Steam curing for precast concrete piles containing silica fume should be permitted by the Florida Department of Transportation, speeding production of
Steam curing for precast concrete piles containing silica fume should be permitted by the Florida Department of Transportation, speeding production of piles at precast plants, according to new peer-reviewed research presented at the 87th annual meeting of the Transportation Research Board meeting in January in Washington, D.C. Following is a summary of the steam curing research, along with other topics of interest to the precast industry. Concrete Products‘ March Technical Talk will explore research in the area of ready-mixed concrete. More information about TRB is posted at www.trb.org.
STEAM CURE FOR CONCRETE PILES WITH SILICA FUME
Florida should approve steam curing of concrete piles containing silica fume, also called microsilica, according to Effect of Steam Curing on Concrete Piles with Silica Fume, by Nur Yazdani, Ph.D. P.E., professor and chairman, Department of Civil & Environmental Engineering, University of Texas at Arlington.
Silica fume is a common addition to high performance concrete mix designs, Yazdani notes, adding that Florida DOT currently allows only a 72-hour continuous, moist-cure process for concrete containing silica fume. Florida does allow steam curing for regular concrete not containing silica fume.
Accelerated curing has been shown to be effective in producing high-performance characteristics at early ages in silica fume concrete, Yazdani reports. However, heat greatly increases the moisture loss from exposed surfaces, which may cause shrinkage problems.
Thus, an experimental study was undertaken to determine the feasibility of steam curing Florida DOT concrete containing silica fume in order to reduce precast turn-around time. Various steam curing durations were utilized with full-scale precast/prestressed pile specimens; and, concrete compressive strength and shrinkage were determined for each duration.
Results indicate that steam-cured silica fume concrete met all FDOT requirements for the 12-, 18- and 24-hour curing periods, Yazdani asserts. No shrinkage cracking was observed in any samples up to one year age. Accordingly, Yazdani recommends that Florida DOT allow 12-hour steam curing for concrete with silica fume.
Silica fume has been used as a concrete property-enhancing material Û and as a partial replacement for portland cement Û for over 25 years, Yazdani says. It is a by-product in the production of silicon metal or ferrosilicon alloys, he explains. Silica fume for use in concrete is available in slurry or dry forms. In either form, it’s a highly reactive pozzolan when used in concrete, due to its fine particles, large surface area, and the high silicon dioxide content.
Silica fume imparts several effects on the properties of fresh and hardened concrete, Yazdani notes. In fresh concrete, he elaborates, silica fume affects water demand and slump. Concrete water demand increases with greater amounts of silica fume, due primarily to the high surface area of the by-product. Fresh concrete containing silica fume is more cohesive and less prone to segregation than concrete without silica fume. Since silica fume is used with other admixtures, such as water-reducing or high-range water-reducing admixtures, slump loss actually is due to the change in cohesivity.
Silica fume also is known to affect the time of setting and bleeding of fresh concrete, Yazdani says. Mechanical properties of silica-fume concrete, such as creep and drying shrinkage, have been known to be lower than that of concrete without silica fume, he observes, adding that at 28 days, the compressive strength of silica fume concrete is significantly higher than that of concrete without silica fume. Moreover, silica fume is linked to decreased permeability, as well as enhanced resistance to chemical attack and chloride ion penetration.
Nonetheless, use of microsilica comes at a cost. The surface of silica fume concrete tends to dry quickly, subsequently causing shrinkage and cracking, Yazdani reports. This is one reason why early-age moist curing of silica fume-concrete is important.
Use of steam curing for silica-fume concrete is convenient, however, and potentially offers significant economic savings for precasters. With a maximum probable steam curing time of 24 hours, precast products could be turned around at a significantly faster rate, resulting in economic benefits, Yazdani contends. It is known that steam curing results in more complete hydration of the pozzolanic materials, resulting in increased strength gain of concrete. Increased cost of the curing process is more than offset by savings in curing time and extra productivity.
Using current technology and facilities available in large precast yards, steam curing of silica-fume precast elements can be conveniently achieved to meet and even exceed Florida DOT specifications regarding the temperature regimens during steam curing.
Steam-cured silica fume concrete can achieve target minimum compressive strengths cited in Florida specifications. In this study, all steam-cured pile specimens reached the 28-day target strength of 41.37 MPa (6,000 psi) for Class V mixes. All steam-cured samples continued to gain in strength with time. At the 365-day age, samples displayed significantly higher compressive strengths than those at 28 days.
Steam-cured samples displayed lower compressive strengths at all ages than their moist-cured counterparts Û results consistent with previous research findings on silica fume concrete.
Steam curing times of 12, 18 and 24 hours do not seem to play a major role in controlling concrete compressive strengths. No consistent pattern of maximum strength was displayed by samples from a single source.
All steam-cured and moist-cured specimens showed a general increase in shrinkage with time. Steam-curing duration did not seem to play a major role in affecting the shrinkage rates. At later stages, e.g., 364-day age, longer steam-curing periods such as 18 and 24 hours accelerated shrinkage growth.
Pile specimen size did not have a significant effect on the shrinkage rate. Larger and smaller pile samples underwent similar shrinkage with time.
During 364 days of monitoring, no distress of prestressed piles was observed due to shrinkage cracking. Visual inspections did not show any shrinkage cracks.
The ACI Branson model for shrinkage prediction underpredicts the shrinkage of steam-cured specimens at 364 days and overpredicts shrinkage for the moist-cured specimens.
Precast yards may adopt a 12- to 24-hour period of steam curing for silica fume concrete elements. Typical current steam-curing duration used by precasters is 10 to eight hours.
FULL-DEPTH PRECAST PANELS SPEED BRIDGE CONSTRUCTION
Full-depth, precast, post-tensioned concrete bridge deck panels can speed new construction or retrofit projects, saving time and money in the process. That conclusion was presented by Erin Santini Bell, Ph.D.; David Salzer; Charles Goodspeed, Ph.D., University of New Hampshire Department of Civil Engineering; and, Rebekah Briggs, PB Americas, Inc., in their peer-reviewed paper, Full Depth Precast Prestressed Post-Tensioned Concrete Bridge Deck Panels for Rapid Bridge Construction.
In 2004, the Federal Highway Administration (FHWA) highlighted bridge construction as a primary source of traffic congestion, proposing a solution in the use of prefabricated bridge elements for rapid construction and rehabilitation. Many contractors are increasingly using precast caps for substructure construction, the researchers assert, and bridge designers are finding ways to integrate precast elements into superstructure and deck construction through the use of precast, prestressed bridge girders and deck panels.
According to FHWA, benefits of using total prefabricated bridge systems include increased work zone safety, improved constructability, lower life-cycle costs, and increased quality through controlled fabrication conditions.
Typical bridge construction or rehabilitation projects can take months to years to complete, causing traffic delays and costing society time and valuable natural resources, the authors write. During this time period, alternate routes, new lane patterns, material delivery, and heavy equipment cause road closures that significantly impact traffic, increase safety hazards, and limit accessibility. Decreasing the length of construction projects without sacrificing quality would also decrease the direct negative impact to the general public.
In response, University of New Hampshire developed a precast, post-tensioned concrete slab system for rapid bridge construction by the New Hampshire DOT. Accordingly, in spring 2006, four full-depth concrete deck panels were produced, modeled and tested, the authors report. The panels were post-tensioned longitudinally with threaded steel rod used to represent the post-tensioning strands, explain Bell, Briggs, Salzer and Goodspeed. These panels are intended for use with precast New England Bulb Tee, steel wide flange, or similar girders.
For shear transfer, one set of panels was fabricated with a tongue-and-groove joint; and, the other set was produced with a butted joint, as requested by the New Hampshire DOT. Both joints were sealed with a high-viscosity thixotropic closed-cellular epoxy. Once leveled, epoxied, post-tensioned, and placed with high-strength grout, the panels were tested in single-point bending.
In field applications, shear studs would be used to connect the slab system to the girders. Strain gauges were used to monitor strains along the transverse joint resulting from longitudinal post-tensioning, the researchers note. The collected data then were used to refine the model of the panel system. This updated model was used to advance the panel system design and will eventually be used to create standard construction details.
The precast, prestressed post-tensioned concrete bridge deck panel system tested as part of this research will be refined on the basis of both laboratory test results and the mathematical model, affirm Bell, Briggs, Salzer and Goodspeed. One area of specific interest is the comparison of a flat-butt joint and the tongue-and-groove joint.
Potential cost savings of using flat-butt joints could greatly impact the cost-effectiveness of precast bridge deck panel systems, the authors say. The research team is planning a fatigue test of both the butted joint and the tongue-and-groove joint in the structural laboratory at the University of New Hampshire. There are also plans for the construction of a pedestrian bridge using this precast prestressed post-tensioned concrete bridge deck panel system.
The mathematical model invites confidence, the authors contend, as model results fell within expected values and closely followed anticipated behavioral trends, with the exception of the support conditions. The usefulness of laboratory tests lies in visual behavior observed during testing for this case, as imperfect control and gauge application led to random error in test data; and, an inadequate number of gauges led to an incomplete data set. Numerical results of the butt joint test offer hope that good correlation potentially can be seen and that slab behavior can be accurately recorded with the evolution of the testing experience and technique at UNH.
This research effort was the first step in developing the slab system design. According to the authors, its continued evolution through testing and model updating will contribute to the development of an effective protocol for integrating rapid bridge construction and structural health monitoring.
COMPOSITE BRIDGES SHOW DEFLECTIONS AFTER SIX YEARS
After six years, deflections have been noted in four composite bridges incorporating fiber-reinforced polymer (FRP) and either precast concrete or steel, write David Holdener, EIT; John Myers, Ph.D., P.E., University of Missouri-Rolla; and, Danielle Kleinhans, Ph.D., P.E., CTL Group, Skokie, Ill., in their paper, Six-Year Performance Evaluation of St. James, Missouri Composite Bridges.
Field validation of FRP bridges through load tests provides a means of measuring the performance of a bridge over time, the authors write. Noncontact optical surveying equipment is one such method that can be utilized to measure the deflection of bridges under a static truck load. In St. James, Mo., four bridges originally constructed in 2001 with FRP technologies were subsequently load tested. Each bridge utilized FRP in its construction in a different way.
One bridge was constructed entirely of FRP sandwich panels, whereas two other bridges utilized FRP panels for the decks and steel beams for girders. The final bridge employed carbon fiber-reinforced polymer (CFRP) and glass-fiber-reinforced-polymer (GFRP) bars within precast concrete panels.
Soon after construction in 2001, deflection readings were obtained using direct current variable transformer (DCVT) transducers mounted on tripods within the creek beds. These bridges were reevaluated in 2007, utilizing high-precision surveying equipment with a similar truck loaded to a comparable weight. The investigation primarily focused on determining if the bridges had undergone any degradation in FRP material properties. Additionally, load distribution between girders and panels was monitored. Results were compared to existing data from the original load tests to determine performance over time and study structural degradation.
The most susceptible part of the bridge is the deck, which typically needs replacement after 15 to 20 years, the authors explain. Loss in structural integrity occurs for myriad reasons. Salt used for deicing contributes heavily to the corrosion of steel in traditional reinforced concrete decks. FRP represents an attractive alternative to the traditional reinforced-concrete deck, they say, in terms of corrosion resistance, easy and quick construction, and a decrease in dead load for seismic considerations. What remains to be seen, however, is the long-range durability of FRP systems, they emphasize. Load testing is one way to ascertain the performance of a bridge structure over time.
The four bridges under examination included a variety of design elements:
St. Francis Street Bridge Û constructed exclusively of glass FRP honeycomb sandwich panels. Four 23.625-in.-deep panels include a 0.375-in. polymer-concrete wearing surface. Bridge span is 26.25 ft. long and 27.33 ft. wide. Four stacked GFRP tubes were used within the joint to transfer load between panels.
St. Johns and Jay Streets Bridges Û featuring glass FRP honeycomb sandwich deck panels, supported by steel girders. The two bridges differ primarily in that the St. Johns Street Bridge includes six lateral panels, whereas the Jay Street Bridge has four longitudinal panels.
Walters Street Bridge Û comprising nine 1-ft.-deep, 2.83-ft.-wide precast panels with FRP bars. Bridge span is 24 ft. long and 25.5 ft. wide. Load transfer at panel joints is accomplished by embedded angles welded together on site; the joint later was filled with grout to seal the connection. Construction was completed in 10 days during June 2001.
Tests on all four bridges Û utilizing a single, loaded, tandem-axle dump truck Û were performed on four consecutive days in October 2001. Repeated testing in May 2007 employed optical laser surveying equipment to record deflection measurements. The Leica TCA 2003 Total Station used for this purpose has been shown to accurately record deflection measurements of 0.005 inches at a distance of 200 ft. to the target or less, the researchers note, which is comparable to traditional monitoring equipment such as DCVTs. The system was used in lieu of traditional methods due to the ease of setup, standing water in the creek bed, and muddy conditions below the bridge.
In May 2007, the weight of the truck’s rear tandem was 32,360 lb.; the weight of the front axle was 15,920 lb.; and, the total weight was 48,040 lb., approximately 0.33 percent heavier than the truck used in 2001 tests. The researchers found:
Higher deflections recorded from Walters Street Bridge tests, as compared to previous testing, was attributed to flexural cracking of the concrete.
Jay and St. Johns streets bridges both exhibited large deflection on one of the FRP deck panels close to midspan, possibly due to truck misplacement, softening of the deck, or slip of the panel-to-girder connection. Beyond that single point, all deflection data from original and recent tests for the Jay and St. Johns Street Bridges matched well.
The St. Francis Street Bridge showed fairly uniform load distribution between panels for the most recent and original load tests.
St. Francis Street Bridge deflection readings were less than the HS20-40 design values.
When comparing test data, the St. Francis Street Bridge exhibited an 18 percent loss in stiffness on average.
On the basis of their research, the author recommend ongoing testing for these bridges, plus coupon level material testing as warranted, to monitor the structures’ future in-situ performance.