ACI 207.1R:2005 pdf free download.Guide to Mass Concrete.
Mass concrete is any volume of concrete with dimensions large enough to require thai measures he taken to cope with the gemieration of hear from hvdraiwn of the cement and aflenclani vo!u,ne chance to mninimi:e cracking. The design of moss concrete structures is generally based on durabilii ecolu,my. and thermal action, with .crrengih often being a secondary concern. ACI 207.1R contains a history of the develoj;mmseni of mass concrete practice and discussion of materials and concrete mixture proportioning. prnperues, construclion mnethod.c. and equipment. Ii coverc traditionally placed and consolidated mass concrete and does not cover rolkr-compacs’d 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 rise: thermal expansion: vibration: volume change.
CHAPTER 1—INTRODUCTION
AND HISTORICAL DEVELOPMENTS
1.1—Scope
Mass concrete is defined in ACI 1 16R 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 based on durability. economy. and thermal action, with strength often being a secondary, rather than a primary, concern. The one characteristic that distinguishes mass concrete from other concrete work is thermal behavior. Because the cement-water reaction is exothermic by nature, the temperature rise within a large concrete mass, where the heat is not quickly dissipated. can be quite high. Significant tensile stresses 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 cause a loss of structural integrity and monolithic action, excessive seepage and shortening of the service 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 he realized.
ACI 207.1R contains a history of the development of mass concrete practice and a discussion of materials and concrete mixture proportioning, properties, construction methods, and equipment. ACI 207.1R 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 darn construction, where temperature-related cracking was first identified, Temperature-related cracking has also been experienced in other thick-section concrete structures, including mat foundations, pile caps. bridge piers. thick walls, and tunnel linings.
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 secondarily 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 Ofl the design and behavior of massive reinforced elements and structures are discussed in ACI 207.2R. Cooling and insulating systems for mass concrete are addressed in ACI 207.4R. Mixture proportioning for mass concrete is discussed in ACI 211.1.
1.2—History
When concrete was first used in dams, the darns were relatively small and the concrete was mixed by hand. The portland cement usually had to he aged to comply with a boiling soundness test, the aggregate was bank-run sand and
2.2—Cements
ACI 207.2R and 207.4R contain additional information on cement types and effects on heat generation. The ftllowing types of hydraulic cement are suitable for use in mass concrete construction:
• Poriland cement—Types 1. II, IV. and V. as covered by ASTM C 150;
• Blended cement—Types P. IP, S. iS. l(PM). and hSM). as covered by ASTM C 595: and
• Hydraulic cement—Types GU, MS. HS, MH. and LH. as covered by ASTM C 1157.
When port land cement is used with pozzolan or with other cements, the materials are batched separately at the mixing plant. Economy and low temperature rise are both achieved by limiting the total cement content to as small an amount as possible.
Type I and GU cements are suitable for use in general construction. They are not recommended for use alone in mass concrete without other measures that help to control temperature problems because of their substantially higher heat of hydration.
Type II (moderate heal) and MH cenients are suitable for mass concrete construction because they have a moderate heat of hydration, which is important to the control of cracking. Type 11 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 11 portland cement require that it contain no more than 8% tricalciurn aluminate (C3A), the compound that contributes substantially to early heat development in concrete. Optional specifications for Type 11 cement place a limit of 589k or less on the sum of C3A and C3S or a limit on the heat of hydration to 70 callg (290 kJ/kg) at 7 days. When one of the optional requiremenis is specified, the 28-day strength requirement for cement paste under ASTM C ISO is reduced due to the slower rate of strength gain of this cement.
Types IV and LH. low-heat cements, may be used where it is desired to produce low heat development in massive structures. They have not been used in recent years because they have been difficult to obtain and, more importantly, because experience has shown that in most cases, heat development 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 C,S of 40%. At the option of the purchaser, the heat of hydration may be limited to 60 cal/g (250 kJ/kg) at 7 days and 70 cal/g (29() Id/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).
design stages of the project. Thcrmal tensile strain developed in mass concrete increases with the magnht ft + r I coefficient of expansion. thermal differential and rate 01 temperature change, and degree of restraint (ACI 207.2R).
Volume changes can also result from chemical reactions, which can be potentially disruptive.
3.6—Permeability
Concrete has an inherently low permeability to water. With properly proportioned mixtures that are compacted by vibration, permeability is not a serious problem. Permeability of concrete increases with increasing w/cm. Therefore. low w/crn and good consolidation and curing are the most important factors in producing concrete with low permeability. Air-entraining and other chemical admixtures pennit the same workability with reduced water content and. therefore, contribute to reduced permeability. Pozzolans usually reduce the permeability of the concrete. Permeability coefficients for some mass concretes are given in Table 3.5.
3.7—Thermal properties
A most important characteristic of mass concrete that differentiates its behavior from that of structural concrete is its thermal behavior. The generally large size of mass concrete structures creates the potential for significant temperature differentials between the interior and the outside surface of the structure. The accompanying volume change differentials, along with restraint, result in tensile strains and stresses that may cause cracking that is detrimental to the structure. Thermal properties that influence this behavior in mass concrete are specific heat, conductivity, and diffusivity. The primary factor affecting the thermal properties of a concrete, however, is the mineralogical composition of the aggregate (Rhodes 1978). Requirements for cement, pozzolan. percent sand, and water content are modifying factors, but offer a negligible effect on thermal properties. Entrained air is an insulator and reduces thermal conductivity, but other considerations that govern the use of entrained air outweigh the significance of its effect on thermal properties. Thermal property values for some mass concrete, an extensive discussion on thermal properties and behavior, and example computations are provided in ACI 207.2R.
3.8—Shear properties
Although the triaxial shear strength may he determined as one of the basic design parameters. the designer usually is required to use an empirical relationship between the shear and compressive strength of concrete. Shear properties for some concrete containing 1-1/2 in. (37.5 mm) maximum size aggregates are listed in Table 3.6. These include compressive strength, cohesion, and coefficient of internal friction, which are related linear functions determined from results of triaxial tests. Linear analysis of triaxial results gives a shear strength slightly above the value obtained from biaxial shear strength (USBR 1992). Past criteria have stated that the coefficient of internal friction can be taken as 1.0 and cohesion as 10% of the compressive strength (USBR 1976).