Michael E. Kreger

INCREASING FLEXURAL CAPACITY OF REINFORCED CONCRETE BEAMS USING CARBON FIBER-REINFORCED POLYMER COMPOSITES

A series of reinforced concrete beams strengthened in flexure using various carbon fiber-reinforced polymer (CFRP) composite systems were fabricated and tested in the lab to examine the effects of the strengthening configuration on the specimen behavior. The main aim was to find strengthening configurations to develop the strength of the composite laminates and preclude failure by debonding of the composite systems from the concrete surface. Results indicate that relying on the contact area between the composite laminates and the concrete surface is insufficient to eliminate debonding. Strengthening configurations involving techniques such as placement of transverse straps along the composite laminates or bonding the composites on the side surface of the specimens controlled debonding and provided a more ductile failure mode than placement on the bottom surface of the beams. Results of this investigation are intended to provide information required for the design of strengthening schemes of existing reinforced concrete bridges using composites.

Fatigue tests of reinforced concrete beams strengthened using carbon fiber-reinforced polymer composites

The use of carbon fiber-reinforced polymer (CFRP) composites has been identified as a viable technique for use in strengthening deteriorating or structurally deficient reinforced concrete bridges. This article presents the results of a study that consisted of repeated-load fatigue testing 10 reinforced concrete beams strengthened using two different types of externally-bonded CFRP composites. The results indicate that the bond between the composite laminates and surface of the concrete can degrade at load amplitudes corresponding to extreme load conditions for a bridge. The stress range applied to the reinforcing steel and maximum stress applied to the composite laminates were the controlling parameters that limited the fatigue life of the specimens under study. The authors conclude that an upper limit on stresses generated along the composite-concrete interface may have to be set during design to avoid premature debonding after a limited number of load cycles.

Retrofit of Non‐Ductile Moment‐Resisting Frames Using Precast Infill Wall Panels

Many existing reinforced concrete moment-resisting frames located in seismic zones lack strength and ductility. One approach for correcting these deficiencies is the construction of infill walls to strengthen and stiffen the structure. Cast-in-place construction is often used; however, there are conditions where cost, time constraints, or limiting disruptions to building operations may dictate other solutions. One possible modification is the use of infill walls constructed of precast concrete panels. A precast infill wall system eliminates the need for large formwork during construction. Elimination or reduction of connection hardware between precast panels or between panels and the existing frame element can provide additional efficiency. Problems associated with casting large quantities of concrete in an existing building are eliminated. Construction time and inconvenience to occupants may be reduced along with the costs. The precast system has the potential of reducing the overall costs of rehabilitating existing structures.

Retrofit of Concrete Columns with Inadequate Lap Splices by the Use of Rectangular Steel Jackets

This paper describes an experimental research program on the use of rectangular steel jackets for seismic retrofit of non-ductile reinforced concrete frame columns. Eleven large scale columns were tested to examine the effectiveness of various types of steel jackets for improving the ductility and strength of columns with an inadequate lap splice in the longitudinal reinforcement. Response of the columns before and after being strengthened with steel jackets was examined. Several types of steel jackets were investigated, including rectangular solid steel jackets with and without adhesive anchor bolts. The test results indicate that a thin rectangular steel jacket combined with adhesive anchor bolts can be a highly effective retrofit measure for reinforced concrete columns with an inadequate lap splice. Design guidelines for the use of rectangular steel jackets as a seismic retrofit for non-ductile reinforced concrete columns are presented.

Stresses in External Tendons at Ultimate

Post-tensioned segmental concrete bridges are often used for long, multispan viaducts or medium-span valley and river crossings. This article presents the research that led to the development of the equation for predicting stresses in unbonded tendons at ultimate; this equation is currently used in the AASHTO LRFD Specifications and AASHTO Guide Specifications for the Design and Construction of Segmental Concrete Bridges. The research, performed by the late Robert J. G. MacGregor, involved the construction and testing of a 1/4 scale model of a three-span, continuous, precast segmental-concrete box girder bridge, erected using span-by-span techniques and post-tensioned with external tendons. The authors discuss the results of the tests to ultimate that were dominated by flexural behavior. Predicted tendon stress increases are then compared to a large data base from other tests of beams and slabs with unbonded tendons. The authors conclude that the equation proposed by MacGregor predicts the tendon stress increases in unbonded tendons conservatively and relatively well, compared to expressions that ignore tendon length.

Grouts for bonded post-tensioning in corrosive environments

Problems with corrosion of prestressing steel in post-tensioned structures due to inadequate grouting have become more apparent in recent years. To improve grouting practices, a series of fresh property tests, accelerated corrosion tests, simulated field tests, and long-term corrosion tests were used in this study to develop optimum grouts for post-tensioning. Grouts possessing adequate workability and bleed resistance were tested in an accelerated corrosion test to evaluate relative corrosion protection properties. This paper focuses on the accelerated corrosion test results. The most promising of these grouts were then tested for placeability under simulated field conditions. A fly ash grout and a thixotropic grout are recommended from this testing program. Fluidity, bleed, and corrosion resistance data was also acquired for a number of different combinations of pozzolans and admixtures.

Strength and ductility of a three-span externally post-tensioned segmental box girder bridge model

This report is the third and final report in a series outlining a major study of the behaviour of post-tensioned concrete box girder bridges with post-tensioning tendons external to the concrete section. It summarizes the design, construction, testing and interpretation of a very comprehensive three-span externally-post-tensioned box girder bridge model. The model was constructed from precast segments using the span-by-span construction procedure. Careful measurements were made during construction to document the actual stresses and prestress losses occurring. One span of the model had dry joints while the other two spans had epoxy joints. Loading was applied at design service load levels, design factored load levels, and ultimate load cycles for both maximum flexure and maximum shear loading configurations. Careful observations were made of deformations, tendon stress changes, joint openings and reaction changes. Companion analysis was performed to assist in development of the model and in the interpretation of the test data. The model bridge was very stiff at service load conditions and exhibited linear behavior to loads higher than the factored design load. The cracking load for epoxy-jointed spans was approximately twice the load required to decompress the flexural tension fiber and begin to open a previously cracked joint. This suggests that epoxied joints can provide a reasonable factor of safety against joint opening and that this same factor of safety can be provided in dry-jointed spans only by applying additional prestress force. Both the dry and epoxy-jointed spans displayed considerable ductility during flexural strength tests (A).

EVALUATION OF ACI 318-95 SHEAR-FRICTION PROVISIONS

Section 11.7 of American Concrete Institute (ACI) 318-95 contains design provisions for conditions where direct shear transfer through shear-friction should be considered. Such conditions include an interface between concretes cast at different times. Shear-friction capacity is defined by this code as a function of steel reinforcement area contained in the interface and treatment provided to the interface before the new concrete is cast against the hardened concrete. The code also considers the presence of permanent net compression across the interface and specifies an upper limit on interface shear capacity. The test results discussed in this paper are used to verify and extend the application of shear-friction provisions currently incorporated in the ACI Code. Modification and expansion of the current code are proposed in light of the additional research information available to date.

Full-scale tests of bridge components strengthened using carbon fiber-reinforced polymer composites

This paper presents results of 4 tests of full-scale reinforced concrete specimens that are representative of common short-span bridges in Texas. The specimens were strengthened to increase flexural capacity using carbon fiber-reinforced polymer laminates. Realistic field conditions were represented to the extent possible in the laboratory. The techniques used to design the strengthening systems are detailed, and necessary assumptions are evaluated using the measured data. Results show that the strengthened specimens failed after composites reached 50-65% of the rupture strength of the laminates.

Mitigation Techniques for Structures with Premature Concrete Deterioration due to ASR/DEF

There was a comprehensive investigation in Texas from 1998 to 2003 on in-service bridge structural performance when premature concrete deterioration was present. Two expansive distress mechanisms are attributed to premature concrete deterioration: delayed ettringite formation (DEF) and alkali-silica reaction (ASR). Moisture is required by both mechanisms, although they are chemically different. There can be extension of deteriorated structure service life through suitable mitigation techniques. The comparative effectiveness of surface treatments is addressed by this study. Existing DEF and ASR ASTM tests were not found to be useful for this purpose. A new test method was found useful and involved controlled wetting and drying cycles. This method was used in comparing proposed surface mitigation treatment effectiveness. Using that comparison as a basis, it is recommended that a surface mitigation treatment consisting of silane followed by a breathable paint appearance coat be used for structural service life extension when premature concrete deterioration due to ASR/DEF is present.