Prestress losses evaluation in prestressed concrete prismatic specimens
Highlights
► Prestress losses have been measured using two testing techniques: forces and strains. ► A coefficient to account for the results from both techniques has been proposed. ► Effects of concrete mix, cross-section size and age at release have been analyzed. ► Comparisons with predicted prestress losses from methods in codes have been analyzed.
Introduction
There are two procedures for prestressing a concrete member through reinforcement: post-tensioning and pre-tensioning. In both cases, the initial tensile stress applied in the prestressing reinforcement decreases through several sources. The difference between initial tensile stress and tensile stress in prestressing reinforcement at any time t is defined as total prestress loss (TPLt). Usually, TPLt is quantified as a percentage over initial tensile stress.
It is generally accepted that prestress losses have little effect on ultimate design strength and on the capacity of pretensioned concrete members, but that prestress losses can affect service conditions [1]. Upon service loads, overestimating prestress losses can lead to excessive camber and inefficient designs, while underestimating prestress losses can result in excessive deflection and unexpected cracks.
Prestress losses can be determined analytically and experimentally. Methods to estimate prestress losses can be classified into the following levels, listed in ascending order in terms of complexity and accuracy [2], [3]: (I) lump-sum or approximate methods to estimate TPL (oversimplified methods for preliminary design); (II) refined or detailed methods to estimate prestress losses separately due to each particular source (commonly used for designs based on elemental information about materials properties and environmental conditions); and (III) accurate determination of cumulative losses by time-step methods, which involves knowledge of the loading history on the member (useful in multi-stage bridge constructions at any critical time).
The experimental techniques used to determine prestress losses include several typologies [4], [5], [6], [7]: (1) monitoring longitudinal concrete strains over time at the level of the center of gravity of the prestressing reinforcement; (2) load testing to determine crack initiation and/or crack re-opening loads to obtain the available compressive stress in the bottom flange of a member; (3) severing the prestressing reinforcement by cutting it into a representative exposed length after placing strain gauges on the reinforcement; (4) relating the tension in the prestressing reinforcement to the vertical deflection recorded when known weights are suspended from it on a representative exposed length; and (5) determining the side pressure to close the induced crack in a small cylindrical hole drilled in the bottom flange of a member.
All these experimental techniques require a back-calculation of the prestress losses from the test data using theory of mechanics concepts. Method 1 requires the instrumentation of the member during casting, and it can be used to determine prestress losses over time. Methods 2 and 3 are destructive tests and provide information only on the existing prestressing reinforcement stress at testing times (prestress losses are frequently obtained by considering theoretical rather than measured initial prestressing reinforcement stress). Method 4 is a semidestructive test and involves accurately determining the exposed length for calculations. Method 5 is a non-destructive technique which involves an appropriate factor by numerical procedures.
The main objective of this experimental research work is to analyze changes in prestress losses over time in pretensioned prestressed concrete using a testing technique that allows the simultaneous application of the aforementioned Method 1 and the continuous measurement of prestressing reinforcement force. To this end, an experimental program has been set up over a 1-year period with several pretensioned prestressed concrete prismatic specimens varying in terms of concrete mix design, specimen cross-section size, and concrete age at the prestress transfer. The ECADA+ test method [8] has been used to measure the effective prestressing force over time. In addition, specimens have been instrumented to determine the longitudinal concrete surface strain by mechanical gauge points. The experimental results have been compared with predicted prestress losses from existing methods in several codes.
Section snippets
Sources of prestress losses
For pretensioned prestressed concrete members, the manufacturing process involves the following main stages:
- (a)
First the prestressing reinforcement is tensioned in the casting bed by stretching it between abutments using provisional end anchorages. Instantaneous anchorage seating elastic loss occurs (prestress loss ranging from fp,jack –initial at jacking- to fp,bed –at anchoring-). This prestress loss can be determined from the equipment and fabrication system characteristics, and very often they
Testing technique
The ECADA+ test method [8] has been used. ECADA+ is a revised, improved version of the original ECADA1 test method [28], and it determines transfer length and development length [29], [30]. Its feasibility has been verified for short-term [31], [32] and long-term [33] analyses.
In this work, only specimens with an embedment
Experimental program
To study the prestress losses changes over a 1-year period on several pretensioned concrete prismatic specimens, an experimental program was carried out by varying the concrete mix design, specimen cross-section size and concrete age at the prestress transfer.
Experimental measurements
For this work, the prestress losses accounted for between jacking and the prestress transfer release were excluded. As the hollow force transducer was placed in the AMA system in contact with the anchorage device, the prestressing reinforcement force just before the prestress transfer release (P0) was known. Furthermore, prestress losses due to prestressing reinforcement relaxation were ruled out by applying a temporary overstressing (see Section 3.2).
By way of example, Fig. 7 shows the
Conclusions
Changes in prestress losses over 1 year in pretensioned prestressed concrete specimens have been analyzed by simultaneously using two measurement techniques: prestressing reinforcement force measurement through the ECADA+ test method; longitudinal concrete strains measurement at the level of the center of gravity of the prestressing reinforcement. The main conclusions drawn from this experimental study are:
- •
A prestress losses underestimation has been obtained from the measured prestressing
Acknowledgments
Funding for this experimental research work has been provided by the Spanish Ministry of Education and Science and ERDF (Project BIA2006-05521 and Project BIA2009-12722). Tests have been conducted at the Institute of Concrete Science and Technology (ICITECH), at the Universitat Politècnica de València (Spain).
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