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10 · Current AASHTO 2007 model for predicting elastic limits are especially critical in deep elements. Such SCC can modulus develop at least 90% in-situ relative compressive strength · Current AASHTO 2007 model for estimating flexural (core results) and modification factor of 1.4 for bond to strength horizontally embedded prestressing strands. · Avoid the use of highly viscous SCC (plastic viscosity The proposed coefficients can be found in Attachment D. greater than 0.073 psis (500 Pas) or T-50 nearing 6 seconds obtained from upright cone position) to ensure adequate 1.5 Validation of Code Provisions to self-consolidation. Estimate Visco-Elastic Properties Creep and shrinkage strains measured in experimental 1.7 Structural Performance factorial design were compared with values predicted by the The structural performance of full-scale precast, prestressed AASHTO 2007, AASHTO 2004, ACI 209, CEB-FIP 1990, and bridge girders constructed with SCC and HPC was investi- GL 2000 (Gardner and Lockman, 2001) models. Coefficients gated. Two SCC and two HPC mixtures with target 56-day of existing models were modified to provide better prediction compressive strengths of 8,000 and 10,000 psi (55 and 69 MPa) of visco-elastic properties for SCC. The following models are were used to cast four full-scale AASHTO-Type II girders. recommended: Constructability, temperature variations, transfer length, cam- ber, flexural cracking, shear cracking, and shear strengths of the · AASHTO 2007 model with suggested modifications to girders were evaluated. More details on the construction and estimate creep testing of the girders are given in Attachment D. · AASHTO 2004 model with suggested modifications to pre- The following findings and observations are made based dict drying shrinkage on the results of these tests: · Current CEB-FIP MC90 model can be used to predict dry- ing shrinkage · With the casting from a single location at midspan of the 31-ft (9.44-m) long girders, no visible segregation was The proposed coefficients can be found in Attachment D. observed in any of the girders. · The maximum temperature rise during the steam-curing 1.6 Homogeneity of In-Situ Strength operation satisfied the maximum temperature limit of and Bond to Reinforcement 150°F (65°C). · There were fewer "bug holes" in the SCC girder than in the Six 60.6 × 84.6 × 7.9 in. (1540 × 2150 × 200 mm) wall ele- HPC girder. ments were cast using a reference HPC concrete of normal · The target 18-hour compressive strengths, required for consistency and five SCC mixtures of different plastic vis- prestress release, were met for the two SCC girders. cosity and static stability levels. The SCC mixtures were pro- · The transfer lengths for the four girders were similar and portioned to yield slump flow consistency of 26.7 ± 0.7 in. considerably shorter than the transfer length values given (680 ± 15 mm) and minimum caisson filling capacity of 80%. in the AASHTO LRFD Specifications  and the ACI The surface settlement of the SCC mixtures ranged between 318-05 code. 0.30% and 0.62% and that of the HPC was 0.23%. · At time of prestress release at 18 hours, the coefficients on Despite the high fluidity of SCC, stable concrete can lead the square root of the compressive strength used to deter- to more homogenous in-situ properties than HPC of normal mine the modulus of elasticity for the two SCC mixtures consistency subjected to mechanical vibration. Although the were about 4% and 11% lower than those for the HPC SCC mixtures exhibited VSI values of 0.5 to 1 and caisson fill- mixtures. ing capacity higher than 80%, the tested mixtures developed · The drying shrinkage for the two SCC mixtures was about various levels of uniformity of core compressive strength and 20% greater than that for the comparable HPC mixtures. pull-out bond strength results. The homogeneity of in-situ · In comparison to HPC girders, the SCC girders exhibited properties was shown to vary with plastic viscosity and static smaller cambers because of the greater elastic shortening stability determined from the surface settlement test. and long-term losses of prestress due to the lower elastic Recommendations to ensure homogenous in-situ proper- modulus and greater drying shrinkage. ties are summarized as follows: · The cracking moments for the SCC girders and the com- panion HPC girders were similar. · Use highly flowable SCC with adequate static stability, · The uncracked and cracked stiffnesses for all four girders maximum surface settlement of 0.5%, column segregation were very similar. index of 5%, and percent static segregation of 15%. These · The cracking shears for all four girders were similar.
OCR for page 11
11 · All four girders failed in shear after developing a significant · The flexural resistances of the SCC girders were within number of wide shear cracks; crack widths just before fail- 1.5% of the theoretical flexural resistance using the approach ure were greater than 0.24 in. (6 mm). provided in the AASHTO LRFD Specifications . · The stirrups developed significant strains beyond strain · The lower ductilities and lower shear resistance of SCC hardening and ruptured at failure. girders compared with the corresponding HPC girders are · The failure shears exceeded the nominal shear resistances due to the lower volume of coarse aggregate that reduces of the girders calculated using the approach given in the aggregate interlock and results in a lower energy absorp- AASHTO LRFD Specifications , probably because of tion capability on the sliding shear failure plane. the strength and stiffness of the top and bottom flanges of the AASHTO girders. The structural performance tests of two SCC girders and · The flexural resistances of the HPC girders exceeded the two HPC girders have highlighted a number of differences nominal resistances calculated using the AASHTO LRFD that could affect design. However, more research is required Specifications . to support any specific changes to the design specifications.