Abstract
Friction in a post-tensioning system has a significant effect on the distribution of the prestressing force of tendons in prestressed concrete structures. However, attempts to derive friction coefficients using conventional electrical resistance strain gauges do not usually lead to reliable results, mainly due to the damage of sensors and lead wires during the insertion of strands into the sheath and during tensioning. In order to overcome these drawbacks of the existing measurement system, the Smart Strand was developed in this study to accurately measure the strain and prestressing force along the strand. In the Smart Strand, the core wire of a 7-wire strand is replaced with carbon fiber reinforced polymer in which the fiber Bragg grating sensors are embedded. As one of the applications of the Smart Strand, friction coefficients were evaluated using a full-scale test of a 20 m long beam. The test variables were the curvature, diameter, and filling ratio of the sheath. The analysis results showed the average wobble and curvature friction coefficients of 0.0038/m and 0.21/radian, respectively, which correspond to the middle of the range specified in ACI 318-08 in the U.S. and Structural Concrete Design Code in Korea. Also, the accuracy of the coefficients was improved by reducing the effective range specified in these codes by 27–34 %. This study shows the wide range of applicability of the developed Smart Strand system.
Highlights
The calculation and control of elongation and the prestressing force during tensioning of tendons are of primary importance in post-tensioned concrete structures
The distribution of the prestressing force obtained at the same Smart Strand as that shown in Fig. 6 is presented in Fig. 7, where the values measured at the load cell, EM sensor, and jack are indicated
In most of the previous studies using the strands with fiber Bragg grating (FBG) sensors, only the distribution of prestressing force considering prestress losses was estimated, and the friction coefficients were not derived (Kim et al 2012; Xuan et al 2009; Zhou et al 2009)
Summary
The calculation and control of elongation and the prestressing force during tensioning of tendons are of primary importance in post-tensioned concrete structures. The relevant design codes and specifications recommend that the friction coefficients be experimentally determined (ACI 2014; KCI 2012), the set-up of test specimens and measurement of forces or strains of tendons required to obtain the coefficients are not easy to carry out. The coefficients show a wide range of differences depending on the provisions, and are sometimes expressed as a range rather than as a specific value. This has caused some confusion and trial-and-error practices for designers and constructors, and has led to the inconsistent use of friction coefficients. An acceptable error limit of ±5 or ±7 % of the jacking force between the measured value in a jack and the calculated value from the elongation of tendons (AASHTO 2014; ACI 2014) may still provide a source of discrepancy from the original calculation sheet in the stress distribution of concrete as well as tendons
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