Showing papers in "Materials in 1996"

Bond strength and load distribution of composite gfrp reinforcing bars in concrete

Abstract: The objective of this investigation was to examine the bond strength of glass fiber reinforced plastic (GFRP) reinforcing bars in concrete. Twenty concrete beams reinforced with four nominal diameters from 12.7 to 25.4 mm of GFRP and steel reinforcing bars were tested to determine the bond strength of GFRP reinforcing bars in comparison with steel reinforcing bars. Additionally, the distribution of tensile and bond stresses along the embedment length of GFRP reinforcing bars was studied by five pullout tests using instrumented 19.1-mm-diameter GFRP reinforcing bars compared with steel reinforcing bars in concrete. Only one type of GFRP reinforcing bar was used in this study. Test results reveal that the diameter effect on bond observed for steel reinforcing bars is also present for GFRP reinforcing bars. The bond strength of GFRP bars is lower than that of steel reinforcing bars. The bond strength from beam tests is lower than that from pullout tests. Furthermore, it was found that the distribution of bond stress along the embedment length of GFRP reinforcing bars is nonlinear.

Statistical analysis of the compressive strength of concrete in structures

Abstract: The relationship between the in-place compressive strength of concrete in structures and specified strength Fc is examined via the use of factors F1 and F2. Factor F1, the ratio of the average strength of standard 28-day-old cylinder specimens to the specified strength, is evaluated using data from 3,756 cylinder tests representing 108 concrete mixes produced in Alberta, Canada, between 1988 and 1993. Factor F2, the ratio of average in-place strength to average cylinder strength, is evaluated using core and cylinder data representing 108 concrete mixes with strengths less than 55 MPa that were studied by others. A statistical description of the compressive strength of concrete in structures is derived that accounts for the inherent randomness of Factors F1 and F2 and also the typical strength variation within a specific structure. The probability of the in-place compressive strength of concrete in a 28-day-old column being less than Fc is approximately 13 percent. It is likely that a recalibration of the load and resistance factors for the design of new structures in Canada based on these findings would yield greater factored concrete strengths than are currently in use.

Mechanical properties of high-performance concrete

Abstract: To understand and document the mechanical properties of high-performance concrete, a test program was initiated. High-performance concretes with 56-day compressive strengths of 65 to 120 MPa were examined. Findings and discussions are presented regarding compressive strength gain with time, effect of type of cement, effect of drying, specimen size effect, static modulus of elasticity, Poisson's ratio, tensile splitting strength, and modulus of rupture. A modification of the American Concrete Institute (ACI) 318-M equation for the modulus of elasticity of normal-weight high-performance concrete is proposed. The validity of equations recommended by ACI 363R for estimating tensile splitting strength and modulus of rupture can be extended for high-performance concretes with or without supplementary cementitious materials and with 28-day compressive strengths up to 120 MPa.

Testing the Bond between RepairMaterials and Concrete Substrate

Abstract: The bond strength of repair materials to the concrete substrate was investigated. This investigation was aimed at studying the effect of the cyclic and static loading on bond as well as the effect of various surface treatments on bond strength. The study was also aimed at investigating the suitability of the slant shear test for evaluating cementitious and cementitious modified materials and resinous materials.

Aggregates and the Deformation Properties of Concrete

Abstract: This paper presents results from two comprehensive series of tests spanning a 5-year period that investigated the influence of 23 different aggregate types on the properties of hardened concrete. The information given is concerned with concrete stiffness and deformation. Tables of design values for estimating the elastic modulus of concrete are given. The influence of aggregate type on concrete shrinkage and creep is discussed with reference to the experimental work. Guidance is provided on the adjustment of code values for the effects of aggregates. The results provide evidence that aggregates exert a profound and important influence on the deformation properties of concrete.

Impact-Echo Studies of Interfacial Bond Quality inConcrete: Part l-Effects of Unbonded Fraction of Area

Abstract: This paper is the first of two papers that summarize results from a research project on the feasibility of using the impact-echo method to determine interfacial bond quality in layered structures, such as overlaid concrete slabs or repaired concrete structures. For purposes of this study, interfacial bond quality is based on unbonded fraction and bond tensile strength. This paper focuses on demonstrating how the unbonded fraction at an interface affects impact-echo response. Results obtained from numerical, experimental, and field studies for concrete structures with concrete overlays or repairs and concrete slabs with asphaltic concrete overlays are presented. Results show that very small unbonded fractions at an interface do not significantly affect impact-echo response. As unbonded fraction increases, a shift in the solid structure's thickness frequency occurs and a small amplitude peak corresponding to reflections from the interface becomes discernible. As the unbonded fraction becomes large, the shift in the thickness frequency becomes more pronounced, and a distinct, larger amplitude peak corresponding to reflections from the interface becomes apparent. Eventually, the response resembles that of a crack at the interface.

Damage in High-Strength Concrete Due toMonotonic and Cyclic Compression-A StudyBased on Splitting Tensile Strength

Abstract: The splitting tension test is used to determine an engineering measure of the damage induced by uniaxial compressive loading. The method is applied to monotonic and cyclic loading. Tests results indicate that damage in high-strength concrete evolves at a much lower rate than in normal concretes. Also, it is observed that tensile strength could be reduced by 25 percent due to compressive stresses applied in a transverse direction.

Interaction between Accelerating Admixtures and PortlandCement for Shotcrete: The Influence of the Admixture’sChemical Base and the Correlation between Paste Testsand Shotcrete Performance

Abstract: This study analyzes the correspondance between initial and final setting times measured using Gillmore needles (commonly specified for shotcrete) with shooting stiffness-measured Proctor needles and compressive strength at early ages (evaluated using nondestructive penetration tests). Two different types of accelerating admixtures (carbonate and aluminate-based) and two different types of Portland cements were used in the tess on cement paste and shotcrete.

Bond Characteristics of Deformed Reinforcing Steel BarsEmbedded in SIFCON

Abstract: The main objective of this research was to study the bond characteristics of deformed reinforcing steel bars embedded in slurry-infiltrated fiber concrete (SIFCON). The several parameters were investigated : bar embedment length, compressive strength of SIFCON, concrete cover, and bar diameter. In addition, two control specimens, one with plain concrete and one with concrete confined by closed stirrups, were tested as a reference base. Both experimental and analytical studies were undertaken. In this paper, only the experimental results are presented.

Stresses in Early-Age Concrete: Comparison of DifferentCreep Models

Abstract: Creep plays a significant role in the evolution of thermal stresses and in the risk of cracking in early-age concrete; therefore, it has to be considered in the calculation of mechanical effects resulting from the temperature fields brought about by the exothermy of the hydration reaction of cement. This paper presents a comparative analysis of different creep models taken from the literature and inserted in a uniaxial calculation of total strains of a concrete member. The calculations are conducted in parallel using the principle of superposition and an incremental model, respectively, and tested on data and results from an existing experimental case.