Self-Consolidating Concrete for Cast-in-Place Bridge Components (2016)

Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

53 Conclusions and Recommendations for Research This chapter summarizes the major findings of the research and presents suggestions for future research. The findings presented in this report were based on the investigations per- formed using specific materials; other materials may result in different findings. 3.1 Mix Proportions and Fresh and Early-Age Concrete Properties The tests on SCC mixtures proportioned for cast-in-place bridge components indicated the following: 1. SCC mixtures with satisfactory properties for cast-in- place bridge components could be proportioned using the procedure proposed by Koehler and Fowler (2007). These mixtures had a water-powder ratio ranging from 0.37 to 0.44, powder content ranging from 650 to 760 lb/cy, and sand-to-aggregate ratio ranging from 0.45 to 0.50. 2. SCC mixtures with satisfactory properties for cast-in-place bridge components could be produced with replacements with 25% Class C fly ash, 25% Class F fly ash, 30% GGBFS, or 20% Class F fly ash and 15% limestone powder. 3. SCC mixtures with satisfactory properties for cast-in- place bridge components could be produced using natu- ral gravel and crushed limestone aggregates with ¾, ½, and 3⁄8 in. NMSA. 4. Workability targets of SCC mixtures depend on the geo- metric characteristics of cast-in-place bridge components. 5. Table 3-1 shows examples of bridge components, their geometric characteristics, and the proposed workability targets of SCC mixtures. 6. SCC mixtures with high filling ability (FA2) are appro- priate for cast-in-place bridge components with intricate shape and/or when high formed surface quality is desired (e.g., for box girders and pier walls); SCC mixtures with low filling ability (FA1) are appropriate for components with simple shape and when formed surface quality is not a concern (e.g., pile caps and floor beams). 7. SCC mixtures with high resistance to segregation (SR2) are appropriate for cast-in-place bridge components that are either deep or long (e.g., arches and abutment walls); SCC mixtures with moderate resistance to segregation (SR1) are appropriate for cast-in-place bridge compo- nents that are shallow and short (e.g., pier caps). 8. SCC mixtures with low passing ability (PA1) are appro- priate for thick components with a low level of reinforce- ment (e.g., footings); SCC mixtures with high passing ability (PA2) are appropriate for thin components and/ or those with a high level of reinforcement (e.g., girders and pier columns). 9. The dosage of HRWRA required to achieve specific work- ability targets varied based on the type of aggregate and SCM/filler. SCC mixtures with crushed limestone aggre- gate and GGBFS required a higher dosage of HRWRA than those with natural gravel aggregate and fly ash to achieve the same workability targets. 10. For SCC mixtures with relatively low powder content and/or high W/P ratio, viscosity-modifying admixture (VMA) was needed to enhance mixture stability. 11. SCC mixtures designed for cast-in-place bridge compo- nents exhibited a wide range of viscosity and yield stress depending on the type of SCM/filler, W/P ratio, and type and size of coarse aggregate. 12. The rate of workability loss of SCC mixtures was directly proportional to the initial slump flow; use of WRAs would help maintain the workability for an extended period of time. A late addition of a small dosage of HRWRA helped improve SCC workability when slump flow was below 22 in. 13. Time of initial setting of SCC mixtures was highly dependent on the temperature and type of SCM/filler. Mixtures containing Class C fly ash had the longest time C H A P T E R 3

54 of initial setting, and those containing Class F fly ash had the shortest time of initial setting. Also, high slump flow SCC mixtures exhibited a longer time of initial set- ting than that of low slump flow SCC mixtures due to the retarding effects of HRWRA. 14. The heat of hydration of SCC mixtures was similar to that of CVC mixtures but SCC experienced a slight delay in reaching the peak temperature (the longest delay was observed in mixtures containing Class C fly ash). 15. The formwork pressure of SCC was slightly less than full hydrostatic pressure. The placement rate, thixotropy, and yield stress of SCC had a significant effect on the maxi- mum formwork pressure. 3.2 Mechanical, Visco-Elastic, and Durability Properties The following findings were based on the properties of hardened SCC mixtures proportioned for cast-in-place bridge components: 1. The compressive strength of SCC at any age was accu- rately predicted using the ACI 209 model for CVC. The ratios of 7-day, 14-day, and 56-day compressive strength to 28-day compressive strength were 0.77, 0.88, and 1.12, respectively for SCC mixtures. 2. MOE of SCC was slightly lower than that predicted by AASHTO LRFD Equation 5.4.2.4-1 for CVC (33,000 K1 wc1.5 √f ′c). A modification factor of 0.96 would account for the effect of high paste-to-aggregate volume of SCC mix- tures on MOE. Also, the MOE of SCC mixtures containing crushed limestone aggregate was slightly higher than that of mixtures containing natural gravel aggregate. Use of the AASHTO aggregate source factor (K1) of 1.0 and 0.95 for limestone and gravel aggregate, respectively, would account for aggregate stiffness. 3. The splitting tensile strength of SCC mixtures was lower than that estimated by AASHTO LRFD Section C5.4.2.7 for CVC (0.23 √f ′c); a correction factor of 0.8 would account for tensile forces that are caused by effects other than flexure, such as anchorage zone design. 4. The MOR of SCC mixtures was within the range pre- dicted by AASHTO LRFD Section C5.4.2.6 for CVC (0.24 √f ′c to 0.37 √f ′c). 5. The pull-out bond strength of vertical reinforcing steel bars cast in SCC was lower than that of bars cast in CVC; a development length modification factor of 1.3 would account for the difference. 6. The pull-out bond strength of horizontal reinforcing steel bars cast in SCC was similar to that of bars cast in CVC, but the top-bar effect was lower in high slump flow SCC than it was in low slump flow SCC and CVC. 7. The interface shear resistance of SCC obtained from push- off testing was lower than that predicted by AASHTO Short Long Shallow Deep Thin Thick Simple Complex Low High Low High FA1 FA2 SR1 SR2 PA1 PA2 Footing Pile Cap Wing Wall Abutment Wall Pier Wall Pier Column Strut or Tie Pier Cap Box Girder Stringer Floor Beam Girder Arch Note: Shaded cells represent selected component geometric characteristics and SCC workability targets. * For deep/long components, SR1 could be acceptable if free-fall height/free-travel distance is controlled (e.g., tremie pipe). Passing Ability Su bs tr u ct ur e Su pe rs tr u ct u re Component Category Filling Ability Segregation Resistance* Formed Surface Quality SCC Workability Targets Bridge Component Length Depth Thickness Shape Intricacy Level of Reinforcement Component Geometric Characteristics Table 3-1. Proposed SCC workability targets for examples of cast-in-place bridge components.

55 LRFD Section 5.8.4.1 for CVC only for compressive strength less than 6 ksi; using a cohesion factor (c) of 0.0 for SCC with compressive strength less than 6 ksi would account for this difference. 8. The nominal shear resistance of SCC beams with differ- ent levels of transverse reinforcement was accurately pre- dicted by AASHTO LRFD Section 5.8.3.3 for CVC. 9. Drying shrinkage of SCC mixtures was higher than that predicted by AASHTO LRFD Equation 5.4.2.3.3-1 for CVC and highly dependent on the type of SCM/filler. A powder composition modification factor for SCC con- taining Class C fly ash, GGBFS, and Class F fly ash with/ without limestone powder would account for this differ- ence. Also, increasing the curing period from 7 to 28 days significantly reduced drying shrinkage of SCC mixtures. 10. Restrained shrinkage of SCC mixtures was highly depen- dent on the type of SCM/filler and NMSA. SCC mixtures containing Class C fly ash and/or 3⁄8 in. NMSA exhibited high cracking potential, but mixtures containing Class F fly ash and/or ¾ in. NMSA exhibited low cracking potential. 11. The creep coefficient of SCC mixtures (except those con- taining limestone powder) was accurately predicted by AASHTO LRFD Equation 5.4.2.3.2-1 for CVC. SCC mix- tures with 20% Class F fly ash and 15% limestone powder exhibited higher creep strains; a modification factor of 1.2 would account for this difference. 12. The air void system of hardened SCC indicated adequate freeze and thaw resistance for most mixtures. However, the wide variation in HRWRA dosages used in SCC mixtures resulted in a large difference in the air content between fresh and hardened conditions. 13. Surface resistivity of SCC mixtures indicated low-to- moderate chloride ion penetrability depending on the type of SCM/filler. SCC mixtures containing Class C fly ash had the lowest surface resistivity (higher penetrability), and mixtures containing GGBFS had the highest surface resistivity (lower penetrability). 3.3 Full-Scale Bridge Components: Constructability and Structural Performance Observations made during the construction and testing of full-scale bridge components indicated the following: 1. SCC proportioned for shallow and short bridge compo- nents (e.g., footings) was satisfactorily placed continu- ously at a high placement rate (e.g., 1.3 cy/min) from one location using the truck chute. 2. SCC proportioned for deep components (e.g., columns) was satisfactorily placed at a high placement rate (e.g., 60 ft/hr) using a crane and bucket with a free-fall height of 15 ft. For deeper components, a tremie pipe would be necessary to reduce the free-fall height. 3. Maximum formwork pressure of SCC was very close to full hydrostatic pressure for high placement rates (e.g., 60 ft/hr). Slower placement rates (e.g., 26 ft/hr) resulted in a lower formwork pressure depending on the tempera- ture and thixotropic characteristics of SCC. 4. Interrupting the placement of SCC for an extended period (e.g., 20 minutes or more) may result in forming pour lines (i.e., lift lines) between consecutive pours due to the thixo- tropic behavior of SCC. Agitating the top surface of the first lift by limited vibration immediately before placing the next lift can reduce or eliminate the formation of pour lines without negatively affecting the stability of SCC. 5. SCC was pumped satisfactorily from one location at one side (i.e., web) of a 40 ft long tub girder that was heavily reinforced. SCC flowed horizontally under its own weight for 20 ft in each direction, filling the bottom flange, encap- sulating reinforcing bars and post-tensioning ducts, and ris- ing up to fill the opposite web up to its mid-height (1.5 ft). 6. A maximum surface void ratio of 0.6% and maximum sur- face void diameter of 3⁄8 in. (i.e., CSC 3) were observed on all formed surfaces of the fabricated SCC bridge components. A lower surface void ratio can be achieved by controlling the direction of flow so that it is bottom-up rather than top-down, further reducing the entrapped air during SCC placement. 7. Visual examination of coarse aggregate distribution in saw cut bridge components at different locations indicated high stability and consolidation of SCC around reinforcing bars, with no signs of segregation or bleeding. 8. No cracking or signs of damage were observed around post-tensioning anchorages of the box girder specimen indicating a satisfactory application of SCC in highly dis- turbed regions (i.e., local zone and general zone). 9. Air void system parameters measured on SCC cylinders and cores extracted from fabricated components were not significantly different (< 1.5%) indicating that placement methods (i.e., truck chute, bucket and tremie pipe, free fall, and pumping) did not significantly affect the air void sys- tem parameters in SCC. 10. A significant difference (>1.5%) was observed between the air content measured at the plant and that measured at the job site in fresh SCC, probably resulting from transporta- tion, haul time, and the addition of HRWRA at the job site to improve workability. 11. Limited structural testing of full-scale bridge components fabricated using four SCC mixtures yielded structural capacities (i.e., flexure resistance and shear resistance) that

56 are different from those predicted by AASHTO LRFD spec- ifications for CVC. 3.4 Recommendations for Future Research NCHRP Project 18-16 examined the properties of SCC intended for use in cast-in-place bridge components. However, further research is needed to address several issues pertaining to SCC applications, including the following: • Investigating the effect of chemical admixtures (e.g., air- entraining admixtures, shrinkage-compensating admix- tures, WRAs, and VMAs) on fresh, early-age, and hardened SCC properties. • Developing specifications for achieving entrained air con- tent and addressing the effect of adding a later dosage of HRWRA on that entrained air content. • Investigating the effect of other SCMs (e.g., silica fume and metakaolin and fillers) on the fresh, early-age, and hard- ened SCC properties. • Investigating the influence of SCM sources and replacement levels on fresh, early-age, and hardened SCC properties. • Developing test methods for evaluating the dynamic stability and thixotropic property of SCC at a job site. • Developing methods for predicting the formwork pressure of SCC considering the rate of placement, temperature, and rheological properties of SCC. • Investigating use of SCC in cast-in-place bridge deck con- struction, drilled shafts, and deep foundations.