Discussion of “Effect of Prestressing Level on the Time-Dependent Behavior of GFRP Prestressed Concrete Beams” by Mohamed Zawam, Khaled Soudki, and Jeffrey S. West

The paper of reference is most interesting and the authors should be congratulated for their work. The relevance of the paper goes beyond prestressing concrete members with glass fiber–reinforced polymer (GFRP) bars. In fact, this research can have important implications on reinforced concrete with unstressed GFRP bars. Presently, the ACI 440.1R (ACI 2015) guidelines impose a very restrictive limitation on the maximum stress under service loads in GFRP reinforcement to account for the possibility of creep rupture, also known as static fatigue. The value in the guidelines is only 0.20 of the design strength of GFRP bars, already reduced by a 0.7 environmental factor.

Some specific aspects of the paper deserving more in-depth insight for future developments are given as follows.

The authors state that the prestressing level does not affect the beam stiffness, which may result in an ambiguous assertion. While the precracking and postcracking stiffnesses are cross-sectional properties, in principle not influenced by different levels of prestressing, the load at which cracking occurs is certainly influenced by the compression forces exerted by the pretensioned GFRP reinforcement on the concrete. In fact, as shown in Fig. 4 of the original paper, the load–deflection curves are clearly distinguishable. As expected, higher displacements are observed for a given load in beams with lower prestressing level. In that sense, the constitutive behavior of the members is strongly influenced by the prestressing level.

The material properties used for design, analysis, and discussion as presented in the paper do not allow for the unequivocal interpretation of the results. It is puzzling to see that data published by the bar manufacturer in 2009 were used for a research project published in 2017. These are guaranteed values, rather than effective values. Without an appropriate assumption of the mechanical properties of the bars used in this research, it is not possible to draw definite conclusions.

As a result, the strength-to-prestressing stress ratio used for experiments should be reflective of the actual mechanical properties measured on the specific production lot. The use of guaranteed values in design is intended to enhance the level of safety. In the absence of specific tests on the production lot, considering the average material properties obtained on different lots from the same pultrusion line (some values are mentioned in the paper) could be a more realistic choice.

It is also unclear how the authors computed the prestressing levels. Which strength and which area were used for reference? It is implied that the values are those listed in Table 2 of the original paper. Did the authors determine the material properties according to

The authors themselves state that the limitations on the maximum stresses imposed by codes are aimed to prevent creep rupture in composite reinforcement. The experimental work performed describes the entity of deferred deformations, over a 1-year period, caused by different levels of sustained loads on beams with different levels of prestressing. Results show that these deformations are caused by concrete creep. In addition, the flexural capacities observed on undisturbed reference members and on sustained load members do not seem to be affected by the load history. Although these results are precious, they do not allow drawing any conclusion on the limitations that may be imposed on maximum stresses in order to prevent the creep rupture of the GFRP bars. This phenomenon can be observed on the basis of creep rupture tests conducted on the bars themselves or, alternatively, creep rupture tests conducted on the whole prestressed member.

Another point to be clarified is whether the deferred effect that can be observed in composites due to sustained load should be considered as a creep or relaxation phenomenon. This remains an unclear aspect in the relevant literature. The authors refer to “creep or relaxation of the GFRP bars” in the abstract, but end up modeling creep in GFRP. Generally, it may be observed that a given deformation (rather than loading) is imposed to the composite tendons through bonding to the surrounding concrete. This probably means that GFRP tendons are subject to relaxation over time, an interpretation coinciding with classical assumptions that are made in the analysis of concrete elements prestressed with steel strands.

Finally, the interpretation of the data reported in Fig. 13 of the original paper is questionable. Based on the limited data points available, the over-time trend of strains observed on the GFRP reinforcement under sustained loads can barely be considered linear in a logarithmic scale.

In conclusion, this paper presents a most valuable contribution to the body of knowledge and this discussion is a testimony of the interest it has generated.