Akanshu Sharma

Studies on Normal Strength Concrete Cubes Subjected to Elevated Temperatures

Concrete in structures is likely to be exposed to high temperatures during fire. The probability of its exposure to elevated temperatures is high due to natural hazards, accidents and sabotages. Therefore, the performance of concrete during and after exposure to elevated temperature is a subject of great importance and interest to the designer. Popular normal strength grades of concrete produced by Ready Mix Concrete (RMC) India, Mangalore have been used in production of test specimens (150 mm cubes), cured and tested by destructive method for gathering data on strength characteristics. Later, these test samples were subjected to elevated temperatures ranging from 100°C to 800°C, in steps of 100°C with a retention period of 2 hours. After exposure, weight losses and the residual compressive strength retention characteristics are studied. Test results indicated that weight and strength significantly reduces with an increase in temperature. Residual compressive strength prediction equations are proposed for...

Pivot hysteresis model parameters for reinforced concrete columns, joints, and structures

Inelasticity in reinforced concrete (RC) frame structures due to seismic loads is generally considered to be concentrated in the form of hinges in beams, columns, and joints. In this paper, the parameters for rectangular RC beams, columns, and nonseismically detailed beam-column joints are suggested, keeping the simplicity of the original pivot hysteresis model. With the proposed parameters, the existing model can be effectively used to perform nonlinear dynamic analysis of nonseismically detailed RC structures. Two major improvements in the parameters for columns as compared to the original model are: 1) new parameters cover the complete range of axial load on the column; and 2) it gives due consideration to the transverse reinforcement on pinching behavior of the members. Additionally, several parameters are proposed for poorly detailed exterior joints. It is shown that the model with the proposed parameters nicely captures the hysteretic behavior of RC members, joints, and structures.

Dynamic Fracture of Concrete–3D Numerical Study of Compact Tension Specimen

The behavior of concrete structures is strongly influenced by the loading rate. Compared to quasi-static loading concrete loaded by impact loading acts in a different way. First, there is a strain-rate influence on strength, stiffness, and ductility, and, second, there are inertia forces activated. Both influences are clearly demonstrated in experiments. For concrete structures, which exhibit damage and fracture phenomena, the failure mode and cracking pattern depend on loading rate. Moreover, theoretical and experimental investigations indicate that after the crack reaches critical speed of propagation there is crack branching. The present paper focuses on 3D finite-element study of the crack propagation of the concrete compact tension specimen. The rate sensitive microplane model is used as a constitutive law for concrete. The strain-rate influence is captured by the activation energy theory. Inertia forces are implicitly accounted for through dynamic finite element analysis. The results of the study show that the fracture of the specimen strongly depends on the loading rate. For relatively low loading rates there is a single crack due to the mode-I fracture. However, with the increase of loading rate crack branching is observed. Up to certain threshold (critical) loading rate the maximal crack velocity increases with increase of loading rate, however, for higher loading rates maximal velocity of the crack propagation becomes independent of the loading rate. The critical crack velocity at the onset of crack branching is found to be approximately 500 to 600 m/s.

Behavior of Anchorages with Supplementary Reinforcement Under Tension or Shear Forces

The behavior of anchorages with multiple headed studs is significantly influenced by the presence of supplementary reinforcement. Under the action of tension forces or the shear forces perpendicular and towards the edge, the behavior of anchorages with supplementary reinforcement can be best described using a strut-and-tie model. The forces applied to the anchorage are resisted by a network of concrete struts taking up the compression forces and tension ties formed by the hanger/surface reinforcement and the stirrups. Thus, in principle, there are three major components in this strut-and-tie model, namely the concrete struts, the tension ties and the nodes. In this work, detailed experimental investigations were carried out on anchorages, using the WELDA® anchor plates from Peikko Group Corporation, with supplementary reinforcement to take up tension forces subjected to tension loads and anchorages with supplementary reinforcement to take up shear forces close to an edge loaded in shear perpendicular and towards the edge. The experimental program was designed to capture the behavior of the different components and the forces taken up by concrete and reinforcement were segregated using the data obtained from strain gauges applied on the stirrups. It was clearly brought out that several assumptions made in the existing models e.g. the models of EN1992-4, ACI 318 or fib Bulletin 58 are not entirely in accordance with the real behavior and therefore the models are either overly conservative or tend to become unconservative depending on the configuration and the amount of supplementary reinforcement. Based on the evaluated results, a new model is developed for anchorages with supplementary reinforcement which is presented in the accompanying paper.

Analytical Model for Anchorages with Supplementary Reinforcement Under Tension or Shear Forces

The models included in the current standards and guidelines (EN1992-4, ACI 318, fib Bulletin 58) to evaluate the failure loads for anchorages with supplementary reinforcement in the form of hanger reinforcement and stirrups subjected to either tension or shear forces are conservative compared with the results of corresponding experiments. In this work, a new analytical model is developed based on experimental investigations performed on anchorages, using WELDA® anchor plates from Peikko Group Corporation, with supplementary reinforcement to take up tension forces subjected to tension loads and anchorages with supplementary reinforcement to take up shear forces close to an edge loaded in shear perpendicular and towards the edge. The model for reinforcement is an adaptation of the model proposed by Schmid (2010) based on research performed at the University of Stuttgart. The mean failure loads calculated by the new model are shown to be in very good agreement with the experimentally obtained mean failure loads. The model is objective and is applicable equally well for anchorages with supplementary reinforcement under either tension or shear forces.

Seismic Performance of Column-Foundation-Joints with Post-installed Rebar Connections: Pre-test Simulations

Seismic strengthening and retrofitting of reinforced concrete structures frequently requires increased sizes (and reinforcement) of the existing columns or providing additional vertical load carrying members (columns). The efficiency of the added columns in carrying the seismic forces depends largely on the effectiveness of the connection between the new columns and the existing structure (foundations, beams and slabs). To form a connection between the existing structural member and the column, post-installed rebar are required. The behavior of these connections can be visualized either from the point of view of the reinforced concrete theory, which requires long bonded lengths (due to equivalence with cast-in reinforcement) or from the point of view of bonded anchor design that requires relatively short bond lengths. Guidelines for application of post-installed reinforcing bars under seismic loads are available in US. In Europe there are no such provisions.

Dynamic Fracture of L and CT Concrete Specimens: Numerical and Experimental Studies

To confirm the findings of recent numerical studies and to obtain the experimental evidence on dynamic fracture of concrete, experimental tests were performed on Land CTspecimens. The experiments fully confirmed the results of previously performed numerical studies. It is shown that inertia effects are responsible for progressive increase of resistance, crack branching and rate dependent crack propagation. The presented, relatively simple, tests can be used to check whether numerical model is able to realistically predict complex phenomena related to dynamic fracture of concrete.

Analytical model for concrete edge failure of multiple row anchorages with supplementary reinforcement

In the accompanying paper, the details and results of the experimental campaign carried out on multiple row anchorages, without and with supplementary reinforcement, loaded in shear towards the edge were presented. It was shown that the models given in standards are insufficient to calculate the failure load for anchorages with supplementary reinforcement failing through concrete edge and reinforcement failure. This paper gives the details of a new analytical model developed to evaluate the failure load of anchorages with multiple anchor rows with supplementary reinforcement. The model is developed on the basis of the detailed evaluation of the results of an experimental campaign carried out on anchorages with up to four anchor rows. It has been shown that with the new model, the failure loads for the anchorages with supplementary reinforcement can be evaluated realistically considering different possible failure modes. In order to investigate the number of anchor rows participating to carry the shear loads, the experimental results are augmented through numerical simulations performed using software MASA at University of Stuttgart.