Catherine French

Strain-softening of concrete in uniaxial compression

0025-5432/97

Stresses in Unbonded Prestressing Tendons at Ultimate: Recommendation

In this research, equations to predict the stress in unbonded tendons at nominal bending resistance of flexural members are given in various codes. They are based on past experience with steel tendons and do not account for a number of key parameters, such as use of fiber-reinforced polymeric (FRP) tendons, partial prestressing, span-depth ratio, loading conditions, and loading patterns in continuous beams. The methodology and prediction equation proposed herein attempt to provide a rational solution to the problem and give a recommendation for code implementation.

TRANSVERSE CRACKING IN CONCRETE BRIDGE DECKS

The dominant parameters that lead to premature transverse cracking in bridge decks are determined, and recommendations to reduce cracking tendency in bridge decks are developed. The project is divided into two parts: a field study and a parametric study. The objective of the field study is to determine the correlation between the observed cracking and available design-, material-, and construction-related data. Seventy-two bridges in the Minneapolis/St. Paul metropolitan area are included in the field study. According to the results of the study and correlation with other research, restrained concrete deck shrinkage is the leading cause of cracking. The dominant factors affecting transverse cracking are the longitudinal end restraint, girder stiffness, cross-frame location, splice location, deck thickness, cutoff length of the deck supplemental reinforcing bar, size of the top transverse bar, concrete shrinkage, deck concrete modulus of elasticity, cement content, aggregate type and quantity, air content, and ambient air temperature at deck placement. Recommended practical improvements to bridge deck construction include reducing the shrinkage of the deck concrete through mix design or better curing practices and minimizing deck restraint.

MECHANICAL PROPERTIES OF HIGH-STRENGTH CONCRETE WITH CONSIDERATION FOR PRECAST APPLICATIONS

This paper presents the results of over 6,000 tests to investigate the mechanical properties (compressive strength, modulus of elasticity, and tensile strength) of high-strength concrete (HSC). HSCs with 28-day compressive strengths in the range of 8,000-18,600 psi (55.2-128 MPa) were produced. Material variables included total amount and composition of cementitious material, type and brand of cement, type and brand of superplasticizer, and coarse aggregate type, source, size, and gradation. Both heat- and moist-curing conditions were investigated. The heat-curing process emulated that used in precast plants. The experimental results are compared with code relations and proposed equations for high-strength concrete. Based on the results of these tests, recommended relationships are given.

FIELD PERFORMANCE OF INTEGRAL ABUTMENT BRIDGE

The behavior of an integral abutment bridge near Rochester, Minnesota, was investigated from the beginning of construction through several years of service by monitoring more than 180 instruments that were installed in the bridge during construction. The instrumentation was used to measure abutment horizontal movement, abutment rotation, abutment pile strains, earth pressure behind abutments, pier pile strains, prestressed girder strains, concrete deck strains, thermal gradients, steel reinforcement strains, girder displacements, approach panel settlement, frost depth, and weather. In addition to determining the seasonal and daily trends of bridge behavior, live-load tests were conducted. All of the bridge components performed within the design parameters. The effects from the environmental loading of solar radiation and changing ambient temperature were found to be as large as or larger than live-load effects. The abutment was found to accommodate superstructure expansion and contraction through horizontal translation instead of rotation. The abutment piles appeared to be deforming in double curvature, with measured pile strains on the approach panel side of the piles indicating the onset of yielding.

TIME-DEPENDENT PROPERTIES OF HIGH-STRENGTH CONCRETE WITH CONSIDERATION FOR PRECAST APPLICATIONS

This paper addresses the creep, shrinkage, and indirect permeability (water absorption) time-dependent properties of high-strength concrete. Tests were conducted on over 250 specimens to investigate the effects of variations in material composition and curing. Results were compared with American Concrete Institute (ACI) 209 relations for normal-strength concrete. Proposed modifications to the ACI 209 relations are given to represent the high-strength concrete data.

Cyclic response of reinforced concrete walls with different anchorage details: Experimental investigation

Previoustestsofstructuralwallshaveroutinelyusedcontinuousreinforcementextendingfromthefoundationtothetopofthespec- imen. This detailing is consistently different from that of multistory walls in the field, which incorporate splices in the wall longitudinal rein- forcement above the wall-foundation interface. As a result, the performance of walls incorporating continuous reinforcement in the laboratory may not be representative of walls in the field that use lap splices or mechanical couplers near the wall base. This paper investigates lateral load behavior of three nominally identical structural walls with continuous reinforcement, lap splices, and mechanical couplers in the plastic hinge region, and quantifies the differences in their responses using force-displacement response, lateral deformation components, and energy dissipation estimated using equivalent viscous damping. DOI: 10.1061/(ASCE)ST.1943-541X.0000732.

Fatigue Evaluation of Transverse U-Bar Joint Details for Accelerated Bridge Construction

When precast full-depth deck panels and/or decked bulb-Ts (DBTs) are used for accelerated bridge construction, transverse joints exist. In this study, continuous transverse U-bar joint details, which can provide negative moment continuity in multispan bridges, have been investigated. Four full-scale specimens connected by the developed U-bar detail together with the selected closure-pour (CP) materials, overnight cure and seven-day cure, were tested. Static and fatigue tests under tension loading were conducted. The loading demand necessary in the testing was determined based on the maximum forces in the transverse joint from an analytical study. Test results were evaluated based on tension capacity, cracking, and steel strain. Based on these test results, the developed transverse U-bar joint detail is a promising connection system.

DUCTILE CONNECTIONS FOR PRECAST CONCRETE FRAME SYSTEMS

This paper describes a research program to investigate the behavior of ductile connections between precast beam-column elements. Eight beam-column connections were tested to characterize the overall behavior of the connection details. Each connection specimen was designed to incorporate one of three behavioral concepts in the connection elements: tension/compression yielding, substantial energy dissipation, or nonlinear-elastic response. Based on the behavioral information collected during connection tests, analytical models were developed to investigate the behavior of complete precast frame systems. Results of the experimental study and preliminary results of the analytical work are presented. The objective of the program is to provide rational design recommendations for engineers to detail precast frame connections for use in regions of seismic risk.

Instrumentation and Modeling of I-35W St. Anthony Falls Bridge

The I-35W St. Anthony Falls Bridge was constructed to replace the steel truss bridge that collapsed on August 1, 2007. The design of the replacement bridge featured a smart-bridge system. The smart-bridge concept included instrumentation for long-term monitoring of the structural behavior of the bridge. Truck-load tests were conducted prior to opening the bridge and 26 months later to measure the response of the structure under controlled loading. The measurements were used to validate a FEM model of the bridge constructed to further investigate the behavior of the structure. The correlation between computed and measured results was found to be good. This paper describes the bridge, the instrumentation installed within the bridge, the FEM model, and validation of the model with respect to the truck-load tests. Recommendations are provided for static instrumentation plans of concrete box-girder structures.