Concrete Cracking in Tension Members and Application to Deck Slabs of Bridges

Currently, estimations of the crack width in the deck slab of bridges given by codes of practice are based on either theoretical or empirical approaches considering mainly the monotonic loading behavior. However, cracking in reinforced tensile members is highly influenced by the loading history (including both the loading and unloading processes) because of the irreversible nonlinear behavior of bond and of tensile response of concrete, resulting into residual cracks of non-negligible width. This paper investigates the influence of this phenomenon and presents a physical model describing it. An analytical model is developed and its results are compared to various tests with good agreement. Finally, a simple design formula is derived and recommendations for its application to practical cases are proposed.     

Performance of Pile-Supported Bridge Approach Slabs

A large number of pile-supported bridge approach slabs in southeastern Louisiana were examined to identify the factors that affect their long-term performance. Design drawings and subsoil conditions at these sites as well as their traffic and maintenance records were compiled, and seven representative test sites were selected for thorough field investigation that included inspection of the approach slabs, bridge decks, bridge abutments, and roadway pavement. Field evaluation included walking profiler, falling-weight deflectometer (FWD), laser profiler, geodetic survey, soil borings, cone penetrometer, and nondestructive testing. Measurements made with the walking profiler agreed well with the geodetic survey. The FWD and nondestructive testing were effectively used to detect voids under the approach slab. Results of the study indicated that the current empirical methodology used by the Louisiana Department of Transportation and Development for design of pile-supported approach slabs yields inconsistent field performance. It was concluded that this inconsistent performance is primarily due to the differences in roadway embankment design and construction and in subsoil conditions, which in turn affect the negative skin friction (downdrag) loads imparted on the piles. Impact of other variables such as ramp type, speed limit, traffic volume, and so on was found to be insignificant. Results of the field study were used to develop a new rating system for approach slabs (IRIS) based on International Roughness Index (IRI) measurements obtained with the laser profiler.     

Behavior, Analysis, and Design of an Instrumented Link Slab Bridge

Presented in this paper are the results of a research project on the monitoring and assessment of the first link slab jointless bridge in the state of North Carolina. The structure was instrumented with a remote data acquisition system and monitored for over a year. In addition, a controlled load test was conducted in an effort to determine the demand on the link slab under known loads. A procedure for the limit-states design of a link slab system is also presented. Results indicate that while the crack size in the link slab exceeded the design level, the link slab fulfilled its function. Furthermore, the rotational demand from the large controlled loads as well as the traffic loads was similar in magnitude to the thermal induced rotations due to the difference in temperature between the top and bottom of the bridge.     

Local Effects Induced by Longitudinal Forces in Composite Girders

Local effects on the shear connection of composite girders induced by longitudinal actions such as the anchorages of prestressing cables, concrete shrinkage, or a uniform thermal action on the slab are analyzed. Closed-form solutions are obtained by using the simple model of a composite beam with a linearly elastic shear connection. Successively, by considering the limit scheme of an infinitely long beam, very simple formulas are derived permitting evaluation of the peak value and extension of the interface shear force distribution induced by the longitudinal actions. Numerical applications are carried out to show the effectiveness of the proposed formulas for a wide range of the shear connection stiffness and for longitudinal forces applied both along the beam axis and at the beam end.     

Nachträgliche Verstärkung gemauerter Tragwerke mit Faserverbundwerkstoffen

Gemauerte Konstruktionen lassen sich sehr effektiv mit Faserverbundwerkstoffen nachträglich verstärken. Dabei ist es sinnvoll, den Faserverbundwerkstoff direkt auf der Mauerwerksoberfläche durch das Einlegen von Fasergelegen in eine Klebstoffmatrix herzustellen. Anwendungsgebiete dieses Verfahrens sind die Umschnürung gemauerter Pfeiler, die Verstärkung von Wänden unter Scheibenbeanspruchung und die zugfeste Bewehrung biegebeanspruchter Bauteile. Die wichtigsten Ergebnisse umfangreicher experimenteller Untersuchungen, die an der Universität Kassel in den vergangenen Jahren durchgeführt wurden, werden vorgestellt und erläutert. Post‐strengthening of masonry structures with fibre reinforced polymers. Fiber reinforced polymeres can be used effectively for post‐strengthening of masonry structures. In this context it is reasonable to manufacture the FRP material by wet‐lay‐up directly on the surface of the masonry structure. Possible applications of the method are the confinement of columns as well as post‐strengthening of in‐plane and out‐of‐plane loaded structures. The main results of experimental research carried out at the University of Kassel during the last years will be presented.     

晶体弧矢弯曲鞍型变形抑制技术的研究

为了降低同步辐射弧矢聚焦单色器第二晶体弯曲过程中鞍型变形对衍射性能的影响,本文重点讨论背面加肋、黄金比例和等分变形槽三种鞍型形变的抑制技术,通过模拟计算、参数优化、试件制作和离线测量,从理论和实验两方面分析比较它们抑制效果。选择黄金比例γ=1.42弯曲模型为NSRL-XAFS光束线弧矢聚焦晶体单色器的改造升级方案。     

FRP-strengthened RC slabs anchored with FRP anchors

An abundance of tests over the last two decades has shown the bending capacity of flexural members such as reinforced concrete (RC) beams and slabs to be enhanced by the bonding of fibre-reinforced polymer (FRP) composites to their tension face. The propensity of the FRP to debond, however, limits its effectiveness. Different types of anchorages have therefore been investigated in order to delay or even prevent debonding. The so-called FRP anchor, which is made from rolled fibre sheets or bundles of lose fibres, is particularly suitable for anchoring FRP composites to a variety of structural element shapes. Studies that assess the effectiveness of FRP anchors in anchoring FRP strengthening in flexural members is, however, limited. This paper in turn reports a series of tests on one-way spanning simply supported RC slabs which have been strengthened in flexure with tension face bonded FRP composites and anchored with different arrangements of FRP anchors. The load-deflection responses of all slab tests are plotted, in addition to selected strain results. The behaviours of the specimens including the failure modes are also discussed. The greatest enhancement in load and deflection experienced by the six slabs strengthened with FRP plates and anchored with FRP anchors was 30% and 110%, respectively, over the unanchored FRP-strengthened control slab. The paper also discusses the strategic placement of FRP anchors for optimal strength and deflection enhancement in FRP-strengthened RC slabs.     

Innovative Hybrid Reinforcement for Flexural Members

The capacity and ductility of RC slabs are affected by the use of fiber-reinforced polymer (FRP) as their main reinforcement for repair and rehabilitation. The balance between increasing the capacity and reducing the ductility of flexural members reinforced with FRP was always a matter of discussion. In this research, innovative hybrid reinforcement system (HRS) was introduced to provide the required increase in capacity while keeping the ductility within an acceptable range. Ten RC one-way slabs were tested in this investigation. They included a control slab which was reinforced with ordinary steel, while the rest of the slabs were reinforced internally with HRS with nine different profiles. The main variables considered in this study were the type of core or perimeter reinforcement and the number of perimeter reinforcing layers. It was revealed that the use of the innovative HRS resulted in remarkable increases in section ultimate load capacity as well as ductility.     

Dynamic response and behavior of reinforced concrete slabs under impact loading

Reinforced concrete slabs are among the most common structural elements. In spite of the large number of slabs designed and built, the effect of their details on their behavior under impact loads are not always appreciated or properly taken into account. This experimental study was aimed at understanding the dynamic behavior of structural concrete slabs under impact loading to improve the state of the art of protective design. This study investigated the effects of different types of slab reinforcements and the applied impact loads on the dynamic response and behavior of reinforced concrete slabs.     

Strengthening of Interior Slab–Column Connections Using a Combination of FRP Sheets and Steel Bolts

The results of an experimental investigation undertaken to evaluate a new technique for strengthening interior slab–column connection in combined flexural and shear modes are presented. The technique consists of using a combination of shear bolts inserted into holes and prestressed against the concrete surface for improving the punching shear capacity, and external [fiber-reinforced polymer (FRP)] reinforcement bonded to the tension face of the slabs in two perpendicular directions for increasing the flexural strength of the slabs. Square slab specimens of 670×670?mm dimensions were tested and the main test variables included the ratio of steel reinforcement (1.0 and 1.5%), span–depth ratio or thickness (55 and 75?mm) of the slabs, area, and configuration of steel bolts, and area of FRP reinforcement. It was found that the use of shear bolts alone improves the punching shear strength and increases the ductility of failure by changing the failure mode from punching to flexural. However, the use of a combination of shear bolts and a moderate amount of FRP reinforcement increased the flexural strength and resulted in a substantial improvement of the punching shear capacity of the slabs. The corresponding increases attained levels between 34 and 77%. A design approach is presented for evaluating the ultimate capacity of the slab–column connections when strengthened using the proposed strengthening technique. Strength results predicted by the proposed approach were in good agreement with the experimental results.