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1 SUMMARY Self-Consolidating Concrete for Precast, Prestressed Concrete Bridge Elements Research Significance The use of self-consolidating concrete (SCC) in the United States has been hindered by concerns about certain design and construction issues that are perceived to influence the performance and structural integrity of bridge structures. The lack of standard tests and training has also hampered widespread use of SCC. Limited and non-systematic information regarding properties of hardened SCC mixtures typically used in precast, prestressed structural applications is available to bridge engineers. There is also uncertainty about key engineering properties of SCC, such as stability, strength development, creep and shrinkage, and durability. Because of the variety of materials em- ployed in the United States in the precast industry (cement type, supplementary cementi- tious materials, and specialty admixtures), there is a need to better understand the influence of these materials combinations on the properties of fresh and hardened concrete used in prestressed concrete construction and to identify reliable test methods and performance specification for mix design and quality control of SCC. NCHRP Project 18-12 was conducted to address these needs. Project Objectives and Scope The objective of this research was to develop guidelines for use of SCC in precast, prestressed bridge elements, including recommended changes to the AASHTO Load and Resistance Factor Design (LRFD) Bridge Design Specifications [2004, 2007] and LRFD Bridge Con- struction Specifications , hereafter referred to as the AASHTO LRFD Specifications. Such guidelines will provide highway agencies with the information necessary for considering SCC mixtures that are expected to produce a uniform product, expedite construction, and yield economic and other benefits. To accomplish this objective, the research included work to: · Develop material properties and performance criteria for SCC used for precast, prestressed concrete bridge elements; · Evaluate key engineering properties, durability characteristics, and structural performance of such concrete; and · Propose relevant changes to AASHTO LRFD Specifications. Specifically, this project: · Developed SCC mixtures that can be produced consistently in the field; · Identified test methods for use in SCC mix design;
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2 · Identified test methods for use for quality control in precasting plants; · Developed specifications and criteria for SCC mixtures for precast, prestressed concrete bridge elements; · Determined the influence of mix parameters, such as raw materials, mixture proportioning, mixing, production, placement, and characteristics of the cast element; · Compared performance of precast, prestressed concrete elements made with SCC with those made with conventional high-performance concrete; · Prepared guidelines for testing, proportioning, and casting SCC bridge elements; and · Investigated applicability of current models recommended by AASHTO LRFD Specifications and suggested revisions whenever applicable. Overview of the Project The influence of different variables on the properties of SCC was evaluated in two parts. In the first part, a parametric study was undertaken to evaluate the influence of binder type, watercementitious material ratio (w/cm), and coarse aggregate type and nominal size on key workability characteristics and compressive strength development of SCC mixtures desig- nated for the construction of precast, prestressed AASHTO girders. Nonair-entrained SCC mixtures were prepared to evaluate workability, rheology, workability loss with time, stability, and strength development. The mixtures were prepared using crushed aggregate and gravel of three different nominal maximum sizes [3/4, 1/2, 3/8 in. (19, 12.5, and 9.5 mm)]; two w/cm (0.33 and 0.38); and three binder compositions (Type I/II cement, Type III cement with 30% slag replacement, and Type III cement with 20% Class F fly ash replacement). Air- entrained SCC mixtures with low w/cm were also prepared to highlight the effect of air entrainment on workability and strength development of SCC. The effect of fluidity level on key workability responses was also investigated. SCC mix- tures were proportioned to have low slump flow values of 23.5 to 25.0 in. (600 to 635 mm) and high slump flow of 28.0 to 30.0 in. (710 to 760 mm) compared with normal slump flow of 26.0 to 27.5 in. (660 to 700 mm). The repeatability of workability tests was also evaluated using SCC mixtures with different slump flow values of 25.0 and 27.5 in. (635 and 700 mm). Based on results of the parametric study, an experimental fractional factorial design was conducted to evaluate and model the effect of mixture proportioning and material char- acteristics on properties critical to the performance of precast, prestressed AASHTO girders. The study included 16 nonair-entrained SCC mixtures and enabled the modeling of plastic viscosity, thixotropy, filling ability, passing ability, filling capacity, static stability, formwork pressure, setting time, compressive and flexural strengths, elastic modulus, auto- genous shrinkage, drying shrinkage, and creep. The air-void system and frost durability were evaluated for selected mixtures. Relevant modifications to current code provisions for mechanical properties, drying shrinkage, and creep were proposed to allow better pre- diction of mechanical and visco-elastic properties of SCC designated for precast, prestressed bridge elements. The effect of SCC static stability and plastic viscosity of SCC on the distribution of pull- out bond strength of horizontally embedded prestressing strands was investigated. Bond strength characteristics and core compressive strength of wall elements cast with SCC were compared with those of similar elements cast with high-performance concrete (HPC) of nor- mal consistency subjected to mechanical vibration. Static stability limits necessary to secure homogenous in-situ properties were recommended. The structural performance of four full-scale AASHTO-Type II precast, pretensioned girders constructed with SCC was investigated. Two girders were constructed with SCC of 8,000 and 10,000 psi (55 and 69 MPa) compressive strength and two girders with HPC of
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3 similar strengths. In particular, the constructability, temperature variations, flexural crack- ing, shear cracking, shear strength, transfer length, and other design issues of precast, pre- tensioned girders made with SCC were evaluated. Based on the results of the material testing and the results of testing the full-scale girders, changes to the AASHTO LRFD Specifications were suggested. Research Findings The major findings of this research are summarized as follows: · The slump flow, J-Ring flow, L-box blocking ratio, and filling capacity tests provide good lev- els of single-operator and multiple-operator repeatability and are recommended for the de- sign and quality control of SCC. · A filling capacity test (the caisson filling capacity) is recommended to evaluate the ability of SCC to fill densely reinforced sections. Combinations of filling ability and passing ability tests are also proposed to estimate the filling capacity of SCC. · A surface settlement test is recommended to evaluate the static stability of SCC. This test re- flects the overall consolidation of plastic concrete, which combines segregation, internal and external bleeding, and loss of air. The rate of surface settlement after 15 minutes can be used to estimate the maximum surface settlement that occurs shortly before the initial setting of concrete. · Concrete mixtures containing high binder content and low w/cm have been shown to develop high autogenous shrinkage, which occurs mostly in the first 28 days of age (85% to 95% of its ultimate values). Autogenous shrinkage of SCC for precast, prestressed applications can vary between 100 and 350 µstrain, depending on mixture composition. · Investigated SCC mixtures have been shown to develop drying shrinkage and creep up to 30% and 20% higher, respectively, after 300 days than those for HPC made with similar w/cm but different paste volume (more detailed information on drying shrinkage and creep can be found in Attachment D). · Based on the comparison of various code provisions, the American Concrete Institute (ACI) 209 and CEB-FIP MC90 models, modified with material coefficients applicable to binder types used in SCC (Type I/II cement and Type III cement with 20% fly ash replacement), are rec- ommended for predicting compressive strength. Similarly, modification to the AASHTO 2007 code equations for predicting elastic modulus and flexural strength is suggested. · Modifications to the AASHTO 2004 and AASHTO 2007 models for drying shrinkage and creep, respectively, are suggested for SCC. Otherwise, the CEB-FIP MC90 model can be used to esti- mate drying shrinkage. · Stable SCC can lead to more homogenous in-situ properties than HPC of normal consistency subjected to mechanical consolidation. A modification factor for bond to prestressing strands of 1.4 can be secured when the static stability of SCC is limited to 0.5%. Use of highly viscous SCC [plastic viscosity greater than 0.0725 psis (500 Pas) or T-50 close to 6 seconds obtained from upright cone position] may lead to inadequate self-consolidation and reduction in bond between concrete and reinforcement (more detailed information on bond to prestressing strands is presented in Attachment D). · Tests on four full-scale AASHTO-Type II girders indicated that the greater shrinkage of SCC (compared with that of HPC) can lead to larger prestressing losses and smaller cambers. SCC and HPC girders of similar compressive strengths exhibit similar transfer lengths, flexural cracking moments, and cracking shears. The shear resistances and displacement ductilities of the SCC girders are less than those of similar HPC girders. The lower displacement ductilities of SCC girders are not expected to have a major effect on performance because all of the specimens
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4 were purposely designed to be "shear critical" and the shear levels reached were considerably above the AASHTO predictions. Based on the findings of this research, some requirements for workability of SCC used in precast, prestressed bridge elements are suggested. Guidelines for material selection and mixture proportioning of SCC for precast, prestressed applications are provided. This in- formation pertains to the effect of w/cm, binder type, and maximum size of aggregate (MSA) and type on workability and early-age strength development.