ACI 207.1R-05 superdes ACI 207.1R-96 and became effective December 1, 2005.Copyright © 2006, American Concrete Institute.
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Reference to this document shall not be made in contract documents. If items found in this document are desired by the Architect/Engineer to be a part of the contract documents, they shall be restated in mandatory language for incorporation by the Architect/Engineer.
Guide to Mass Concrete
Reported by ACI Committee 207
ACI 207.1R-05
Mass concrete is any volume of concrete with dimensions large enough to require that measures be taken to cope with the generation of heat from hydration of the cement and attendant volume change to minimize cracking.The design of mass concrete structures is generally bad on durability,economy, and thermal action, with strength often being a condary concern.This document contains a history of the development of mass concrete practice and discussion of materials and concrete mixture proportioning,properties, construction methods, and equipment. It covers traditionally placed and consolidated mass concrete and does not cover roller-compacted concrete.Keywords : admixture; aggregate; air entrainment; batch; cement; compressive strength; cracking; creep; curing; durability; fly ash; formwork; grading;heat of hydration; mass concrete; mixing; mixture proportion; modulus of elasticity; placing; Poisson’s ratio; pozzolan; shrinkage; strain; stress;temperature ri; thermal expansion; vibration; volume change.
CONTENTSpets报名
Chapter 1—Introduction and historical developments, p. 207.1R-21.1—Scope 1.2—History
1.3—Temperature control
1.4—Long-term strength design
Chapter 2—Materials and mixture proportioning, p. 207.1R-52.1—General 2.2—Cements
2.3—Pozzolans and ground slag 2.4—Chemical admixtures 2.5—Aggregates 2.6—Water
2.7—Selection of proportions 2.8—Temperature control
Chapter 3—Properties, p. 207.1R-123.1—General 3.2—Strength
3.3—Elastic properties 3.4—Creep
3.5—Volume change 3.6—Permeability
3.7—Thermal properties 3.8—Shear properties 3.9—Durability
Chapter 4—Construction, p. 207.1R-194.1—Batching 4.2—Mixing 4.3—Placing 4.4—Curing 4.5—Forms
4.6—Height of lifts and time intervals between lifts 4.7—Cooling and temperature control 4.8—Instrumentation
4.9—Grouting contraction joints
Jeffrey C. Allen Robert W. Cannon John R. Hess
Tibor J. Pataky Terrence E. Arnold Teck L. Chua Rodney E. Holderbaum Steven A. Ragan Randall P. Bass Eric J. Ditchey Allen J. Hulshizer Ernest K. Schrader J. Floyd Best Timothy P. Dolen David E. Kiefer Gary P. Wilson
Anthony A. Bombich
Barry D. Fehl
Gary R. Mass
Stephen B. Tatro
Chair
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拉丁语系Chapter 5—References, p. 207.1R-27
5.1—Referenced standards and reports
5.2—Cited references
CHAPTER 1—INTRODUCTION
AND HISTORICAL DEVELOPMENTS
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1.1—Scope
Mass concrete is defined in ACI 116R as “any volume of concrete with dimensions large enough to require that measures be taken to cope with generation of heat from hydration of the cement and attendant volume change to minimize cracking.” The design of mass concrete structures is generally bad on durability, economy, and thermal action, with strength often being a condary, rather than
a primary, concern. The one characteristic that distinguishes mass concrete from other concrete work is thermal behavior. Becau the cement-water reaction is exothermic by nature, the temperature ri within a large concrete mass, where the heat is not quickly dissipated, can be quite high. Significant tensile stress and strains may result from the restrained volume change associated with a decline in temperature as heat of hydration is dissipated. Measures should be taken where cracking due to thermal behavior may cau a loss of structural integrity and monolithic action, excessive epage and shortening of the rvice life of the structure, or be aesthetically objectionable. Many of the principles in mass concrete practice can also be applied to general concrete work, whereby economic and other benefits may be realized. This document contains a history of the development of mass concrete practice and a discussion of materials and concrete mixture proportioning, properties, construction methods, and equipment. This document covers traditionally placed and consolidated mass concrete, and does not cover roller-compacted concrete. Roller-compacted concrete is described in detail in ACI 207.5R.
Mass concreting practices were developed largely from concrete dam construction, where temperature-related cracking was first identified. Temperature-related cracking has also been experienced in other thick-ction concrete structures, including mat foundations, pile caps, bridge piers, thick walls, and tunnel linings.
ultimate是什么意思High compressive strengths are usually not required in mass concrete structures; however, thin arch dams are exceptions. Massive structures, such as gravity dams, resist loads primarily by their shape and mass, and only condarily by their strength. Of more importance are durability and properties connected with temperature behavior and the tendency for cracking. The effects of heat generation, restraint, and volume changes on the design and behavior of massive reinforced elements and structures are discusd in ACI 207.2R. Cooling and insulating systems for mass concrete are addresd in ACI 207.4R. Mixture proportioning for mass concrete is discusd in ACI 211.1.
迈锐宝广告歌曲1.2—History
tourism in chinaWhen concrete was first ud in dams, the dams were relatively small and the concrete was mixed by hand. The portland cement usually had to be aged to comply with a boiling soundness test, the aggregate was bank-run sand and gravel, and proportioning was by the shovelful (Davis 1963). Tremendous progress has been made since the early 1900s, and the art and science of dam building practiced today has reached a highly advanced state. Prently, the lection and proportioning of concrete materials to produce suitable strength, durability, and impermeability of the finished product can now be predicted and controlled with accuracy. Covered herein are the principal steps from tho very small beginnings to the prent. In large dam construction, there is now exact
and automatic proportioning and mixing of materials. Concrete in 12 yd3 (9 m3) buckets can be placed by conventional methods at the rate of 10,000 yd3/day (7650 m3/day) at a temperature of less than 50 °F (10 °C) as placed, even during extremely hot weather. Grand Coulee Dam still holds the all-time record monthly placing rate of 536,250 yd3 (410,020 m3), followed by the more recent achievement at Itaipu Dam on the Brazil-Paraguay border of 440,550 yd3 (336,840 m3) (Itaipu Binacional 1981). The record monthly placing rate of 328,500 yd3 (250,200 m3) for roller-compacted concrete was achieved at Tarbela Dam in Pakistan. Lean mixtures are now made workable by means of air entrainment and other chemical admixtures and the u of finely divided pozzolanic materials. Water-reducing, strength-enhancing, and t-controlling chemical admixtures are effective in reducing the required cement content to a minimum and in controlling the time of tting. Placing rates for no-slump concrete, by using large earth-moving equipment for transportation and large vibrating rollers for consolidation, appear to be limited only by the size of the project and its plant’s ability to produce concrete.
1.2.1 Before 1900—Before to the beginning of the twentieth century, much of the portland cement ud in the United States was imported from Europe. All cements were very coar by prent standards, and quite commonly they were underburned and had a high free lime content. For dams
of that period, bank-run sand and gravel were ud without the benefit of washing to remove objectionable dirt and fines. Concrete mixtures varied widely in cement content and in sand-coar aggregate ratio. Mixing was usually done by hand and proportioning by shovel, wheelbarrow, box, or cart. The effect of the water-cement ratio (w/c) was unknown, and generally no attempt was made to control the volume of mixing water. There was no measure of consistency except by visual obrvation of the newly mixed concrete.
Some of the dams were of cyclopean masonry in which “plums” (large stones) were partially embedded in a very wet concrete. The spaces between plums were then filled with concrete, also very wet. Some of the early dams were built without contraction joints and without regular lifts. There were, however, notable exceptions where concrete was cast in blocks; the height of lift was regulated, and concrete of very dry consistency was placed in thin layers and consolidated by rigorous hand tamping.
Generally, mixed concrete was transported to the forms by wheelbarrow. Where plums were employed in cyclopean masonry, stiff-leg derricks operating inside the work area moved the wet concrete and plums. The rate of placement
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was, at most, a few hundred cubic yards (cubic meters) a day. Generally, there was no attempt to moist cure.
An exception to the general practices was the Lower Crystal Springs Dam, completed in 1890. This dam is located near San Mateo, California, about 20 miles (30 km) south of San Francisco. According to available information, it was the first dam in the United States in which the maximum permissible quantity of mixing water was specified. The concrete for this 154 ft (47 m) high structure was cast in a system of interlocking blocks of specified shape and dimensions. An old photograph indicates that hand tampers were employed to consolidate the dry concrete (concrete with a low water content and presumably very low workability). Fresh concrete was covered with planks as a protection from the sun, and the concrete was kept wet until hardening occurred.
1.2.2 1900 to 1930—After the turn of the century, construction of all types of concrete dams was greatly accel-erated. More and higher dams for irrigation, power, and water supply were built. Concrete placement by means of towers and chutes became common. In the United States, the portl
and cement industry became well established, and cement was rarely imported from Europe. ASTM specifications for portland cement underwent little change during the first 30 years of the century, aside from a modest increa in fineness requirement determined by sieve analysis. Except for the limits on magnesia and loss on ignition, there were no chemical requirements. Character and grading of aggregates were given more attention during this period. Very substantial progress was made in the development of methods of proportioning concrete. The water-cement strength relationship was established by Abrams and his associates from investigations before 1918, when Portland Cement Association (PCA) Bulletin 1 appeared (Abrams 1918). Nevertheless, little attention was paid to the quantity of mixing water. Placing methods using towers and flat-sloped chutes dominated, resulting in the u of excessively wet mixtures for at least 12 years after the importance of the w/c had been established.
Generally, portland cements were employed without admixtures. There were exceptions, such as the sand-cements ud by the U.S. Reclamation Service (now the U.S. Bureau of Reclamation [USBR]) in the construction of the Elephant Butte Dam in New Mexico and the Arrowrock Dam in Idaho. At the time of its completion in 1915, the Arrowrock Dam, a gravity-arch dam, was the highest dam in the world at 350 ft (107 m). The dam was constructed with lean interior concrete and a richer exterior
face concrete. The mixture for interior concrete contained approximately 376 lb/yd3 (223 kg/m3) of a blended, pulverized granite-cement combination. The cement mixture was produced at the site by intergrinding approximately equal parts of portland cement and pulverized granite so that no less than 90% pasd the No. 200 (75 µm) mesh sieve. The interground combination was considerably finer than the cement being produced at that time. Another exception occurred in the concrete for one of the abutments of Big Dalton Dam, a multiple-arch dam built by the Los Angeles County Flood Control District during the late 1920s. Pumicite (a pozzolan) from Friant, California, was ud as a 20% replacement by mass for portland cement. During this period, cyclopean concrete went out of style. For dams of thick ction, the maximum size of aggregate for mass concrete was incread to as large as 10 in. (250 mm). The slump test had come into u as a means of measuring consistency. The testing of 6 x 12 in. (150 x 300 mm) and 8 x 16 in. (200 x 400 mm) job cylinders became common prac-tice in the United States. European countries generally adopted the 8 x 8 in. (200 x 200 mm) cube for testing the strength at various ages. Mixers of 3 yd3 (2.3 m3) capacity were commonly ud near the end of this period, and there were some of 4 yd3 (3 m3) capacity. Only Type I cement (normal portland cement) was available during this period. In areas where freezing-and-thawing conditions were vere, it was common practice to u a concrete mixture containing 564 lb/yd3 (335 kg/m3) of cement for the entire concrete mass. The construction practice of using an
interior mixture containing 376 lb/yd3 (223 kg/m3) and an exterior face mixture containing 564 lb/yd3 (335 kg/m3) was developed during this period to make the dam’s face resistant to the vere climate and yet minimize the overall u of cement. In areas of mild climate, one class of concrete that contained amounts of cement as low as 376 lb/yd3 (223 kg/m3) was ud in some dams.
An exception was the Theodore Roovelt Dam built during the years of 1905 to 1911 in Arizona. This dam consists of a rubble masonry structure faced with rough stone blocks laid in portland cement mortar made with a cement manufactured in a plant near the dam site. For this structure, the average cement content has been calculated to be approximately 282 lb/yd3 (167 kg/m3). For the interior of the mass, rough quarried stones were embedded in a 1:2.5 mortar containing approximately 846 lb/yd3 (502 kg/m3) of cement. In each layer, the voids between the cloly spaced stones were filled with a concrete containing 564 lb/yd3 (335 kg/m3) of cement, into which rock fragments were manually placed. The conditions account for the very low average cement content. Construction was slow, and Roovelt Dam reprents perhaps the last of the large dams built in the United States by this method of construction. 1.2.3 1930 to 1970—This was an era of rapid development in mass concrete construction for dams. The u of the tower and chute method declined during this period and was ud only on small projects. Concrete was typically placed using
large buckets with cranes, cableways, railroad systems, or a combination of the. On the larger and more cloly controlled construction projects, the aggregates were carefully procesd, ingredients were proportioned by weight, and the mixing water was measured by volume. Improvement in workability was brought about by the introduction of finely divided mineral admixtures (pozzolans), air entrainment, and chemical admixtures. Slumps as low as 3 in. (76 mm) were employed without vibration, although most projects in later years of this era ud large spud vibrators for consolidation.
A study of the records and actual inspection of a considerable number of dams shows that there were differences in condition that could not be explained. Of two structures that appeared to
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be of similar quality subjected to the same environment, one might exhibit excessive cracking while the other, after a similar period of rvice, would be in near-perfect condition. The meager records available on a few dams indicated wide internal temperature variations due to cement hydration. The degree of cracking was associated with the temperature ri.
ACI Committee 207, Mass Concrete, was organized in 1930 (originally as Committee 108) for the purpo of gath-ering information about the significant properties of mass concrete in dams and factors that influence the properties. Bogue (1949) and his associates, under the PCA fellowship at the National Bureau of Standards, had already identified the principal compounds in portland cement. Later, Hubert Woods and his associates engaged in investigations to deter-mine the contributions of each of the compounds to heat of hydration and to the strength of mortars and concretes.
By the beginning of 1930, the Hoover Dam in Nevada was in the early stages of planning. Becau of the unprecedented size of the Hoover Dam, investigations much more elaborate than any previously undertaken were carried out to determine the effects of factors, such as composition and fineness of cement, cement factor, temperature of curing, and maximum size of aggregate, on the heat of hydration of cement, compressive strength, and other properties of mortars and concrete.
The results of the investigations led to the u of low-heat cement in the Hoover Dam. The investigations also furnished information for the design of the embedded pipe cooling system ud for the first time in the Hoover Dam. Low-heat cement was first ud in the Morris Dam, near Pasadena, Calif., which was started a year before the Hoover Dam. For the Hoover Dam, the constr
uction plant was of unprecedented capacity. Batching and mixing were completely automatic. The record day’s output for the two concrete plants, equipped with 4 yd3 (3 m3) mixers, was over 10,000 yd3 (7600 m3). Concrete was transported in 8 yd3 (6 m3) buckets by cableways, and compacted initially by ramming and tamping. In the spring of 1933, large internal vibrators were introduced and were ud thereafter for compacting the remainder of the concrete. Within approximately 2 years, 3,200,000 yd3 (2,440,000 m3) of concrete were placed. Hoover Dam marked the beginning of an era of improved practices in large concrete dam construction. Completed in 1935 at a rate of construction then unprecedented, the practices employed there, with some refinements, have been in u on most of the large concrete dams that have been constructed in the United States and in many other countries since that time.大学英语2课后答案
The u of a pozzolanic material (pumicite) was given a trial in the Big Dalton Dam by the Los Angeles County Flood Control District. For the Bonneville Dam, completed by the Corps of Engineers in 1938 in Oregon, a portland cement-pozzolan combination was ud. It was produced by intergrinding the cement clinker with a pozzolan procesd by calcining an altered volcanic material at a temperature of approximately 1500 °F (820 °C). The proportion of clinker to pozzolan was 3:1 by weight. This type of cement was lected for u at Bonneville on the basis of test results
on concrete that indicated large extensibility and low temperature ri. This is the earliest known concrete dam in the United States in which an interground portland-pozzolan cement has been ud. The u of pozzolan as a parate cementing material to be added at the mixer, at a rate of 30% or more of total cementitious materials, has come to be regular practice by the USBR, the Tenne Valley Authority (TVA), the United States Army Corps of Engineers (USACE), and others.
The chemical admixtures that function to reduce water in concrete mixtures, control tting, and enhance strength of concrete began to be riously recognized in the 1950s as materials that could benefit mass concrete. In 1960, Wallace and Ore published their report on the benefit of the materials to lean mass concrete. Since this time, chemical admixtures have been ud in most mass concrete.
Around 1945, it became standard practice to u intentionally entrained air for concrete in most structures that are expod to vere weathering conditions. This practice was applied to the concrete of expod surfaces of dams as well as to concrete pavements and reinforced concrete in general. Air-entraining admixtures introduced at the mixer have been ud for both interior and exterior concretes of practically all dams constructed since 1945.
Placement of conventional mass concrete has remained largely unchanged since that time. The major new develop-ment in the field of mass concrete is the u of roller-compacted concrete.
1.2.4 1970 to prent—During this era, roller-compacted concrete was developed and became the predominant method for placing mass concrete. Becau roller-compacted concrete is now so commonly ud, a parate report, ACI 207.5R, is the principal reference for this subject. Traditional mass concrete methods continue to be ud for many projects, large and small, particularly where roller-compacted concrete would be impractical or difficult to u. This often includes arch dams, large walls, and some foundation works, particularly where reinforcement is required.
The continuing development of chemical admixtures has allowed the placement of very large underwater placements where the concrete flows laterally up to 100 ft. Float-in construction methods where structural elements are precast or prefabricated and later filled with underwater concrete have been developed. Construction of dam ctions and power-hous has been done in this manner.
1.2.5 Cement content—During the late 1920s and early 1930s, it was practically an unwritten law that no mass concrete for large dams should contain less than 376 lb/yd3 (223 kg/m3) of cement. Some authorities of that period believed that the cement factor should never be less than 564 lb/yd3
(335 kg/m3). The cement factor for the interior concrete of Norris Dam (Tenne Valley Authority 1939) constructed by the Tenne Valley Authority (TVA) in 1936, was 376 lb/yd3 (223 kg/m3). The degree of cracking was excessive. The compressive strength of the wet-screened 6 x 12 in. (150 x 300 mm) job cylinders at 1 year of age was 7000 psi (48.3 MPa). Similarly, core speci-mens 18 x 36 in. (460 x 910 mm) drilled from the first stage concrete containing 376 lb/yd3 (223 kg/m3) of cement at Grand Coulee Dam tested in excess of 8000 psi (55 MPa)
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at the age of 2 years. Judged by composition, the cement was of the moderate-heat type corresponding to the prent Type II. Considering the moderately low stress within the two structures, it was evident that such high compressive strengths were quite unnecessary. A reduction in cement content on similar future constructions might be expected to substantially reduce the tendency toward cracking.
For Hiwa Dam, completed by TVA in 1940, the 376 lb/yd3 (223 kg/m3) cement-content barrier was broken. For that structure, the cement content of the mass concrete was only 282 lb/yd3 (167 k
g/m3), an unusually low value for that time. Hiwa Dam was singularly free from thermal cracks, which began a trend toward reducing the cement content, which is still continuing. Since that time, the Type II cement content of the interior mass concrete has been approximately 235 lb/yd3 (140 kg/m3) and even as low as 212 lb/yd3 (126 kg/m3). An example of a large gravity dam for which the Type II cement content for mass concrete was 235 lb/yd3 (140 kg/m3) is Pine Flat Dam in California, completed by the USACE in 1954. In arch-type high dams where stress are moderately high, the cement content of the mass mixture is usually in the range of 300 to 450 lb/yd3 (180 to 270 kg/m3), with the higher cement content being ud in the thinner and more highly stresd dams of this type. Examples of cementitious contents, including pozzolan, for more recent dams are:
•Arch dams—282 lb/yd3 (167 kg/m3) of cement and pozzolan in Glen Canyon Dam, a relatively thick arch dam in Arizona, completed in 1963; 373 lb/yd3 (221 kg/m3) of cement in Morrow Point Dam in Colorado, completed in 1968; and 303 to 253 lb/yd3 (180 to 150 kg/m3) of portland-pozzolan Type IP cement in El Cajon Dam on the Humuya River in Honduras, completed in 1984.•Straight gravity dams—226 lb/yd3 (134 kg/m3) of Type II cement in Detroit Dam in Oregon, completed in 1952; 194 lb/yd3 (115 kg/m3) of Type II cement and fly ash in Libby Dam in Montana, completed in 1972; and 184 lb/yd3 (109 kg/m3) of Type II cement and calcined clay in Ilha Solteira Dam in Brazil, completed in 1973.
1.3—Temperature control
The practice of precooling concrete materials before mixing to achieve a lower maximum temperature of interior mass concrete during the hydration period began in the early 1940s, and has been extensively ud in the construction of large dams. The first practice of precooling appears to have occurred during the construction of Norfork Dam from 1941 to 1945 by the USACE. The plan was to introduce crushed ice into the mixing water during the warmer months. By so doing, the temperature of freshly mixed mass concrete could be reduced by approximately 10 °F (5.6 °C). Not only has crushed ice been ud in the mixing water, but coar aggre-gates have been precooled either by cold air or cold water before batching. Recently, both fine and coar aggregates in a moist condition have been precooled by various means, including vacuum saturation and liquid nitrogen injection. It has become almost standard practice in the United States to u precooling for large dams in regions where the summer temperatures are high to ensure that the temperature of concrete, as it is placed, does not exceed approximately 50 °F (10 °C). On some large dams, including Hoover (Boulder) Dam, a combination of precooling and postcooling refrigeration by embedded pipe has been ud (USBR 1949). A good example of this practice is Glen Canyon Dam, where the ambient temperatures can be greater than 100 °F (38 °C) during the summer months. The
temperature of the precooled fresh concrete did not exceed 50 °F (10 °C). Both refrigerated aggregate and crushed ice were ud to achieve this low temperature. By means of embedded-pipe refrigeration, the maximum temperature of hardening concrete was kept below 75 °F (24 °C). Postcooling is sometimes required in gravity and in arch dams that contain transver joints so that transver joints can be opened for grouting by cooling the concrete after it has hardened. Postcooling to control cracking is also done for control of peak temperatures.
1.4—Long-term strength designnaru
A most significant development of the 1950s was the abandonment of the 28-day strength as a design requirement for dams. Maximum stress under load do not usually develop until the concrete is at least 1 year old. Under mass curing conditions, with the cement and pozzolans customarily employed, the gain in concrete strength between 28 days and 1 year is generally large. ACI 232.2R reports that the gain can range from 30 to more than 100%, depending on the quantities and proportioning of cementitious materials and properties of the aggregates. It has become the practice of some designers of dams to specify the desired strength of mass concrete at later ages, such as at 1 or 2 years. For routine quality control in the field, 6 x 12 in. (150 x 300 mm) cylinders are normally ud with aggregate larger than 1-1/2 in.
(37.5 mm). The aggregate larger than 1-1/2 in. (37.5 mm) is removed from the concrete by wet-screening. Strength requirements of the wet-screened concrete are correlated with the specified full-mixture strength by laboratory tests.
CHAPTER 2—MATERIALS AND
MIXTURE PROPORTIONING
2.1—General
As is the ca with other concrete, mass concrete is compod of cement, aggregates, and water, and frequently pozzolans and admixtures. The objective of mass concrete mixture proportioning is the lection of combinations of materials that will produce concrete to meet the requirements of the structure with respect to economy; workability; dimensional stability and freedom from cracking; low temperature ri; adequate strength; durability; and, in the ca of hydraulic structures, low permeability. This chapter describes materials that have been successfully ud in mass concrete construction and the factors influencing their lection and proportioning. The recommendations contained herein may need to be adjusted for special us, such as for massive precast beam gments, tremie placements, and roller-compacted concrete. Guidance in proportioning mass concr
ete can also be found in ACI 211.1, particularly
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Appendix 5, which details procedures for mass concrete proportioning.
2.2—Cements
ACI 207.2R and 207.4R contain additional information on cement types and effects on heat generation. The following types of hydraulic cement are suitable for u in mass concrete construction:
•Portland cement—Types I, II, IV, and V, as covered by ASTM C 150;
•Blended cement—Types P, IP, S, IS, I(PM), and I(SM), as covered by ASTM C 595; and
新纪元英语•Hydraulic cement—Types GU, MS, HS, MH, and LH, as covered by ASTM C 1157.
When portland cement is ud with pozzolan or with other cements, the materials are batched par
ately at the mixing plant. Economy and low temperature ri are both achieved by limiting the total cement content to as small an amount as possible.
Type I and GU cements are suitable for u in general construction. They are not recommended for u alone in mass concrete without other measures that help to control temperature problems becau of their substantially higher heat of hydration.
Type II (moderate heat) and MH cements are suitable for mass concrete construction becau they have a moderate heat of hydration, which is important to the control of cracking. Type II must be specified with the moderate heat option as most Type II and MS cements are designed for moderate sulfate resistance and do not have moderate heat properties. Specifications for Type II portland cement require that it contain no more than 8% tricalcium aluminate (C3A), the compound that contributes substantially to early heat development in concrete. Optional specifications for Type II cement place a limit of 58% or less on the sum of C3A and C3S or a limit on the heat of hydration to 70 cal/g (290 kJ/kg) at 7 days. When one of the optional requirements is specified, the 28-day strength requirement for cement paste under ASTM C 150 is reduced due to the slower rate of strength gain of this cement.
Types IV and LH, low-heat cements, may be ud where it is desired to produce low heat development in massive structures. They have not been ud in recent years becau they have been difficult to obtain and, more importantly, becau experience has shown that in most cas, heat develop-ment can be controlled satisfactorily by other means. Type IV specifications limit the C3A to 7%, the C3S to 35%, and place a minimum on the C2S of 40%. At the option of the purchar, the heat of hydration may be limited to 60 cal/g (250 kJ/kg) at 7 days and 70 cal/g (290 kJ/kg) at 28 days. Type IV cement is generally not available in the United States.
Type V and HS sulfate-resistant cements are available in areas with high-sulfate soils, and will often have moderate heat characteristics. They are usually available at a price higher than Type I. They are usually both low alkali (less than 0.6 equivalent alkalies) and low heat (less than 70 cal/g at 7 days).
Type IP portland-pozzolan cement is a uniform blend of portland cement or portland blast-furnace slag cement and fine pozzolan. Type P is similar, but early strength requirements are lower. They are produced either by intergrinding portland cement clinker and pozzolan or by blending portland cement or portland blast-furnace slag cement and finely divided pozzolan. The pozzolan constituents are between 15 and 40% by weight of the portland-pozzolan cement, with Type P generally having th
e higher pozzolan content.
Type I(PM) pozzolan-modified portland cement contains less than 15% pozzolan, and its properties are clo to tho of Type I cement. A heat of hydration limit of 70 cal/g (290 kJ/kg) at 7 days is an optional requirement for Types IP and I(PM) by adding the suffix (MH). A limit of 60 cal/g (250 kJ/kg) at 7 days is optional for Type P by adding the suffix (LH). Type IS portland blast-furnace slag cement is a uniform blend of portland cement and fine blast-furnace slag. It is produced either by intergrinding portland cement clinker and granulated blast-furnace slag or by blending portland cement and finely ground-granulated blast-furnace slag. The amount of slag ud may vary between 25 and 70% by weight of the portland blast-furnace slag cement. This cement has sometimes been ud with a pozzolan. Type S slag cement is a finely divided material consisting of a uniform blend of granulated blast-furnace slag and hydrated lime in which the slag constituent is at least 70% of the weight of the slag cement. Slag cement is generally ud in a blend with portland cement for making concrete.
Type I(SM) slag-modified portland cement contains less than 25% slag, and its properties are clo to tho of Type I cement. Optional heat-of-hydration requirements can be applied to Types IS and I(SM), similar to tho applied to Types IP, I(PM), and P.
Low-alkali cements are defined by ASTM C 150 as portland cements containing not more than 0.60% alkalies calculated as the percentage of Na2O plus 0.658 times the percentage of K2O. The cements can be specified when the cement is to be ud in concrete with aggregate that may be deleteriously reactive. The u of low-alkali cement may not always control highly reactive noncrystalline siliceous aggregate. It may also be advisable to u a proven pozzolan to ensure control of the alkali-aggregate reaction.
2.3—Pozzolans and ground slag
lofaceA pozzolan is generally defined as a siliceous or siliceous-and-aluminous material that posss little or no cementitious value but will, in finely divided form and in the prence of moisture, chemically react with calcium hydroxide at ordinary temperatures to form compounds posssing cementitious properties. Pozzolans are ordinarily governed and classified by ASTM C 618 as natural (Class N) or fly ash (Class F or C). There are some pozzolans, such as the Class C fly ash, that contain significant amounts of compounds like tho of portland cement. The Class C fly ashes likewi have cementitious properties by themlves that may contribute significantly to the strength of concrete. Pozzolans react chemically with the calcium hydroxide or hydrated lime liberated during the hydration of portland
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