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

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5The major findings of the research study are described in this chapter. Further details of the experimental work, analyses of the data, and conclusions are presented in Attachment D. 1.1 Test Methods and Mixture Requirements Various test methods are used to assess the workability characteristics of SCC. The most promising test methods that are relevant for the fabrication of precast, prestressed concrete bridge elements (details of test are given in Attachment D) are: • Filling ability: slump flow and T-50 (ASTM C 1611); • Passing ability: J-Ring (ASTM C 1621) and L-box; • Filling capacity: caisson test (filling vessel); and • Segregation resistance: column segregation (ASTM C 1610), visual stability index (VSI), surface settlement, and rate of settlement. The use of a combination of test methods is necessary to reduce the time and effort required for quality control in the precasting plant. The caisson filling capacity test (modified from initial value) is found to be promising to evaluate both the filling ability and passing ability of SCC. This test can be especially useful for SCC cast in densely reinforced sections. A mean caisson filling capacity value of 80% (75% to 90%) is considered as a lower limit for precast, prestressed concrete applications. A lower limit of 70% can be tolerated for rela- tively simple elements. Values greater than 90% can be secured for highly flowable and stable mixtures. The L-box blocking ratio (h2/h1) index, J-Ring flow, or the difference between slump flow and J-Ring flow can be com- bined with slump flow testing to evaluate the filling capacity of SCC. The recommended combined test methods for evalu- ating the filling capacity of SCC are (1) slump flow and L-box blocking ratio (h2/h1) and (2) slump flow and J-Ring flow. SCC mixtures suitable for use in precast, prestressed concrete girders should exhibit slump flow of 23.5 to 29 in. (600 to 735 mm), L-box blocking ratio (h2/h1) greater than 0.5, J-Ring flow of 21.5 to 26.0 in. (545 to 660 mm), filling capacity greater than 70%, and a difference in slump flow and J-Ring flow values lower than 4 in. (100 mm). Regardless of the MSA, stable SCC should develop a col- umn segregation index (C.O.V.) less than 5% and percent static segregation lower than 15. The recommended limits for surface settlement depend on the MSA. SCC proportioned with 3⁄4 in. (19 mm) and 1⁄2 or 3⁄8 in. (12.5 or 9.5 mm) MSA should have maximum rates of settlement at 30 minutes of 0.12%/h and 0.27%/h, respectively. SCC mixtures investigated in this study developed yield stress values varying between 0.00145 and 0.01885 psi (10 and 130 Pa). SCC made with crushed aggregate should develop plas- tic viscosity of 100 to 225 Pas at the time of casting to ensure adequate passing ability and static stability. This range can be 0.0145 to 0.0326 psis (100 to 400 Pas) for SCC made with gravel having 1⁄2 in. (12.5 mm) MSA. The lower limit of plastic viscosity is necessary to secure a maximum rate of settlement of 0.27%/h at 30 minutes of testing and a maximum C.O.V. of 5%. The upper limit of plastic viscosity of 250 and 400 Pas is necessary for the SCC with slump flow consistency of 26.0 to 27.5 in. (660 to 700 mm) to achieve adequate passing ability (minimum L-box blocking ratio of 0.5). Based on the proper- ties of SCC made with different viscosity levels cast in experi- mental wall elements, plastic viscosity higher than 500 Pas should be avoided to ensure proper self-consolidating proper- ties and homogeneity distribution of in-situ properties. 1.2 Selection of Concrete Constituents Effect of Binder Type The binder content and composition were shown to have direct influence on high-range water-reducing admixture (HRWRA) demand, fluidity retention, temperature rise, early- age strength development, and mechanical properties at 28 and C H A P T E R 1 Findings

56 days. Among three binder types used in the parametric study, SCC mixtures made with Type III cement and 20% Class F fly ash exhibited better workability than that for simi- lar mixtures prepared with Type I/II cement or Type III cement and 30% slag. SCC containing 20% Class F fly ash developed high fluidity retention, high passing ability and filling capacity, as well as a high level of static stability. The concrete propor- tioned with Type III cement and 30% slag exhibited relatively low passing ability [difference between slump flow and J-Ring flow diameters larger than 4 in. (100 mm)]. The evaluated mixtures developed similar compressive strengths after 18 hours of steam curing regardless of binder type. However, concrete made with Type III cement and 20% Class F fly ash developed slightly higher 56-day moist- cured compressive strength than that of concrete made with Type I/II cement. Based on this evaluation, a mixture of Type III cement and 20% Class F fly ash was selected for the experimental evalua- tion that was performed to model the performance of SCC for precast and prestressed girder elements. Effect of Type and Maximum Size of Coarse Aggregate The maximum size of coarse aggregate and coarse aggre- gate type had a marked effect on passing ability, filling capac- ity, and static stability of SCC. The MSA should be selected with consideration of the minimum clear spacing between the reinforcing steel bars and prestressing strands, the cover over the reinforcement, and the geometry of the elements to be cast. The reduction in MSA is required to enhance sta- bility. From a workability point of view, SCC mixture made with crushed aggregate of 3⁄8 in. (9.5 mm) MSA exhibited greater passing ability [difference between slump flow and J-Ring flow diameters lower than 2 in. (50 mm)] and higher filling capacity (caisson filling capacity higher than 90%). In particular, mixtures containing 3⁄4 in. (19 mm) MSA exhib- ited a relatively low level of filling capacity (caisson filling capacity less than 70%) and relatively low resistance to seg- regation (column segregation index higher than 5%). The SCC mixtures made with 3⁄8 in. (9.5 mm) MSA exhibited surface settlement and column segregation index values similar to those for mixtures made with larger MSA. As in the case of fresh properties, SCC mixtures made with crushed aggregate of 3⁄8 in. (9.5 mm) MSA developed similar or higher compressive strengths after 18 hours of steam cur- ing and 56 days of moist curing than those for mixtures made with 1⁄2 or 3⁄4 in. (12.5 and 19 mm) MSA. SCC proportioned with gravel developed better passing ability and filling capac- ity than similar concrete made with crushed aggregate of the same MSA [1⁄2 in. (12.5 mm)]. The former had high passing ability [h2/h1 greater than 0.7 and difference between slump flow and J-Ring flow diameters less than 2 in. (50 mm)] and high filling capacity (caisson filling capacity greater than 90%). Both SCC types exhibited similar segregation resist- ance (column segregation index of 2% to 5%). However, mixtures made with gravel developed lower compressive strength and modulus of elasticity (e.g., up to 25% and 16% lower, respectively, under moist curing conditions at 56 days) than those for mixtures made with crushed aggregate of the same MSA. In terms of hardened concrete properties, mix- tures made with crushed aggregate exhibited better overall performance than those made with gravel. Effect of w/cm and Air Entrainment In general, SCC mixtures with 0.38 w/cm exhibited better workability than those with 0.33 w/cm in terms of passing abil- ity, filling capacity, and fluidity retention. However, SCC mix- tures made with 0.33 w/cm developed greater static stability and higher 18-hour and 56-day compressive strengths under steam-cured and moist-cured conditions. Also, air-cured SCC mixtures made with 0.33 w/cm exhibited lower 18-hour com- pressive strength than the latter concrete under the same cur- ing regime, possibly due to the relatively higher dosage of HRWRA necessary to achieve the target slump flow. No sig- nificant difference was found in the 18-hour modulus of elas- ticity between the 0.33 and 0.38 w/cm mixtures. SCC with 0.38 w/cm will attain a minimum release compres- sive strength of 5,000 psi (34.5 MPa) and ultimate compressive strength of 8,000 psi (55.2 MPa). Such concrete can be used for casting highly reinforced and restricted sections because of its good filling capacity. Higher strength may require the use of mixtures with lower w/cm (e.g., 0.32 to 0.35). In general, air-entrained SCC exhibited superior passing ability and filling capacity than SCC without air entrainment because of its lower viscosity and greater paste content. How- ever, air-entrained concrete developed lower static stability and lower compressive strength and modulus of elasticity, both under steam-curing and moist-curing conditions. Effect of Fluidity of SCC Workability responses and mechanical properties of SCC designed for relatively high, medium, and low slump flow values of 28 to 30 in. (710 to 760 mm), 25 to 28 in. (640 to 710 mm), and 23.5 to 25 in. (600 to 640 mm), respectively, are compared. SCC mixtures with low and medium slump flow had similar levels of passing ability (medium), filling capac- ity (medium), and resistance to surface settlement (high). Mixtures with high fluidity (slump flow) exhibited high pass- ing ability and filling capacity, but relatively medium to low static stability. As expected, SCC with high fluidity developed lower compressive strengths at 18 hours of steam curing and 6

56 days of moist curing and lower 18-hour modulus of elastic- ity than similar concrete having low slump flow and lower HRWRA content. In general, SCC mixtures with medium flu- idity level are recommended for casting precast, prestressed concrete girder elements. Effect of Viscosity-Modifying Admixture For a given slump flow, SCC designed with low to moderate dosage thickening-type viscosity-modifying admixture (VMA) had greater HRWRA demand. Higher dosage of HRWRA improves retention of workability but reduces early-age development of mechanical properties. The incorporation of thickening-type VMA considerably improves static stability. In general, SCC designed with 0.40 w/cm and low HRWRA content exhibited better static stability when the thickening- type VMA was incorporated. SCC containing thickening-type VMA had lower early-age mechanical properties. In general, the use of VMA is not necessary in SCC propor- tioned with low w/cm and high binder content because such concrete can develop proper stability. On the other hand, SCC made with relatively high w/cm and/or low binder content should incorporate a VMA to secure adequate stability and robustness. It is important to note that the incorporation of a low dosage of VMA can enhance robustness, even in SCC made with relatively low w/cm. Guidelines for Materials Selection and Mix Design Based on the results of the parametric investigation, guide- lines for the selection of material constituents, mixture pro- portioning, and fluidity level necessary to ensure adequate performance of plastic and hardened SCC properties for pre- cast, prestressed concrete bridge elements are recommended. SCC mixtures proportioned with w/cm of 0.33, crushed aggre- gate with 1⁄2 in. (12.5 mm) MSA, and Type III cement with 20% Class F fly ash can develop the properties required for this application. 1.3 Factorial Design to Model Fresh and Hardened Concrete Properties Factorial design was carried out to model the effect of mix- ture parameters and material properties on workability char- acteristics, mechanical properties, and visco-elastic properties of SCC. The modeled parameters included binder content (BC), binder type (BT), w/cm, dosage of thickening-type VMA, and sand–to–total aggregate volume ratio (S/A). This design enabled the evaluation of the five selected parameters with each evaluated at two distinct levels of −1 and +1 (minimum and maximum levels). In total, 16 SCC mixtures were used in the factorial design. The derived models that yielded high correlation coeffi- cients (R2) are summarized in Tables 1 to 3. All factors are expressed in terms of coded values: • Coded BC = (absolute BC − 793) / 50 • Coded w/cm = (absolute w/cm − 0.37) / 0.03 • Coded VMA = (absolute VMA − 0.75) / 0.75 • Coded S/A = (absolute S/A − 0.50) / 0.04 The estimated values in the models (e.g., −1.06, −0.33, +0.33, etc. in the HRWRA demand model) reflect the level of significance of each response. A negative estimate signifies that an increase in the modeled parameter can lead to a reduction in the measured response. Based on the derived statistical models, the following obser- vations can be made for proportioning SCC mixtures for use in precast and prestressed bridge elements: • Fresh concrete properties – Typical w/cm for precast, prestressed applications can range between 0.34 and 0.40. The selected value should secure the targeted stability, mechanical properties, visco-elastic properties, and durability requirements. – 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 that made with Type I/II cement. – HRWRA demand decreases with the increase in w/cm and binder content. The higher HRWRA demand required for SCC made with Type III cement and 20% Class F fly ash than that required for SCC prepared with Type I/II cement can reduce early-age compressive strength if the concrete is not heat cured. For steam-cured concretes, no difference in 18-hour compressive strength between SCC made with either type of cements should be expected. – Better slump flow retention can be obtained with SCC made with a lower w/cm because of the higher HRWRA demand. – A low S/A value (e.g., 0.46 to 0.50) will result in ade- quate workability. – Coarse aggregate with 1⁄2 in. (12.5 mm) MSA is rec- ommended. – VMA should be used in SCC made with relatively high w/cm and/or low binder content to secure stability and homogenous in-situ hardened properties. The use of thickening-type VMA at low dosage can enhance static stability. VMA can also be used in stable SCC (e.g., low w/cm) to enhance robustness. – Incorporation of thickening-type VMA can delay setting and the elapsed time to attain peak temperature. 7

– Use air-entraining admixture where required for frost durability. It is important to note that the use of polycarboxylate-based HRWRA can lead to air entrain- ment, but it does not necessarily produce an adequate air-void system to secure frost durability. – 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. Higher thixotropy can be detrimental to surface finish and advantageous to formwork pressure. 8 Table 1. Derived statistical models for fresh concrete [slump flow  26.8  0.8 in. (680  20 mm)]. Table 2. Derived statistical models for mechanical properties. Modeled response Derived equations R² [HRWRA demand] 0.5 (fl oz/cwt) 4.76 – 1.06 w/cm – 0.33 BC + 0.33 BT + 0.11 VMA + 0.13 (w/cm · BT) 0.97 Filling ability Slump flow loss (in.) 0.16 – 0.84 BT – 0.57 BC + 0.42 w/cm+ 0.16 S/A – 0.54 (BT · S/A) + 0.49 (BC· w/cm) – 0.42 (BC · BT) 0.84 [L-box blocking ratio] 1.4 0.69 + 0.13 w/cm + 0.12 BC – 0.13 (BC · w/cm) 0.93 Passing ability [J-Ring flow] 3 (in.) 16,329 + 1,344 w/cm + 1,324 BC + 814 BT – 729 S/A – 465 VMA – 1,140 (BC · BT) – 1,136 (BC · w/cm ) + 824 (BT · S/A) – 650 ( w/cm · BT) – 465 ( w/cm · S/A) + 351 (VMA · S/A) – 291 (BC · S/A) 0.99 Caisson filling capacity (%) 92 + 4.38 BC + 3.75 w/cm + 3.63 BT – 3.63 (BT · w/cm) – 2.63 (w/cm· BT) – 2.50 (BC · BT) 0.92 Filling capacity Slump flow – J-Ring flow (in.) 1.42 – 0.70 BC – 0.63 BT – 0.55 w/cm + 0.26 S/A + 0.63 (BC · w/cm) + 0.50 (BC · BT) + 0.40 (w/cm · BT) – 0.26 (BT · S/A) 0.94 [Surface settlement] 0.5 (%) 0.677 + 0.037 w/cm + 0.036 BC – 0.024 BT 0.86 Stability Column segregation (C.O.V.) 3.25 – 0.30 BC – 0.61 (BC · BT) + 0.44 (BT · S/A) + 0.42 (w/cm · BT) – 0.39 (BC · VMA) – 0.36 VMA + 0.30 (w/cm · S/A) 0.89 Plastic viscosity (Pa·s) 298 – 133.4 w/cm– 105.3 BC + 53.7 S/A + 49.7 (BT · w/cm) – 27.6 (w/cm · S/A) 0.93 Thixotropy (A b ) (J/m 3 ·s) 586 – 323.4 w/cm – 181.8 BC + 71.1 (BC · w/cm) 0.95 Rheology and formwork pressure Initial form pressure at 3.3 ft (1 m) (K 0 ) 0.90 + 0.027 BC + 0.027 w/cm – 0.014 S/A – 0.023 (BC · w/cm) – 0.013 (BT · w/cm) + 0.11 (S/A · w/cm) 0.96 Property Age Derived equations R² 18 hours 4,752 – 293 w/cm – 111 BT – 81 VMA + 153 (w/cm · BT) – 128 (VMA · S/A) – 97 (w/cm · VMA) 0.96 Compressive strength (psi) 56 days 9,176 – 773 w/cm + 290 BT + 220 BC – 368 (BC · w/cm) 0.87 18 hours 4,419 – 268 w/cm – 103 BT – 86 S/A – 78 BC – 158 (BC · w/cm) – 96 (BT · S/A) 0.89 Modulus of elasticity (ksi) 56 days 5,554 – 311 w/cm – 166 S/A + 69 BT + 79 (BC · BT) 0.87 7 days 1,036 + 123 S/A – 90 BC – 58 w/cm – 126 (BC · w/cm) 0.76 Flexural strength (psi) 56 days 1,128 – 110 w/cm + 48 S/A + 35 (BC · BT) 0.83

– 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. The relative pressure increases with the increase in binder content and w/cm but decreases with the increase in S/A. • Mechanical properties – Mechanical properties, including compressive strength, modulus of elasticity (MOE), and flexural strength, increase with the decrease in w/cm. – Increase in binder content can lead to higher 56-day compressive strength but 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), but leads to higher flexural strength. – 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 those for concrete made with Type I/II cement mainly because of delayed setting resulting from greater HRWRA demand. • Visco-elastic properties – The increase in binder content increases drying shrink- age and creep. – Theoretically, for a given binder content, drying shrink- age increases with increase in w/cm; however, the derived statistical models show an opposite trend because dry- ing 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 con- crete has better workability and higher mechanical prop- erties than the former 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 values of autogenous shrink- age, most of which occurs in the first 28 days and can vary between 100 and 350 µstrain. – 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 that for concrete with Type I/II cement. – For a given w/cm, SCC made with high binder content can exhibit high drying shrinkage that can range between 500 and 1000 µstrain after 300 days. – SCC exhibits 5% to 30% higher drying shrinkage at 300 days than that of HPC made with similar w/cm (more detailed information on drying shrinkage can be found in Attachment D). – The increase in S/A can lead to higher long-term drying shrinkage. – The binder type does not have significant effect on dry- ing shrinkage but can significantly affect creep. SCC made with Type III cement and 20% Class F fly ash exhibited higher creep compared with similar SCC proportioned with Type I/II cement, regardless of the binder content, w/cm, S/A, and use of VMA. – The w/cm does not have considerable effect on creep because other parameters (binder content, binder type, and S/A) have more predominant influence on creep. – SCC exhibited 10% to 20% higher creep after 300 days than that for HPC made with similar w/cm (more detailed information on creep can be found in Attachment D). 1.4 Validation of Code Provisions to Estimate Mechanical Properties Coefficients of prediction models in current codes and pro- cedures were modified to provide better prediction of mechan- ical properties of SCC for precast, prestressed concrete bridge elements. The following codes and models are recommended: • ACI 209 and CEB-FIP codes with suggested changes to coefficients for predicting compressive strength 9 Table 3. Derived statistical models for visco-elastic properties. Property Age Derived equations R² 7 days 134 – 42.4 w/cm + 37.4 BT – 21.6 (BC · w/cm) – 20.1 (w/cm · BT) –15.9 (BC · BT) 0.96Autogenous shrinkage (µstrain) 56 days 201 + 67.1 BT – 40.6 w/cm – 18.8 (BC · w/cm) + 17.8 (BC · S/A) 0.93 28 days 308 – 71.1 w/cm + 35 BC + 48.4 (w/cm · VMA) + 30.8 (VMA · BT) 0.78Drying shrinkage (µstrain) 112 days 554 – 58.1 w/cm + 48.4 BC + 37.4 S/A + 46.2 (w/cm · VMA) + 41.9 (w/cm · BT) – 40.6 (BC · VMA) + 30.8 (VMA · BT) 0.96 28 days 680 + 79.3 BT – 37.5 w/cm + 30.6 (VMA · BT) + 28.8 (w/cm · BT) 0.75Creep (µstrain) 112 days 1,036 + 73.6 BT + 38.8 BC + 40.7 (VMA · BT) + 34.9 (w/cm · BT) – 32.9 (BC · S/A) 0.89

• Current AASHTO 2007 model for predicting elastic modulus • Current AASHTO 2007 model for estimating flexural strength The proposed coefficients can be found in Attachment D. 1.5 Validation of Code Provisions to Estimate 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 1990, and GL 2000 (Gardner and Lockman, 2001) models. Coefficients of existing models were modified to provide better prediction of visco-elastic properties for SCC. The following models are recommended: • AASHTO 2007 model with suggested modifications to estimate creep • AASHTO 2004 model with suggested modifications to pre- dict drying shrinkage • Current CEB-FIP MC90 model can be used to predict dry- ing shrinkage The proposed coefficients can be found in Attachment D. 1.6 Homogeneity of In-Situ Strength and Bond to Reinforcement Six 60.6 × 84.6 × 7.9 in. (1540 × 2150 × 200 mm) wall ele- ments were cast using a reference HPC concrete of normal consistency and five SCC mixtures of different plastic vis- cosity and static stability levels. The SCC mixtures were pro- portioned to yield slump flow consistency of 26.7 ± 0.7 in. (680 ± 15 mm) and minimum caisson filling capacity of 80%. The surface settlement of the SCC mixtures ranged between 0.30% and 0.62% and that of the HPC was 0.23%. Despite the high fluidity of SCC, stable concrete can lead to more homogenous in-situ properties than HPC of normal consistency subjected to mechanical vibration. Although the SCC mixtures exhibited VSI values of 0.5 to 1 and caisson fill- ing capacity higher than 80%, the tested mixtures developed various levels of uniformity of core compressive strength and pull-out bond strength results. The homogeneity of in-situ properties was shown to vary with plastic viscosity and static stability determined from the surface settlement test. Recommendations to ensure homogenous in-situ proper- ties are summarized as follows: • Use highly flowable SCC with adequate static stability, maximum surface settlement of 0.5%, column segregation index of 5%, and percent static segregation of 15%. These limits are especially critical in deep elements. Such SCC can develop at least 90% in-situ relative compressive strength (core results) and modification factor of 1.4 for bond to horizontally embedded prestressing strands. • Avoid the 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) to ensure adequate self-consolidation. 1.7 Structural Performance The structural performance of full-scale precast, prestressed bridge girders constructed with SCC and HPC was investi- gated. Two SCC and two HPC mixtures with target 56-day compressive strengths of 8,000 and 10,000 psi (55 and 69 MPa) were used to cast four full-scale AASHTO-Type II girders. Constructability, temperature variations, transfer length, cam- ber, flexural cracking, shear cracking, and shear strengths of the girders were evaluated. More details on the construction and testing of the girders are given in Attachment D. The following findings and observations are made based on the results of these tests: • With the casting from a single location at midspan of the 31-ft (9.44-m) long girders, no visible segregation was observed in any of the girders. • The maximum temperature rise during the steam-curing operation satisfied the maximum temperature limit of 150°F (65°C). • There were fewer “bug holes” in the SCC girder than in the HPC girder. • The target 18-hour compressive strengths, required for prestress release, were met for the two SCC girders. • The transfer lengths for the four girders were similar and considerably shorter than the transfer length values given in the AASHTO LRFD Specifications [2007] 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 deter- mine the modulus of elasticity for the two SCC mixtures were about 4% and 11% lower than those for the HPC mixtures. • The drying shrinkage for the two SCC mixtures was about 20% greater than that for the comparable HPC mixtures. • In comparison to HPC girders, the SCC girders exhibited smaller cambers because of the greater elastic shortening and long-term losses of prestress due to the lower elastic modulus and greater drying shrinkage. • The cracking moments for the SCC girders and the com- panion HPC girders were similar. • The uncracked and cracked stiffnesses for all four girders were very similar. • The cracking shears for all four girders were similar. 10

• All four girders failed in shear after developing a significant number of wide shear cracks; crack widths just before fail- ure were greater than 0.24 in. (6 mm). • The stirrups developed significant strains beyond strain hardening and ruptured at failure. • The failure shears exceeded the nominal shear resistances of the girders calculated using the approach given in the AASHTO LRFD Specifications [2007], probably because of the strength and stiffness of the top and bottom flanges of the AASHTO girders. • The flexural resistances of the HPC girders exceeded the nominal resistances calculated using the AASHTO LRFD Specifications [2007]. • The flexural resistances of the SCC girders were within 1.5% of the theoretical flexural resistance using the approach provided in the AASHTO LRFD Specifications [2007]. • The lower ductilities and lower shear resistance of SCC girders compared with the corresponding HPC girders are due to the lower volume of coarse aggregate that reduces aggregate interlock and results in a lower energy absorp- tion capability on the sliding shear failure plane. The structural performance tests of two SCC girders and two HPC girders have highlighted a number of differences that could affect design. However, more research is required to support any specific changes to the design specifications. 11