Robert W Barnes

Properties of Self-Consolidating Concrete for Prestressed Members

This paper evaluates self-consolidating concrete (SCC) mixtures for use in prestressed concrete applications. In the laboratory, 21 SCC mixtures are created with varying water-to-cementitious materials ratios, sand-to-total aggregate ratios, and cementitious material combinations (e.g., Type III cement, Class C fly ash, ground-granular blast-furnace slag, silica fume). Prestress transfer compressive strengths of between 5470 and 9530 psi (38 and 66 MPa) are shown for the SCC mixtures. The authors note that the moduli of elasticity of the SCC mixtures show reasonable agreement with the elastic stiffness assumed to be present in the design of conventional slump concrete structures. Also noted is that long-term drying shrinkage strain for all the SCC mixtures show approximately the same or less shrinkage strain than those measured for the control mixtures. Moreover, long-term drying shrinkage is not significantly affected by a change in sand-to-total aggregate ratio. And, at later ages of 56 and 112 days, measured drying shrinkage corresponds reasonably well to drying shrinkage predicted by the American Concrete Institute (ACI) Committee 209 procedure.

Early-Age Cracking Tendency and Ultimate Degree of Hydration of Internally Cured Concrete

Early-age cracking in bridge decks is a severe problem that may reduce functional life of the structure. The effect of lightweight fine aggregate on the cracking tendency of bridge deck concrete was evaluated using cracking-frame testing techniques. Water for internal curing is provided by incorporating prewetted absorptive lightweight aggregate (LWA) into the mixture during batching. The absorbed water within the LWA is desorbed at early ages as hydration progresses. The release of the internal curing water increases cementing material hydration and reduces capillary stress caused by self-desiccation. Cracking frames measure the development of stresses due to thermal and autogenous shrinkage effects from setting until the onset of cracking. Restrained concrete specimens were tested under temperature conditions that match those of an in-place bridge deck and under isothermal curing conditions to determine the stress development due to autogenous shrinkage. Internal curing from LWA increased the time to initial cracking, reduced autogenous shrinkage stress development, and increased the degree of hydration in bridge deck concretes.

Cracking Tendency of Bridge Deck Concrete

Early-age cracking can adversely affect the behavior and durability of bridge deck concrete. Cracking of hardening concrete occurs when the induced tensile stress exceeds the tensile strength of the concrete. The development of in-place stresses is affected by the shrinkage, modulus of elasticity, coefficient of thermal expansion, setting characteristics, restraint conditions, stress relaxation, and temperature history of the hardening concrete. Tensile strength increases as the hydration of the cementitious system progresses. Rigid cracking frame (RCF) testing techniques capture the combined effects of modulus of elasticity, creep and relaxation, coefficient of thermal expansion, thermal conductivity, autogenous shrinkage, and tensile strength on the cracking potential of a mixture in a specific application. This paper describes an experimental evaluation of the effect of supplementary cementing materials, water-to-cement ratio (w/c), and placement temperature conditions on the early-age cracking tendency of bridge deck concrete through the use of RCF testing techniques. Specimens were tested under temperature conditions that match those in an 8-in.-thick bridge deck to explore early-age cracking mechanisms. The laboratory testing program revealed that the placement temperature and curing temperature significantly affected the time to cracking of all the mixtures. Use of either fly ash or ground-granulated blast-furnace slag was effective in reducing the heat generation and rate of stiffness development in bridge deck concretes and thus in significantly reducing restraint stresses and delayed the occurrence of cracking at early ages. A decrease in w/c resulted in increased stresses, and it accelerated the occurrence of cracking at early ages.

Fiber-Reinforced Polymer Strengthening of Concrete Bridges that Remain Open to Traffic

This brief discussion article comments on a study of traffic loads applied to a reinforced concrete bridge during and after its strengthening with externally bonded fiber-reinforced polymer (FRP) reinforcement (W. Reed et al, 2005). The study included eight beams: seven of them were strengthened with a precured, unidirectional, carbon-fiber laminate strip that represented the materials used on an actual bridge. The variables tested included the intensity and frequency of load cycles applied during the epoxy-curing period, the thickness of the epoxy layer, and the thickness of the FRP strip. The study showed that for all of the traffic loads applied during and after installation no reduction in strengthening was seen. The experiment concluded that concrete bridges can be strengthened effectively even if they remain open to traffic during the procedure. In this commentary, Barnes reminds readers of the importance of considering the effects of traffic vibration during curing. Traffic vibration would have an effect on the strength of the adhesives typically used for this application. However, as the failure plane in an FRP strengthened concrete beam is typically within the concrete, the weakening of the adhesive does not affect the overall load capacity of the strengthened beams. Barnes concludes by cautioning that in other situations, such as steel beams, vibration from traffic loads may indeed lead to a reduction in ultimate capacity of the strengthened member.

Assessment of Stability Test Methods for Self-Consolidating Concrete

Five test methods were evaluated that assess the stability of fresh self-consolidating concrete (SCC): the visual stability index (VSI) test; column segregation test; rapid penetration test; sieve stability test; and surface settlement test. These tests were performed on nine precast, prestressed-suitable SCC mixtures each placed in four walls of varying heights, and the stability test results were compared to the results of ultrasonic pulse velocity (UPV) and pullout testing conducted on the hardened concrete walls. The surface settlement, VSI, and sieve stability test results were best correlated to the hardened concrete uniformity test results. Therefore, the surface settlement test should be used to prequalify precast, prestressed SCC mixtures prior to production, while the combined use of the VSI and sieve stability tests is recommended to determine batch acceptance during precast, prestressed concrete production.

LIVE-LOAD RESPONSE OF ALABAMA'S HIGH-PERFORMANCE CONCRETE BRIDGE

Results are reported from live-load tests performed on Alabama’s high-performance concrete (HPC) showcase bridge. Load distribution factors, deflections, and stresses measured during the tests are compared with values calculated using the provisions of the AASHTO LRFD Bridge Design Specifications and AASHTO Standard Specifications for Highway Bridges. Measured dynamic amplification of load effects was approximately equal to or less than predicted by both specifications. Distribution factors from both specifications were found to be conservative. Deflections computed according to AASHTO LRFD Bridge Design Specifications suggestions matched best with the measured deflections — overestimating the maximum deflections by 20% or less. Bottom flange stresses computed with AASHTO distribution factors were significantly larger than measured values. AASHTO LRFD Bridge Design Specifications provisions suggest a special procedure for computing exterior girder distribution factors in bridges with diaphragms. When two or more lanes were loaded, this special procedure did not reflect the actual behavior of the bridge and resulted in very conservative distribution factors for exterior girders. Further research is recommended to correct this deficiency.

Cracking Tendency of Lightweight Aggregate Bridge Deck Concrete

Early-age cracking in bridge decks is a severe problem that may reduce the functional life of the structure. In this project, the effect of using lightweight aggregate on the cracking tendency of bridge deck concrete was evaluated using testing frames that restrain movement due to volume change effects from placement to cracking. Expanded shale, clay, and slate lightweight coarse and fine aggregates were used to produce internal curing, sand-lightweight, and all-lightweight concretes to compare their behavior relative to a normalweight concrete in bridge deck applications. Specimens were tested under temperature conditions that simulate summer and fall placement scenarios. Increasing the amount of pre-wetted lightweight aggregate in the concrete systematically decreased the density, modulus of elasticity, and coefficient of thermal expansion of the concrete. When compared to a normalweight concrete, the use of lightweight aggregates in concrete effectively delays the occurrence of early-age cracking in bridge deck applications.

Full-Scale Implementation and Testing of Full-Depth Precast Bridge Deck Panels

A bridge deck panel system using nonprestressed full-depth precast concrete bridge deck panels with continuous shear pockets was investigated. First, the research team performed conceptual improvement, design, detailing, and fabrication studies on a specific deck replacement system (System CD-2) previously proposed by NCHRP Project 12-65 researchers. Key improvements to the CD-2 deck panel system included modifications to the transverse joint coupler for ease of construction and the addition of a longitudinal staged-construction joint to expedite bridge deck replacement projects. Next, an experimental program was carried out to construct and perform service-level load testing on a full-size precast deck panel assemblage that incorporated the refinements. On the basis of static and cyclic loading test results, it was found that the modified CD-2 deck panel system as a whole performed satisfactorily with regard to AASHTO serviceability requirements.