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B-1 ATTACHMENT B Recommended Guidelines for Use of Self-Consolidating Concrete in Precast, Prestressed Concrete Bridge Elements These proposed guidelines are the recommendations of the NCHRP Project 18-12 staff at the University of Sherbrooke. These guidelines have not been approved by NCHRP or any AASHTO committee nor formally accepted for adoption by AASHTO.
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B-3 CONTENTS B-5 Introduction B-6 Glossary B-9 B.1 Guidelines for Selection of Constituent Materials B-9 B.1.1 General B-9 B.1.2 Cement and Cementitious Materials B-9 B.1.2.1 Cement and Blended Cement B-10 B.1.2.2 Fly Ash B-10 B.1.2.3 Silica Fume B-10 B.1.2.4 Ground Granulated Blast-Furnace Slag B-11 B.1.2.5 Fillers B-11 B.1.2.6 Other Supplementary Cementitious Additions B-11 B.1.3 Aggregate Characteristics B-11 B.1.3.1 Coarse Aggregate B-12 B.1.3.2 Fine Aggregate B-12 B.1.4 Chemical Admixtures B-12 B.1.4.1 High-Range Water-Reducing Admixtures B-13 B.1.4.2 Viscosity-Modifying Admixtures B-13 B.1.4.3 Air-Entraining Admixtures B-13 B.1.4.4 Set-Retarding and Set-Accelerating Admixtures B-13 B.1.4.5 Shrinkage-Reducing Admixtures B-14 B.1.4.6 Other Admixtures B-14 B.1.5 Fibers B-14 B.2 Guidelines for Selection of Workability Test Methods B-14 B.2.1 General B-14 B.2.2 Filling Ability B-14 B.2.2.1 Significance B-15 B.2.2.2 Test Methods to Assess Filling Ability B-15 B.2.2.3 Special Issues B-16 B.2.3 Passing Ability B-16 B.2.3.1 Significance B-16 B.2.3.2 Test Methods to Assess Passing Ability B-16 B.2.3.3 Special Issues B-16 B.2.4 Filling Capacity B-16 B.2.4.1 Significance B-17 B.2.4.2 Test Method to Assess Filling Capacity B-17 B.2.4.3 Special Issues B-18 B.2.5 Static Stability B-18 B.2.5.1 Significance B-18 B.2.5.2 Test Methods to Assess Static Stability B-18 B.2.5.3 Special Issues
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B-4 B-18 B.2.6 Dynamic Stability B-18 B.2.6.1 Significance B-18 B.2.6.2 Test Methods to Assess Dynamic Stability B-19 B.2.7 Rheology B-19 B.2.7.1 Significance B-19 B.2.7.2 Test Methods to Assess Rheological Parameters B-19 B.2.7.3 Special Issues B-20 B.3 Guidelines for Mix Design B-20 B.3.1 General B-20 B.3.2 Mix Design Principles B-22 B.3.2.1 Minimum Free Water Content B-22 B.3.2.2 Moderate Water Content and Medium Concentration of VMA B-22 B.3.2.3 Low Water Content and Low Concentration of VMA B-22 B.3.3 Cementitious Materials Content and Water-Cementitious Material Ratio B-22 B.3.4 Nominal Size of Coarse Aggregate B-23 B.3.5 Air-Entrainment and Air-Void Stability B-23 B.3.6 Mixture Robustness B-23 B.3.7 Trial Batches B-23 B.3.8 Recommended Range of Workability Characteristics B-24 B.3.9 Quality Confirmation of SCC B-26 B.4 Guidelines for Early-Age and Hardened Properties B-26 B.4.1 General B-26 B.4.2 Setting B-26 B.4.3 Temperature Development B-27 B.4.4 Release Compressive Strength B-28 B.4.5 Flexural Strength B-28 B.4.6 Modulus of Elasticity B-29 B.4.7 Creep B-30 B.4.8 Autogenous Shrinkage B-30 B.4.9 Drying Shrinkage B-31 B.4.10 Durability and Air-Void System B-32 B.4.11 Bond to Prestressing Strands B-33 B.5 Guidelines for Production and Control B-33 B.5.1 General B-33 B.5.2 Control of Raw Materials B-34 B.5.3 Mixing Process and Sequence B-34 B.5.4 Transport B-34 B.5.5 Site Acceptance of Plastic Concrete B-34 B.5.6 Placement Techniques and Casting Considerations B-35 B.5.7 Temperature Control B-36 B.5.8 Formwork Considerations and Lateral Pressure B-36 B.5.9 Finishing B-36 B.5.10 Curing B-38 References
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B-5 Introduction · Greater flexibility to produce a wide variety of architectural finishes The competitive situation in the precast concrete construc- · Increased assembly rates and reduced labor for placement, tion market is significantly affected by price and cost factors as vibration and finishing well as by productivity and quality. This environment is char- · Improved working environment and safety acterized by ever-shorter construction times, rising labor · Lower capital investment costs and higher service life of the costs, as well as greater demand for high workability, strength, formwork elements and durability. Technological developments and methods of · Improved surface quality with greater uniformity and fewer production that can lead to improved concrete quality and surface imperfections savings are therefore becoming increasingly important. Self- consolidating concrete (SCC) represents a significant advance- Use of SCC in the precast, prestressed applications can ment in concrete technology that provides great potential for result in specific advantages. Complex precast concrete mem- efficiency and economy in concrete construction. bers can be prefabricated with greater ease, speed, economy, SCC is a highly workable concrete that can flow through and higher surface quality. This can be achieved even in tightly densely reinforced or geometrically complex structural ele- spaced areas or congested reinforcement--such as columns, ments under its own weight without mechanical consolidation cap beams, and superstructure elements--and lead to provid- and adequately fill the formwork with minimum risk of segre- ing uniform and aesthetically pleasing surfaces. The quality gation. The flowability of SCC is higher than that of normal control and quality assurance measures used for producing high-performance concrete typically used in precast, pre- SCC will help achieve structures with the desired durability stressed concrete plants. This characteristic of SCC, coupled and service life. with the absence of the noise associated with vibration, make These guidelines provide the information necessary for SCC a desirable material for fabricating prestressed bridge ele- considering use of SCC in precast, prestressed bridge girders. ments. More specifically, SCC offers the following advantages: The guidelines include information on the selection of con- crete constituents and proportioning of concrete mixtures, · Simplification of the concreting procedure and ability to workability characteristics, testing methods, mechanical prop- produce heavily reinforced precast elements with virtually erties, visco-elastic properties, production and control issues, any cross-sectional shape and durability of SCC.
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B-6 Glossary portland cement paint or other materials to reduce shrinkage, improve workability, or act as an extender or material used to The following definitions may be referred to in these fill an opening in a form (ACI 116). guidelines. Some of them are general and apply to conven- Filling ability--The ability of SCC to flow into and fill tional concrete while others are specific to SCC. Some of these completely all spaces within the formwork, under its own definitions are based on definitions given in ACI and PCI weight, also referred to as deformability or non-restricted de- technical documents. formability (ACI 237). Admixture--A material, other than water, aggregates, hy- Filling capacity--The ability of SCC to flow into and fill draulic cement, and fiber reinforcement, used as an ingredi- completely all spaces within the formwork. ent of a cementitious mixture to modify its freshly mixed, set- Flowability--The ability of fresh concrete to flow in con- ting, or hardened properties and that is added to the batch fined or unconfined form of any shape, reinforced or not, before or during its mixing (ACI 116). under gravity and/or external forces, assuming the shape of Autogenous shrinkage--The shrinkage occurring in the its container (PCI 2003). absence of moisture exchange due to the hydration reactions Fluidity--The ease by which fresh concrete flows under taking place inside the cement matrix (ACI 209). gravity (PCI 2003). Fluidity is the reciprocal of dynamic Binder--A cementing material, either a hydrated cement or viscosity. reaction products of cement or lime and reactive siliceous ma- Fly ash--The finely divided residue that results from the terial; also materials such asphalt, resins, and other materials combustion of ground or powdered coal and that is trans- forming the matrix of concretes, mortars, and sanded grouts. ported by flue gasses from the combustion zone to the parti- Bingham fluid--A fluid characterized by a yield stress and cle removal system (ACI 116). Because of its spherical shape a constant plastic viscosity, regardless of flow rate (PCI 2003). and fineness, fly ash can improve the rheology of SCC. Bleed water--The water that rises to the surface subse- Formwork pressure--Lateral pressure acting on vertical quent to the placing of the concrete. The rise of mixing water or inclined formed surfaces, resulting from the fluid-like be- within, or its emergence from, newly placed concrete, caused havior of the unhardened concrete confined by the forms by settlement and consolidation of the plastic concrete (PCI (ACI 116). 2003). Ground granulated blast-furnace slag (GGBFS)--A fine Bleeding test--The standard test for determining the rel- granular, mostly latent hydraulic binding material that can be ative quantity of mixing water that will bleed from a sample added to SCC to improve workability of the material (PCI of freshly mixed concrete (ASTM C 232). 2003). GGBFS is also referred to in some cases as slag cement Blocking--The condition in which coarse aggregate parti- (a waste product in the manufacture of pig iron and chemi- cles combine to form elements large enough to obstruct the cally a mixture of lime, silica, and alumina). flow of the fresh concrete between the reinforcing steel or High-range water-reducing admixture (HRWRA)-- other obstructions in the concrete formwork (PCI 2003). A water-reducing admixture capable of producing large Cohesiveness--The tendency of the SCC constituent ma- water reduction or greater flowability without causing undue terials to stick together, resulting in resistance to segregation, set retardation or entrainment of air in mortar or concrete settlement, and bleeding (PCI 2003). (ACI 116). Consistency--The relative mobility or ability of freshly J-Ring test--Test used to determine the passing ability of mixed concrete or mortar to flow (ACI 116). SCC, or the degree to which the passage of concrete through Consolidation--The process of inducing a closer arrange- the bars of the J-Ring apparatus is restricted (ASTM C 1621). ment of the solid particles in freshly mixed concrete or mortar J-Ring flow--The distance of lateral flow of concrete using during placement by the reduction of entrapped voids (ACI the J-Ring in combination with a slump cone (ASTM C 1621). 116). In SCC, consolidation is achieved by gravity flow of the L-box test--Test used to assess the confined flow of SCC material without the need of vibration, rodding, or tamping. and the extent to which it is subject to blocking by reinforce- Creep--Time-dependent deformation due to sustained ment (ACI 237). load (ACI 209). Metakaolin--Mineral admixture used as binding material Deformability--The ability of SCC to flow under its own (supplementary cementitious material) in concrete (PCI 2003). mass and fill completely the formwork. Mixture robustness--The characteristic of a mixture that Drying shrinkage--Shrinkage occurring in a specimen encompasses its tolerance to variations in constituent char- that is exposed to the environment and allowed to dry (ACI acteristics and quantities, as well as its tolerance to the effects 209). of transportation and placement activities (PCI 2003). Fillers--Finely divided inert material, such as pulverized Passing ability--The ability of SCC to flow under its own limestone, silica, or colloidal substances, sometimes added to weight (without vibration) and completely fill all spaces
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B-7 within intricate formwork, containing obstacles, such as re- as movement of aggregate particles within fresh concrete inforcement (ASTM C 1621). (consolidation). Paste volume--Proportional volume of cement paste in Shear stress--The stress component acting tangentially to concrete, mortar, or the like, expressed as volume percent of a plane (ACI 116). the entire mixture (ACI 116). Silica fume--Very fine non-crystalline silica produced in Plastic viscosity--The resistance of the plastic material to electric arc furnaces as a byproduct of the production of ele- undergo a given flow. It is computed as the slope of the shear mental silicon or alloys containing silicon (ACI 116). Silica fume stress versus shear rate curve measurements. Mixtures with can be added to SCC to improve the rheological properties. high plastic viscosity are often described as "sticky" or "cohe- Slump flow--Test method used (upright or inverted) to sive." Concrete with higher plastic viscosity takes longer to measure mixture filling ability (ASTM C 1611). flow. It is closely related to T-50 and V-funnel time (higher Slump flow retention--The ability of concrete to main- plastic viscosity: higher T-50 and V-funnel time). tain its slump flow over a given period of time. Powder (also referred to as graded powder)--Includes ce- Slump flow spread--The distance of lateral flow of con- ment, fly ash, GGBFS, limestone fines, material crushed to crete during the slump-flow test (ASTM C 1611). Slump flow less than 0.125 mm (No. 100 sieve), or other non-cementi- spread is the numerical value in inches (mm) of flow deter- tious filler (ACI 237). mined as the average diameter of the circular deposit of SCC Powder-type SCC--SCC mixtures that rely extensively on at the conclusion of the slump flow test. the amount and character of the fines and powder included Stability--The ability of a concrete mixture to resist seg- in the mixture for meeting workability performance require- regation of the paste from the aggregates (ASTM C 1611). ments (stability) (PCI 2003). Stability, Dynamic--The resistance to segregation when ex- Pumpability--The ability of an SCC mixture to be pumped ternal energy is applied to concrete, namely during placement. without significant degradation of workability (PCI 2003). Stability, Static--The resistance to segregation when no Rheological properties--Properties dealing with the de- external energy is applied to concrete, namely from immedi- formation and flow of matter (PCI 2003). ately after placement and until setting. Rheology--The science of dealing with flow of materials, T-50 measurement (also referred to as the T-20 in. time including studies of deformation of hardened concrete, the in North America)--The time for the concrete to reach the handling and placing of freshly mixed concrete, and the be- 500 mm (20 in.) diameter circle drawn on the slump plate, after havior of slurries, pastes, and the like (ACI 116). In the con- starting to raise the slump cone (ASTM C 1611). text of SCC, rheology refers to the evaluation of yield stress, Texture--The pattern or configuration apparent in an ex- plastic viscosity, and thixotropy to achieve desired levels of posed surface, as in concrete and mortar, including roughness, filling ability, passing ability, and segregation resistance. streaking, striation, or departure from flatness (ACI 116). Segregation--The differential concentration of the com- Thixotropy--The property of a material that enables it to ponents of mixed concrete, aggregate, or the like, resulting in stiffen in a short period while at rest, but to acquire a lower non-uniform proportions in the mass (ACI 116). In the case viscosity when mechanically agitated, the process being re- of SCC, segregation may occur during transport, during flow versible, a material having this property is termed thixotropic into the forms, or after placement when the concrete is in a or shear thinning (ACI 116). Thixotropy indicates formwork plastic state. This results in non-uniform distribution of in-situ pressure and segregation resistance of SCC. properties of the concrete. Transportability--The ability of concrete to be trans- Segregation resistance--The ability of concrete to remain ported from the mixer to the placement site while remaining uniform in terms of composition during placement and until in a homogeneous condition (PCI 2003). setting (PCI 2003). Segregation resistance encompasses both V-funnel--Device used to determine the time for a given dynamic and static stability. volume of concrete to flow out through a funnel opening Self-consolidating concrete (SCC) (also self-compacting (PCI 2003). concrete)--A highly flowable, non-segregating concrete that Viscosity--The resistance of a material to flow under an can spread into place, fill the formwork, and encapsulate the re- applied shearing stress (ASTM 1611). inforcement without any mechanical consolidation (ACI 237). Viscosity-modifying admixture (VMA)--An admixture Service life--The time during which the structure performs used for enhancing the rheological properties of cement- its design functions without unforeseen maintenance or repair. based materials in the plastic state to reduce the risk of segre- Settlement--The condition in which the aggregates in gation and washout (ACI 237). SCC tend to sink to the bottom of the form resulting in non- Visual Stability Index (VSI)--A test that involves the vi- homogeneous concrete (PCI 2003). Surface settlement can sual examination of the SCC slump flow spread resulting also be caused by bleeding of free water and loss of air as well from performing the slump flow test (ACI 237).
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B-8 Water-cementitious material ratio (w/cm)--The ratio of ability, passing ability, and segregation resistance, and it is af- the mass of water, exclusive only of that absorbed by the ag- fected by rheology. gregate, to the mass of cementitious material (hydraulic) in Yield stress--The minimum shear stress required to initi- concrete, mortar, or grout, stated as a decimal (ACI 116). ate (static yield stress) or maintain (dynamic yield stress) flow Workability--That property of freshly mixed concrete or (ACI 237). The yield stress is closely related to slump flow mortar that determines the ease with which it can be mixed, (lower yield stress results in higher slump flow); it is calcu- placed, consolidated, and finished to a homogenous condi- lated as the intercept of the shear stress versus shear rate plot tion (ACI 116). For SCC, workability encompasses filling from rheometer flow curve measurements.
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B-9 Guidelines Commentary B.1 Guidelines for Selection of Constituent Materials B.1.1 General The production of SCC requires uniform quality of all con- stituent materials, and it is therefore necessary that these ma- terials meet standard specifications. A choice of suitable con- stituent materials is vital to the optimization of SCC mix design for different applications. Constituent material qualification for SCC designated for precast, prestressed concrete bridge elements generally follows the requirements of AASHTO LRFD Bridge Design  and Construction  Specifications. Except mixing water and materials mentioned in the following Sections B.2.2 to B.2.4, no materials may be incorporated into the concrete without the authorization of the Engineer. It is important to continually check for any change in materials or proportions that will affect surface appearance, strength, or other charac- teristics of SCC that may affect its overall performance. B.1.2 Cement and Cementitious Materials One must ensure that material additions do not adversely affect the desired architectural appearance, where appearance is a design requirement. B.1.2.1 Cement and Blended Cement All cements that conform to the AASHTO M 85 or ASTM Selection of the type of cement will depend on the overall C 150 standard specifications can be used for the production requirements for the concrete, such as compressive strength of SCC. The correct choice of cement type is normally dic- at early and ultimate ages, mechanical properties, durability, tated by the specific requirements of each application or by and color considerations in architectural applications where the availability. color and color uniformity are important. For SCC applications where visual appearance is impor- Blended hydraulic cements that conform to the AASHTO tant, adequate cement content and uniform w/cm should be M 240 or ASTM C 595M can also be used. Unless otherwise adopted to minimize the color variation. Therefore, the ce- specified, Types I, II, or III cement; Types IA, IIA, or IIIA air- ment should be from the same mill and of the same type, entrained cement; or Types IP (portland-pozzolan cement) or brand, and color. IS (portland blast-furnace slag cement) blended hydraulic ce- ments can be used for the construction of precast, prestressed concrete elements. Types I, II or III cements can be used with some replacements by supplementary cementitious materials and other hydraulic binders. In general, fly ash and slag re- placement values shall not exceed 20% and 40%, respectively, to ensure high-early strength for satisfactory release of strands. The total content of cementitious materials used in pre- stressed concrete for a 28-day design compressive strength of 4,000 to 8,000 psi (28 to 55 MPa) can vary from 600 to 1,000 lb/yd3 (356 to 593 kg/m3) [PCI, 1997]. The AASHTO LRFD Bridge Design Specifications  suggest that the sum of portland cement and other cementitious materials should
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B-10 Guidelines Commentary not exceed 800 lb/yd3 (475 kg/m3), except for Class P concrete where the total cementitious materials should not exceed 1,000 lb/yd3 (593 kg/m3). These values for SCC designated for precast, prestressed applications shall range between 650 and 800 lb/yd3 (386 and 475 kg/m3) [ACI Committee 237, 2007 (237R-07)]. B.1.2.2 Fly Ash Pozzolans and slag meeting ASTM C 618, C 989, or C 1240 In some cases, higher level of fly ash replacement may re- are supplementary cementitious material and may be added duce the ability of SCC to flow. The replacement rate of fly to portland cements during mixing to produce SCC with im- ash also affects strength and durability. Contribution of fly proved workability, increased strength, reduced permeability ash delays the hydration process and strength development. and efflorescence, and improved durability. In general, Class Fly ash can also affect air entrainment since the carbon pres- F fly ash has been shown to be effective in SCC providing in- ent in fly ash can absorb air-entraining admixture and ad- creased cohesion and robustness to changes in water content versely affect the ability to entrain air. Therefore, specific lim- [European Guidelines, 2005]. its on LOI which is indicative of the carbon content need to be stated. Fly ash shall not be used with Type IP or IS cements. Fly ash should conform to the AASHTO M 295 or ASTM C 618 [AASHTO, 1998, 2007]. In general, the content of ce- ment replaced with fly ash is 18% to 22% by mass [Florida DOT, 2004]. B.1.2.3 Silica Fume Silica fume conforming to AASHTO M 307 or ASTM C In some cases, a high level of silica fume addition can cause 1240 can be used as supplementary cementitious material in rapid surface crusting that leads to cold joints or surface de- the proportioning of SCC for improved strength and dura- fects if delays occur in concrete delivery or surface finish (and bility. Silica fume also improves resistance to segregation and also increases cost). According to Florida DOT , the bleeding. Special care should be taken to select the proper sil- quantity of cement replacement with silica fume should be ica fume content. 7% to 9% by mass of cementitious materials. B.1.2.4 Ground Granulated Blast-Furnace Slag Ground granulated blast-furnace slag (GGFBS) meeting AASHTO M 302 or ASTM C 989 may be used as supplemen- tary cementitious materials. GGBFS provides reactive fines and due to large replacement rate usually about 40% enables a low heat of hydration. Cement replacement by GGBFS is based on the severity of the environment to which the concrete is exposed. The level of GGBFS addition is 25% to 70% for slightly and moderately aggressive environments, and 50% to 70% by mass when used in extremely aggressive environments. When used in combination with silica fume and/or A high proportion of GGBFS (e.g., exceeding 40%) may metakaolin, GGBFS content should be limited to 50% to 55% however affect stability of SCC resulting in reduced robust- of the total cementitious content, by mass of binder [Florida ness with problems of consistency control while delayed set- DOT, 2004]. However, in precast, prestressed members, the ting can increase the risk of static segregation. amount of slag is usually 40%. GGBFS shall not be used with Type IP or Type IS cements.
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B-11 Guidelines Commentary B.1.2.5 Fillers The particle-size distribution, shape, and water absorption of fillers may affect the water demand/sensitivity and suit- ability for use in the production of SCC. Calcium carbonate- based mineral fillers can enhance workability and surface fin- ish. The fraction below 0.005 in. (0.125 mm) shall be of most benefit to SCC flow properties. Contents of fillers should be evaluated to ensure adequate performance of concrete, in- cluding strength development and durability. B.1.2.6 Other Supplementary Cementitious Additions Metakaolin, natural pozzolan, ground glass, air-cooled slag and other fine fillers have also been used or considered as ad- ditions for SCC, but their effects need to be carefully evalu- ated for both short- and long-term effects on the fresh and hardened concrete. B.1.3 Aggregate Characteristics A well-graded combined aggregate with sufficient inter- mediate sizes is highly desirable for improved stability. Also, if the proper particle shape and texture are selected, com- bined aggregate grading can lead to large reductions in water, paste, and cement contents leading to improved hardened concrete properties. The moisture content, water absorption, grading and Gravel, crushed stone, or combinations can be used as variations in fines content of all aggregates should be coarse aggregate. In the case of fine aggregate, natural sand or closely and continuously monitored and must be taken into manufactured sand can be used. Coarse and fine aggregates account in order to produce SCC of constant quality. should conform to the grain-size distribution recommenda- Changing the source of supply for aggregates is likely to tions of the project specifications. make a significant change to the concrete properties and should be carefully and fully evaluated [European Guide- lines, 2005]. B.1.3.1 Coarse Aggregate Unless otherwise specified in the contract documents, the Slightly gap-graded aggregates may lead to greater flow- recommendation is to use normal-density coarse aggregate ability than continuously graded aggregate. Gap-graded ag- meeting the requirements of AASHTO M 80 or ASTM C 33. gregate can, however, increase the risk of bleeding and segre- The use of continuously graded aggregates is recommended. gation, and proper measures are needed to ensure adequate The nominal maximum-size of coarse aggregate (MSA) static stability of the concrete. should be selected based on mix requirements and minimum In the design of SCC, typically the MSA values are smaller clear spacing between the reinforcing steel and prestressing than those of conventional vibrated concrete. The MSA is strands, cover of the reinforcement steel, and thickness of the generally limited to 1/2 to 3/4 in. (12.5 to 19 mm). In the member. The recommendations given in the PCI Bridge De- placement of SCC in highly congested and restricted section, sign Manual  apply. MSA value of 3/8 in. (9.5 mm) can be used.
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B-12 Guidelines Commentary If aggregates susceptible to alkali-aggregate reactivity are used, special precautions must be observed. These include the use of low-alkali cement, blended cements, or pozzolans and GGBFS. B.1.3.2 Fine Aggregate For normal weight concrete, fine aggregates conforming to It may be beneficial to blend natural and manufactured AASHTO M 6 are appropriate for the production of SCC. sand to improve plastic properties of SCC. Common concrete The fine aggregate component should be well-graded con- sand, including crushed or rounded sand and siliceous or cal- crete sand. careous sand, can be used in SCC. Fine aggregates for SCC should conform to the gradation requirements of AASHTO M 6 or ASTM C 33, as presented in Table B.1. Table B.1. Grading requirements for fine aggregates. Percent passing Percent passing Particle size fractions of less than 0.005 in. (0.125 mm) Sieve (AASHTO M 6) (ASTM C 33) should be considered as powder material in proportioning 3 8 in. (9.5 mm) 100 100 No. 4 (4.75 mm) 95 to 100 95 to 100 SCC. Such fine content can have marked effect on rheology. No. 8 (2.36 mm) 80 to 100 80 to 100 No. 16 (1.18 mm) 50 to 85 50 to 85 No. 30 (600 m) 25 to 60 25 to 60 No. 50 (300 m) 10 to 30 5 to 30 No. 100 (150 m) 2 to 10 0 to 10 B.1.4 Chemical Admixtures Chemical admixtures are used in precast, prestressed con- crete to reduce water content, improve filling ability and sta- bility, provide air entrainment, accelerate strength develop- ment, enhance workability retention, and retard setting time. Because chemical admixtures can produce different results Incompatibility of admixtures with binders can lead to im- with different binders, and at different temperatures, the se- proper air void system and delayed or accelerated setting lection of the admixtures should be based on the plant mate- time. Therefore, before the start of the project, concrete with rials and conditions that will be utilized in production. the job materials, including the admixtures, should be tested For prestressed concrete, chloride-ion content in chemical to ensure compatibility. Such testing should be repeated admixtures should be limited to 0.1%, by mass of the admix- whenever there is a change in the binder and admixtures. ture [AASHTO, 2004]. B.1.4.1 High-Range Water-Reducing Admixtures High-range water-reducing admixtures (HRWRA) shall The required consistency retention will depend on the ap- conform to the requirements of ASTM C 494 Type F (water- plication. Precast concrete is likely to require a shorter reten- reducing, high range) or G (water-reducing, high range, and tion time than cast-in-place concrete. retarding) or ASTM C 1017. The admixture should enable the required water reduction and fluidity during transport and placement. The use of Type F or G HRWRA is essential to achieve SCC fluidity. Such HRWRA can be used in combination with reg-
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B-28 Guidelines Commentary Type III + 20% FA (moist-cured): A = 2.15; B = 0.89; R2 = 0.95 CEB-FIP MC90 28 1 2 fcm ( t ) = exp s 1 - fcm t t1 fcm(t) = mean compressive strength at t days (in psi); fcm = mean 28-day compressive strength; s = coefficient depending on cement type (0.20 for high early-strength cement, 0.25 for normal-hardening cement, and 0.38 for slow-hardening cement); t1 = 1 day. 16 non-AEA SCC + 4 AEA SCC + 2 HPC: s = 0.19 Type I/II cement; R2 = 0.95 s = 0.20 Type III + 20% FA; R2 = 0.92 16 nonair-entrained SCC: s = 0.20 Type I/II cement; R2 = 0.95 s = 0.23 Type III + 20% FA; R2 = 0.93 B.4.5 Flexural Strength For precast and structural civil engineering applications, The flexural strength of SCC depends on the w/cm, coarse SCC mixtures are typically proportioned with relatively low aggregate volume, and quality of the interface between the ag- w/cm of 0.32 to 0.36 and with supplementary cementitious gregate and cement paste. The curing method of SCC can sig- materials and fillers and are expected to achieve higher flex- nificantly influence the flexural strength. Moist-cured speci- ural strength and flexural-to-compressive ratio than conven- mens can exhibit higher flexural strength because the samples tional slump concrete [ACI Committee 237, 2007]. do not develop surface drying that could lead to premature The flexural strength can be determined by testing in ac- microcracking development. cordance with ASTM C 293 and C 78-02 or can be estimated from the compressive strength. For SCC used for precast, pre- stressed applications, the flexural strength can be estimated with the AASHTO 2007 model, given by: fr = 0.97 fc = specified compressive strength of concrete (MPa) fc B.4.6 Modulus of Elasticity In applications where the modulus of elasticity (MOE) is The MOE is used to calculate camber of prestressed mem- an important design parameter, the MOE should be deter- bers at the release of the prestressing load, elastic deflections, mined and considered in the design of the prestressed con- axial shortening and elongation, and prestress losses. crete member. In the absence of measured data, the equation The MOE is related to the compressive strength of the con- proposed by the AASHTO LRFD Bridge Design Specifica- crete, type and content of aggregate, as well as unit weight of the tions  is recommended to estimate the elastic modulus concrete. The modulus of elasticity is related to compressive
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B-29 Guidelines Commentary of concrete having a unit weight of 2,427 to 4,214 lb/yd3 strength and unit weight of the concrete, aggregate type and (1,440 and 2,500 kg/m3) and specified compressive strength content, and testing parameter, including loading rate, mois- of up to 15,230 psi (105 MPa). For an accurate prediction, de- ture and temperature conditions of the test specimen, as well as termine the MOE in conformance with ASTM C 469 using specimen size and shape. The content and MOE of the aggre- the job-specific materials. gate have the largest influence on the MOE of the concrete. The modulus of elasticity for SCC used for precast, pre- Selecting an aggregate with high modulus of elasticity will stressed applications can be estimated using the AASHTO increase the modulus of elasticity of the concrete. Increase in 2007 equation: sand-to-coarse aggregate ratio can decrease the modulus of elasticity of the concrete. Ec = 0.043 1 c .5 fc In some cases, SCC mixtures can develop modulus of elas- ticity that can be up to 20% lower than typical values found c = unit weight of concrete (kg/m3); in high-performance concrete of normal consistency, which = specified compressive strength of concrete (MPa) fc is mainly due to the lower coarse aggregate volume, increase in paste content, and higher content of ultra-fine materials. At equivalent strength, SCC made with different cement types should develop similar modulus of elasticity when cured and tested under identical conditions. B.4.7 Creep Incorrect or inaccurate design for creep and shrinkage may Length changes of prestressed members due to time- have important undesirable consequence on stability and dependent deformation, creep, and shrinkage play a crucial performance of the structure. role in the design of concrete structures and on structural be- In applications where creep characteristics are important havior, especially at long term. design parameters, this aspect should be considered in the de- Creep behavior is related to the compressive strength of the sign and confirmed for the mixture used in the production of matrix, coarse aggregate type, relative content of aggregate, as precast members. well as magnitude of applied load and age of loading. Creep Perform creep testing in accordance with ASTM C 512 takes place in the cement paste and is influenced by the cap- using the job-specific materials. The age when the load is ap- illary porosity of the paste. Cement type and w/cm can affect plied affects creep values. For SCC used in precast, pre- creep. High early-strength cement can lead to lower creep. stressed elements load should be applied at an early age cor- The presence of aggregate restrains creep deformation in the responding to prestress release time. paste. Therefore, an increase in the volume and elastic mod- In the absence of measured data, the modified AASHTO ulus of the aggregate can lower creep. 2007 prediction model can be used to predict the creep of SCC. Due to the higher volume of cement paste and fines and smaller MSA of SCC, creep potential of SCC can be higher AASHTO 2007 than conventional concrete made with the same raw materi- ( t , t i ) = 1.9kvs khc k f ktd t i-0.118 × A als and having the same 28-day design compression strength. in which: kvs = 1.45 - 0.0051(V / S) 0.0 khc = 1.56 - 0.008H 35 t kf = , ktd = 7 + fci 61 - 0.58 fci + t where: H = relative humidity (%). In the absence of better in- formation, H may be taken from Figure 188.8.131.52.3-1 of AASHTO Bridge Design Specifications . kvs = factor for the effect of the volume-to-surface ratio of the component kf = factor for the effect of concrete strength
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B-30 Guidelines Commentary khc = humidity factor for creep ktd = time development factor t = maturity of concrete (day). Defined as age of concrete between time of loading for creep calculations, or end of curing for shrinkage calculations, and time being considered for analysis of creep or shrinkage effects ti = age of concrete when load is initially applied (day) V / S = volume-to-surface ratio (mm) i = specified compressive strength of concrete at time of fc prestressing for pretensioned members and at time of initial loading for non-prestressed members. If concrete age at time of initial loading is unknown at i may be taken as 0.80 f c design time, f c i (MPa) A = factor for the effect of cement type: 1.19 for Type I/II cement and 1.35 for Type III + 20% FA binder which may be used for P(SCC) B.4.8 Autogenous Shrinkage SCC and conventional concrete used in precast applica- Autogenous shrinkage corresponds to the macroscopic vol- tions proportioned with relatively low w/cm (0.32 to 0.36) ume reduction due to cement hydration (chemical shrinkage) and high content of cement and supplementary cementitious as well as self-desiccation of the cement paste. The volume of materials could exhibit high autogenous shrinkage. This is es- the hydration products is less than the original volume of un- pecially the case when capillary porosity is refined when using hydrated cement and water. Such reduction in volume can silica fume. Cement type has a considerable effect in the de- lead to tensile stresses in the cement paste and microcracking. velopment of autogenous shrinkage. Higher surface area of The reduction of relative humidity in capillary pores due to ce- the cement can activate the reactivity of the binder, hence in- ment hydration can also result in negative pressure in the cap- creasing the degree of autogenous shrinkage. illary pores, leading to the formation of meniscus and the de- velopment of tensile stresses in the cement paste. In the case of concrete proportioned with high w/cm (higher than 0.40), autogenous shrinkage is low given the ample presence of water in capillary pores. B.4.9 Drying Shrinkage In prestressed applications, shrinkage should be considered Drying shrinkage must be taken into consideration to in the mix design and taken into consideration in the struc- avoid cracking and excessive deflection resulting from time- tural design of the member. Proportion SCC with relatively dependent concrete deformation and loss of prestress. Drying low binder content and w/cm to reduce drying shrinkage. shrinkage is caused by the loss of water from the concrete to Drying shrinkage can be evaluated in accordance with the atmosphere. The increased volume of paste in SCC and re- ASTM C 157 (AASHTO T 160). In the absence of measured duction in aggregate content and size can increase the poten- data, the modified AASHTO 2004 or CEB-FIP MC90 shrink- tial for drying shrinkage. The presence of aggregate restrains age models can be used to estimate drying shrinkage of SCC, shrinkage of the cement paste; therefore, the increase in ag- as indicated below. For steam cured concretes devoid of gregate volume reduces drying shrinkage. A decrease in the shrinkage-prone aggregates, the strain due to shrinkage, sh, MSA can necessitate higher paste volume, thus leading to at time, t, may be taken as: higher shrinkage. Drying shrinkage increases with the increase in powder material content, which is particularly high in SCB. The use of fly ash in normal proportions does not signifi- cantly influence drying shrinkage of concrete. The use of limestone powder with Blaine fineness greater than that of
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B-31 Guidelines Commentary AASHTO 2004 portland cement can reduce drying shrinkage of SCC. This can be explained by the denser matrix obtained when fine t sh = -ks kh × 0.56 × 10-3 × A ( steam-cured ) limestone powder is used [Holschemacher and Klug, 2002]. 55 + t The effect of HRWRA and VMA on shrinkage of SCC is re- t ported to be beneficial. Indeed, the use of HRWRA reduces 26e 0.0142(V S ) + t 1064 - 3.70 (V S ) ks = the surface tension of the water, thus decreasing the capillary t 923 tension of pore water [Ulm et al., 1999; Acker, 1988; Acker 45 + t and Bazant, 1998; Neville, 1981; Wittman, 1976; Neville and Meyers, 1964]. However, the air content may increase when t = drying time (day) using polycarboxylate-based HRWRA, which could lead to ks = size factor greater shrinkage. kh = humidity factor V / S = volume-to-surface ratio A = cement factor: 0.918 for Type I/II cement and 1.065 for Type III + 20% FA binder which may be used for P(SCC) CEB-FIP MC90 (t - t c ) cso = s ( f cm )( RH ) 2 Ac 2 350 + ( t - t c ) 100 s(fcm) = [160 + 10sc(9 - 0.1 fcm)] × 10-6 RH = -1.55ARH; ARH = 1 - (RH/100)3 cso = drying shrinkage (mm/mm) s = drying shrinkage obtained from RH-shrinkage chart sc = cement type factor RH = relative humidity factor fcm = mean 28-day compressive strength (MPa) Ac = cross-sectional area (mm2) µ = perimeter (mm) tc = age at which drying commenced (day) t = age of concrete (day). B.4.10 Durability and Air-Void System It is essential to proportion SCC with adequate stability to Segregation and bleeding have significant negative effect ensure high performance, including durability, of the hard- on permeability and quality of the interfacial zone between ened concrete. The durability of a concrete structure is closely cement paste and aggregate, embedded reinforcement, and associated to the permeability of the surface layer and curing. existing surface, and hence on durability of the concrete. The most significant durability characteristics affecting the Higher air content (6% to 9%) may be necessary in durability of SCC used in precast, prestressed elements pro- most severe frost environments, especially when using duction include: w/cm, cement content, degree of consolida- polycarboxylate-based HRWRA, which could result in some tion, curing, cover over the reinforcement, and reactivity of entrapment of relatively large air voids. Coalescence of small aggregate-cement combinations. air bubbles during agitation can occur when high air contents Bridge structures constructed in environments prone to are obtained and when concrete is retempered with water. freezing and thawing may become critically saturated, thus The dosage of AEA in SCC prepared with polycarboxylate- necessitating air entrainment when exposed to cycles of freez- based HRWRA can be quite low compared with values used ing and thawing. In some cases, the bridge deck can shelter for conventional concrete of normal consistency. Still, it is
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B-32 Guidelines Commentary some of the precast beam elements, thus reducing the rate of critical to incorporate an AEA in concrete subjected to mod- saturation and exposure to any deicing salt applied on the erate frost exposure conditions to secure stable and closely bridge deck. In most cases, bridge girders can be considered to spaced air bubbles (adequate spacing factor). In general, for be subjected to moderate exposure conditions that correspond mixtures made with a relatively low content of cementitious to situations where deicing salts are not used or where the con- materials and a high w/cm, the air-void stability increases crete is only occasionally exposed to moisture prior to freezing when a VMA is incorporated [Khayat, 1995]. and do not get critically saturated. Therefore, under these con- Air entrainment is necessary to stabilize small, closely ditions, prestressed bridge girders require sufficient air content spaced, and well-distributed voids in concrete. Such voids can for moderate exposure conditions. For example, under these be obtained when the SCC is proportioned with an effective conditions, SCC proportioned with 1/2 in. (12.5 mm) nomi- AEA that is compatible with the HRWRA and other chemical nal MSA would then require 4% to 7% air volume in the fresh admixtures in use. concrete to provide adequate frost resistance. ASTM C 457 can be used to test the air-void parameters of the concrete, and ASTM C 666, Procedure A (AASHTO T 161, Method A), is used to test resistance to freezing and thawing. B.4.11 Bond to Prestressing Strands Ensuring proper stability of SCC is essential to ensure ho- Bond between the strand and concrete is affected by the mogenous in-situ properties, including bond to embedded re- position of the embedded reinforcements and quality of the inforcement, which is critical for structural performance of pre- cast concrete. Bond to prestressed tendons can be influenced cast, prestressed applications [Moustafa, 1974; Logan, 1997]. by the flow properties of the SCC, grading of the aggregate In general, adequate concrete cover is necessary to prop- and content of fines in the matrix [Holschemacher and Klug, erly transfer bond between prestressed tendons and concrete. 2002]. Despite the high fluidity of SCC, high static stability of the A surface settlement of 0.5% corresponds to 1.4 modifica- SCC after placement can lead to more homogenous in-situ tion factor of prestressing strands [Khayat et al., 1997; Petrov properties and denser matrix at the interface between the ce- et al., 2001]. It is important to note that selection of highly ment paste and reinforcement, thus enhancing bond strength viscous SCC can result in some lack of consolidation of the compared with normal conventional concrete subjected to concrete, which can in turn affect bond stresses between the mechanical vibration. On the other hand, bond can be sig- concrete and prestressing strand. nificantly affected by excessive segregation found in poorly designed SCC. As indicated in Table B.10, in order to secure adequate static stability, the SCC should have maximum sur- face settlement of 0.5%, column segregation index of 5%, or percent static segregation of 15%. Table B.10. Recommendations to secure homogenous in-situ properties of SCC. Material properties Recommended values Maximum surface settlement 0.5% Static stability Column segregation index (Iseg) 5% Percent static segregation (S) 15% Plastic viscosity 0.0725 psi·s (500 Pa·s) Viscosity (Modified Tattersall two-point rheometer with vane device) Core-to-cylinder compressive strength 90% (similar curing Mechanical properties conditions) Bond strength modification factor 1.4
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B-33 Guidelines Commentary B.5 Guidelines for Production and Control B.5.1 General The need for adequate quality control is much more criti- cal with SCC than in the case of conventional concrete. In order to maintain a given workability, it is essential to main- tain constant quality of all concrete constituent during SCC production. Successful production of SCC requires greater competence and proper control of materials and equipments used for production. SCC intended for use in precast plants should meet the tech- nical requirements of the fresh concrete. The mixture needs to be tested to ensure that required properties are achieved given the performance specifications, casting conditions, and geom- etry of the cast element. Before selecting the raw material and finalizing the mix design, several factors should be known, including the size and shape of elements to be cast. Laboratory trials should be used to validate the material se- lection and verify the properties of the mix design to achieve the targeted properties. Once the optimum properties are achieved, proper quality control for material properties should be observed to eliminate fluctuations in fresh and hardened properties of the concrete. Any changes in raw materials properties should be immediately identified to allow necessary adjustments of the mix to meet the specified properties. B.5.2 Control of Raw Materials Depending on the mix design, SCC may be less robust than The moisture content, water absorption, aggregate grada- conventional concrete. SCC may therefore undergo greater tion, and variations in fines content of the aggregate should changes in workability given small variations in the physical be continuously monitored and must be taken into account properties of its constituents, especially in the moisture con- to produce SCC with constant characteristics. Changing the tent of the sand, fine particle content in sand, as well as grad- source of supply for aggregate is likely to significant change ing and shape of the sand and coarse aggregate. This would the concrete properties and should be carefully and fully eval- necessitate frequent controls to check for any changes in ma- uated [European Guidelines, 2005]. terial properties that could affect the performance of SCC. The maximum deviation of the sand moisture should not It is preferable to control the moisture of sand before every exceed ± 0.2% in order to minimize the variations in fresh batch of SCB. The moisture content in coarse aggregate must be properties of SCC. The water content of sand should be de- also taken into account and should be determined at least twice termined just before production of SCC. a day, at the beginning of the first and second production shifts. Changes in coarse aggregate physical characteristics (shape, When designing SCC, some factors should be taken into texture, gradation) can affect workability. Inspection at the consideration to a greater degree than when designing con- storage location should be conducted on coarse aggregate to ventional concrete to ensure good filling capacity, such as the characterize their physical characteristics for every aggregate geometry configuration of cast elements and placement con- delivery. ditions. Indeed, the nominal maximum size of coarse aggre- gate should be selected based on mix requirements and the minimum clear spacing between the reinforcing steel, cover to reinforcing steel, and thickness of the member. The thick- ness of the cast element and the congestion level of the rein- forcement are key factors affecting workability of SCC.
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B-34 Guidelines Commentary B.5.3 Mixing Process and Sequence The mixing process should be properly determined given The batch volume is typically limited to 80% to 90% of the the conditions at hand. For example, the batch volume maximum capacity of the mixer to allow efficient mixing en- should be determined in consideration of the type of SCC ergy [JSCE, 1999]. When the mixer is alternatively used for (consistency level), efficiency of the mixer to produce a well- mixing normal concrete and SCC, testing should be per- dispersed and homogeneous mix, and transportation rate formed to verify that this does not result in any adverse effect from the plant to the casting site. on SCC properties. Just prior to mixing of the first batch, the mixer should be Mixing equipment and mixing sequence should be vali- pre-wetted or "buttered" with SCC of approximately similar dated by testing consistency and self-consolidation properties consistency. for a given mix design. Necessary adjustments to time and Suitable mixing sequence should be determined given the speed of mixing should be carried out until consistent and mixing and storage equipments available at the plant. Cement compliant results are obtained. particles should be wetted before contact with HRWRA. Dry mixing before water introduction is not recommended, be- cause it may lead to build-up of fine materials in the mixer. All batching water should be added at the same time. For example, it has been shown that the introduction of VMA at the end of the mixing sequence and of air-entraining admixture at the be- ginning can provide good performance [Khayat 1995; Khayat and Assaad, 2002]. The addition sequence of VMA should be evaluated given the mix design and admixture in use. B.5.4 Transport SCC should be delivered in a continuous and timely man- ner to ensure continuous placement of precast members with the workability-retention period of the mixture. This is nec- essary to avoid lift lines and other surface defects. Transport method shall be confirmed in order to provide SCC at the casting location that is sufficiently homogeneous to allow successful placement in the precast element and to achieve the targeted properties. Mixer trucks have proven to be the best method of delivery of SCC when transporting over rough terrain or long transport distance [PCI, 2003]. B.5.5 Site Acceptance of Plastic Concrete The producer should determine the frequency of perform- ing quality control testing based on available experience mix- tures [PCI, 2003]. The quality control tests should include visual inspection Quality control for frequently used SCC is less critical than of every batch of the concrete and any specific tests and com- in the case of SCC that is occasionally produced. pliance parameters. For example, the slump flow and VSI tests can be adopted. The T-50 can also be run at least once on new mixtures and used to check the performance in the event of mix performance problems. B.5.6 Placement Techniques and Casting Considerations Prior to the production process, full-size mock-ups should In a placement case that will require multiple batches, be cast for final approval. Placement method should be selected mixing facilities are required to ensure that concrete will be
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B-35 Guidelines Commentary given the production capacity and transport rate to the cast- available within a short time frame with proper workability ing point. characteristics as specified in performance specifications. Placement techniques should be selected based on the Placement techniques of SCC can have a significant impact total volume of the concrete to be discharged, the transporta- on the required fluidity level and flowing performance of the tion rate, and whether the placement process is continuous concrete. For example, in the case of higher energy involved or discontinuous. during placement, lower fluidity level for the SCC will be In the case of placement technique involving higher en- required to achieve a given flow and filling performances. ergy, extra care should be taken with regard to stability char- acteristics. Relative energy involved during each placement technique is summarized in Table B.11 [PCI, 2003]. Table B.11. Summary of different placement techniques for SCC [Bury and Bühler, 2002]. Single Relative Placement Discharge Discharge discharge energy technique rate type volume delivered Truck discharge High Continuous High High Pumping Medium/High Continuous Medium High/Medium Crane and bucket High Discontinuous Low Medium Auger (Tuckerbuilt) Low/Medium Continuous Medium Low/Medium discharge Placement of SCC in horizontal elements can be done by starting at one end of the mold, with the discharge as close to the form surfaces as possible. It is recommended to discharge the SCC in the direction of desired flow to maximize the travel distance. The recommended maximum flowing distance should be between 10 and 33 ft (3 and 10 m), depending on the geometry of the element [RILEM, 2000]. As in the case of conventional concrete, the free-fall dis- Free-fall distance should be fixed given the element depth tance should be controlled to avoid concrete segregation. For to be cast and static and dynamic stability of the concrete. example, based on the Norwegian experience, the free-fall distance should be limited to 6.5 ft (2 m) when casting wall and beam elements. B.5.7 Temperature Control The mix design should be tailored to achieve the targeted properties specified in the performance specifications. When the use of steam curing is required to achieve the targeted early-age strength, the temperature of the concrete should not exceed 160°F (71°C) [AASHTO, 1998]. Furthermore, according to the AASHTO LRFD Bridge Construction Spec- ifications , the temperature within the curing cham- ber shall increase at a rate not exceeding 72°F (22°C) per hour.
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B-36 Guidelines Commentary B.5.8 Formwork Considerations and Lateral Pressure Formwork for SCC can be constructed of different materi- als, including wood, steel, plastic, fiberglass, or combination of these materials. Formwork made with wood often leads to less pores and bubble formation than smooth formwork. Be- cause of the high fluidity of SCC compared with conventional concrete, formwork should be rigid enough with accommo- date variations in product dimensions and form, and to with- stand lateral form pressure exerted by the plastic concrete. Given the high fluidity of SCC compared with conventional Before applying the release agent, the wood of the form- concrete, extra care should be taken to avoid any leakage. work should be dry to ensure good release performance and Formwork joints should be adequately sealed. Vegetable oil has avoid appearance of air-bubbles at the formed surface of the been shown to be a good release agent as it reduces the amount cast element. of pores on the concrete surface [Brite-EuRam, 1998]. Depending on the casting rate and thixotropy of SCC, lat- Experience has shown that for a given casting rate, concrete eral pressure can be lower than the theoretical hydrostatic with a higher level of thixotropy can develop lower lateral pressure. This is especially the case when the casting rate ex- pressure, faster decay in lateral pressure, and shorter time to ceeds 10 ft/h (3 m/h). pressure cancellation [Assaad et al., 2004]. SCC cast at 16 ft/h Lateral pressure can be 50% to 80% of the calculated pres- (5m/h) is shown to develop maximum initial lateral pressure sure for conventional vibrated bridge concrete with a slump of 90% of hydrostatic pressure. In general, sections measur- consistency of 4 in. (100 mm) [RILEM, 2000]. ing up to 10 ft (3 m) in height should be designed for full hydrostatic pressure. Lateral pressure developed by SCC cast from the top of the formwork is lower than in the case when the concrete is pumped from the bottom. SCC pumped from the bottom should be designed for full hydrostatic pressure. B.5.9 Finishing Finishing of SCC is easier and faster than for conventional Given the concrete properties and ambient conditions, concrete. Finishing practices employed with conventional some surfaces may require only nominal screeding and float- concrete can be employed with SCC. However, finishing op- ing, while other surfaces may require mild vibratory screed- erations should be delayed slightly more than for conven- ing [ACI Committee 237, 2007; PCI, 2003]. tional superplasticized concrete [PCI, 2003]. Surface drying during finishing should be prevented. Fog Because of the relatively higher content of fines and even- misting to increase the relative humidity would minimize rate tual presence of VMA, SCC mixtures develop little or no of evaporation and reduce the risk of plastic shrinkage. bleed water compared with conventional concrete. SCC exposed surfaces may dry faster than those of normal It is important to begin the finishing of the surface with superplasticized concrete. This can happen when casting at light vibrating screeds, or other manual equipment, as soon hot temperature or windy conditions. Also, depending on the as the correct level of the concrete in the formwork has been SCC mixture proportioning, stiffening can increase rapidly in reached. the period 10 to 40 minutes after casting. Setting time of the SCC mixture should be adjusted to allow necessary time to carry out the placement process. B.5.10 Curing Membrane curing, matting, foils, or appropriate materials should be left in place for at least 4 days for cast-in-place
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B-37 Guidelines Commentary concrete elements [Swedish Concrete Association, 2002]. This measure should be applied to SCC with low w/cm and SCC made with high fines content or VMA. During hot or windy weather conditions, moisture should be added by wa- tering or by protecting the surface with wetted membranes for proper curing. AASHTO LRFD Bridge Construction Specifications  During the waiting period, the temperature within the cur- recommend that for concrete cured by other than steam or ra- ing chamber shall not be less than 10°C. During the applica- diant heat methods, whenever there is a probability of air tem- tion of steam, the ambient temperature within the curing perature below 36°F (2°C) during the curing period, the con- chamber shall not increase at an average rate greater than crete shall be maintained at a temperature of not less than 45°F 22°C/h until the targeted temperature value is reached. (7°C) for the first 6 days after placement. This period must be extended if pozzolans are used as partial replacement of ce- ment. If the compressive strength of 65% of the specified 28-day design strength is achieved in 6 days, an extended period of controlled temperature may be waived [AASHTO, 1998]. Due to the specification in terms of early-age compressive strength, steam curing or radiant heat curing can be used for precast concrete members. The initial application of steam or heat shall be from 2 to 4 hours after the initial placement of concrete to allow the initial set of the concrete to take place [AASHTO, 1998]. In the case of concrete incorporating a set retarder, the waiting period shall be increased to between 4 and 6 hours after casting.
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B-38 References Khayat, K. H. (1999), "Workability, Testing, and Performance of Self- Consolidating Concrete." ACI Materials Journal, Vol. 96, No. 3, Acker, P. (1988), Mechanical Behavior of Concrete: Physico-Chemical pp. 346353. Approach, Etudes et Recherches des LPC, 152, Laboratoire Central Khayat, K. H., and Assaad, J. (2002), "Air-Void Stability of Self- des Ponts et Chaussées, 121 p. (In French) Consolidating Concrete." ACI Materials Journal, Vol. 99, No. 4, Acker, P., and Bazant, Z. P. (1998), "Measurement of Time-Dependent pp. 408416. Strains of Concrete, RILEM Draft Recommendations." Material and Khayat, K. H., Manai, K., and Trudel, A. (1997), "In-Situ Mechanical Structures, RILEM, Vol. 31, No. 8 pp. 507512. Properties of Wall Elements Cast Using Self-Consolidating Con- American Association of State Highway Transportation Officials crete." ACI Materials Journal, Vol. 94, No. 6, pp. 491500. (AASHTO) (1998), AASHTO LRFD Bridge Construction Specifica- Khayat, K. H., Mitchell, D. Long, W. J., Lemieux, G., Hwang, S.-D., tions, 1st edition. Yahia, A., Cook, W. D., Baali, L. (2007), Self-Consolidating Con- American Association of State Highway Transportation Officials crete for Precast, Prestressed Concrete Bridge Elements. Draft final (AASHTO) (2004), AASHTO LRFD Bridge Design Specifications, report, NCHRP Project 18-12. University of Sherbrooke, Quebec, 3rd edition. Canada. American Association of State Highway Transportation Officials Logan, D. (1997), "Acceptance Criteria for Bond Quality of Strand for Pre- (AASHTO) (2007), AASHTO LRFD Bridge Design Specifications, 4th edition. tensioned Prestressed Concrete Applications." PCI Journal, Vol. 42, American Concrete Institute (ACI) Committee 116 (2000), Cement and No. 2, pp. 5290. Concrete Terminology. Neville, A. M. (1981), Properties of Concrete, 3rd Edition, Pittman, Lon- American Concrete Institute (ACI) Committee 237 (2007), Provisional don, 779 p. Guidelines on Self-Consolidating Concrete. 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