The 82nd annual meeting of the Transportation Research Board (TRB) held Jan. 12-16 in Washington, D.C., highlighted one of today's hottest topics in precast/prestressed concrete — self-consolidating mixes. Because its faster placement and greater flowability has the potential to reduce costs, self-consolidating concrete (SCC) is of increasing interest to producers. Headlining TRB offering was a Virginia Transportation Research Council paper examining SCC for bridge components.

Once again, a record crowd of over 8,000 delegates traveled to the nation's capital to attend the TRB meeting. TRB is a unit of the National Research Council, a private, nonprofit institution that is the principal operating agency of the National Academy of Sciences and the National Academy of Engineering. Among the many topics covered at the event, those of particular interest to the readers of Concrete Products are summarized below. More information can be obtained by contacting the Transportation Research Board, 2001 Wisconsin Avenue, N.W., Green Building, Washington, D.C.20007 or visiting TRB's web site at http://trb.org.

SCC PROVES IDEAL FOR HEAVILY REINFORCED BRIDGE COMPONENTS

SCC can enhance strength and durability of concrete used in precast bridge elements, according to Celik Ozyildirim, Ph.D., P.E., principal research scientist, and D. S. Lane, senior research scientist, Virginia Transportation Research Council, in their paper, “Investigation of Self-Consolidating Concrete.” The researchers explain, “Conventional concrete tends to present a problem with regard to adequate consolidation in thin sections or areas of congested reinforcement, which leads to a large volume of entrapped air voids and compromises the strength and durability of the concrete. Using [SCC] can eliminate the problem, since it was designed to consolidate under its own mass.”

Examining several mix designs in the laboratory, their goal was to create mixtures with desirable flow characteristics that did not require additional consolidation, yet provided adequate compressive strength, low permeability, shrinkage control, and resistance to freeze/thaw cycles. The study was designed to determine the feasibility of producing SCC on a commercial scale using locally available materials at two concrete plants. In a field application, SCC from one plant was used for a small bridge in a residential area. Results demonstrated — with some tweaking of the mix proportions — that SCC can be produced successfully to provide many benefits for transportation agencies and the construction industry.

SCC is an import from the Far East, Ozyildirim and Lane observe. Because of a decline in the skilled construction labor force, they contend, researchers at the University of Tokyo started to develop SCC in 1986. Since its debut, the material has been used in many structures, including buildings, bridge towers, and bridge girders. “Positive attributes of SCC include safety, reduced labor and construction time, and improved quality of finished product,” the researchers affirm.

SCC differs from conventional concrete in that it has a lower viscosity and, thus, a greater flow rate when pumped, they report. As a consequence, the pumping pressure is lower, reducing wear and tear on pumps and the need for cranes to deliver concrete in buckets at the job site.

To achieve a high flow rate and avoid obstruction by closely spaced reinforcing, SCC is designed with limits on the nominal maximum size (NMS) of the aggregate, the quantity of aggregate, and aggregate grading, say VTRC's Ozyildirim and Lane. However, when the flow rate is high, the potential for segregation and loss of entrained air voids increases.

Such problems can be alleviated by designing a concrete mix with a high fine-to-coarse aggregate ratio, a low water-to-cementitious material ratio (w/cm), good aggregate grading, and a high-range water-reducing admixture (HRWRA). Viscosity modifying admixtures (VMA) are also used to reduce the tendency for segregation and enhance the stability of the air-void system.

SCC MIX PROPORTIONS
(OZYILDIRIM AND LANE)

MATERIAL		DESCRIPTION	AMOUNT
PLANT NO. 1
Cement		Type II/III	205 kg
Slag		40%, ASTM C 989, Grade 120	137 kg
Fine Aggregate		Natural sand	704 kg
Coarse Aggregate		Granite, 12.5 mm NMS	610 kg
Water		—	122 kg
Test 1 Admixtures
	AEA	Neutralized Vinsol resin	0.08 mL/kg
	HRWRA	Polycarboxylate	7.82 mL/kg
Test 2 Admixtures
	AEA	Neutralized Vinsol resin	0.08 mL/kg
	WRA	Sugar and lignin solution	9.13 mL/kg
	HRWRA	Polycarboxylate	3.26 mL/kg
PLANT NO. 2
Cement		Type III	216 kg
Pozzolans		Natural, ASTM C 618, Class N	93 kg
Fine Aggregate		Natural sand	631 kg
Coarse Aggregate		Granite, 19 mm NMS	703 kg
Water		—	126 kg
AEA		Sodium-salt type soap	0.20 mL/kg
HRWRA		Polycarboxylate	5.22 mL/kg

Another negative attribute of SCC is shrinkage, which may result in more cracks in the restrained concrete elements, accelerating the deterioration of both concrete and reinforcement.

For the laboratory phase of this research, all mixtures contained Type II portland cement and Class F fly ash, which was added at 20 percent of the total cementitious material. The coarse aggregate, consisting of crushed granite gneiss with an NMS of 25 mm, was prepared by blending aggregates retained on the 19.0-, 12.5-, 9.5-, and 4.75-mm sieves, each 25 percent by weight. The fine aggregate was natural sand. Several admixtures were included in the mixture: a saponified-resin, air-entraining admixture (AEA) complying with the requirements of ASTM C 260; a lignin, regular water-reducing admixture (WRA) complying with the requirements of ASTM C 494, Type A; and, polycarboxylate HRWRA complying with the requirements of ASTM C 494, Type F, Ozyildirim and Lane state.

During the field phase of this project, SCC mixtures were produced at a precast plant and a prestressing plant, designated as P1 and P2, respectively. To verify self-consolidation, these specimens were compared to additional samples subjected to rodding, the method of consolidation in the field. The strength and permeability tests of the consolidated samples provided a baseline for evaluating the need for consolidation of the remaining specimens. Freeze/thaw resistance was also determined in the field mixtures. Moist-cured beams were tested. The plant mixtures had smaller aggregates than the laboratory mixtures in order to maximize paste content and improve flow characteristics, the researchers explain.

For the precast plant (P1) mixture, the slump flow values ranged from 572 to 660 mm. Slump flow values for the mixture at the prestressing plant (P2) were considerably lower, starting at 483 mm for Batch 1 and increasing to 572 mm with the combination of WRA and HRWRA in Batch 2.

Lab and field results supported the feasibility of SCC, prompting a field application involving an arch bridge in Fredericksburg, Va. With arches comprising heavily reinforced, thin, curved sections difficult to construct of conventional concrete, the researchers note, this project was an excellent candidate for SCC.

The bridge carries traffic over a small creek in a residential area. A total of 25 precast arch segments placed side by side creates a single 9.14-m span across the creek. Each segment is an ellipsoidal arch measuring 2.29 m wide and 254 mm thick, with an arc length of 13.72 m.

Ozyildirim and Lane offered several conclusions supported by their research:

• SCC that flows into formwork and through reinforcement under the influence of its own weight can be made such that no external vibration is required.
• Although careful proportioning and batching are needed, SCC can be produced with locally available materials.
• SCC can have high compressive strength and low permeability for use in bridge structures.
• Concrete with high-slump flow is prone to segregation and bleeding. Tests should be conducted with the material used for a specific project to establish sufficient flow of SCC without segregation, bleeding, or the need for additional consolidation.
• Increased drying shrinkage, improper air-void systems, and reduced freeze/thaw resistance can occur, but are not necessarily intrinsic to SCC. Using the correct proportion of materials can mitigate these problems.
• To obtain high-slump flow without segregation, the amount of fine material should be increased by reducing the NMS, increasing the fine-aggregate-to-coarse-aggregate ratio, and increasing the amount of cementitious material. Without such modifications, the use of a VMA may be necessary.

“SCC is particularly applicable to thin sections and areas with dense reinforcement because of its high workability,” the researchers emphasize.

Precast slabs offer hassle-free, full-depth pavement repairs

Precast slabs for full-depth, portland cement concrete (PCC) repairs may speed construction, with attendant reduction in road user delays and work zone exposure, contend Neeraj Buch, Michigan State University, Vernon Barnhart, Michigan DOT, and Rahul Kowli, MSU, in their paper, “Pre-Cast Concrete Slabs as Full-Depth Repairs.” The authors state, “The use of precast [PCC] panels eliminates the time required for curing and offers numerous other benefits, such as excellent quality of concrete (strength and durability), minimal variability in slab thickness, and minimal negative impacts from built-in curl.”

In October 2001 and summer 2002, MSU in cooperation with the Michigan DOT installed 21 full-depth, precast patches along I-94 BL (Benton Harbor) and I-196 (South Haven) in southwestern Michigan. According to the authors, the performance of full-depth PCC pavement repairs is inconsistent; poorly designed and poorly constructed repairs account for the lack of consistency. “The pressure for early opening to traffic — an increasing number of state highway agencies are specifying six to eight hours opening time — has compounded the problem, leading to the development of fast-track concrete mixtures that can satisfy specified mechanical properties,” they observe. “In many instances, the result has been the construction of full-depth repairs and complete slab replacements with PCC that possessed adequate initial strength, but poor long-term durability characteristics.”

An alternative to cast-in-place concrete patches is the use of precast patches to address issues related to joint and slab deterioration, they propose. However, very limited laboratory or field data is available on the construction and performance of these precast patches.

Using precast PCC panels has the potential to address the key issues of urban pavement renewal, such as minimized construction time and reduced user delay costs, as well as enhanced long-term pavement performance. Such benefits are among the many that precast PCC panels offer due to the elimination of time required for PCC curing:

• Excellent quality concrete batched in factory conditions, with high strength, low shrinkage and superior durability;
• Control of built-in curling, yielding no slabs with excessive built-in curling; and,
• Greatly reduced construction variability, including uniform thickness and material quality.

“The complete development of this preventive maintenance treatment could greatly enhance the capability of [state highway agencies] to effectively extend the usable service life of older concrete pavements,” the authors affirm. “By delaying expensive major rehabilitation or reconstruction, millions of dollars can be transferred annually to fund additional preventive projects on the state network.”

When user costs are figured in, as required for major projects by federal law when federal funds are involved, the economics of using precast slabs for full-depth repairs become even more compelling, conclude Buch, Barnhart and Kowli.

Tennessee evaluates its two high-performance concrete bridges

Tennessee has studied its high-performance concrete (HPC) jointless bridges featuring integral abutments, write David J. Knickerbocker, Prodyot K. Basu, D.Sc., Vanderbilt University, Department of Civil & Environmental Engineering, and Mark A. Holloran, P.E., and Edward P. Wasserman, P.E., Tennessee DOT, in their paper, “Recent Experience with High Performance Concrete Jointless Bridges in Tennessee.” According to the authors, the two bridges were built in Tennessee as part of the Federal Highway Administration's (FHWA) nationwide initiative to implement HPC in bridge structures. Performance of the bridges was observed through all stages of construction and service to date, via material testing, bridge instrumentation for both short- and long-term performance monitoring, and live-load testing.

“Local fabricators were found to be capable of producing concrete to meet increased requirements in strength and durability parameters,” they contend. “In addition, new insights were derived about HPC behavior in such applications, identifying areas that require updating of current practice,” the authors note. Load test data revealed that load distribution among the girders differed markedly from codes of practice. Thermal response of the bridges indicated longitudinal flexibility offered by the jointless construction.

The Porter Road Bridge and the Hickman Road Bridge were erected in September 2000. “Both bridges have pretensioned concrete girders and cast-in-place, reinforced concrete deck slab,” the writers report. “As in the case of the majority of recent Tennessee bridges, these two are jointless with integral abutments.

HPC MIX DESIGN, TENNESSEE BRIDGE GIRDERS
(KNICKERBOCKER, ET AL.)

MATERIALS		DENSITY
		U.S. Custom	Metric
Fine aggregate (red sand)		974 lbs/yd3	578 kg/m3
Coarse aggregate
	#67	1,439 lbs/yd3	854 kg/m3
	#11	481 lbs/yd3	285 kg/m3
Cement Type I		747 lbs/yd3	443 kg/m3
Flyash Type C		249 lbs/yd3	148 kg/m3
High-range water reducer		5-15 oz/yd3	148-444 ml/m3
Water		29.75 gal/yd3	147 l/m3
Remarks: Retarder to be added when ambient temperature is 75° or higher. Maximum slump not to exceed 8 in. after addition of HRWR.

“This project takes full advantage of the superior properties of HPC by using it in pretensioned girders and reinforced deck slab, coupling it with jointless construction by making the girders and deck monolithic with the abutment wall and the deck slab continuous over the intermediate bent,” the authors observe. “It is expected that this synergy — the ideal combination of material and structural system — will lead to more dramatic short- and long-term benefits.” The design permits some of the longest spans of integral abutment, prestressed concrete highway bridges to be put into service.

First Missouri HPC bridge tested with live loads

Live-load tests prove that Missouri's first high-performance concrete (HPC) bridge is meeting AASHTO guidelines, say Yumin Yang, graduate research assistant, and John J. Myers, Ph.D., P.E., assistant professor of civil engineering, University of Missouri-Rolla, in their paper, “Live Load Test Results of Missouri's First High Performance Concrete Superstructure Bridge.” The authors state, “For its significant economical savings and greater design flexibility, high performance concrete (HPC) is becoming more widely utilized in highway bridge structures. High performance bridges with HPC and large-diameter prestressed strands are becoming attractive to designers.”

With design compressive strengths between 69 MPa (10,000 psi) and 90 MPa (15,000 psi), HPC has been successfully produced with conventional materials and concrete production methods. Bridge A6130 is the first bridge in Missouri whose superstructure is fully constructed of HPC, report Yuan and Myers. The bridge has high-strength concrete (HSC) girders with a high performance concrete cast-in-place deck. The HSC precast/prestressed girders utilize 15.2-mm (0.6-in.)-diameter strand. Strain gauges and thermocouples embedded in the concrete were used to monitor early-age and later-age behavior of the bridge components from construction through service.

To investigate the overall behavior of the bridge under live load, a static, live-load test was developed and implemented last June. During the live load test, 64 embedded vibrating-wire strain gauges and 14 embedded electrical-resistance strain gauges were used to monitor the changing strain rate in the bridge caused by varying live load conditions, Yang and Myers write.

Girder deflections and rotations were also recorded using external sensors and a data acquisition system. Based on the test results, load distribution to the girders was determined. Comparing the measured value to AASHTO specifications, the live-load distribution factor (AASHTO, 1994) recommended for design was found to be overly conservative.

By contrast, AASHTO-recommended LRFD live-load distribution factors (AASHTO, 2002) were found to be comparable to measured values. Two finite element models developed using ANSYS were compared with measured values to investigate the continuity level of the Missouri Department of Transportation interior bent detail.

CORROSION MARS FLEXURAL REINFORCEMENT IN PRECAST BEAMS

Potentially serious deterioration is plaguing Arkansas' older precast bridges, and they're not alone, say Stephan A. Durham, E.I.T., Ernest Heymsfield, Ph.D, P.E., and John J. Schemmel, Ph.D, P.E., University of Arkansas-Fayetteville, in their TRB paper, “The Structural Evaluation of Precast Concrete Channel Beams in Bridge Superstructures.” From the mid-1950s through the mid-1970s, a large number of bridges was constructed throughout Arkansas using the then-standard, 5.79-m (19-ft.)-long precast, nonprestressed, concrete channel beam. A survey of highway departments has identified 12 states that used a similar bridge element in the past. It has been determined that nearly 400 of these bridges remain in use in Arkansas alone.

Recently, the Arkansas State Highway and Transportation Department (AHTD) discovered that a number of these sections are exhibiting potentially serious deterioration. It appears to have been initiated by the corrosion of the flexural reinforcement in the beam stems.

An additional source of concern is that these beams were fabricated without any shear reinforcement. Moreover, some sections are also showing signs of concrete degradation. “The need to determine the in-place load capacity, serviceability, and durability of these sections has reached a critical level,” the authors attest. To date, 20 beams have been removed from existing structures and tested for their flexural load capacity as well as the material properties of the concrete and longitudinal reinforcement.

Results have varied, depending upon the extent of any deterioration, they report. “However, in nearly every case, shear failure has controlled the load capacity of a section. Based on this research, a draft field guide, intended for use by inspection crews, is being prepared. This guide will aid inspectors in prioritizing sections for repair, rehabilitation, or removal.”

Results from the study will be summarized in a document titled “A Guide for the Field Evaluation of Precast Concrete Channel Beams in Arkansas.” The authors note, “The intent of this manual is to provide the Arkansas HTD with a resource to relate the physical appearance of a precast channel section to beam load-carrying capacity.”