Mary Beth D. Hueste

Mechanical Properties of High-Strength Concrete for Prestressed Members

High-strength concrete (HSC) is widely used in prestressed concrete bridges (PCBs). Current design guidelines for PCB structures such as the AASHTO LRFD specifications, however, were developed based on mechanical properties of normal-strength concrete (NSC). As a first step toward evaluating applicability of current AASHTO design provisions for HSC prestressed bridge members, statistical parameters for the mechanical properties of plant-produced HSC were determined. In addition, prediction equations relating mechanical properties with the compressive strength were evaluated. HSC samples were collected in the field from precasters in Texas and tested in the lab at different ages for compressive strength, modulus of rupture, splitting tensile strength, and modulus of elasticity. Statistical analyses were conducted to determine the probability distribution, bias factors (actual mean-to-specified design ratios), and coefficients of variation for each mechanical property. It was found that for each mechanical property, the mean values are not significantly different among the considered factors (precaster, age, specified strength class) or combination of these factors, regardless of specified design compressive strength. Overall, 28-day bias factors (mean-to-nominal ratios) decrease with increases in specified design compressive strength due to relative uniformity of mixture proportions provided for the specified strength range. Still, the 28-day bias factors for compressive strength are higher than those used to calibrate AASHTO LRFD specifications. With few exceptions, the coefficients of variation were uniform for each mechanical property. In addition, the coefficients of variation for the compressive strength and splitting tensile strength of HSC in this study are lower than those for NSC used in the development of the AASHTO LRFD specifications.

Parameters Influencing Corrosion and Tension Capacity of Post-Tensioning Strands

A 12-month long strand corrosion test program with 298 specimens was conducted to identify and quantify parameters influencing corrosion and tension capacity of strands in post-tensioned bridges. The parameters investigated were grout class, moisture content, chloride concentration, void type, and stress level. The test specimens were 41 in. (1041 mm) long, in unstressed or stressed conditions, partially or completely embedded in cementitious grout, and exposed to various environmental conditions representing possible field conditions. After the exposure period, the grout material was removed and the strand surfaces were cleaned and visually evaluated for corrosion damage. The tension capacities of the strands were then determined. Results indicate that the corrosion was most severe at or near the grout-air-strand (GAS) interface. Corrosion evaluation and statistical analysis of the strand tension capacity results show that orthogonal, inclined, and bleedwater void conditions caused more corrosion and tension capacity loss than parallel and no-void conditions. The change in grout class did not result in statistically significant changes in the tension capacity of the strand samples evaluated. Statistically significant changes in tension capacity were observed with changes in the GAS interface, stress level, moisture content, and chloride concentration.

Evaluation of a four-story reinforced concrete building damaged during the Northridge Earthquake

A four-story reinforced concrete (RC) frame structure damaged during the Northridge Earthquake was evaluated to determine if punching shear failures that occurred at interior slab-column connections could be post-calculated using analytical methods. The building was evaluated using: 1) a code level strength analysis, 2) a static nonlinear (“pushover”) analysis, and 3) a dynamic nonlinear response analysis using ground motions recorded within 1 km (0.6 mi.) of the building. Based on design material properties and field observations, both the static and dynamic nonlinear analyses were able to successfully post-calculate the observed punching shear failures.

Probabilistic Capacity Models for Corroding Posttensioning Strands Calibrated Using Laboratory Results

The presence of air voids, moisture, and chlorides inside tendons or ducts was cited as a reason for the early age strand corrosion and failure in the Mid-bay, Sunshine Skyway, and Niles Channel posttensioned (PT) bridges in Florida, United States. Although rare, these incidents call for frequent inspection and structural reliability assessment of PT bridges exposed to moisture and chlorides. This paper develops and presents probabilistic strand capacity models that are needed to assess the structural reliability of such PT bridges and recommends a time frequency of inspection. A total of 384 strand test specimens were exposed to various void, moisture, and chloride concentration conditions for 12 and 21 months; the remaining tension capacities were then determined. Using this experimental data and a Bayesian approach, six probabilistic capacity models were developed based on the void type. The mean absolute percentage errors of these models are less than 4%, indicating that reasonably accurate prediction of the strand capacity is possible, when void, aggressive moisture, and chloride conditions are present.

Probabilistic Models for the Tensile Strength of Corroding Strands in Posttensioned Segmental Concrete Bridges

The presence of air voids, moisture, and chlorides inside tendon systems on segmental posttensioned (PT) bridges has been cited as a reason for the early age corrosion and failure of strands in these bridges. This paper develops probabilistic models to predict the time-variant tension capacity of PT strands exposed to wet-dry conditions. A total of 384 unstressed and 162 stressed strand test specimens were exposed to various void, moisture, and chloride conditions for 0, 12, 16, and 21 months; the residual tension capacities of the strands were then determined. Using these experimental data, a Bayesian approach is used to develop probabilistic capacity models for unstressed and stressed strands. The tension capacities of stressed strands under potential void, wet-dry, and chloride conditions in the field are predicted using the developed models. Probabilistic time-variant models are formulated in such a way that they can be updated by other researchers using additional information from the testing of unstressed strands only, avoiding expensive and cumbersome testing of stressed strands. The mean absolute percentage errors of these models are less than 3.2%, indicating good overall model accuracy.

Experimental Study on Creep and Durability of High-Early-Strength Self-Consolidating Concrete for Precast Elements

Self-consolidating concrete (SCC) typically has high cement paste volumes to achieve the desired fresh characteristics. These high paste volumes may lead to increased creep, which can increase the concrete compressive strain in prestressed concrete members. This increased compressive strain can lead to a reduction in the prestressing force for these elements. Reduced prestressing forces can result in higher deflections and reduced capacities. Accurate estimates of the creep of SCC prestressed members are needed so that these structural elements can be properly designed. The creep of high-early-strength (HES) SCC mixtures was assessed in this study. In addition to the creep characteristics, the durability characteristics of HES SCC need to be assessed. Because sustainability and durability are critical for long-lasting, economically viable systems, implementing the use of a new material that exhibits poor durability can be costly and unsafe. The permeability, diffusivity, and freezing-and-thawing resistance of HES SCC mixtures were assessed in this study. An assessment of the creep compliance of HES SCC values indicates that the 2006 American Association of State Highway Transportation Officials (AASHTO) Load and Resistance Factor Design Specifications predicted the creep compliance with the highest accuracy among all the models considered in this research. The 2004 AASHTO, ACI 209, and CEB-FIP MC90-99 models provide fairly good predictions of the creep compliance for both the conventional concrete (CC) and HES SCC mixture prediction equations. It is noted that for more accurate predictions, however, all models should be calibrated with data from full-scale structural elements. Durability measurements indicate similar performances for the HES SCC mixtures relative to the CC mixtures; however, the 5 ksi (35 MPa) release strength CC and HES SCC mixtures exhibited low resistance to freezing-and-thawing cycles.

Shear Characteristics and Design for High-Strength Self-Consolidating Concrete

To achieve adequate flow and homogeneous concrete for precast, prestressed members, self-consolidating concrete (SCC) typically has higher paste and lower coarse aggregate volumes than conventional concrete (CC). The lower aggregate content of SCC can affect the shear capacity of concrete systems. This research performed 48 push-off tests to investigate the influence of SCC aggregate and paste volumes on the shear capacity and these results were compared with those obtained from similar CC samples. The variables included coarse aggregate type (river gravel and limestone), three coarse aggregate volumes for the SCC mixtures, and two target 16-h release strengths [34 and 48 MPa (5 and 7 ksi)]. The aggregate type, aggregate volume, and concrete strength were found to have significant effects on the aggregate interlock. Test results were used to propose new aggregate interlock models based on the modified compression field theory adopted in the AASHTO Load and Resistance Factor Design Specifications. More appropriate expressions have been developed to determine the limiting value of concrete shear strength for CC and SCC precast, prestressed concrete girders with similar mixture proportions, and a 28-day compressive strength greater than 70 MPa (10 ksi).

Seismic design criteria for slab-column connections

Two-way slabs without beams are popular floor systems because of their relatively simple formwork and the potential for shorter story heights. Earthquakes, however, have demonstrated that slab-column frames are vulnerable to brittle punching shear failures in the slab-column connection region and dropping of the slab, which are costly to repair. This paper focuses on the behavior and design of slab-column connections under combined gravity and lateral loading and reviews current design procedures, performance-based design approaches, and relevant experimental data. An equation relating the gravity shear ratio at a slab-column connection to drift capacity is presented. Finally, practical recommendations are provided for defining specific performance objectives.

Bond Performance in Self-Consolidating Concrete Pretensioned Bridge Girders

This study investigated the potential application of high-early-strength self-consolidating concrete (HES SCC) in precast, prestressed bridge girders. This paper discusses the findings from tests performed to evaluate the transfer length, development length, and prestress losses of prestressed bridge girder-deck systems made with HES SCC. These results were compared to control girder-deck systems made with conventional concrete (CC). In addition, pullout tests were performed to investigate the top-bar effect on the tension development length of mild steel reinforcement. The results indicate that the bond performance of the HES SCC girders is similar to or better than that of the companion control CC girders. The ACI 318-08 and American Association of State Highway and Transportation Officials (AASHTO) Load and Resistance Factor Design (LRFD) equations for strand transfer and development length are appropriate for HES SCC girder-deck systems. The evaluated elastic shortening and gains are reasonably estimated using the AASHTO and ACI 318-08 expressions. The ACI 318-08 and AASHTO top-bar multipliers are appropriate for the specimens evaluated in this study.

Cracking in reinforced concrete bent caps

This paper presents findings from an experimental investigation into the causes of unexpected cracking in reinforced concrete bent caps at outside column locations during service load conditions. 16 full-scale bent cap specimens were constructed and tested under quasistatic monotonic loading. Several bent cap parameters were evaluated with regard to their influence on cracking. Cracks were measured and reinforcement strain data was recorded throughout the load history. The experimental program confirmed that flexural crack widths during service loading are directly proportional to the level of stress in the longitudinal reinforcement, and that the allowable service stress limit should be lowered to reduce such cracking. In addition, increasing the shear strength of the bent cap led to reduced inclined flexure-shear crack widths.