Prestressed Concrete Box Girder with High-Capacity Strands-Monitoring and Analysis during Fabrication

Round 1

Reviewer 1 Report

Dear Authors,

the article is very interesting and deals with current problems. The results are interesting and will interest many readers, designers and technologists.

However, to improve the quality of the material, I suggest:

1. expand the section on literature review. Especially from recent years.

2. because the composition of the concrete is atypical, e.g. for other areas in the world, please enclose information about the properties of the cement: chemical composition, water demand, strength after 2 and 28 days, the amount of heat released over time.

3. Please add information on the grain size composition of the aggregates and properties such as water absorption, frost resistance or alkaline reactivity.

4. Please add information about admixtures. Are air-entraining admixtures not used in the recipe? Was it not necessary to use concrete resistant to variable freezing / thawing temperatures?

5. Please provide information on concrete durability requirements. Is concrete with such a composition resistant to moisture and gas diffusion? Was there any additional protection, e.g. impregnation of elements?

6. It would be good to write if max. the temperature in the element did not cause microcracks. Have there been studies of the microstructure of concrete?

Author Response

Dear Reviewer,

Thanks for your advice. We appreciate the reviewer’s insightful suggestion on material properties and microstructural analysis. However, these are not our research objective. Instead, the goal is to alleviate the structural concerns using high capacity strands, especially the detension timing, so that the strands may be widely applied in real bridge structures. Thus, we monitored the behavior of the girder in the fabrication factory while we only conducted the traditional tests of concrete strength and modulus of elasticity in the lab.

Please find my itemized responses in below and my corrections in the re-submitted files.

1. References in recent years have been added.
2. The strength of concrete test cube after 2d and 28d was 22.5 and 62.0 MPa, respectively. The heat emission of concrete was indirectly measured based on the monitoring of concrete temperature using 16 gauges embedded in the girder, and the curves of temperature variation over time were given in section 6.
3. The maximum size of coarse aggregate was 25 mm, and river sand was used as fine aggregate. The material properties of frost resistance or alkaline reactivity are not our research concerns.
4. The mix design of concrete has been given in Table 1. We only conducted the traditional tests of concrete strength and modulus of elasticity in the lab. Only these two material properties were required in the design. The girder was cast in the fall, and there was no freezing / thawing problem.
5. This is beyond our research.
6. No cracks were detected in the girder. The microstructure of concrete was not studied. Instead, we focused on the structural behavior of the box girder.

Reviewer 2 Report

The paper presents a highly interesting study of prestress losses in the production phase of a 32.6 m pretensioned concrete box girder bridge. The aim is to alleviate certain production concerns of prestressed concrete bridge girders using high capacity strands. Deformations, temperatures and forces were monitored, as well as the concrete- and steel properties.

The overall quality of the paper is high. The method, results and conclusions are well defined and explained.

Some general points are: The aim of the paper should be mentioned in the Abstract. The readers should be referred to Figure 2 and 3 in section 2 where you are detailing the dimensions. In section 4 you should mention that the water to binder ratio of the concrete was 0,30 and maybe give some comment on the reason for using both fly ash and slag in addition to cement. On line 147 (page 4) yo mention that the 6-d strength was 42,2 MPa, which is exactly the same as the 4-d strength. I assume it is a misspelling for the 6-d strength since you say it is higher than the 28-d strength (50 MPa). (There is an extra the in the same sentence). Figure 4 to 7 are very interesting. You could perhaps mention why there are no results for T-E-2 and 3 in Fig. 4b and T-E-7 in Fig. 6b. The calculations in section 7 and 8 seems correct (I checked by back calculations). The conclusion that the detensioning could theoretically be done at 4-d instead of 6-d provides important knowledge and understanding to the bridge manufacturers. As you mention in section 9, the concrete shall according to current requirements be at least 5 days at detensioning, but your findings provide understanding that this time could be reduced.

Good job authors!

Author Response

Dear Reviewer,

Thank you for your advice. The paper has been revised according to your suggestion. The typo has been revised. In section 6, “Notably, three concrete sensors at the beam-end (T-E-2, T-E-3, T-E-8) failed during the fabrication.” The data of these three sensors were not available.