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7 56 days of moist curing and lower 18-hour modulus of elastic- maximum levels). In total, 16 SCC mixtures were used in the ity than similar concrete having low slump flow and lower factorial design. HRWRA content. In general, SCC mixtures with medium flu- The derived models that yielded high correlation coeffi- idity level are recommended for casting precast, prestressed cients (R2) are summarized in Tables 1 to 3. All factors are concrete girder elements. expressed in terms of coded values: · Coded BC = (absolute BC - 793) / 50 Effect of Viscosity-Modifying Admixture · Coded w/cm = (absolute w/cm - 0.37) / 0.03 For a given slump flow, SCC designed with low to moderate · Coded VMA = (absolute VMA - 0.75) / 0.75 dosage thickening-type viscosity-modifying admixture (VMA) · Coded S/A = (absolute S/A - 0.50) / 0.04 had greater HRWRA demand. Higher dosage of HRWRA improves retention of workability but reduces early-age The estimated values in the models (e.g., -1.06, -0.33, development of mechanical properties. The incorporation of +0.33, etc. in the HRWRA demand model) reflect the level of thickening-type VMA considerably improves static stability. significance of each response. A negative estimate signifies In general, SCC designed with 0.40 w/cm and low HRWRA that an increase in the modeled parameter can lead to a content exhibited better static stability when the thickening- reduction in the measured response. type VMA was incorporated. SCC containing thickening-type Based on the derived statistical models, the following obser- VMA had lower early-age mechanical properties. vations can be made for proportioning SCC mixtures for use in In general, the use of VMA is not necessary in SCC propor- precast and prestressed bridge elements: tioned with low w/cm and high binder content because such concrete can develop proper stability. On the other hand, SCC · Fresh concrete properties made with relatively high w/cm and/or low binder content Typical w/cm for precast, prestressed applications can should incorporate a VMA to secure adequate stability and range between 0.34 and 0.40. The selected value should robustness. It is important to note that the incorporation of a secure the targeted stability, mechanical properties, low dosage of VMA can enhance robustness, even in SCC visco-elastic properties, and durability requirements. made with relatively low w/cm. 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 Guidelines for Materials Selection Type I/II cement. and Mix Design HRWRA demand decreases with the increase in w/cm and Based on the results of the parametric investigation, guide- binder content. The higher HRWRA demand required for lines for the selection of material constituents, mixture pro- SCC made with Type III cement and 20% Class F fly portioning, and fluidity level necessary to ensure adequate ash than that required for SCC prepared with Type I/II performance of plastic and hardened SCC properties for pre- cement can reduce early-age compressive strength if the cast, prestressed concrete bridge elements are recommended. concrete is not heat cured. For steam-cured concretes, no SCC mixtures proportioned with w/cm of 0.33, crushed aggre- difference in 18-hour compressive strength between SCC gate with 1/2 in. (12.5 mm) MSA, and Type III cement with 20% made with either type of cements should be expected. Class F fly ash can develop the properties required for this Better slump flow retention can be obtained with SCC application. 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- 1.3 Factorial Design to Model quate workability. Fresh and Hardened Coarse aggregate with 1/2 in. (12.5 mm) MSA is rec- Concrete Properties ommended. Factorial design was carried out to model the effect of mix- VMA should be used in SCC made with relatively high ture parameters and material properties on workability char- w/cm and/or low binder content to secure stability and acteristics, mechanical properties, and visco-elastic properties homogenous in-situ hardened properties. The use of of SCC. The modeled parameters included binder content thickening-type VMA at low dosage can enhance static (BC), binder type (BT), w/cm, dosage of thickening-type VMA, stability. VMA can also be used in stable SCC (e.g., low and sandtototal aggregate volume ratio (S/A). This design w/cm) to enhance robustness. enabled the evaluation of the five selected parameters with each Incorporation of thickening-type VMA can delay setting evaluated at two distinct levels of -1 and +1 (minimum and and the elapsed time to attain peak temperature.
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8 Table 1. Derived statistical models for fresh concrete [slump flow 26.8 0.8 in. (680 20 mm)]. Modeled response Derived equations RČ 0.5 [HRWRA demand] 4.76 1.06 w/cm 0.33 BC + 0.33 BT + 0.11 VMA 0.97 (fl oz/cwt) + 0.13 (w/cm · BT) 0.16 0.84 BT 0.57 BC + 0.42 w/cm+ 0.16 S/A Filling Slump flow loss (in.) 0.54 (BT · S/A) + 0.49 (BC· w/cm) 0.84 ability 0.42 (BC · BT) [L-box blocking 0.69 + 0.13 w/cm + 0.12 BC 0.13 (BC · w/cm) 0.93 ratio]1.4 Passing 16,329 + 1,344 w/cm + 1,324 BC + 814 BT 729 ability [J-Ring flow]3 S/A 465 VMA 1,140 (BC · BT) 1,136 (BC · 0.99 (in.) w/cm) + 824 (BT · S/A) 650 (w/cm · BT) 465 (w/cm · S/A) + 351 (VMA · S/A) 291 (BC · S/A) Caisson filling 92 + 4.38 BC + 3.75 w/cm + 3.63 BT 3.63 (BT · 0.92 capacity (%) w/cm) 2.63 (w/cm· BT) 2.50 (BC · BT) Filling capacity 1.42 0.70 BC 0.63 BT 0.55 w/cm + 0.26 S/A Slump flow J-Ring flow (in.) + 0.63 (BC · w/cm) + 0.50 (BC · BT) + 0.40 (w/cm · 0.94 BT) 0.26 (BT · S/A) [Surface settlement]0.5 0.677 + 0.037 w/cm + 0.036 BC 0.024 BT 0.86 (%) Stability 3.25 0.30 BC 0.61 (BC · BT) + 0.44 (BT · S/A) Column segregation + 0.42 (w/cm · BT) 0.39 (BC · VMA) 0.36 0.89 (C.O.V.) VMA + 0.30 (w/cm · S/A) Plastic viscosity 298 133.4 w/cm 105.3 BC + 53.7 S/A 0.93 (Pa·s) + 49.7 (BT · w/cm) 27.6 (w/cm · S/A) Rheology and Thixotropy (Ab) 586 323.4 w/cm 181.8 BC + 71.1 (BC · w/cm) 0.95 formwork (J/m3·s) pressure Initial form pressure 0.90 + 0.027 BC + 0.027 w/cm 0.014 S/A 0.023 at 3.3 ft (1 m) (BC · w/cm) 0.013 (BT · w/cm) + 0.11 (S/A · 0.96 (K0 ) w/cm) Use air-entraining admixture where required for Plastic viscosity decreases with the increase in binder frost durability. It is important to note that the use of content and w/cm but increases slightly with the increase polycarboxylate-based HRWRA can lead to air entrain- in S/A. ment, but it does not necessarily produce an adequate Thixotropy or structural build-up at rest of the SCC air-void system to secure frost durability. decreases with the increase in binder content and w/cm. Surface settlement of SCC increases with the increase in Higher thixotropy can be detrimental to surface finish binder content and w/cm. and advantageous to formwork pressure. Table 2. Derived statistical models for mechanical properties. Property Age Derived equations RČ 4,752 293 w/cm 111 BT 81 VMA + 153 (w/cm · BT) 18 hours 0.96 Compressive 128 (VMA · S/A) 97 (w/cm · VMA) strength (psi) 56 days 9,176 773 w/cm + 290 BT + 220 BC 368 (BC · w/cm) 0.87 4,419 268 w/cm 103 BT 86 S/A 78 BC 18 hours 0.89 Modulus of 158 (BC · w/cm) 96 (BT · S/A) 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