Sustained Stress Levels Research Articles

Effect of Sustained Load and Environment on Long-Term Tensile Properties of Glass Fiber-Reinforced Polymer Reinforcing Bars

Glass fiber-reinforced polymer (GFRP) bars are gaining popularity as reinforcement for concrete bridge deck slabs and other concrete structures. This article reports on a study of the creep behavior of GFRP bars in different environments under sustained load. Twenty GFRP bars (E-glass in vinylester matrix) 9.5 mm in diameter in four series were tested for over 417 days (10,000 h) under combinations of different sustained load levels and surrounding mediums in ambient temperature. The bars were subjected to two levels of sustained tensile stress at 25% and 38% of guaranteed tensile strength while being surrounded by either alkaline solution (pH 12.8) or de-ionized water (pH 7.0). Axial strain in the central conditioned part of the bars was monitored with time to evaluate the creep behavior. Following the extended creep test, the GFRP bars were tested in axial tension until failure for residual tensile strength, elastic modulus, and axial strain. Results showed that the tested GFRP bar performed very well under these extreme loading and environmental conditions. Creep strain in the GFRP bars is less than 5% of the initial value after 10,000 h of sustained tensile loading. The average residual tensile strength was found to be 139% and 144% of the design tensile strength for bars conditioned in de-ionized water at 25% and 38% stress level, respectively. Alkaline solution tends to have more harmful effects on the bars than de-ionized water at higher stress levels. In alkaline solution, this range was 126% and 97% at 25% and 38% stress level, respectively. More importantly, the modulus of elasticity of the bars is very stable and almost unaffected by the conditions and sustained stress levels used. The authors note that this finding is critical in the design of concrete elements reinforced with FRP bars because the modulus is directly related to the crack width, deflection, and other serviceability concerns.

Long-Term Performance of High Strength Concrete

This paper describes an experimental investigation of how time-dependent deformations of high strength concretes are affected by maximum size of coarse aggregate, curing time, and relatively low sustained stress level. A set of high strength concrete mixes, mainly containing two different maximum sizes of coarse aggregate, have been used to investigate drying shrinkage and creep strain of high strength concrete for 7 and 28-day moist cured cylinder specimens. Based upon one-year experimental results, drying shrinkage of high strength concrete was significantly affected by the maximum size of coarse aggregate at early age, and become gradually decreased at late age. The larger the maximum size of coarse aggregate in high strength concrete shows the lower the creep strain. The prediction equations for drying shrinkage and creep coefficient were developed on the basis of the experimental results, and compared with existing prediction models.

Durability of Glass Fiber-Reinforced Polymer Reinforcing Bars in Concrete Environment

Accelerated aging tests are being conducted on more than 20 types of glass fiber-reinforced polymer (GFRP) reinforcing bars, which are produced from different combinations of constituent materials, manufacturing parameters, sizes and shapes, and surface coatings. The specimens are being subjected to various sustained tensile loading (22 to 68% of ultimate strength) in three types of alkaline environments: NaOH, simulated pore-water solution, and embedded in concrete. Time to rupture or residual strength, as applicable, have been determined. Additionally, stress corrosion mechanisms were evaluated by various microstructural analyses. The results showed clearly that alkaline ions and moisture could penetrate or diffuse through the resin (or through cracks and voids) to the interphases and the fibers. For GFRP bars embedded in moist concrete under various sustained stress levels, three types of stress corrosion mechanisms have been identified: stress dominated, crack propagation dominated, and diffusion dominated.

Variation of differential strain ratio in normal-and high-strength concrete under short-term uniaxial compression

The creep behaviour of concrete test specimens of three different mix proportions and strength levels was studied at various sustained stress levels. The experimental results of short-term sustained loading tests were analysed to provide a quantitative description of the observed behaviour. The creep behaviour of normal- and high-strength concrete and the companion mortar specimens was compared. A method based on the variation of differential strain ratio (the rate of change of transverse strain with respect to axial strain) for determining whether a concrete specimen has reached the point of creep fracture is proposed. Test results showed that the stress–strength ratio of the creep strength of concrete increased with the strength of concrete. The transverse and axial strains of concrete at this critical stress–strength ratio did not vary much with the strength of concrete, but averaged about 320–350 and 1430–1470 microstrain respectively.

Fatigue test of a three-span composite bridge

The feasibility of conducting a fatigue test on an existing three-span continuous composite highway bridge has been evaluated. An analytical study, in which the bridge was treated as a multi-degree of freedom system, showed that critical fatigue zones occur in the girder flanges within the end spans. The base metal under the top-flange shear connectors between a field splice and the end of a cover plate, and the base metal at the end of a cover plate in the top and bottom flanges, are susceptible to fatigue failure. This anticipated test behavior simulates the concern of the current AASHO Specifications. The sustained stress levels will be equal to the design live load plus impact stresses. Fatigue failure is probable within 600,000 cycles at a stress range of approximately 14 ksi (96×106 Pa). A series of steady-state resonance tests were conducted on a single-span composite model to assess the practicability of the proposed fatigue-testing technique. These laboratory tests verified that an electrohydraulic actuator with an attached weight would perform satisfactorily as a loading device to induce resonant vibrations. The performance of the bridge-shaker system was analyzed to insure the capability of this technique to produce the required stress levels for the fatigue test on the prototype bridge. The test was scheduled for the summer of 1975.

Rheological properties of beech wood in the ammonia-plasticized state

The rheological properties of ammonia platicized beech wood are presented in ways that permit evaluation of the effect of time and degree of plasticization. In particular 30 minute-creep curves are shown for various levels of sustained stress and directions of loading. The results clearly show the importance of the time-factor in evaluations of the mechanical properties of ammonia plasticized wood. Further, the results show that ammonia affects the compressive mechanical behavior to a much higher extent than the tensile behavior. Commercially, the rheological results can be utilized to devise better processes for industrial forming of wood products.