E-mail address. Cart Check out. All new products. All best sellers. The effect of the gain that could delay form stripping ACI R. The most common weathering action is fect crack control. Concretes differ widely in the amount of multiple cycles of freezing and thawing of water near the tensile strain they can withstand before cracking. For strain surface that fracture and deteriorate the concrete over time. Naturally occurring sulfate can combine For strain that is applied slowly, the creep or relaxation of with calcium and alumina in hardened concrete to form the concrete is important.
Abrasion on dams and shown that tensile failure occurs without much plastic strain. It is accurate to assume that the tensile-strain capacity Mass concrete structures, normally unreinforced, can have an is inversely proportional to the modulus of elasticity of the outer layer of temperature reinforcement that requires corrosion concrete. The modulus of elasticity of the aggregate is important protection similar to that of reinforced structural concrete.
Therefore, in the extreme case where a crushed aggregates are apt to be superior to natural aggregates concrete has a low tensile strength, a high modulus of for crack prevention.
Table 7. On the other elasticity of a particular concrete after various ages of curing. More crete can withstand varies greatly with the composition of data on the thermal expansion of concrete can be found in the concrete and the strain rate. When strain is applied ACI Therefore, concrete in the interior of a large mass that straint of thermal contraction, and to a far lesser degree, from must cool slowly can undergo a large strain before cracking.
Drying shrinkage is important be- If concrete contains rough-textured aggregate with small cause it can cause shallow cracks to occur at the surfaces. There is an Temperature change is a principal contributor to tensile optimum, however, with respect to aggregate size. Concrete strain in mass concrete. Predicting probable strain requires with smaller aggregates requires more cement for a given the expected temperature, which can be determined if the strength. This results in more heat, a higher maximum tem- adiabatic temperature curve for the concrete is known, as perature, and greater subsequent strain due to cooling.
The temperature rise that would occur if there oped from the use of smaller aggregates. The main problem is choosing the point flexural loading. It is satisfactory to use air cross section and to mm 64 to in. For ten used Houghton Strain capacity is determined dams, measured reservoir temperatures are used.
Refer to from these tests under both rapid and slow loading to simulate ACI Expressed as extreme fiber stresses, the loading rates are The heat-producing characteristics of cement play an im- generally 0. Type II moderate-heat cement is also recommend- surface or embedded strain gages or meters Houk, Borge, ed, and it usually is available.
Pozzolans can be used to re- and Houghton ; Houghton Embedded meters are place a portion of the cement to reduce the peak temperature best for long-term tests.
The strain can also be determined due to the heat of hydration ACI In some cases, from deflection measurements. Some of the more common poz- prevent loss of moisture. Testing should be conducted at a zolans used in massive concrete include calcined clays, di- constant temperature for maximum accuracy in measure- atomaceous earth, volcanic tuffs and pumicites, and fly ash.
Detailed test procedures can be found in Houk, Borge, The actual type of pozzolan to be used and its appropriate re- and Houghton and McDonald, Bombieh, and Sulli- placement percentage are determined by test, cost, and avail- van In the preliminary studies of temperature and ability. Figure 7. The data show the effect of pozzolan replace- due to temperature change; their determination is detailed in ment of cement on temperature reduction. The approximate range of thermal properties is shown in Table 7.
The amount of strain that a temperature crete structures are required to be safe, economical, durable, change will produce is directly proportional to the coefficient and aesthetically pleasing.
Each of these requirements influ- of thermal expansion of the concrete. The average coefficient ences the crack resistance. The panels should be of good quality tions. Aesthetics will come from good workmanship, for durability and lightweight to provide thermal insulation. It is important to have a comprehensive materials improves crack control. More information on the use of precast test program to establish necessary control before preparing panels for protecting mass concrete can be found in ACI R.
Precooling sures are taken to reduce the amount and rate of temperature concrete before placement is accomplished by a variety of change.
Measures commonly used include precooling; post- methods, including evaporative cooling of the stockpiled ag- cooling, or a combination of the two; and thermal insulation gregates using sprinklers, cooling all ingredients of the mix- to protect exposed surfaces. The degree of temperature con- ture by liquid nitrogen, or using small ice particles as a trol necessary to control cracking varies with factors such as replacement of part of the mixing water. When these precau- location, height, and thickness of the structure, character of tions and natural cooling are ineffective in reducing crack- the aggregate, properties of the concrete, and the external re- ing, postcooling is prescribed, especially for large concrete straints.
Although a large amount of the data for this chapter dams. Postcooling concrete is accomplished by circulating has been obtained by experience from using mass concrete in cool liquids usually water through pipes embedded in the dams, it applies equally well in mass concrete used in other concrete. The water source from a nearby lake or river can be structures, such as steam powerplants, powerhouses, bridge directly circulated or refrigerated as needed. Roller-compacted concrete is such as lift height, pipe spacing, water temperature, flow discussed in ACI Details on pipe cooling are given in mass, should be avoided.
This is particularly true in smaller Section 7. A vulnerable location in pipe cooling systems Even though the more crack-resistant concrete can be too is centered at the cooling coils where sharp gradients and costly to be used throughout the structure, it can be used to cracking can be induced if termination of cooling water limited extent without serious effect on cost. Casting thin circulation is not timely. In mass concrete, a maximum sufficiently dissipated or slow concrete placement to a rate rate of cooling of 0.
Cooling should continue until the prescribed temperature Thermal couples should be used in sufficient numbers to is reached. This temperature is dependent on the type of permit adequate monitoring and control of the internal con- structure and loading. The prescribed temperature can be the crete temperatures. They should be located to record repre- mean annual temperature or other temperature as a compromise sentative temperatures.
For example, a grid that records for the seasonal variations. Spacing of the thermal the concrete cools. In some dams especially arch type , couples should be closer near the dam faces. Grout and isolated concrete lifts, pipe diameter, risers, and pipe techniques used for post-tensioned concrete should be used.
In most of the dam, a uniform 7. These concentrations tend to occur at the downstream of time. If the insulation is sufficient to allow slow cooling, face of the dam where inlets and outlets to cooling pipes are the tensile strain need never exceed the cracking limit. The located, adjacent to openings in the dam, and at isolated and concrete can relax at the same rate as the tensile stress tends sloping lifts of concrete.
Proper planning will alleviate many to develop until stable temperatures are reached. If the con- of the undesirable conditions that can result from these con- crete has a very slow relaxation rate or creep rate , however, centrations. For example, it should be determined to what the amount of insulation and the long protection time re- extent the cost-saving procedure of concentrating cooling quired can make this measure difficult.
Also, it should be decided if cooling tion will be required during severely cold months, it may be pipes to isolated areas in the foundation and at openings, necessary to remove the insulation in stages as the warmer such as galleries, can extend from the downstream face of the months approach.
Temperatures within the concrete just be- dam or if a vertical riser should be used. Aluminum tubing is lightweight could induce cracking at or below the surface with possible and easy to handle; however, breakdown from corrosion is a subsequent deeper penetrations into the mass.
Avoid using potential problem if cooling activities are carried on over a too much insulation or leaving it in place too long, which period of several months. In this case, steel tubing is pre- could result in stopping the desired cooling of the interior ferred. Surface connections to the cooling pipe should be re- mass and possibly cause the interior temperature to begin to movable to a depth of to mm 4 to 6 in.
Insulation can be obtained in a variety of forms and materials. Coils should be pressure tested for leaks at the maximum It can be either semirigid panel-type material, or foamed spray- pressure they will receive from the cooling system before on material that becomes semirigid in place. The semirigid placing concrete. Pressure should also be maintained during panels are usually installed on the inside face of the forms. The insulation can be easily removed from embedded pipe coil to permit ready observance of cooling the surface.
Roll-out application is particularly applicable for water flow. In addition to regular observance of flows, water use on horizontal lift joints. It is easy to install and remove temperatures, and pressures, concrete temperatures should and can be reused many times. Spray-on insulation can be be observed and recorded at least once daily while the lift is used on either horizontal or vertical surfaces.
This type of in- being cooled. Thermometers are placed at the inlet and outlet sulation is particularly useful for increasing the thickness of the cooling water system to record the temperature and and effectiveness of insulation already in place and for insu- change throughout the placement. Insulation that permits transmission of light The refrigeration plant for cooling water can be located rays should not be used because a temperature rise occurs be- centrally, or several complete smaller portable plants can be tween the insulation and the concrete when the insulation is sub- used to permit moving the refrigeration system as the dam jected to direct sunlight.
Precast panels made of low- progresses upward. Sufficient standby components, equal in conductance lightweight concrete or regular-weight concrete capacity to the largest individual refrigeration units, should cast with laminated or sandwich layers of low-conductance be provided. Insufficient plant size can cause premature cellular concrete are also acceptable for insulation. These pan- shutdown of cooling before the heat of hydration has been els would then serve as both forms and face concrete.
Measurement is usually made by means of embedded Sustained modulus strain meters, although any reliable method of measuring Es at time of loading in days, GPa psi strain can be employed. Butyl rubber is satisfactory for seal- Time after loading, d 1 3 7 28 ing the specimens, but neoprene should be avoided because 0 Specimens should be , 1,, 2,, 4,, loaded at the same ages as specified for the modulus of Again, the specimens should be large enough 3 , 1,, 2,, 3,, to permit concrete like that used in the structure.
Frequently, 7 Useful Table 7. T able 7. ACI R presents a discussion of the sustained elastic modulus. A high rate of creep for concrete is helpful Cooling mass concrete structures can be permitted without when the tensile strain is applied gradually.
Because the tensile insulation in relatively thin sections of about 7 m 20 ft in strength of concrete is nearly independent of prior loading, moderate climates where the natural dissipation of heat is creep tends to increase the strain capacity. In the case of Dwor- gradual and the thermal gradient is not steep. Ex- the stiffness of the aggregate. When the modulus is high, the pansion can be helpful in preventing cracks, but contraction creep is low and vice versa.
The importance of aggregate ri- increases the tendency to crack. Autogenous volume change gidity on concrete creep can be illustrated by two examples. When compressive stress is loss of moisture, and the concrete kept at constant tempera- applied, the stress and corresponding strain will be the same ture. Measurements begin as soon as the specimens are hard- in the aggregate as in the cement paste. The aggregate does ened and sealed, and are continued periodically for months.
This imposes After construction, the structure is exposed to the annual cy- an elastic strain on the aggregate that accounts for a large cle of load combinations for which it is designed.
During the ini- part of the concrete creep. The amount of elastic strain is di- tial operation period, which may last for a few months or even rectly related to the modulus of elasticity of the aggregate; years, the structure and the immediate foundation will undergo the more rigid the aggregate, the lower the creep.
Next, some elastic movement superimposed on a permanent set. Dur- assume that the aggregate has a much higher modulus ing the succeeding years, small permanent deformations occur than the cement paste.
When compressive stress is applied, in the structure, foundation, or both, that are categorized as the average stress in the aggregate will be higher than that in creep. Creep can initiate unanticipated cracking by causing a the paste and the paste will creep less than it did when the load shift to structural areas not intended to deform or carry moduli were equal.
The elastic strain in the aggregate due to more of the loads. These conditions can be somewhat anticipat- the creep of the paste will then be less than it was when the ed with analyses that include concrete creep and designing the moduli were equal. Therefore, an increase in the rigidity of the structure to accommodate these movements. Concrete creep in tension is difficult to measure; therefore, 8.
If all parts of the con- developed. Cracking is usually inevitable. Due consideration same degree to volume changes. Consequently, differential should also be given for temperature drops in wall construc- strains develop and tensile stresses are induced. When these tion. New concrete should be vals ranging from one for high walls to three for low protected for as long as possible from the loss of moisture or walls times the height of the wall.
These considerations can result in The interior and exterior concrete, particularly in larger, stresses capable of causing cracks at an early age but that thicker sections, can be subjected to changes in temperature might be sustained at greater maturity.
Concrete should have or moisture content at different rates and different degrees. Therefore, a low modulus of elasticity in tension is or concrete from shrinking, and tensile stresses develop that desirable. This occurs when the surface cools while the interior is still warm from the heat of hydra- 8.
It is often feasible to protect the surface at early structure and its concrete elements should perform. Reinforcement provided for both thermal surface dryness; mixture proportions, especially mixing wa- contraction and drying shrinkage can only partially restrain ter; the character and degree of restraint; and the extensibili- the contraction of surface concrete, but more and narrower ty tensile-strain capacity of the concrete.
The extensibility cracks can result. The latter is largely related to the tions. If feasible, a contraction joint can be used to relieve the composition of the aggregate and can vary widely.
Typical- restraint. Restraint of flatwork results from anchoring slab ly, some concretes of highly quartzitic gravels have a low reinforcements in perimeter concrete footings. When a slab strain capacity and a high modulus of elasticity. Concretes is free to shrink from all sides toward its center, there is min- with a low strain capacity are much more sensitive to shrink- imal cracking. Contraction joints and perimeter supports age due to drying and to a decrease in temperature and will should be designed accordingly.
Walls, slabs, and tunnel linings placed against the irregular A prime objective of crack-control procedures is to keep surface of a rock excavation are restrained from moving the concrete wet as long as possible so that the concrete will when the surface expands or contracts in response to changes have time to develop more strength to resist cracking forces.
Closely spaced contraction As described in Chapter 3 and Carlson , there are joints or deep grooves should be provided to hide the cracks some cases where prolonged moist curing is not beneficial. In tunnel linings, the con- The importance of moist curing will vary with the weather traction in the first few weeks is primarily thermal, and the and the time of year.
At ally. By the time drying is significant, the concrete lining has some depth, concrete loses moisture slowly. Where surface a greater tensile strain capacity and therefore is able to resist drying can be rapid, more care should be devoted to uninter- shrinkage cracking. Circumferential cracks in tunnel linings rupted curing to improve surface strength. Cracking will be and other cast-in-place concrete conduits and pipe lines, further reduced by creep if the surface is prevented from drying however, can be greatly reduced in number and width if a quickly at the end of the curing period.
To accomplish slow bulkhead is used to prevent air movement through the tunnel, drying, wet curing should remain for several days without and shallow ponds of water are placed in the invert as soon wetting after the specified curing period preferably 7 to as possible after placing the lining. They should remain there 10 days until the cover and the concrete under it appear until the tunnel is put into service USBR The con- to be dry.
If these procedures are required to be used, they crete can become much stronger in the humid environment should be included in project contract documents.
If the tunnel carries wa- cracks commonly occur in the surfaces of floors and slabs ter, there will be no further drying shrinkage. While prop- water. These cracks occur before the start of curing and can er construction performance can contribute a great deal as occur either before or after final finishing. As the moisture is will be discussed , the contractor cannot be expected to use removed, the surface concrete contracts, resulting in tensile the best procedures unless these procedures are included in stresses in the weak, stiffening, plastic concrete that cause the contract documents.
These cracks are usually wide at the surface but only a few millimeters inches in 8. The The cracks range from a few millimeters inches to a few use of the lowest practical slump adds in the control of crack- meters feet in length and are a few millimeters inches to ing.
This means avoiding over- to be removed in later floating or troweling operations. Insufficient plant size can cause premature cellular concrete are also acceptable for insulation. These This report presents the principal causes of cracking and a smaller microcracks have a surface density that is two to detailed discussion of crack-control procedures.
Specific recommendations and information on concrete beams the use of shrinkage-compensating concrete are contained A number of equations have been proposed for predicting in ACI R. Although a large amount of the data for this chapter dams. Contraction joints and perimeter supports age due to drying and to a decrease in temperature and will should be designed accordingly.
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Halvorsen y N. Bums, eds. Abeles, P. II y Morrow, J. S1-S90," Concrete International, V. Atimtay, E. Attiogbe, E. Bascoul, A. Shah and S. Swartz, eds. Bate, S. Shah, ed. K y Pfeiffer. Beckett, D. Beeby, A. Bennett, E. Bishara, A. Brendel, G. Broms, B. Brookhaven National Laboratory, Brooks, J. Bryant, A. Burgess, J. Burns, eds. Carino, N. Carlson, R. Carrasquillo, R. Castro-Montero, S. Catalano, D. Clear, K.
Cook, D. Darwin, D. EM2, Abril, pp. Derucher, K. Dolan, C. SetiembreOctubre, pp. Feldman, R. Frantz, G. Frosch, R. Gergely, P.
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