Self-Consolidating Concrete for Precast, Prestressed Concrete Bridge Elements (2009)

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23 This chapter presents the major conclusions of the research effort and provides suggestions for future research. 4.1 Test Methods and Material Requirements The use of proven combinations of test methods is neces- sary to reduce time and effort required for quality control of SCC used in precast, prestressed bridge elements. These methods include the components required for evaluating the deformability, passing ability, and resistance to segregation of the concrete. The most promising SCC test methods for these evaluations are: • Filling ability (slump flow and T-50); • Passing ability (J-Ring and L-box); • Filling capacity [caisson test (filling vessel)]; and • Segregation resistance (VSI, surface settlement and rate of settlement, and column segregation). Recommended acceptance values for these tests are summa- rized in Table 10. These tests are appropriate for material selec- tion and mix design as well as for quality control (QC) testing. 4.2 Material Constituents and Mix Design Based on the results derived from the factorial design, the relative influence of various mixture parameters on the mod- eled properties of SCC are summarized in Table 11. Table 12 gives recommendations for proportioning of SCC mixtures for use in precast, prestressed applications. Regarding the fresh SCC properties, the following recom- mendations and observations are made: • A w/cm should be selected to obtain the targeted stability, mechanical properties, visco-elastic properties, and dura- bility (typical w/cm for precast, prestressed applications can range between 0.34 and 0.40). • Low S/A values (e.g., 0.46 to 0.50) should be used to obtain adequate workability. • Coarse aggregate with 1⁄2 in. (12.5 mm) MSA is recom- mended to achieve adequate workability and mechanical properties. • Use of thickening-type VMA is required for SCC made with moderate and relatively high w/cm and low binder content to enhance stability and obtain homogenous in- situ properties. The use of thickening-type VMA at a low level can enhance static stability (lower column segregation index). VMA can also be used in highly stable SCC (e.g., with low w/cm) to enhance robustness. • Use of air entrainment is required for frost durability (use of air-entraining admixture will help stabilize small air bubbles). • SCC made with Type III cement and 20% Class F fly ash can exhibit better slump flow retention, higher passing ability, and higher filling capacity than SCC made with Type I/II cement. • The HRWRA demand decreases with the increase in w/cm and binder content. The use of Type III cement and 20% Class F fly ash necessitates higher HRWRA demand than that required for SCC prepared with Type I/II cement (thus resulting in lower early-age compressive strength). • Better slump flow retention can be obtained with SCC made with low w/cm because of the higher HRWRA demand required to achieve 26.0 to 27.5 in. (660 to 700 mm) slump flow. • Surface settlement of SCC increases with the increase in binder content and w/cm. • Plastic viscosity decreases with the increase in binder content and w/cm but increases slightly with the increase in S/A. • Thixotropy or structural build-up at rest of the SCC decreases with the increase in binder content and w/cm. C H A P T E R 4 Conclusions and Suggested Research

24 Table 10. Recommended test methods and target values. Table 11. Relative significance of modeled SCC parameters. Property Test method Target value D es ig n QC Filling ability Slump flow T-50 (ASTM C 1611) 23.5–29 in. (600–735 mm) 1.5–6 sec (upright cone position) J-Ring flow (ASTM C 1621) Slump flow – J-Ring flow 21.5–26 in. (545–660 mm) 0–3 in. (0–75 mm) Passing ability L-box blocking ratio (h2/h1) 0.5–1.0 Filling capacity 70%–100% Slump flow and J-Ring flow Filling capacity Slump flow and L-box tests Surface settlement Rate of settlement, 25–30 min (value can decrease to 10–15 min) – MSA of and ½ in.3 8 (9.5 and 12.5 mm) ≤ 0.27%/h (Max. settlement ≤ 0.5%) – MSA of ¾ in. (19 mm) ≤ 0.12%/h (Max. settlement of 0.3%) Column segregation (ASTM C 1610) Column segregation index (C.O.V.) ≤ 5% Percent static segregation (S) ≤ 15% Static stability VSI (ASTM C 1611) 0–1 (0 for deep elements) Air volume AASHTO T 152 4%–7% depending on exposure conditions, MSA, and type of HRWRA. Ensure stable and uniform distribution of small air voids. Binder content w/cm VMA content Binder type S/A Lo w M ed iu m H ig h Lo w M ed iu m H ig h Lo w M ed iu m H ig h Lo w M ed iu m H ig h Lo w M ed iu m H ig h HRWRA demand Slump flow retention J-Ring Slump flow – J-Ring flow L-box blocking ratio (h2/h1) Caisson filling capacity Maximum surface settlement Column segregation index Plastic viscosity Thixotropy (Ab) Form pressure 18-hour ' cf 56-day ' cf 18-hour MOE 56-day MOE 7-day flexural strength 56-day flexural strength Autogenous shrinkage at 7 days Autogenous shrinkage at 56 days Drying shrinkage after 28 days of exposure Drying shrinkage after 112 days of exposure Creep after 28 days of loading Creep after 112 days of loading Darkened areas indicate high degree of influence for the modeled mixture parameter.

Higher thixotropy can be detrimental to surface finish and advantageous to formwork pressure. • Initial relative form pressure at 3.3 ft (1 m) in height cast at 13.1 to 16.4 ft/h (4 to 5 m/h) varies between 0.80 and 1.00 of hydrostatic pressure; it increases with the increase in binder content and w/cm but decreases with the increase in S/A. • Incorporation of thickening-type VMA in the mixture could delay setting and increase the time to attain peak tem- perature, thus leading to some delay in early-age strength development. In that case, steam curing could be used to accelerate the strength development. Regarding the mechanical properties, the following rec- ommendations and observations are made: • Mechanical properties increase with the decrease in w/cm. • Increase in binder content can lead to higher 56-day com- pressive strength but to lower 18-hour MOE and 7-day flexural strength. • The increase in S/A results in lower MOE at 18 hours (steam curing) and 56 days (moist curing) and higher flex- ural strength. • SCC made with Type III cement and 20% Class F fly ash exhibits lower early-age compressive strength than that made with Type I/II cement (due to higher HRWRA demand). • SCC made with Type III cement and 20% Class F fly ash can develop higher compressive strength and MOE at 56 days but lower mechanical properties at 18 hours than for concrete made with Type I/II cement (mainly because of delayed setting resulting from greater HRWRA demand). Regarding the visco-elastic properties, the following rec- ommendations and observations are made: • The increase in binder content increases drying shrinkage and creep. • Although for a given binder content drying shrinkage is expected to increase with increased w/cm, for the derived statistical models an opposite trend appears because the drying shrinkage also includes autogenous shrinkage that decreases with the increase in w/cm. • SCC mixtures made with Type I/II cement develop less creep and shrinkage than those prepared with Type III cement and 20% Class F fly ash. However, the latter concrete has better workability and higher mechanical properties than the for- mer SCC. Therefore, use of Type III cement and 20% Class F fly ash will require reduction of binder content to ensure better overall performance. • Concrete mixtures containing high binder content and low w/cm can exhibit high autogenous shrinkage; the majority (85% to 95%) of which occurs in the first 28 days (values after 56 days can vary between 100 and 350 µstrain depend- ing on mixture composition). • Autogenous shrinkage is mostly affected by binder type and paste volume. SCC made with Type III cement and 20% Class F fly ash can develop higher autogenous shrinkage and creep than SCC made with Type I/II cement. • For a given w/cm, increasing binder content can result in higher drying shrinkage (500 and 1000 µstrain after 300 days is possible). • SCC exhibits up to 30% higher drying shrinkage at 300 days than HPC made with similar w/cm but different paste 25 Table 12. Recommendations for proportioning SCC mixtures. w/cm Binder type Binder content S/A VMA 0. 34 0. 40 I/I I II I + 2 0% fly a sh 74 2 lb /y d³ (44 0 k g/m ³) 84 3 lb /y d³ (50 0 k g/m ³) 0. 46 0. 54 0 M od er at e Filling ability retention Passing ability Filling capacity Static stability 18-hour 'cf 56-day 'cf 18-hour MOE 56-day MOE Flexural strength Autogenous shrinkage Drying shrinkage Creep Darkened areas indicate better performance for each property.

volume. More detailed information on drying shrinkage can be found in Attachment D. • Increase in S/A can lead to higher long-term drying shrinkage. • The binder type does not have significant effect on drying shrinkage but can significantly affect creep (e.g., SCC made with Type III cement and 20% fly ash exhibited higher creep than similar SCC proportioned with Type I/II cement regardless of the binder content, w/cm, S/A, and use of thick- ening-type VMA). • The w/cm does not have considerable effect on creep because of the more predominant influence of other parameters such as binder content, binder type, and S/A. • SCC exhibits up to 20% higher creep after 300 days than HPC made with similar w/cm but different paste vol- ume. More detailed information on creep can be found in Attachment D. 4.3 Code Provisions for Estimating Mechanical and Visco-Elastic Properties Mechanical Properties Material coefficients of existing prediction models were modified to provide better prediction of mechanical proper- ties of SCC for precast, prestressed concrete bridge elements. The following codes are recommended: • ACI 209 and CEB-FIP codes with modified coefficients for predicting compressive strength • Current AASHTO 2007 model for predicting elastic modulus • Current AASHTO 2007 model for estimating flexural strength Visco-Elastic Properties Creep and shrinkage strains measured in experimental factorial design were compared with values predicted by the AASHTO 2007, AASHTO 2004, ACI 209, CEB-FIP MC90, and GL 2000 (Gardner and Lockman, 2001) models. Coeffi- cients of the following models were modified to provide better prediction of visco-elastic properties for SCC: • AASHTO 2004 model for estimating drying shrinkage • AASHTO 2007 model for estimating creep 4.4 Homogeneity of In-Situ Strength and Bond to Reinforcement • Highly flowable SCC should have adequate static stabil- ity with maximum surface settlement, column segregation index, and percent static segregation of 0.5%, 5%, and 15%, respectively, particularity for deep elements. • Highly flowable SCC can develop at least 90% in-situ rela- tive compressive strength (core results) and modification factor of 1.4 for bond to horizontally embedded prestress- ing strands. More detailed information on bond to pre- stressing strands is presented in Attachment D. • Use of highly viscous SCC [plastic viscosity greater than 0.073 psis (500 Pas) or T-50 nearing 6 seconds (obtained from upright cone position)] should be avoided to ensure adequate self-consolidation. 4.5 Structural Performance of AASHTO-Type II Girders The following conclusions and observations are based on the construction and testing of the full-scale precast, pretensioned girders: • With the casting from only a single location at midspan of the 31 ft (9.44 m) long girders, no visible segregation was observed and fewer “bug holes” were observed in the SCC concrete than in the HPC. • The transfer lengths were similar for the four concrete mix- tures and were considerably shorter than the values given in the 2007 AASHTO LRFD Specifications and the ACI 318-05 code. • At time of prestress release at 18 hours, the coefficients on the square root of the compressive strength used to determine the modulus of elasticity for the SCC mixtures were about 4% and 11% lower than those for the HPC mixtures. • Due to the low elastic modulus and greater drying shrink- age, greater elastic shortening losses and greater long-term losses of prestress occurred, resulting in smaller cambers for the SCC girders. • The cracking moments for the SCC girders and the com- panion HPC girders were similar, and the uncracked and cracked stiffnesses for all four girders were very similar. • The cracking shears for all four girders were similar. • The four girders failed in shear after developing a significant number of wide shear cracks; shear crack widths just before failure were greater than 0.24 in. (6 mm). The failure shears exceeded the nominal shear resistances predicted using the approach given in 2007 AASHTO LRFD Specifications, probably because of the strength and stiffness of the top and bottom flanges of the girders. • The flexural resistances of the HPC girders exceeded that predicted using the 2007 AASHTO LRFD Specifications. • The flexural resistances of the SCC girders were within 1.5% of the flexural resistance calculated using the approach pro- vided in the 2007 AASHTO LRFD Specifications. • The HPC girders exhibited higher ductilities than the cor- responding SCC girders. • The lower shear resistance and lower ductility experienced by the SCC girders are probably due to the lower volume 26

of coarse aggregate that reduces aggregate interlock and results in lower energy absorption capability on the sliding shear failure plane. 4.6 Recommendations for Future Research The following recommendations are made for future research related to SCC used in precast, prestressed applications. Constructability • Short term – Validation of the recommended workability character- istics proposed in Table 13 using full-scale prestressed girders. – Evaluation of the effect of horizontal flow distance and free-fall distance of concrete in the formwork on segre- gation potential and in-situ properties of the hardened concrete. – Evaluation of the effect of concrete workability, place- ment techniques, casting rate, and form release material on surface finish of SCC. – Evaluation of the effect of rheological properties of SCC, delivery period, and delay between successive place- ment on cold joint formation and surface defects. Test Methods • Short term – Development of a reliable test method to determine bond-strength and modification factor to prestressing strands. • Medium term – Development of a QC test method to evaluate plastic viscosity of SCC. 27 Table 13. Workability values of SCC used in precast/prestressed applications. Slump flow (ASTM C 1611/C 1611 M-05 ) J-Ring (Slump flow– J-Ring flow) (ASTM C 1621) L-box blocking ratio (h2/h1) Caisson filling capacity Relative values 23 .5 -2 5 in . 25 -2 7. 5 in . 27 .5 -2 9 in . 3- 4 in . 2- 3 in . ≤ 2 in . 0. 5- 0. 6 0. 6- 0. 7 ≥ 0. 7 70 % -7 5% 75 % -9 0% ≥ 90 % Low Medium High Reinforce- ment density Small Moderate Congested Shape intricacy Shallow Moderate Deep Depth Short Moderate Long Length Thin Moderate El em en t c ha ra ct er ist ic s Thick Thickness Low Medium High Coarse aggregate content 1 in. = 25.4 mm Shaded zones indicate suggested workability characteristics. All SCC mixtures must meet requirements for static stability.

– Development of a QC test method to determine struc- tural build-up and evaluate its effect on consolidation level, surface quality, and cold joint formation. – Development of a dynamic stability test to assess segre- gation resistance of SCC subjected to horizontal flow and free-fall into the formwork. Material Selection and Mix Design • Short term – Evaluation of the effect of shrinkage-reducing admix- tures on shrinkage and creep of SCC used in precast, prestressed applications (relevant because of the higher drying shrinkage and creep of SCC). – Investigation of the compatibility issues between chemi- cal admixtures (in particular HRWRA, VMA, shrinkage- reducing admixtures, and air-entraining admixtures) on flow properties and strength development of SCC. – Determination of key factors affecting robustness of SCC and ways to enhance it in order to ensure consis- tent concrete quality and productivity. • Medium term – Extension of the modeled region of the factorial design beyond the range of −1 to +1 as well as incorporating other parameters in order to take into consideration the quadratic effect of various parameters in the derived models, in particular those of the visco-elastic proper- ties and formwork pressure. – Investigation of the influence of mixture proportioning and material characteristics that were not considered in this research {e.g., MSA [3⁄8 and 1⁄2 in. (9.5 and 12.5 mm)], combined sand and coarse aggregation content and gra- dation, sand type [crushed vs. natural], and paste volume} on workability, mechanical properties, and visco-elastic properties of SCC. – Investigation of the effect of finely ground limestone fillers on fresh and hardened concrete properties of SCC, in particular stability, temperature rise, strength development, and visco-elastic properties. Structural Performance • Short term – Determination of modification factor (top-bar effect) of reinforcing bars in structural elements cast with SCC of different workability characteristics (especially static stability) and element depth. – Evaluation of the effect of SCC on transfer length with simple transfer length specimens. – Evaluation of the influence of coarse aggregate content and MSA on aggregate interlock (direct shear “push-off” specimens) and companion tests to investigate the shear behavior of simple SCC elements (non-prestressed rec- tangular beam specimens). • Medium term – Evaluation of key engineering properties, durability characteristics, and structural performance of SCC with high-release strength [e.g., 7,000 psi (48 MPa)] and design compressive strength greater than 12,000 psi (83 MPa). – Evaluation of the use of steel and synthetic fibers in SCC mixtures. – Extensive testing and evaluation of full-scale specimens to provide definitive information on structural per- formance, including the contribution of the presence of top and bottom flanges to the shear resistance. 28