N. Banthia

Impact behaviour of concrete beams

An instrumented impact machine was used to carry out impact tests on concrete beams, 100×125 mm in cross-section and 1400 mm long. The simply supported beams were struck at their midpoints by a 345 kg mass impact hammer, dropped from various heights. The instrumentation included strain gauges mounted on the striking end of the hammer, strain gauges mounted on one support anvil, and three accelerometers placed at various locations along the beam. The data were collected using a 5-channel data acquisition system. Normal strength, high strength, and fibre reinforced concrete beams were tested. In general, it was found that the properties of concrete under the high stress rates associated with impact loading could not be predicted from conventional static tests.

The behaviour of concrete under impact loading: Experimental procedures and method of analysis

The behaviour of concrete beams under impact loading was studied, using an instrumented drop-weight impact apparatus capable of dropping a 345 kg mass from heights of up to 3 m. Both plain and conventionally reinforced beams, with overall dimensions (length x width x depth) of 1,400×100×125 mm, were tested on a 960 mm span, with the impact load applied at mid-span. The load on the instrumented striking tup, and the acceleration of the beam itself, were measured as a function of time; the entire impact events had a duration of 10 to 70 ms. This paper describes the methods used to analyze the impact data, as well as some preliminary results.RésuméOn a étudié le comportement de poutres de béton sous charge de choc à l’aide d’un appareil à mouton en chute libre qui peut libérer une masse de 345 kg avec des hauteurs de chute jusqu’à 3 m. On a essayé à la fois des poutres de béton non armé et armé conventionnellement, de dimensions générales (longuer x largeur x profondeur) de 1400×100×125 mm, la charge de choc étant appliquée au milieu d’une portée d’essai de 960 mm. On a mesuré comme une fonction du temps la charge au moment de sa mise en action et au moment de l’impact; la durée totale de l’action était de 10 à 70 m/s.Sur la base des résultats et de l’analyse, on peut tirer les conclusions suivantes:1.Même pour des vitesses de choc relativement basses adoptées dans ces essais (∼3 m/s), la valeur de pic se produisait dans un délai de ` m/s après l’impact.2.L’utilisation des accéléromètres montés sur les poutres peut servir à évaluer la charge d’inertie, ainsi que les vitesses et accélérations repaires.3.La charge d’inertie correspondant à la charge de pic mesurée peut s’élever à plus des 2/3 de la charge totale.4.La rupture à imputer à la charge d’inertie peut conduire à des conclusions erronées.5.Les évaluations de l’énergie à partir des charges appliquées par le mouton mis en œuvre ne concordent pas bien avec la somme de l’énergie cinétique calculée et de l’énergie dépensée pour fléchir et fracturer la poutre.

Restrained shrinkage cracking in fibre-reinforced cementitious composites

The influence of seven types of fibres on restrained shrinkage cracking in Portland cement pastes and mortars is investigated at various fibre volume fractions. Linear specimens with one-dimensional restraint were subjected to a drying environment soon after casting. Fibres belonging to two major categories-macro (large) and micro (fine)-were investigated. These two categories of fibres were found to result in distinctly different cracking patterns due, in part, to their different reinforcing mechanisms. An attempt is made to relate the observed cracking patterns with the perceptible micromechanical processes. A ‘fibre efficiency factor’ is proposed which appears to be an appropriate basis for grading the fibres.ResumeOn étudie l’influence de sept sortes de fibres sur la fissuration due au retrait pour différentes proportions en volume de fibres dans des mélanges de pâtes de ciment Portland et de mortiers. Des échantillons linéaires, avec retrait empêché uniaxial, ont été soumis à un environnement sec peu après leur moulage. L’étude a été effectuée avec deux catégories de fibres-macro (larges) et micro (fines).En raison de mécanismes de renforcement différents, ces deux catégories de fibres ont donné lieu à des schémas de fissuration distincts. L’utilisation des fibres a entraîné une réduction significative de la largeur des fissures. Les macro-fibres ont provoqué davantage de fissurations multiples dans l’échantillon que les micro-fibres; dans le cas des composites renforcés de micro-fibres, habituellement une seule fissure étroite apparaît sur les échantillons.Suite à l’observation de ces mécanismes, on propose un facteur d’efficacité de fibre (défini par le rapport de la longueur de fissuration cumulative sur la largeur de fissuration cumulative observée dans cet essai). Ce paramètre semble être un indicateur approprié de l’efficacité d’un type de fibre destiné à renforcer une matrice cimentaire dans des conditions de retrait empêché.Finalement, l’espacement des fissures dans les composites renforcés de macro-fibres est calculé de façon analytique en présumant certaines valeurs de la résistance de l’adhérence fibre-matrice et les résultats sont comparés avec les valeurs expérimentales. L’étude démontre que les valeurs théoriques d’espacement des fissures correspondent raisonnablement aux valeurs expérimentales.

Durability of microfiber-reinforced mortars

Abstract The frost durability of steel and carbon micro-fiber reinforced mortars was assessed. Tests of freezing and thawing and surface scaling were performed on the mortars. The effect of drying on the frost durability was also studied. The results show that the use of steel and particularly carbon microfibers improve the frost and deicer salt scaling resistance of mortars. The improvement is however in part due to the “air entrainment” properties of the micro-fibers.

The response of reinforced concrete beams with a fibre concrete matrix to impact loading

Abstract The behaviour under impact loading of reinforced concrete beams with a fibre concrete matrix was studied using an instrumented drop weight impact machine. The concrete matrix was of two types: normal strength and high strength, and the fibres were fibrillated polypropylene. The fibres had only a small effect in static loading, but in impact they increased the toughness of the reinforced beams by more than a factor of 2. The presence of the fibres in reinforced beams that were predamaged in static loading was found to have a considerable positive effect on their response to impact loading.

Energy Balance in Instrumented Impact Tests on Plain Concrete Beams

Impact tests were carried out on plain concrete beams, 100 mm × 125 mm in cross-section and 1400 mm long, using an instrumented drop weight impact machine. The machine, capable of dropping a 345 kg mass through heights of up to 2.3 m, had strain gauges attached to the striking end of the hammer, and also to one of the support anvils. In addition, in order to record the beam response during the impact, three accelerometers were mounted along the length of the beam. Normal strength and high strength concretes were tested. For three different drop heights, the energy lost by the hammer was compared to the energy gained by the beam in various forms. It was found that up to the peak load, the energy gained by the beam was only a small percentage of the energy lost by the hammer. However, by the end of the impact event, most of the energy lost by the hammer could be accounted for.

Crack Development in Cementitious Materials under Impact Loading

Flexural specimens of hardened cement paste, fibre reinforced concrete, and conventionally reinforced concrete were tested in an instrumented dropweight impact machine, employing a 345 kg mass impact hammer dropped from a height of 500 mm. The crack development in the specimens was monitored using high speed motion picture photography, carried out at about 10,000 frames per second. It was found that, for all three specimens, some crack branching occurred. The cracks did not propagate in a continuous fashion; they appeared to arrest occasionally, and then began to grow again. However, it appeared as if the crack velocities reached a maximum value very soon after the impact occurred; they then decreased, and finally increased again just prior to failure. The average crack velocities were in the range of about 75 to 115 m/s.

A Study of FRP–Concrete Bond under Impact

Our understanding of how the bond between FRP and concrete performs under impact loading is severely limited. In this paper, bond performance of Sprayed FRP with concrete was studied under impact loading. A novel 550 mm × 150 mm × 150 mm notched specimen was developed and a 75 mm wide strip of Sprayed FRP was applied to strengthen the specimen. A novel test set-up designed to minimize the influence of specimen rebound after impact as well as eliminate the need to correct the load data for inertial effects was developed. FRP strains were measured using strain gauges carefully installed on the FRP, and the bond stress was calculated using the differential strain values. A kinematics analysis was also performed to translate the vertical deflections into strain values. Three types of surface preparation methods (water jetting, sand blasting, and jack hammering) were studied. Specimens are tested under four different strain rates (four different hammer heights of 250, 500, 750, and 1000 m) and the results were compared with quasi static values using loading rates of 0.005, 0.05, and 0.5 mm/min. Results showed that sand blasting is the most effective surface preparation method resulting in the highest bond strength values. Impact results were characterized using a Dynamic Improvement Factor (DIF). Results indicate that the FRP–concrete bond is highly strain sensitive, and in general the bond strength increases and the fracture energy decreases under higher rates of strain. Untreated specimens are shown to be more strain-rate sensitive than the surface treated ones.

Response to Discussion of “Influence of Feedback Control on Flexural Toughness of Fiber Reinforced Concrete in ASTM C1399 Tests” by N. Banthia, S. Mindess, and Z. Jiang

We would like to express our sincere thanks to Prof. Zollo for his very thoughtful comments on our paper. We are very aware of the seminal leadership of Prof. Zollo on the Task Group that developed C1399. We also fully agree with Prof. Zollo that these discussions are useful in improving our understanding of the limitations of C1399.

Mitigating Impact Frailty of Concrete with Fiber Reinforcement

Since 9/11, there has been an increased interest in developing a better understanding of the properties of concrete structures under impact and blast loading. Although concrete, as a material, demonstrates extreme brittleness under dynamically applied loads, fortunately, fiber reinforcement significantly enhances such resistance. Yet, the dynamic properties of both concrete and fiber reinforced concrete (FRC) remain poorly understood. This paper provides a historical perspective of our efforts aimed at understanding the impact resistance of fiber reinforced concrete, highlights some of the issues and challenges encountered and identifies the emerging areas where further research is necessary.