it interferes with proper finishing. If the bleed water is corded should clearly show the temperature history of mixed into the concrete during trowelling, the resulting each section of concrete cast.
A copy of the tempera- surface will have a lower strength and may be prone to ture readings should be included in the permanent job dusting and subsequent freeze-thaw damage if exposed. It is preferable to measure the temperature of Thus, during cold weather, the concrete mixture should concrete at more than one location in the section cast be proportioned so that bleeding is minimized as much and use the lowest reading to represent the temperature as practicable.
If bleedwater is present on flatwork, it of that section. Internal temperature of concrete should should be skimmed off prior to trowelling by using a be monitored to insure that excessive heating does not rope or hose. The gate, mixing may be more difficult. To facilitate mix- placement temperature of concrete should be deter- ing, about three-fourths of the added hot water should mined according to ASTM C The more massive be placed in the drum either ahead of the aggregates or the concrete section, the less rapidly it loses heat; with them.
To prevent packing at the end of the mixer, therefore, lower minimum placement temperatures are coarse aggregate should be added first. The cement recommended as concrete sections become larger. For should be added after the aggregates. As the final in- massive structures, it is especially beneficial to have low gredient, the remaining one-fourth of the mixing water placement temperatures see ACI Concrete should be placed into the drum at a moderate rate.
If loss of effectiveness of the In addition, higher temperatures require more mix- air-entraining admixture is noted due to an initial con- ing water, increase the rate of slump loss, may cause tact with hot water, the admixture must be added to the quick setting, and increase thermal contraction. Rapid batch after the water temperature has been reduced by moisture loss from exposed surfaces of flatwork may contact with the cooler solid materials. Rapid moisture loss can occur from surfaces exposed to cold weather because 3.
There- the desired temperature of the concrete during mixing fore, the temperature of concrete as placed should be can usually be obtained by heating only the mixing wa- kept as close to the recommended minimum value as is ter, but when air temperatures are consistently below practicable.
Placement temperatures should not be 25 F - 4 C , it is usually necessary to also heat the ag- higher than these minimum values by more than 20 F gregates. Heating aggregates to temperatures higher 11 C. If the coarse aggregate is dry 3. As the ambient air temperature de- dom has to be above about F 40 C , if mixing wa- creases, the concrete temperature during mixing should ter is heated to F 60 C.
Seasonal variations must be increased to offset the heat lost in the interval be- be considered, as average aggregate temperatures can tween mixing and placing.
The mixing temperature be substantially higher than air temperature during au- should not be more than 15 F 8 C above the recom- tumn, while the reverse may occur during spring. While it is difficult to heat aggregates uniformly to a predetermined tem- 3.
For small jobs, aggregates may be crete temperature within 10 F 5 C of the required thawed by heating them carefully over culvert pipes in temperature.
When aggregates are thawed or heated by circulating steam in pipes, ex- 3. Steam jets liberated in aggregate may cause concrete from batch to batch. Since the temperature of troublesome moisture variation, but this method is the concrete affects the rate of slump loss and may affect most thermally efficient procedure to heat aggregate. If the performance of admixtures, temperature fluctua- steam is confined in a pipe-heating system, difficulties tions can result in variable behavior of individual from variable moisture in aggregates are avoided, but batches.
ACI 306R-10 Guide to Cold Weather Concreting
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