Showing papers in "Materials in 1988"

Factors Affecting Bond Between New and Old Concrete

Abstract: When new concrete is placed next to old concrete, attempts are usually made to bond the two concretes together: ofen a cement paste or mortar joint is used, as in masonry. The service stress states in bonds can vary tremendously so no single test method can replicate all these states. A method of test for bond should reflect a typical in-service stress state yet also be sensitive to variation in the strength of the bond. Four tests were evaluated, and a slant shear test was found to be the most appropriate. The effects of various parameters were evaluated theoretically with the finite element and/or experimentally. A bond material with a modulus of elasticity similar to the adjacent concrete was found to be desirable, as was consistency in the bond material properties. Thick bond layers were found to reduce bond strength considerably and copolymer polyvinyl acetate was found to be a poor bonding agent over a wide range of curing conditions and mortar mix designs.

Engineering Properties of Concrete Affected by Alkali-Silica Reaction

Abstract: A detailed study of the effects of alkali-silica reaction (ASR) on the engineering properties of concrete such as compressive and tensile strength, elastic modulus, and pulse velocity is presented. Two types of reactive aggregate - a naturally occurring Beltane opal and synthetic fused silica - were used. The tests wee carried out at 20 C and 96 percent relative humidity (RH). The results showed that losses in engineering properties do not all occur at the same rate or in proportion to the expansion undergone by the ASR-affected concrete. The two major properties affected by ASR were flexural strength and dynamic modulus of elasticity. Compressive strength was not a good indicator of ASR, but the flexural strength proved to be a reliable and sensitve test for mointoring ASR. Nondestructive tests like dynamic modulus and pulse velocity were also able to identify deterioration of concrete by ASR. The data indicate that critical expansion limits due to ASR would vary depending on the type and use of a concrete structure.

Influence of mix proportions and construction operations on plastic shrinkage cracking in thin slabs

Abstract: Plastic shrinkage cracking in mortar panels, simulating cracking in concrete slabs, was investigated using the procedure developed by Kraai. For the specific conditions of the testing program, the incidence of plastic shrinkage cracking increased with the paste volume fraction. It was also observed that the orientation and severity of the cracks were influenced more strongly by the direction and speed of strikeoff operations than for all other variables studied. While it is believed that some threshold evaporation rate is necessary to initiate cracking, for the tests conducted there was no direct correlation between the severity of the cracking and the rate of evaporation.

Properties of steel fiber reinforced concrete under cyclic loading

Abstract: An experimental investigation of the behavior of steel fiber reinforced concrete under cyclic compressive loading is presented. Cylindrical specimens were cast using normal- and high-strength concrete mixes, four types of steel fiber, and three different volume fractions. Stress-strain responses were obtained for three cyclic loading regimes as well as for monotonic loading. The envelope curve is shown to govern cyclic response. Toughness under cyclic loading is found to be at least as great as that under monotonic loading. The behavior of fiber reinforced concrete under cyclic loading, when normalized by its monotonic behavior, is very similar to that of plain concrete or concrete confined by steel spirals, indicating that the fibers primarily influence the envelope curve.

SettingTime and Strength of Concrete Using the Impact-Echo Method

Abstract: Tests were performed to evaluate the feasibility of using the impact-echo method to determine setting time and to monitor strength development of concrete. In the impact-echo method, the test object is subjected to point impact and the surface displacement is monitored at a point adjacent to the impact. From the measured displacement waveform and the thickness of the object, the P-wave velocity is determined. Changes in the P-wave velocity with time reveal information about the development of mechanical properties as the concrete matures. The results are discussed of setting-time tests on concrete mixtures of two water-cement ratios both with and without set-controlling admixtures. The results are also discussed of tests that examined the relationship between P-wave velocity, as determined by the impact-echo method, and the compressive strength of concrete. It is concluded that the impact-echo method is a promising nondestructive technique for monitoring the development of mechanical properties in concrete from initial setting to ages of several days.

Properties of Fiber Reinforced Concrete: Workability, Behavior Under Long-Term Loading, and Air-Void Characteristics

Abstract: This paper presents the results of an experimental invesigation of the behavior of steel fiber reinforced concrete. The properties investigated were: setting times, slump and air content loss with time, creep and shrinkage, and air-void characeristics including bubble size distribution. To compare the behavior of fiber reinforced concrete with plain concrete, all of the proceding properies were also investigated for plain concrete mixtures of similar composition. Two mixture proportions with cement contents of 611 and 799 lb/yd sup 3 (363 and 474 kg/m sup 3) were investigated. The lower cement content that was used with a water-cement ratio of 0.4 resulted in a highly workable medium-strength concrete. The compressive strength was in the range of 6000 psi (41 MPa). The higher cement content, wih a 0.3 water-cement ratio, resulted in a relatively stiff concrete with an average compressive strength of 7000 psi (48 MPa). High-range water reducers and air-entraining admixtures were used for all the mixtures. Collated 50 mm long steel fibers with hooked ends were used for the fiber concrete. The experiments were conducted using the appropriate ASTM standards. The air-void characteristics were studied using the linear traverse method. The results indicate that setting times are about the same for both concretes. Fiber concrete has lower slump and air content; the rate of loss of these parameters with time is also higher. Shrinkage of fiber reinforced concrete is slightly less, but it undergoes slightly more creep deformations. In the area of aid-void characteristics, the specific surface of air bubbles is lower for fiber reinforced concrete, and it has relatively less number of chord-intercepts in the 0 to 0.002-in. (0 to 0.05-mm) range.

Ordinary Portland Cement Mixes With Selected Admixtures Subjected to Sulfuric Acid Attack

Abstract: Several 102-mm cubes were cast from cement paste, mortar, and concrete mixes containing ordinary portland cement and suspended in a channel containing either a 1 or 3 percent nominal solution of continuously flowing sulfuric acid. The parameters investigated included cement content, water-cement ratio (W/C), additions of pulverized fuel ash (PFA), polyvinyl alcohol (PVA), a latex (styrene butadiene), and a high-range water-reducing admixture (HRWA). The changes in weight with time for each cube were determined continuously up to a maximum exposure period of 93 days. The results indicated that the rate of deterioration of the cubes increased with an increase in cement content, w/c, or acid concentration. The addition of polyvinyl alcohol appeared to enhance the resistance and integrity of the cubes.

Bubble Characteristics as They Pertain to Compressive Strength and Freeze-Thaw Durability

Abstract: There are numerous chemical agents to achieve the specified air entrainment system. Generally, these are organic chemicals which are broadly classified as soaps and detergents. One of the best known chemical agents of this type is known in the art as Vinsol resin, manufactured by Hercules, Inc., which is a wood resin salt and is the standard against which other air-entraining agents are tested under ASTM C-233.

Realistic strength of air-entrained concretes with and without fly ash

Abstract: Effects of various curing regimes on the strength development of both plain and fly ash concretes were studied. Three grades of air-entrained concretes with 28 day nominal strengths of 20, 30, and 40 MPa (2900, 4350, and 5800 psi) were cast. For each grade, 0, 20, 35, and 50 percent of the weight of cement was replaced by two subbituminous fly ashes. Cylinders were cured at 100 and 50 percent relative humidities and under outdoor ambient conditions--including a Canadian winter climate. Structural-size members were also exposed to the elements and strength was estimated by testing extracted cores. Results indicate exposure of test cylinders to site conditions does not adequately reflect strength development of the in situ concrete; this is particularly true if test cylinders are allowed to dry. Prediction of in situ strength from site-cured cylinders is particularly inaccurate for the fly ash concrete. For in situ concretes with a low total cementitious centent, the inclusion of fly ash in appreciable quantities is likely to result in slower strength gain than plain concrete. However, for higher grade concretes (higher total cementitious content) even with high ash content, early exposure to the elements is no more detrimental than it is to plain concrete.

Les affiches de "mai 68"

Abstract: Gervereau Laurent. Les affiches de "mai 68". In: Materiaux pour l'histoire de notre temps, n°11-13, 1988. Mai-68 : Les mouvements etudiants en France et dans le monde, sous la direction de Stephane Courtois. pp. 160-171.

Les groupes politiques d'extrême-gauche à Nanterre

Abstract: Duteuil Jean-Pierre Les groupes politiques d'extreme-gauche a Nanterre In: Materiaux pour l'histoire de notre temps, n°11-13, 1988 Mai-68 : Les mouvements etudiants en France et dans le monde, sous la direction de Stephane Courtois pp 110-115

L'automne de Mexico (crépuscule à Tlatelolco).

Abstract: Lafaye Jacques. L'automne de Mexico (crepuscule a Tlatelolco).. In: Materiaux pour l'histoire de notre temps, n°11-13, 1988. Mai-68 : Les mouvements etudiants en France et dans le monde, sous la direction de Stephane Courtois. pp. 60-65.

La sérigraphie à l'école des Beaux-Arts. Entretien avec Rougemont

Abstract: Gervereau Laurent, Rougemont . La serigraphie a l'ecole des Beaux-Arts. Entretien avec Rougemont. In: Materiaux pour l'histoire de notre temps, n°11-13, 1988. Mai-68 : Les mouvements etudiants en France et dans le monde, sous la direction de Stephane Courtois. pp. 180-183.