Group effect in axially loaded chemical anchors embedded in low strength concrete

The use of chemical anchors for the connection of existing structural elements with the new elements during strengthening of existing structures is quite a preferred method. Since in our country, chemical anchors are being widely used for repair and strengthening works, there should be a standard of design and application on this topic. Up to now, research done on the topic of chemical anchors, are mostly carried out on concrete blocks of compressive strength 20 MPa and higher. Since parameters such as distance from the edges, embedment depth and group effect were mostly ignored, the resulting behavior was brittle, instead of the desired ductile behavior. This is particularly true for low strength concretes, in which concrete related damages increase the probability of an overall brittle behavior. In the scope of this study, 100 epoxy bonded anchors were embedded into concrete blocks of strength between 5.8-16.4 MPa. Among these anchors, 22 were single anchors, and 26 were group of three anchors. In these experiments, 12 mm S420a and 20 mm S420b rebars were used. The behavior of the anchors was investigated by varying embedment depth and distance from edges and corners. It was observed that stress concentrations in projected failure area are more significant for greater bar diameters. And increase in expected stress levels, increases the possibility of having concrete governing failure. Therefore, it is suggested to put an upper limit in codes for bar diameters. It was observed that stress concentrations in projected failure area are more significant for greater bar diameters. And increase in stress levels, increases the possibility of having brittle concrete breakout failure. Therefore, it is suggested to put an upper limit for bar diameter to limit this possibility. Besides, findings about stress concentrations show that ACI318 formulation yields safer design strength for small-diameter anchors with respect to large-diameter anchors. To compensate this, some modification factors have been proposed for the calculation of projected breakout failure area according to ACI318. Calculated design strengths of specimens per ACI318 were lower than anchor capacities obtained from tests. It is observed that average factor of safety for majority of the experiments is around 2. Therefore, it is concluded that ACI318 design strength can safely be used for most of the anchor configurations. However, in some experiments, ultimate capacity of specimens were very close to ACI318 design strength. This is especially observed for the cases where stress concentrations occur, for example group anchors with large diameter bars located parallel to edges