Showing papers in "Aci Materials Journal in 1997"

Packing of aggregates : An alternative tool to determine the optimal aggregate mix

Abstract: Concrete mix designs include a number of subdesigns of, for example, the aggregate mix. The aggregates are usually selected and combined according to simple mix design rules, which requires each of the aggregates to be graded. This procedure limits the use of natural aggregate grades and does not always lead to the best quality of concrete. The selection and combination of the aggregates has a dominating influence on the quality and price of the concrete and should receive more thorough attention. An alternative mix design may often lead to a better and cheaper concrete. An alternative approach for the aggregate selection and combination is to aim for the aggregate mix that has the lowest void content or the maximal packing degree. This will lead to the lowest cement consumption, porosity, and shrinkage and thus the concrete with the highest performance and the lowest price. This paper presents a model that has been used for more than 5 years on a large number of industrial projects. The model is verified by comparison with more than 800 test results from different sources as being reliable for large ranges of materials.

Mechanical properties and durability of two industrial reactive powder concretes

Abstract: Two reactive powder concretes (RPC) were produced on an industrial scale at the Universite de Sherbrooke and in a nearby precast plant. A 2.6 cubic meter mix was prepared in the central mixer of the precast plant. The ready mix RPC was sampled before and after the addition of steel fibers while the one produced at the precast plant was sampled only at the end of the mixing process. These RPCs were tested for compressive strength, modulus of elasticity, freezing and thawing cycling resistance, scaling resistance to deicing salts, and resistance to chloride ion penetration. The results show that a 200 MPa compressive strength could be achieved in both cases: after curing in hot water at 90 degrees C or in the low pressure steam chambers at the precast plant. Confinement of the RPC in a steel tube greatly increases its compressive strength and its ductility.

Use of viscosity-modifying admixture to enhance stability of fluid concrete

Abstract: This paper presents the results of a study carried out to investigate the effects of viscosity-modifying admixture concentration, placement height, and mode of consolidation on enhancing the stability of mixtures made with various water-to-cementitious material ratios and consistency levels. In the first phase, bleeding and settlement are determined using 70-cm high columns cast with concrete containing various viscosity-modifying admixture dosages, water-to-cementitious material ratios (0.50 to 0.70), and slump values (140 to 220 mm). In the second phase, bleeding, settlement, and segregation are evaluated for concretes with 220-mm slump cast in 50-, 70-, and 110-cm high columns. The mixtures are made with a water-to-cementitious material ratio of 0.50 and various contents of viscosity-modifying admixture. The effect of excessive external vibration on stability is also evaluated.

The influence of aggregate on the compressive strength of normal and high-strength concrete

Abstract: This paper presents a comprehensive theory describing the influence of aggregate on the compressive strength of concrete. A first distinction is made between topological and mechanical aspects. The former, called confining effect, includes the effect of the volume and the maximum size of the aggregate, which are best described by means of a single physical parameter, the maximum paste thickness (MPT). MPT is defined as the mean distance between two adjacent coarse aggregates. Equations are given to calculate MPT and its effect on compressive strength. The second type of effect concerns the bond between past and aggregate (bond effect), and a limitation of the strength that originates in the intrinsic strength of the rock (ceiling effect). Experiments have been performed on 13 mixtures made up with five sources of aggregate, in order to compare the strength development on pure paste versus composites (mortar and concrete). The quantitative assessment of the three effects of aggregate on compressive strength gives an accuracy close to 2.2 MPa on the studied mixes. Such a model is suitable to be incorporated into software for computer-aided mixture-proportioning and quality-control of structural concrete, up to the high-performance concrete range.

Effect of Welan Gum-High-Range Water Reducer Combinations on Rheology of Cement Grout

Abstract: The effects of combined additions of welan gum, a commonly used rheology modifier, and naphthalene-based high-range water reducer on the rheological properties of cement grouts are investigated for mixtures made with 0.40 water-to-cement ratios. Grouts with dosages of rheology-modifying admixture varying from 0 to 0.075 percent by mass of cement were prepared. For each group of grout, the concentration of high-range water reducer was varied to obtain four mixtures of various fluidity levels. Measured properties included apparent viscosities at different shear rates, and estimates of plastic viscosity and yield value. Other measurements of consistency included the ease of spread and flow of grout evaluated using the mini-slump and Marsh cone tests, respectively. The grout stability was evaluated by measuring its resistance to water dilution when cast in water as well as its ability to retain water when subjected to sustained pressure (forced bleeding). Initial setting times were determined for selected mixtures. In all, a total of 27 grout mixtures were evaluated. Test results show that the increase in the dosage of rheology-modifying admixture increases significantly the yield value and plastic and apparent viscosities of cement grouts. Combined with an adequate dosage of high-range water reducer, losses in fluidity are regained without significant reduction in stability. With the increase in high-range water reducer dosage, the apparent viscosity at low rates of shear decreases more dramatically than that at high rates of shear due to the pseudo-plastic behavior of such grouts. The combined use of proper dosages of rheology-modifying admixture and high-range water reducer is shown to clearly contribute to securing high-performance cement grout that is highly fluid, yet cohesive enough to reduce water dilution and enhance water retention. For equal fluidity level, greater stability is obtained with mixtures containing high contents of viscosity modifying admixture. The initial setting time is shown to be delayed by the incorporation of high-range water reducer and rheology-modifying admixture with the latter additive exhibiting greater influence on retardation of setting.

In situ mechanical properties of wall elements cast using self-consolidating concrete

Abstract: The use of self-consolidating concrete (SCC) can facilitate the placement of concrete in congested members and in restricted areas. Given the highly flowable nature of such concrete, care is required to ensure adequate stability. The objective of this paper is to evaluate the uniformity of in situ mechanical properties of SCC used to cast experimental wall elements. Eight optimized SCC mixtures with slump flow values greater than 630 mm and a control concrete with a slump of 165 mm were investigated. The SCC mixtures incorporated various combinations of cementitious materials and chemical admixtures. The water-cementitious materials ratios ranged between 0.37 and 0.42. Experimental walls measuring 95 cm in length, 20 cm in width, and 150 cm in height were cast. No consolidation was used for the SCC mixtures, while the medium fluidity control concrete received thorough internal vibration. Cores were obtained to evaluate the uniformity of compressive strength and modulus of elasticity along the height of each wall. Bond strengths were also determined for 12 horizontal reinforcing bars embedded at various heights of each wall. All SCC mixtures exhibited small variations in compressive strength and modulus of elasticity in relation to height and were similar to those obtained with the medium fluidity control concrete. Considerable reductions were, however, obtained between compressive strength values determined on core samples and those of cast cylinders. Such reduction was approximately 10% and 20% for SCC mixtures made with 10- and 20-mm maximum size aggregate, respectively, and 10-15% for the control concrete. The top-bar factor for reinforcing bars positioned approximately at 140 cm from the bottom of the experimental walls was 1.4 plus or minus 0.2 for seven of SCC mixtures and approximately 2.0 for the control concrete and one SCC. The optimized SCC mixtures are therefore highly stable despite their flowing nature and can ensure uniform in situ properties when cast in deep structural elements.

The alkali-silica reaction, part I: Use of the double-layer theory to explain the behavior of reaction-product gels

Abstract: An understanding of the expansion mechanisms resulting from alkali-silica reaction is necessary to assess the susceptibility of a concrete structure to deterioration by these processes and to the planning and implementation of preventive measures. As a result of the alkali-silica reaction between certain reactive aggregates and the highly alkaline pore fluids in a cement paste, a reaction-product gel develops that, in the presence of water, expands and may cause cracking of mortar or concrete. To explain the volume change behavior of mortar bars containing a reactive aggregate, a theoretical model is proposed in this paper. The expansion of the alkali-silica reaction-product gels is attributed to swelling caused by electrical double-layer repulsive forces. For a given colloidal system, double-layer theory indicates that the larger the valence of the counterions in the double layer, or the larger the concentration of these ions, the smaller the double-layer thickness and the repulsive forces that may be generated in the presence of water. Results of experiments from the literature support the double-layer model. According to these results, the expansion of mortar bars in the American Society for Testing and Materials (ASTM) C 1260 test is related to the composition of the reaction product gels. The reaction-product gels containing larger amounts of equivalent sodium oxide (Na2Oe) and smaller CaO/Na2Oe cause larger expansions in the mortar bars.

Microstructural and Mechanical Effects of Latex, Methylcellulose, and Silica Fume on Carbon Fiber Reinforced Cement

Abstract: The effect of methylcellulose, silica fume, and latex on the degree of dispersion of short carbon fibers in cement paste was assessed. This degree, as indicated by the ratio of the measured volume of electrical conductivity to the calculated value, and the effectiveness of the fibers in enhancing the tensile/flexural properties attained by using methylcellulose and silica fume were higher than those attained by using methylcellulose alone or latex. Methylcellulose was superior to latex in giving a high degree of fiber dispersion at fiber volume fractions < 1 percent, as measured by this technique. Latex resulted in superior tensile-flexural properties and lower content and size of air voids than methylcellulose. With the fiber content fixed at 0.53 vol. percent, the degree of fiber dispersion, as measured by this technique, decreased with increasing latex-cement ratio. As a result the flexural toughness decreased monotonically with increasing latex-cement ratio and the flexural strength attained a maximum at an intermediate latex-cement ratio of 0.15. In contrast, both flexural toughness and strength increased monotonically with increasing latex-cement ratio when fibers were absent.

Aggregate Bond and Modulus of Elasticity of Concrete

Abstract: The difference between the modulus of elasticity of aggregate and of hardened cement paste is shown to influence the bond stress between the two materials and therefore the shape of the stress-strain relation. Better monolithic behavior of concrete, which is beneficial, for example, when temperature cycling is expected, is achieved when the difference between the moduli is reduced. A discussion of the behavior of high-strength concrete and of lightweight aggregate concrete gives a pointer for the choice of mix ingredients.

Concrete strength estimation at early ages: modification of the method of equivalent age

Abstract: This paper discusses the estimation, at early ages, of the compressive strength of a concrete mix subjected to a temperature history. The method of "equivalent age" based on the Arrhenius law is used. It involves a new definition of the rate of hydration linked to the concrete strength. This rate of hydration includes a model of a linear decrease of the long-term strength of concrete with increasing temperature. Tests at constant and variable temperatures were carried out. They enable, first, the proposed method applicable to the problem to be verified, and second, the accuracy of the new approach to be compared with that of the one currently used on construction sites.

A New Rheometer for Soft-to-Fluid Fresh Concrete

Abstract: This article describes the development of apparatus designed to characterize the rheological behaviour of fresh concrete , both in the lab and on site, in the range of consistency from soft to fluid. The principles that governed the design of the rheometer are first recalled. It is a torsional rheometer, in which a sample shaped like a hollow cylinder is sheared between two horizontal planes. The conduct of the tests is then described, followed by their processing, which yields, in particular, two physical quantities to characterize a concrete, the shear yield stress and the plastic viscosity. These parameters may be measured with or without vibration, giving different values in each case. Later, the results of experiments are described. They deal with the detection, with the detection, with only one sample, of the phenomenon of loss of workability, and with the pumpability of the concrete on site, respectively. The article concludes with a look at the prospects for industrial use of the BT RHEOM rheometer.

Mechanical properties of pva fiber reinforced cement composites fabricated by extrusion processing

Abstract: Discontinuous polyvinyl alcohol (PVA) fiber reinforced cement matrix composites fabricated by an extrusion process were investigated. The extruded composites have exhibited a postpeak strain hardening type of response with an enhanced composite strength. Tensile strength of extruded composites was dependent on the fiber fraction used, while the flexural strength and deflection at the peak load were influenced by both the fiber fraction and the matrix composition. Larger fiber volume fraction, longer fiber length, and higher cement content lead to a higher flexural strength, a larger deflection at peak, and a higher elastic modulus. Fracture process was studied using laser Moire interferometry and scanning electron microscopy. It was observed that sequential multiple cracking was associated with the strain hardening type of response and that the spatial distribution of fibers can control the sequence of multiple cracking. Interface between fiber and matrix was also studied using continuous, aligned fibers and with the help of Moire interferometry.

Use of reclaimed asphalt pavement as an aggregate in portland cement concrete

Abstract: The main objective of this research is to investigate the potential use of reclaimed asphalt pavement (RAP) as an aggregate in portland cement concrete (PCC). RAP aggregate finer than 4.75 mm (#4) sieve was treated as fine materials while those particles passing 19.05 mm (3-quarters of an inch) and retained on #4 sieve were treated as coarse aggregates. One set of concrete cylinders was prepared using 100, 75, 50, 25, and 0 percent RAP as coarse aggregate while fine aggregate was 100 percent RAP. Another set of samples was made using 100, 75, 50, 25, and 0 percent RAP as fine aggregate while coarse aggregate was 100 percent RAP. Samples were prepared using water-cement ratios of 0.40 and 0.50. In addition, control mixtures containing 100 percent conventional aggregate were made for comparison purposes. Higher compressive strength was obtained as the percentage of RAP aggregate was decreased for all mixtures. The control specimens yielded the highest compressive strength. However, the strength was sufficient to qualify the use of RAP aggregate in concrete applications such as barriers, sidewalks, driveways, pipes, curbs, and gutters. Furthermore, mixtures containing RAP aggregate had enhanced ductility and showed excellent shatter resistance properties. Additional studies should investigate the use of additives and admixtures with RAP aggregate.

Influence of Concrete and Fiber Characteristics on Behavior of Steel Fiber Reinforced Concrete under Direct Shear

Abstract: Results of an experimental investigation carried out to assess the effect of various concrete strength levels on strength and ductility behavior of steel fiber reinforced concrete (SFRC) under direct shear is reported. The principal variables of the testing program were compressive strength of the concrete, volume of the steel fibers, and the aspect ratio of the steel fibers. The concrete strengths investigated include 28 MPa for low strength, 44 MPa for normal strength, 54 MPa for medium strength, and 72 MPa representing high-strength concrete. Fiber content ranges from zero to 1.5% by volume of matrix. In total, 28 push-off type specimens were tested. Formulations are proposed to predict the shear response of the test specimens. Addition of fibers regardless of concrete strength led to shear strength, ductility, and toughness enhancement of SFRC; however, the improvement for higher strength concrete was more pronounced than that for lower strength concrete. This is attributed to the improved bond characteristics associated with the use of fibers in conjunction with high-strength concrete. Fibers with larger aspect ratio provided higher toughness and higher ultimate shear strength concrete. The influence of fiber volume on shear behavior of concrete was much higher for high-strength concrete. In general, good agreement between the shear behavior predictions and the test results was found.

Can steel fibers replace transverse reinforcements in reinforced concrete beams

Abstract: Steel fiber reinforced concretes currently used do not have enough post-cracking tensile strength to be used alone, without main reinforcement (rebars), in a beam. On the other hand, replacement of the secondary reinforcement (stirrups) seems promising. The use of fibers would save considerable time and would facilitate placement in highly reinforced structures. A large experimental program was carried out to quantify the mechanical contribution of fibers in beams. In a first step, five large scale double-T-girders were tested, with the transverse reinforcement varied - concrete only, stirrups, or fibers. A global optical method, stereophotogrammetry, was used to monitor cracking. It showed a mainly mode I crack opening process. Furthermore, these tests suggested a scale effect linked to the height of the beam: the higher the height of the beam, the wider the crack opening at failure, i.e. the smaller the residual stress carried by the fibers. These findings lead to optimize both material and structure in regard to shear behavior. A high-strength steel fiber reinforced concrete was used in small rectangular beams, 0.25-m high, to provide high residual stress of the material and small crack opening at failure. For both types of beams--T-girders and rectangular beams--the mix was optimized using the Baron-Lesage method and the mechanical characterization of the material was conducted using a uniaxial tensile test monitored by the rate of the crack opening. A method of analysis based on a block mechanism is proposed and compares well with experiments.

High performance multimodal fiber reinforced cement composites (hpmfrcc): the lcpc experience

Abstract: By using, on the one hand, a solid suspension model and a maximum paste thickness concept to optimize the matrix composition and, on the other hand, a "two levels" concept to optimize the mixture of different dimensions of fibers introduced in the matrix, it is possible to obtain a high performance multimodel fiber reinforced cement composite (HPMFRCC) This paper presents results concerning the strength and the ductility of one of the HPMFRCCs developed at Laboratoire Central des Ponts et Chaussees (LCPC)

Influence of Fly Ash on Setting and Hardening Characteristics of Concrete Systems

Abstract: This project was carried out to investigate the effect of fly ash obtained from various sources A reference mixture without fly ash was proportioned to have 28-day design strength of 35 MPa Fly-ash mixtures were proportioned to contain fly ash in the range of 0 to 100 percent by mass of the cementitious medium The ratio of fly ash to cement was kept at about 125 In general, initial and final times of setting of concretes were greatly affected by both source and fly ash content The times of setting were generally delayed up to a certain level of cement replacement with fly ash Beyond this level, which was about 60%, rapid setting occurred

Concrete and Sustainable Development

Abstract: The first signs of global warning caused by the greenhous effect are now apparent. In the near future, a new evaluation of building materials in light of their ability to fulfill the requirements of sustainable development will be required. In this paper, the energy consumption and greenhouse gas emissions of concrete in residential buildings will be examined, taking into consideration production and operational phases, as well as traffic-induced energy consumption and emissions in residential areas. The massiveness of concrete buildings causes significant energy and emission savings compared to buildings comprised of lighter materials. This improves the ecological balance of concrete and lifts it to the group of building materials which burden the environment least.

Methods for the determination of water permeability of concrete

Abstract: Two test methods have been successfully used to determine the water permeability of different concretes. The methods are based on the determination of coefficient of permeability using either a constant flow or a depth of penetration technique. The flow method has generally been found to suit concretes with higher permeability, while the penetration method is used for concretes with very low permeability. Presently no clear guidelines exist for the selection of the appropriate method for a particular type of concrete. This study was carried out to examine the correlation between the two methods. A broad guideline has also been established for the selection of the appropriate method for a particular concrete with respect to its binder composition, 28-day compressive strength, and age. The concretes examined were prepared from five types of binders and with a grade range of 35-50 MPa.

Properties of high-calcium dry bottom ash concrete

Abstract: A study of various fresh and hardened properties of bottom ash concrete is reported. Standardized tests were conducted on four series of concrete mixtures having a constant volume of solid constituents and a uniform slump of 101.6 +/- 1/4 mm. Series A and B were mixes of natural sand (control mix) and bottom ash concretes, respectively. Series C contained mixes in which the fine aggregate component of the matrix consisted of equal volumes of natural sand and bottom ash. The mixes of Series D incorporated a high range of water-reducing admixture into the mixtures of Series C. The test results indicated that the mixing water requirement increases rapidly when bottom ash is used in concrete. When 50% of the total dry volume of the natural fine aggregate was replaced by bottom ash or a high-range water reducing admixture was used in conjunction with the combined bottom ash and natural sand mixtures, the engineering characteristics were similar and in most cases superior to those of reference mixes.

Low-cycle fatigue of plain and fiber-reinforced concrete

Abstract: This paper reports on a study which is part of an investigation into the fatigue reliability of plain and fiber-reinforced concrete structures, underway at Columbia University since 1989. The ultimate purpose of this research is to develop practical tools for damage assessment and prediction of remaining life of concrete structures. The scope of the research reported herein is limited to a few key tasks. Several hundred 4-inch cube specimens were tested uniaxially under cyclic compression with constant amplitude. The variables studied were the concrete strength (r, 5, and 7 ksi cylinder strength), type of fiber (steel and polypropylene), and fiber volume (0.00, 0.25, 0.50, 0.75, and 1.00 percent). The emphasis of this study was to examine the effect of various parameters on energy absorption characteristics, fatigue life and the damage accumulation history.

Soil physics principles validated for use in predicting unsaturated moisture movement in portland cement concrete

Abstract: Moisture movement in concrete is of primary concern for many aspects of long-term concrete performance. While the field of soil physics has advanced considerably in the modeling of unsaturated moisture movement in soil and other porous solids, differences in pore systems between concrete and soils have prevented general acceptance of the application of soil physics principles to concrete. The purpose of this study was to determine if soil physics principles can be applied to unsaturated moisture movement in concrete. A desorption environment, assuming 1-D moisture movement, was imposed on concrete specimens from four different mix proportions. Five different desorption environments were used in the experiments, those being 97, 92, 75, 53, and 31 percent relative humidity. Each represented a different constant boundary condition. Intermediate and equilibrium moisture contents were determined using mass loss measurements, and a closed-form solution was developed using van Genuchten moisture characteristics parameters along with other fundamental physical relationships. Validation was achieved by comparing experimentally determined moisture contents against those predicted by the closed-form analytical solution. Prediction of experimentally determined moisture contents was very good, with the percentage difference between measured and predicted values being less than 10 percent for all combinations of boundary conditions and mix proportions. In most cases, variations were 5 percent or less. Hence, the success of the general prediction equation indicated a valid application of soil physics principles to unsaturated moisture movement in portland cement concrete.

Freeze-thaw and freeze-deicing salt resistance of concretes containing cement rich in granulated blast furnace slag

Abstract: Whereas blast furnace cement concretes have proved successful in structures subjected to freeze-thaw attack, their use in structures subjected to freeze-deicing salt attack is still a problem. The present paper discusses the causes of this controversy. It was found that the freeze-deicing salt resistance of blast furnace cement concretes is closely related to the carbonation of the surface area. The carbonation of blast furnace cement concretes does not only lead to an increase in capillary porosity but also to metastable calcium carbonates soluble in NaCl. Based on the results of investigations, ways of improving the freeze-deicing salt resistance of blast furnace cement concretes are proposed.

A Study of Corrosion Inhibitor Performance in Chloride Contaminated Concrete by Electrochemical Impedance Spectroscopy

Abstract: The deterioration of the highway and bridge infrastructure has received significant attention in recent years. A major contributing factor to this deterioration is the reinforcing steel corrosion. Electrochemical impedance spectroscopy and linear polarization techniques were used to study 5-year-old "lollipop-like" concrete specimens containing sodium nitrite and dinitrobenzoic acid. An equivalent circuit model considering the physical characteristics of the rebar/concrete (RC) interface was used to simulate the impedance spectra. The RC parameters obtained from the impedance spectra simulation including the maximum phase angle shift and polarization resistance were used to characterize the rebar corrosion. The effectiveness of the corrosion-inhibiting additives in the presence and absence of chloride ions was evaluated. The corrosion current densities estimated by impedance measurement were confirmed by those determined using linear polarization techniques. The purpose of this study was to evaluate the long-term performance potential of sodium nitrite and dinitrobenzoic acid used as the corrosion-inhibiting additives in chloride contaminated reinforced concrete.

Mechanical properties of concrete in uniaxial compression

Abstract: Pre- and postpeak response phenomena are examined when cylindrical concrete specimens are loaded under deformation control in uniaxial compression. The experiments focus on degradation of stiffness and strength during unloading and reloading cycles, considering cylindrical specimens of different height. Fracture energy concepts are used to interpret the postpeak response as a function of specimen height in order to extract a characteristic length measure. In conclusion, the existence of a unique focal point (pole) is explored for the secant modulus description of elastic stiffness degradation and permanent (plastic) deformation.

Effect of fibers on development/splice strength of reinforcing bars in tension

Abstract: The effects of fibers on development/splice strength of reinforcing bars in tension were experimentally investigated. Three series of tests, with five beams each, lap spliced at midspan, were carried out. The research parameters were: bar diameter (d sub b) fiber type, fiber volume fraction (V sub f), and fiber reinforcing index (V sub f L/d sub f). The beams were loaded in positive bending with constant moment along the splice region. The current experimental results demonstrated clearly that the use of hooked steel fibers in concrete matrixes increases significantly the development/splice strength of reinforcing bars in tension. While the use of polypropylene fibers, added in 0.6 percent by volume, improved the bond performance in the post-splitting range, they were not as effective as steel fibers in increasing the development/splice strength of the reinforcing bars.

Experimental investigation of multiaxial compressive strength of concrete under different stress paths

Abstract: Many multiaxial failure criteria of concrete have been suggested based on the experimental results under proportional loading paths. To verify whether these failure criteria of concrete are applicable for nonproportional loading paths or whether multiaxial strengths of concrete are stress paths dependent, the strength behavior of concrete under nonproportional loading was investigated. An extensive experimental program involving the testing of 105 specimens was conducted. The stress paths used included four biaxial and two triaxial compressive stress paths. The multiaxial compressive strengths of concrete under these stress paths were compared with those under proportional loading. The condition in which the multiaxial compressive strength of concrete is not influenced by the stress path is discussed and concluded.

Assessing quality of concrete with wave propagation techniques

Abstract: An instrument for monitoring the quality and thickness of slabs has been designed. The quality is assessed by estimating the slab's Young's and shear moduli from ultrasonic surface wave and ultrasonic body wave velocities. The thickness is determined from results obtained with the impact echo and above tests. In this paper, the conceptual and actual designs of the device are described, the advantages of, and problems with the approaches reported, the theoretical limitations of the methodologies expanded, and the results from several field-testing programs discussed. Based on test results, the device is quite promising, and its capabilities are more diverse than those of existing devices.

A procedure for determining p-wave speed in concrete for use in impact-echo testing using a p-wave speed measurement technique

Abstract: The dilatational or P-wave speed in concrete is needed in impact-echo testing if the dimensions of the structural elements or the location of flaws are to be determined. Previously, the P-wave speed was determined from cores or from performing a test on a portion of the structure having known dimensions and no flaws. In cases where neither approach was possible, an estimate had to be made of the wave speed. This paper presents the details of a method for independently determining P-wave speeds in concretes by determining the time it takes the P-wave to travel between two points along the surface of a structure. This procedure is more accurate than procedures that involve the use of Rayleigh wave speed measurements, and thus it is a better technique for use in quality control situations and other testing situations where more accurate results are needed.

Effects of shape, size, and casting direction of specimens on stress-strain curves of high-strength concrete

Abstract: An experimental investigation conducted to study the effects of the shape, size, and casting direction of speciments on the complete stress-strain relationship of high-strength concrete in compression is reported in this paper. Both cylindrical and prismatic specimens were used for this purpose, and the concrete grade was varied from 50 MPa to 120 MPa. Tests results reveal that the effect of specimen size vanishes below a certain size and that the effects of the shape and casting direction of specimens are significant, particularly for the descending branch of the stress-strain curve. Based on test data, a simple model is proposed to generated the complete stress-strain relationship of high-strength concrete in compression that is applicable to horizontally cast prisms. The proposed model has been found to give a good representation of the actual stress-strain response, and a comparison with a similar model proposed earlier for cylindrical specimens shows that the horizontally cast prisms are more ductile than corresponding cylinders.