Lateral Load Resistance of Diaphragms in Prestressed Concrete Girder Bridges, HR-319, 1991

Each year several prestressed concrete girder bridges in Iowa and other states are struck and damaged by vehicles with loads too high to pass under the bridge. Whether or not intermediate diaphragms play a significant role in reducing the effect of these unusual loading conditions has often been a topic of discussion. A study of the effects of the type and location of intermediate diaphragms in prestressed concrete girder bridges when the bridge girder flanges were subjected to various levels of vertical and horizontal loading was undertaken. The purpose of the research was to determine whether steel diaphragms of any conventional configuration can provide adequate protection to minimize the damage to prestressed concrete girders caused by lateral loads, similar to the protection provided by the reinforced concrete intermediate diaphragms presently being used by the Iowa Department of Transportation. The research program conducted and described in this report included the following: A comprehensive literature search and survey questionnaire were undertaken to define the state-of-the-art in the use of intermediate diaphragms in prestressed concrete girder bridges. A full scale, simple span, restressed concrete girder bridge model, containing three beams was constructed and tested with several types of intermediate diaphragms located at the one-third points of the span or at the mid-span. Analytical studies involving a three-dimensional finite element analysis model were used to provide additional information on the behavior of the experimental bridge. The performance of the bridge with no intermediate diaphragms was quite different than that with intermediate diaphragms in place. All intermediate diaphragms tested had some effect in distributing the loads to the slab and other girders, although some diaphragm types performed better than others. The research conducted has indicated that the replacement of the reinforced concrete intermediate diaphragms currently being used in Iowa with structural steel diaphragms may be possible.

Investigation of the Loss of Prestress in Prestressed Concrete Beams due to Steam Curing, HR-62, 1961

Mass production of prestressed concrete beams is facilitated by the accelerated curing of the concrete. The ·method most commonly used for this purpose is steam curing at atmospheric pressure. This requires concrete temperatures as high as 150°F. during the curing period. Prestressing facilities in Iowa are located out of doors. This means that during the winter season the forms are set and the steel cables are stressed at temperatures as low as 0°F. The thermal expansion of the prestressing cables should result in a reduction of the stress which was placed in them at the lower temperature. If the stress is reduced in the cables, then the amount of prestress ultimately transferred to the concrete may be less than the amount for which the beam was designed. Research project HR-62 was undertaken to measure and explain the difference between the initial stress placed in the cables and the actual stress which is eventually transferred to the concrete. The project was assigned to the Materials Department Laboratory under the general supervision of the Testing Engineer, Mr. James W. Johnson. A small stress bed complete with steam curing facilities was set up in the laboratory, and prestressed concrete beams were fabricated under closely controlled conditions. Measurements were made to determine the initial stress in the steel and the final stress in the concrete. The results of these tests indicate that there is a general loss of prestressing force in excess of that caused by elastic shortening of the concrete. The exact amount of the loss and the identification of the factors involved could not be determined from this limited investigation.

Fatigue and Residual Stress Investigation of Composite Prestressed Steel Beams, HR-74, 1963

Two composite, prestressed, steel beams, fabricated by slightly different methods, were fatigue tested to destruction. Stresses and deflections were measured at regular intervals, and the behavior of each beam as failure progressed was recorded. Residual stresses were then evaluated by testing segments of each beam. An attempt was made to assess the effects of the residual stresses on fatigue strength.

Field Observation of Five Lightweight Aggregate Pretensioned Prestressed Bridge Beams, HR-104, 1969

The use of lightweight aggregates in pretensioned prestressed concrete beams is becoming more advantageous as our design criteria dictate longer span concrete bridges. Bridge beams of greater lengths have been restricted from travel on many of our highways because the weight of the combined beams and transporting vehicle was excessive, making hauls of any distance prohibitive. This, along with the fact that new safety requirements necessitate the use of longer spans in grade separation structures over major highways, prompted the State of Iowa to investigate the use of lightweight aggregate bridge beams. The objective of this project is the collection of field deflection measurements for five pretensioned prestressed lightweight aggregate concrete bridge beams fabricated by conventional plant processes; also the comparison of the actual cambers and deflections of the beams with that predicted from the design assumptions.

The Flexural Fatigue Strength of Prestressed Steel I-Beams, HR-73, 1962

The purpose of this investigation was to study the flexural fatigue strength of two prestressed steel I-beams which had previously been fabricated in connection with a jointly sponsored project under the auspices of the Iowa State Highway Commission. The beams were prestressed by deflecting them under the action of a concentrated load at the center of a simple span, then welding unstressed high strength steel plates to the top and bottom flanges to retain a predetermined amount of prestress. The beams were rolled sections of A36 steel and the plates were USS "T-1" steel. Each of the two test specimens were subjected to an identical repeated loading until a fatigue failure occurred. The loading was designed to produce stresses equivalent to those which would have occurred in a simulated bridge and amounted to 84 percent of a standard H-15 live load including impact. One of the beams sustained 2,469,100 repetitions of load to failure and the other sustained 2,756,100 cycles. Following the fatigue tests, an experimental study was made to determine the state of stress that had been retained in the prestressed steel beams. This information, upon which the calculated stresses of the test could be superimposed, provided a method of correlating the fatigue strength of the beams with the fatigue information available on the two steels involved.

Live Load Deflections in a Prestressed Steel Beam Bridge, HR-74, 1963

This report describes the measurement of dynamic (live load) deflections in a 240' x 30' three span continuous prestressed steel bridge, skewed 30 degrees. The design assumptions and prestressing procedure are described briefly, and the instrumentation and loading are discussed. The actual deflections are presented in tabular form, and the deflections due to the design live load are calculated. The maximum deflections are presented as a ratio of the span length, and the further use of prestressed steel beams is recommended.

Construction of the Iowa Highway 60 Precast Prestressed Concrete Pavement Bridge Approach Slab Demonstration Project, HR-1085, 2007

Reconstruction of bridge approach slabs which have failed due to a loss of support from embankment fill consolidation or erosion can be particularly challenging in urban areas where lane closures must be minimized. Precast prestressed concrete pavement is a potential solution for rapid bridge approach slab reconstruction which uses prefabricated pavement panels that can be installed and opened to traffic quickly. To evaluate this solution, the Iowa Department of Transportation constructed a precast prestressed approach slab demonstration project on Highway 60 near Sheldon, Iowa in August/September 2006. Two approach slabs at either end of a new bridge were constructed using precast prestressed concrete panels. This report documents the successful development, design, and construction of the precast prestressed concrete bridge approach slabs on Highway 60. The report discusses the challenges and issues that were faced during the project and presents recommendations for future implementation of this innovative construction technique.

Nonlinear Analysis of Shear Dominant Prestressed Concrete Beams using ANSYS

This study reports the details of the finite element analysis of eleven shear critical partially prestressed concrete T-beams having steel fibers over partial or full depth. Prestressed T-beams having a shear span to depth ratio of 2.65 and 1.59 that failed in shear have been analyzed using the ‘ANSYS’ program. The ‘ANSYS’ model accounts for the nonlinearity, such as, bond-slip of longitudinal reinforcement, postcracking tensile stiffness of the concrete, stress transfer across the cracked blocks of the concrete and load sustenance through the bridging action of steel fibers at crack interface. The concrete is modeled using ‘SOLID65’- eight-node brick element, which is capable of simulating the cracking and crushing behavior of brittle materials. The reinforcement such as deformed bars, prestressing wires and steel fibers have been modeled discretely using ‘LINK8’ – 3D spar element. The slip between the reinforcement (rebars, fibers) and the concrete has been modeled using a ‘COMBIN39’- nonlinear spring element connecting the nodes of the ‘LINK8’ element representing the reinforcement and nodes of the ‘SOLID65’ elements representing the concrete. The ‘ANSYS’ model correctly predicted the diagonal tension failure and shear compression failure of prestressed concrete beams observed in the experiment. The capability of the model to capture the critical crack regions, loads and deflections for various types of shear failures in prestressed concrete beam has been illustrated.

Impact of Bottom Flange Confinement Reinforcement on Performance of Prestressed Concrete Bridge Girders

For many years AASHTO provided no recommendation to state DOT’s on bottom flange confinement reinforcement for their bridge superstructures. The 1996 edition of AASHTO Standard Specification for Highway Bridges stated that nominal reinforcement be placed to enclose the prestressing steel from the end of the girder for at least a distance equal to the girder’s height. A few years later the 2004 AASHTO LRFD Bridge Design Specification changed the distance over which the confinement was to be distributed from 1.0h to 1.5h, and gave minimum requirements for the amount of steel to be used, No.3 bars, and their maximum spacing, not to exceed 6”. Research was undertaken to study what impact, if any, confinement reinforcement has on the performance of prestressed concrete bridge girders. Of particular interest was the effect confinement had on the transfer length, development length, and vertical shear capacity of the fore mentioned members. First, an analytical investigation was performed on the subject, and then an experimental investigation followed which consisted of designing, fabricating, and testing eight tee-girders and three NU1100 girders with particular attention paid to the amount and distribution of confinement reinforcement placed at the end of each girder. The results of the study show: 1) neither the amount or distribution of confinement reinforcement had a significant effect on the initial or final transfer length of the prestress strands; 2) at the AASHTO calculated development length, no significant impact from confinement was found on either the nominal flexural capacity of bridge girders or bond capacity of the prestressing steel; 3) the effects from varied confinement reinforcement on the shear resistance of girders tested was negligible, however, distribution of confinement did show to have an impact on the prestressed strands’ bond capacity; 4) confinement distribution across the entire girder did increase ductility and reduced cracking under extreme loading conditions.

Flexural strengthening of reinforced concrete beams with carbon fibers reinforced polymer (CFRP) sheet bonded to a transition layer of high performance cement-based composite

Resistance to corrosion, high tensile strength, low weight, easiness and rapidity of application, are characteristics that have contributed to the spread of the strengthening technique characterized by bonding of carbon fibers reinforced polymer (CFRP). This research aimed to develop an innovate strengthening method for RC beams, based on a high performance cement-based composite of steel fibers (macro + microfibers) to be applied as a transition layer. The purpose of this transition layer is better control the cracking of concrete and detain or even avoid premature debonding of strengthening. A preliminary study in short beams molded with steel fibers and strengthened with CFRP sheet, was carried out where was verified that the conception of the transition layer is valid. Tests were developed to get a cement-based composite with adequate characteristics to constitute the layer transition. Results showed the possibility to develop a high performance material with a pseudo strain-hardening behavior, high strength and fracture toughness. The application of the strengthening on the transition layer surface had significantly to improve the performance levels of the strengthened beam. It summary, it was proven the efficiency of the new strengthening technique, and much information can be used as criteria of projects for repaired and strengthened structures.

Studio dell’effetto di cicli termici su materiali CFRP (Carbon Fiber Reinforced Polymer) attraverso prove di caratterizzazione statica

L’intento di questa tesi è fornire un andamento di alcune proprietà dei materiali compositi in fibra di carbonio, CFRP, utilizzati soprattutto nell’ambito aeronautico e navale, esposti quindi a condizioni ambientali specifiche di variazione ciclica della temperatura. Lo studio è effettuato sulle prove di caratterizzazione statica, di compressione, flessione in tre punti e taglio interlaminare, che generano risultati sulla resistenza delle fibre e della matrice e sul modulo elastico a compressione e trazione del composito.

Utilizzo industriale di cfrp (carbon fiber reinforced polymers) nanomodificati con nanofibre prodotte per elettrospinning

Attraverso questo studio sono state indagate le proprietà di compositi laminati in fibra di carbonio (CFRP) nano-modificati con nanofibre in Nylon 6.6, in termini di resistenza al danneggiamento da impatti a bassa velocità (con caratterizzazione Drop Weight at Low Velocity) e di smorzamento della vibrazione (con caratterizzazione a damping). Sono stati indagate due configurazioni di nanorinforzo differenti, confrontate con le prestazioni di provini vergini laminati tradizionalmente. Sono infine state operate delle analisi grafiche delle micrografie di campioni sezionati per trarre conclusioni di carattere tecnologico.

Monitoraggio strutturale (structural health monitoring) di laminati CFRP tramite reticoli di Bragg in fibra ottica

Lo studio si propone di valutare la capacità dei reticoli di Bragg in fibra ottica, di individuare e monitorare danneggiamenti presenti all'interno di laminati in composito, tramite l'analisi degli spettri riflessi dai reticoli stessi. Sono stati utilizzati due tipi diversi di stratificazione dei laminati, per valutare il diverso comportamento dei reticoli di Bragg inseriti all'interno dei laminati. Le proprietà dei materiali e degli spettri riflessi dai reticoli, sono state messe in relazione qualitativamente e quantitativamente, al fine di sviluppare un valido metodo di Structural Health Monitoring.

A new method for continuous quality control of nsm cfrp systems

In the last decade the near-surface mounted (NSM) strengthening technique using carbon fibre reinforced polymers (CFRP) has been increasingly used to improve the load carrying capacity of concrete members. Compared to externally bonded reinforcement (EBR), the NSM system presents considerable advantages. This technique consists in the insertion of carbon fibre reinforced polymer laminate strips into pre-cut slits opened in the concrete cover of the elements to be strengthened. CFRP reinforcement is bonded to concrete with an appropriate groove filler, typically epoxy adhesive or cement grout. Up to now, research efforts have been mainly focused on several structural aspects, such as: bond behaviour, flexural and/or shear strengthening effectiveness, and energy dissipation capacity of beam-column joints. In such research works, as well as in field applications, the most widespread adhesives that are used to bond reinforcements to concrete are epoxy resins. It is largely accepted that the performance of the whole application of NSM systems strongly depends on the mechanical properties of the epoxy resins, for which proper curing conditions must be assured. Therefore, the existence of non-destructive methods that allow monitoring the curing process of epoxy resins in the NSM CFRP system is desirable, in view of obtaining continuous information that can provide indication in regard to the effectiveness of curing and the expectable bond behaviour of CFRP/adhesive/concrete systems. The experimental research was developed at the Laboratory of the Structural Division of the Civil Engineering Department of the University of Minho in Guimar\~aes, Portugal (LEST). The main objective was to develop and propose a new method for continuous quality control of the curing of epoxy resins applied in NSM CFRP strengthening systems. This objective is pursued through the adaptation of an existing technique, termed EMM-ARM (Elasticity Modulus Monitoring through Ambient Response Method) that has been developed for monitoring the early stiffness evolution of cement-based materials. The experimental program was composed of two parts: (i) direct pull-out tests on concrete specimens strengthened with NSM CFRP laminate strips were conducted to assess the evolution of bond behaviour between CFRP and concrete since early ages; and, (ii) EMM-ARM tests were carried out for monitoring the progressive stiffness development of the structural adhesive used in CFRP applications. In order to verify the capability of the proposed method for evaluating the elastic modulus of the epoxy, static E-Modulus was determined through tension tests. The results of the two series of tests were then combined and compared to evaluate the possibility of implementation of a new method for the continuous monitoring and quality control of NSM CFRP applications.