The object of keen interest again this year, self-consolidating concrete was one of a variety of topics covered at the 85th Transportation Research Board
The object of keen interest again this year, self-consolidating concrete was one of a variety of topics covered at the 85th Transportation Research Board (TRB) meeting, January 22-26, in Washington, D.C. Accordingly, a discussion of optimum SCC mix composition for prestressed plant use was included among presentations. Following is a roundup of precast-related studies reviewed at the TRB event. More information can be obtained at www.trb.org.
SCC MIX FOR PRESTRESSED
While self-consolidating concrete offers significant advantages for precast/prestress plant operations, the right mix is essential to reap benefits of the technology, attest Anton K. Schindler, Robert W. Barnes, James B. Roberts, Department of Civil Engineering, Auburn University; and, Sergio Rodriguez, Alabama Department of Transportation, in their paper Properties of Self-Consolidating Concrete (SCC) for Use in Prestressed Applications. The American Concrete Institute (ACI) defines SCC as a highly flowable, nonsegregating concrete that can spread into place, fill the formwork, and encapsulate reinforcement without any mechanical consolidation.
The use of self-consolidating concrete may help precast/prestressed plants produce high-quality prestressed concrete members at reduced labor costs, the authors write. Contractors are currently exploring the use of SCC, as it may produce members with homogeneous quality, even in highly congested narrow components, such as prestressed concrete members.
Using SCC may also decrease construction costs due to the reduced number of laborers required for placement. The researchers note, Alabama producers using conventional-slump girder mixes have experienced many consolidation and finish problems that could be mitigated with SCC. Yet, the Alabama DOT has not allowed the use of SCC for prestressed girder applications, mainly due to a lack of standardized test procedures and performance data, as well as uncertainty regarding the applicability of current design procedures to SCC-fabricated members.
Responding to such conditions, the authors evaluated SCC mixtures for use in prestressed concrete applications. The experimental program involved cylinders match-cured to a temperature history typical of prestressed concrete operations in the Southeast. Twenty-one air-entrained SCC mixtures were prepared with varying water-to-powder ratios of 0.28, 0.32, 0.36, and 0.40; varying sand-to-aggregate ratios of 0.38, 0.42, and 0.46; and, different powder combinations including Type III cement, Class C fly ash, ground granulated blast-furnace slag, and silica fume. Match-curing the cylinders to a temperature history typical of prestressing operations permitted comparison of SCC properties to those of two conventional-slump prestressed concrete mixes.
The SCC mixtures achieved 18-hour (prestress transfer) compressive strengths between 5,470 and 9,530 psi, the authors report. Compressive strengths at prestress transfer and at later ages were not sensitive to changes in the sand-to-aggregate ratio. All SCC mixtures had lower 18-hour moduli of elasticity than the control mixture with a water-to-powder ratio of 0.37. The SCC mixtures’ moduli of elasticity are in reasonable agreement with the elastic stiffness assumed during the design of conventional-slump concrete structures.
The 112-day drying shrinkage strain for all the SCC mixtures are of the same order of magnitude or less than those recorded for the control mixtures, according to Schindler, Barnes, Roberts and Rodriguez. A change in sand-to-aggregate ratio from 0.38 to 0.46 had no significant effect on the 112-day drying shrinkage strain of the SCC mixtures, they observe. At later ages of 56 and 112 days, the AASHTO LRFD procedure overestimated the measured drying shrinkage of the SCC mixtures.
In addition, the authors note:
- Air-entraining admixture dosage was significantly affected by the powder type, the high-range water reducer (HRWR) admixture dosage, the water-to-powder ratio (w/p), and the mixing sequence. Increased HRWR admixture dosages generally tended to increase the SCC mixtures’ total air content, thus requiring a reduction in air-entraining admixture to meet the target air content.
- The use of ground granulated blast furnace (GGBF) slag required much higher dosages of air-entraining admixture.
- Increased HRWR admixture dosage was required for the GGBF slag and silica fume mixtures to obtain a slump flow within the desired range.
Further research is required to evaluate the creep behavior of the SCC mixtures discussed in this paper, the authors contend, and this work is ongoing. The effect of these SCC mixtures on the transfer and development length of prestressed tendons is also being examined. Further, they add, The performance of some of these SCC mixtures should be evaluated during the construction of full-scale, plant-cast prestressed members.
PRECAST CHANNEL BRIDGE INSPECTION
Precast channel bridges (PCBs) are sufficiently unique to require inspection and testing to ensure they are able to maintain their function, write Terry Wipf, P.E., Wipf, Klaiber, Ingersoll and Wood, and professor of Civil Engineering, Iowa State University; F. Wayne Klaiber, P.E., Iowa State University; J. Scott Ingersoll, P.E., WHKS and Company; and, Douglas Wood, Iowa State University, in their paper, Field and Laboratory Testing of Precast Concrete Channel Bridges.
The precast channel bridge (PCB) is a short-span bridge that was commonly constructed on Iowa’s secondary roads approximately 40 years ago, the authors explain. Each PCB span consists of eight to 10 simply supported precast panels ranging in length from 19 ft. to 36 ft. In cross section, they observe, the panels resemble a steel channel, the web oriented horizontally to form the roadway deck with the vertical legs acting as shallow beams. Bundled reinforcing bars in each leg act as the primary flexural reinforcement.
Many of the approximately 600 PCBs in Iowa show signs of significant deterioration, the researchers emphasize. Typical deterioration consists of spalled concrete cover and corrosion of the bundled primary reinforcement. Our objective was to assess the structural sufficiency of the deteriorated PCBs through field and laboratory testing.
To that end, four deteriorated PCBs were instrumented with gauges to measure strains in both the concrete and reinforcing steel and with transducers to measure vertical deflections. Responses from loaded trucks were recorded and analyzed, the authors note. Test results revealed that all measured strains and corresponding stresses were well within acceptable limits. Likewise, measured deflections were much less than the recommended AASHTO values.
Lab testing consisted of loading 12 deteriorated panels to failure in a four-point bending arrangement, as well as stressing a four-panel model bridge. Though all panels exhibited significant deflection prior to failure, the experimental capacity of 11 panels exceeded their theoretical capacity, the authors report. The experimental capacity of the twelfth panel, an extremely distressed panel, was only slightly below its theoretical capacity.
According to the researchers, the most common form of deterioration exhibited on PCBs was corrosion of primary reinforcement and spalling of the concrete cover. Through laboratory and field tests, it was determined that this deterioration had minimal effect on the performance of these bridges, they affirm. Hooks on the ends of the bottom layer of the primary reinforcement reduced the need for bond along the span. However, the authors add, Another less common form of deterioration, deck delamination, did cause a decrease in the ultimate strength of the panels.
Based on lab results, the researchers concluded that load ratings of PCBs exhibiting the type of wear observed in panels tested in the investigation need not be adjusted due to the deterioration. Using the design capacity in load rating calculations will result in a conservative load rating, they maintain. However, care must be taken when using this information on other bridges; one needs to ascertain that corrosion of the reinforcement has not caused a significant reduction in area and that the hooked ends of the reinforcement are anchored in sound concrete.
NEW INSPECTION CRITERIA FOR SIDE-BY-SIDE, BOX-BEAM BRIDGES
Common forms of deterioration in side-by-side box-beam bridges were inventoried and the structural impact of these distresses were assessed by Christopher G. Gilbertson, P.E., Michigan Technological University; Upul Attanayake, Wayne State University; Dr. Theresa M. Ahlborn, P.E., Michigan Technological University; and, Dr. Haluk Aktan, P.E., Wayne State University, in their paper, Prestressed Concrete Box-Beam Bridge Performance: Condition Assessment and Design Analysis. The outcome of their research is an inspection handbook that field inspectors should find especially useful.
Side-by-side, prestressed concrete box-beam bridges have been Û and continue to be Û a popular choice in the state of Michigan and throughout the U.S., the authors contend. Various forms of deterioration have been identified specific to this type of construction; however, due to the nature of the design, only the bottoms of the beams and exterior fascia beams are available for visual inspection.
In view of that dilemma, 15 bridges of varying ages were carefully selected and inspected for distress. Common forms of deterioration present in box-beam bridges were identified and presented in an inspection handbook.
Finite element analysis was performed to assess box-beam designs that may cause future deterioration, the authors report. Load ratings were performed to assess the impact of common forms of distress, using the established AASHTO method as well as the box-beam specific finite element model.
In 1955, Michigan built its first side-by-side prestressed concrete box-beam bridge, the researchers assert. Beginning in the 1970s, side-by-side box-beam bridges became widely used due to construction advantages, they add. In fact, such structures are the bridge of choice for spans less than 110 ft. in Michigan, where the state DOT currently has jurisdiction over 235 of these bridges, while an additional 1,818 are owned and operated by local agencies.
Construction of a side-by-side, box-beam bridge typically involves placing precast/prestressed box-beams adjacent to each other, mortar grouting full-depth shear keys, applying transverse post-tensioning, and casting a 6-in.-thick reinforced concrete deck. The resulting superstructure is expected to behave as a plate simply supported at two ends.
The integrity of the plate becomes compromised when longitudinal cracks form along the shear keys, allowing surface water to penetrate and become trapped between the box-beams, the authors state. Water saturated with deicing salts penetrates along the full length of the beams and initiates corrosion of prestressing tendons. Though significant cost and construction advantages attend this design, premature deterioration may cause highway agencies to reconsider the use of this particular type of bridge superstructure.
The longitudinal cracking observed along the bottom flange of box-beams in bridges built before 1974 may have occurred due to pressure exerted by corroding strands or by freezing of water collected inside the box-beam cavity, the authors write. Although expanded polystyrene is now used to form the cavity, moisture will certainly continue to collect between the expanded polystyrene and concrete on the inside of the bottom flange, they maintain. Hence, prestressing strands near the cavity may be subjected to more severe exposure than that of the outmost layer because of less concrete cover and prolonged exposure to moisture. Current Michigan box-beam standard designs show that concrete cover to the prestressing tendons near the beam cavity is less than that of the outmost tendon layer. It is recommended that the concrete cover above the prestressing tendons near the top of the bottom flange be increased to accommodate severe corrosive exposure conditions.
According to the authors, side-by-side prestressed concrete box-beam bridges are cost effective, eliminate the need for shoring a cast-in-place concrete deck, and present a low profile, making them advantageous where clearance requirements apply. But, they emphasize, Deterioration of box-beams is a significant problem and adherence to proper inspection and maintenance programs is essential to maintain the box-beam bridges currently in service. Yet, continuing research is promoting the development of additional maintenance and repair programs, along with recommended design changes, to minimize the likelihood of current forms of distress appearing on future side-by-side prestressed concrete box-beam bridges.
PRECAST BARRIER CONNECTION PROVIDES ROOM FOR WORKERS
Temporary barriers provide positive protection for motorists and workers in a highway work zone. Because most highway work zones are restricted in the lateral space available for accommodation of traffic and work activity, minimizing potential deflection of work zone barriers is desirable. Accordingly, a new connection designed to reduce the dynamic deflection of portable concrete traffic barriers was developed through a program of finite element simulation and full-scale crash testing, write Roger P. Bligh, Nauman M. Sheikh, Dean C. Alberson and Akram Y. Abu-Odeh, Texas Transportation Institute at Texas A&M University System, in their paper Low-Deflection Portable Concrete Barrier.
The new cross-bolted (or X-bolt) connection incorporates two threaded rods in different horizontal planes across the barrier joint to form a tight, moment connection. That configuration achieves the objective of low dynamic barrier design deflection without sacrificing constructability, the authors assert. In addition to being easy to install, the new barrier system is perceived as easily inspected and repaired.
Through full-scale crash testing using 10- and 30-ft. segment lengths, the researchers report, the barrier connection’s crashworthiness and design deflection were verified. An F-shaped barrier with X-bolt connection was found to have the lowest deflection of any approved portable concrete barrier.
Predictive LS-DYNA computer simulations were performed to design the barrier, quantify its deflection characteristics, and assess its ability to meet NCHRP Report 350 impact-performance criteria. Simulation provided a more detailed understanding of the barrier’s three-dimensional impact response prior to conducting full-scale crash testing, the authors explain. Once factored to account for an anticipated level of concrete damage, predicted deflections for the 10-ft.-segment barrier were consistent with deflections measured in the crash test.
Subsequent to its design and simulation, the new X-bolt connection was subjected to two full-scale crash tests: impact performance was assessed, and the cross-bolted F-shaped barrier’s design deflection was quantified for two different segment lengths. In both tests, structural integrity of the barrier and its connections was maintained, as the barrier successfully contained and redirected the test vehicle in an upright manner, the researchers affirm. The occupant risk factors were within preferred limits specified in NCHRP Report 350, and all relevant evaluation criteria were met.
For those interested in using self-consolidating concrete, the list of admixture contacts in Concrete Products‘ February 2006 Technical Talk (p. 60) can be expanded to include:
10600 High Way 62, Gate 19, Unit 7
Charlestown, IN 47111-0459
PROPERLY REINFORCED PRECAST SUBSTRUCTURES SUIT SEISMIC ZONES
Appropriate reinforcement may allow the use of precast bridge substructures in seismic risk regions where they are not currently employed, write Ou, Chiewanichakorn, Ahn, Aref, Chen, Filiatrault and Lee, Department of Civil, Structural, and Environmental Engineering, University at Buffalo, State University of New York, in their paper, Cyclic Performance of Precast Concrete Segmental Bridge Columns.
Prefabricated bridge elements and systems improve work-zone safety, minimize traffic disruption, increase speed of construction, reduce life-time cost, and minimize project environmental impact. The precast substructure system has been used in regions with low seismicity, but its application in moderate-to-high seismic regions such as the state of California is not actively considered, the engineers assert. The lack of information concerning its seismic behavior is the main reason why such a system is avoided in engineering practice.
Addressing that deficiency, the authors examined the seismic behavior of the precast concrete segmental column, raising the possibility of its use in regions of moderate-to-high seismicity for accelerated bridge construction. The columns examined were assumed to have single curvature under lateral load.
Among several issues in bridge-column seismic design, the authors evaluated self-centering and energy-dissipation capacities. Self-centering capability denotes a member’s ability to recover its original position after cyclic loadings, they explain. A bridge column with large residual displacement after an earthquake is difficult to repair and may need to be rebuilt, increasing cost and time to regain its functionality.
The researchers studied the cyclic performance of a precast segmental bridge column system in which unbonded post-tensioning provides a self-centering capability, and mild steel components extended across segment joints enhance energy dissipation. Based on the analytical model, it is suggested that the mild steels used for energy dissipation be continued from the column base to a height above the decompression region to maximize inelastic straining of the mild steels at the base segment joints, the authors conclude. A limitation of the steel ratio is also suggested such that the tendon prestressing force and the gravity load from the superstructure are able to push the column back to the pre-decompression stage, thus minimizing residual displacement and preserving the self-centering feature.
The study also revealed that increasing the steel ratio will increase the equivalent damping ratio as well as the residual displacement. The system with a steel ratio determined by the proposed equation results in significant increase in equivalent damping ratio, the authors note. Also, the system maintains small residual displacement after loading reversals. The sufficiency of the energy-dissipation capability of the system with the steel ratio suggested by this paper depends on the design ground motions considered in each specific project. Supplemental energy-dissipation devices may be warranted if the system fails to meet required seismic performance levels.