Search

The long-term deformation of a 17.5 m deep excavation constructed with the bottom-up technique in silty clay in Xi’an was monitored in this study. The retaining wall was built with one or two rows of contiguous bored concrete piles and tie-back prestressing tendons. The monitoring of settlement and lateral deflection at the top of the retaining structure, and the settlement of the ground surface and of the adjacent building lasted for more than 4 years. After the final depth was reached, the excavation was surveyed for more than 3.1 years. It was observed that the deformation of the excavation increased with the increase in the excavation depth, and a significant increase in displacement was recorded after the excavation reached the final depth. The averaged time-dependent rates of the deflection of the retaining wall, and the settlement of the ground surface and of the adjacent building when the excavation remained partially unfilled were around 0.23–0.25 mm/month in this study. The time-dependent behavior of the excavation could lead to excessive deformation and cause severe safety risks to the retaining system. The recorded data, together with a comparison with cases reported in the open literature, are presented in this paper.

The contribution of near-surface mounted (NSM) fiber-reinforced polymer (FRP) bars to the shear strength of unreinforced masonry (URM) walls built with concrete masonry units (CMU) was eval...

This research investigates the feasibility of using prestressed glass fiber reinforced polymer (GFRP) bars for enhancing the in-plane shear strength of unreinforced masonry (URM) walls. In this application, URM walls were strengthened with prestressed GFRP bars by using the near-surface mounted reinforcement (NSM) technique. Prestressed GFRP bars would be used for repointing/closing wide open cracks in existing masonry structures. In total, eight URM walls were tested under in-plane diagonal compression. Experimental results validated the higher shear capacity of the strengthened URM walls and this increase was more pronounced for walls strengthened with prestressed GFRP bars. Furthermore, the calculated drift ratio in strengthened walls was considerably higher when failure was along stair-stepped diagonal cracks. Experimental results were used to modify and calibrate a theoretical shear friction model for predicting the in-plane shear capacity of strengthened URM walls with NSM prestressed GFRP bars. The model was capable of predicting reasonably well the shear capacity of strengthened URM walls.

A model proposed in the literature for the evaluation of the in-plane shear capacity of unstrengthened and strengthened concrete and clay brick unreinforced masonry (URM) walls was modified and calibrated following the results from an experimental research program. The tested walls were strengthened with grids made from glass fiber-reinforced polymer (GFRP) embedded within a rapid-setting sprayed polyurea. Various GFRP grid reinforced polyurea layouts were investigated, and consisted of strips oriented in either the vertical or horizontal direction and installed on one or both faces. The prediction models proposed in this paper were subsequently evaluated using a probabilistic Monte Carlo simulation (MCS) by considering the uncertainty and variability of the independent variables, which were assumed to follow a truncated normal distribution. Corroborated by the MCS, test results clearly show that the failure modes of the strengthened URM walls were affected by the strengthening scheme. Experimental and simulation results are presented and discussed in this paper.

A mechanical device specially designed for the application of low-level post-tensioning forces to glass fiber reinforced polymer (GFRP) bars has been developed at the University of Missouri-Rolla. Some of the advantageous features of this device are that it is simple to assemble and the low-level post-tensioning forces can be applied manually and safely without the need for hydraulic jacks or heavy equipment. This device has been conceived with the main objective of retrofitting masonry buildings, some of which remain in service despite large, open cracks leading to considerable instability and serviceability concerns. According to the method derived in this paper, GFRP bars are installed in artificially imposed grooves and then post-tensioned with low-level stresses with the main objective to partially close these cracks, such that the serviceability and in-plane capacity of un-reinforced masonry (URM) buildings may be regained. In this paper we describe the mechanical components of this device, along with its advantageous features and potential application for the retrofit of URM walls.

In an accompanying paper in the same issue of this journal, the development of a prototype mechanical device for prestressing carbon fiber-reinforced polymer (CFRP) sheets was described. To investigate the feasibility of using this device, seven reinforced concrete (RC) beams were strengthened with prestressed CFRP sheets. This paper describes theoretical and experimental results related to prestressing levels that can lead to debonding of the sheets immediately after transfer and during flexural testing. An investigation regarding the increase in the flexural capacity that results from prestressing and anchoring the CFRP sheets at the beam ends with U-wraps is also discussed. Analytical and experimental results suggest that values of the tensile strength of concrete are likely to be higher than suggested by ACI. The predominant premature failure mode in RC beams prestressed with CFRP sheets is by concrete cover separation unless the CFRP sheets are effectively anchored with U-wraps. For the prestressed sheets anchored with U-wraps, concrete cover separation was averted and the predicted theoretical capacity was achieved. Results also showed that the addition of prestressed and non-prestressed sheets increased the cracking load by nearly 3 to 6 times, respectively compared with the control beam. The yield and ultimate loads were increased by nearly 25%, but displacement ductility capacity of the system was reduced by nearly 5 times.

A mechanical device for prestressing carbon fiber-reinforced polymer (CFRP) sheets was developed, and its application for the flexural strengthening of reinforced concrete (RC) beams was investigated in this research program. Significant features of this mechanical device are 1) the CFRP sheets are directly anchored to the device itself; 2) prestressing is easily achieved while the device is not yet attached to the RC beam; 3) the prestressing is applied with a manual torque wrench without the need for power-operated hydraulic jacks; 4) fiber-reinforced polymer (FRP) U-wraps can be easily attached to the end of the prestressed CFRP sheets; and 5) the prestressing transfer is accomplished under slow strain rates. This paper describes in detail the main features and the application procedures for this mechanical device. To investigate the feasibility of using this device for flexural strengthening, the experimental program consisted of testing seven RC beams under a four-point bending test setup. Test results indicate that the device was efficient in applying prestressing to the CFRP sheets and, as predicted, in comparison with the control beams, the flexural capacity and serviceability of the beams were increased after strengthening with the prestressed CFRP sheets. In a second accompanying paper, the experimental and analytical results obtained from the flexural program are discussed in further detail.