Evaluation of Design Provisions for Horizontal Shear Strength in Composite Precast Concrete Beams with Different Interface Conditions

Round 1

Reviewer 1 Report

The research is interesting. My suggestions about your article are:

- I would explain better the specimens that you have tested.

- I would explain the conclusions explaining why it is valuable for the day by day practice. For example, in which type of structures/load situations is it possible to apply these solutions, and why not in some cases, etc. why is valuable.

- I would say in some point in the text (it could be in the Introduction) why this type of construction is better than all precast or all cast in situ. Advantages, disadvantages, challenges, problems to solve of Composite Precast Concrete Beams

- Also, I would explain in the conclusions the limitations and assumptions of the study and future research that propose (which is the research line to follow).

- I suggest to update the references (more references, the crucial ones and also new ones).

I hope it helps. Best regards.

Author Response

Your suggestion is right and valuable for our research.

"Please see the attachment"

Author Response File: Author Response.docx

Reviewer 2 Report

The paper presents problem of the load capacity of interfaces in composite elements made of concrete cast at different times, which is very important from the point of view of engineering practice. The analysis of the results of 95 composite beams tests, collected by the Authors, seems particularly interesting in this regard. Unfortunately, according to the reviewer, the paper in its current form lacks a lot of important information. Before publication, it should be supplemented and corrected.

 

The Authors have treated the background very briefly, basically limiting themselves only to listing selected works on composite elements (beams and push-off specimens). The included works on shear strength by Mattock et al., although important in the context of the discussed issue, are quite old. Why did the authors not present more recent works, concerning both push-off elements (e.g. (Randl and Wicke, 2000), (Ahmad, Bhargava and Chourasia, 2018), (Jiang et al., 2016), (Júlio et al., 2010)) and composite beams (e.g. (Halicka, 2011), (Fang et al., 2018)). The introduction in section 2 is too concise and does not allow the reader to get an idea of what has been done on the issue of shear transfer in composite RC beams. The authors should briefly describe the most important conclusions from the previous research at least.

 

Below, the reviewer provides more detailed comments on the manuscript:

 

The following statement is questionable: „Composite PC beams should be designed to have a horizontal shear strength greater than the horizontal shear force that may cause the interface sliding under the bending deformation. Otherwise, brittle failure may occur due to excessive horizontal cracks generated across the interface of the composite beam”

In the case of properly design shear reinforcement, it is possible to achieve ductile failure, which will be associated with slip in the interface. What did the authors mean?

 

Please explain PC and CIP abbreviations (line 35) – they are only described in abstract and should also be explained in the main text

 

Please provide the references to the mentioned works (line 40)

 

There is no quantitative description of the effect of the interface location on behavior of the composite beams (for example the effect on the load-carrying capacity) (lines 54-59). Please provide additional information.

 

Figure 1 adds little to the issue under consideration and the reviewer suggests removing it. It would be much more useful to present test specimens considered in the research discussed in section 3 (reinforcement, cross section, test setup) in more detail.

 

Figure 3 requires change and improvement – the descriptions are adapted to Figure 2 from (Moon and Oh, 2013), which was used in the crippled version in the paper under consideration

 

The authors refer in the paper to the results of their own research, but the description contained in lines 117-134 is far from sufficient. It will be difficult for the reader to find all of the important the information as some works are not publicly available and are also written in Korean. This section needs to be expanded.

 

Additionally, in Table 1:

• clarify the surface classification (intermediate, rough, keyed) – are there measured any roughness parameters, such as e.g. mean peak-to-valley height?
• the presented compressive strengths are characteristic (as it would appear from the description of f_ck) or mean values?
• please explain what mean the values ​​(loads) indicated in the table
• how the load carrying capacity of 2V_n was established?
• why was the nominal value for the yield strength introduced? For example, according to (Moon and Oh, 2013) f_y = 428 MPa for SF-1027 specimen
• what were the experimental loads? Why the information that appears, for example, in table 7 of (Moon and Oh, 2013),was not included?
• please check forces 2V_n and 2M_n/a - what are the reasons for the differences in relation to the values ​​given in the paper (Moon and Oh, 2013)?

 

Figure 4 shows the failure of selected elements, but once again, there is no more detailed commentary. In the opinion of the reviewer, it would be interesting to discuss, for example, the crack pattern – was the first crack observed at the interface or resulted initially from bending and formed in diagonal crack in the further stage? What slip was recorded in the tests and how did it develop depending on the roughness of the interface?

 

When discussing the issue of behavior of interface located in the tension zone (so-called Case 2), the Authors use the shear-friction theory and compare the results of research on composite beams with the results of tests on push-off elements, e.g. (Mattock and Hawkins, N. M., 1972). The results are shown in Fig. 5, but the commentary is unsatisfactory. In Figure 5 there is a clear difference between horizontal shear stress for composite beams and push-off elements at the same clamping stress level. What, according to the Authors, is the explanation for this fact?

It should also be noted that the test by Mattock et al. concerned initially monolithic elements that were cracked before the test. Therefore, they do not fully represent contact between concretes cast at different times.

 

What works the authors mean: “A number of computational equations have been proposed for the horizontal shear strength of the interface based on various research results” ? – please provide suitable references.

 

Model Code 2010 is cited incorrectly - fib instead of CEB should appear.

The sentence "The CEB model code (MC 2010 [3]) provides the interface shear strength in Equation (5) while accounting the adhesion, shear friction, and dowel action together, although defining the friction factor of 0.7 for roughened surface" is not entirely true.

The fib MC 2010 procedure clearly distinguishes the interfaces with rigid and non-rigid behavior. In the first case, the forces of adhesion and friction are taken into account, while in the second one - aggregate interlock, friction and dowel action. Thus, the principles of Eurocode 2, in which simultaneous action of adhesion, friction and dowel action is assumed, were very rightly departed from. The dowel action effect requires significant slip when the adhesive forces break long ago.

The quoted τ_c factor (see line 170) is not the coefficient of adhesion, as stated by the Authors, but is the coefficient for aggregate interlock effects at rough interfaces - see Equation (7.3-51) of fib Model Code 2010 (coefficient c_r)

Equation (5) also needs to be improved - instead of f_c(cube), f_cd should appear

 

Why are the more recent push-off tests not included in the statistical analysis? Only few works are included (Hofbeck, Ibrahim & Mattock [14] (1969), Mattock, Li & Wang [15] (1976))

 

Table 4 - there is no explanation of the substrate and overlay concrete strength

 

Figures 8 and 9 – as a function of what compressive strength were the charts drawn? Substrate or overlay?

 

The work also considers monolithic uncracked push-off specimens – what do they relate to the elements cast at different times, mentioned in the paper?

 

There is no commentary from the Authors, what results in significant differences between the results of the analyses for push-off specimens and composite beams (case 2). In both cases, the authors use the same calculation procedures, but in the case of push-off elements the average ratio of the experimental to theoretical load was 1.29-1.91 with COV = 14.1-23.0%, while in the case of beams (case 2 only) it was 0.93-1.35 with COV = 34.8-40.7%. The obtained results require a commentary.

 

The conclusions refer only to the codes of practice. Why did the Authors do not relate in any way to the experimental results? The title of the manuscript clearly suggests that it concerns Effects of Interface Conditions on the Horizontal Shear Strength for Composite Precast Concrete Beams. If the authors intend to limit themselves to a detailed description of the design procedures, then it is necessary to change the title of the manuscript. The current title suggests discussing horizontal shear strength in a much broader context.

 

References

Ahmad, S., Bhargava, P. and Chourasia, A. (2018) ‘Shear transfer strength of uncracked interfaces: A simple analytical model’, Construction and Building Materials. Elsevier Ltd, 192, pp. 366–380. doi: 10.1016/j.conbuildmat.2018.10.094.

Fang, Z. et al. (2018) ‘Horizontal Shear Behaviors of Normal Weight and Lightweight Concrete Composite T-Beams’, International Journal of Concrete Structures and Materials. Springer Singapore, 12(1). doi: 10.1186/s40069-018-0274-3.

Halicka, A. (2011) ‘Influence new-to-old concrete interface qualities on the behaviour of support zones of composite concrete beams’, Construction and Building Materials. Elsevier Ltd, 25(10), pp. 4072–4078. doi: 10.1016/j.conbuildmat.2011.04.045.

Jiang, H. et al. (2016) ‘Interface shear behavior between high-strength precast girders and lightweight cast-in-place slabs’, Construction and Building Materials. Elsevier Ltd, 128, pp. 449–460. doi: 10.1016/j.conbuildmat.2016.10.088.

Júlio, E. N. B. S. et al. (2010) ‘Accuracy of design code expressions for estimating longitudinal shear strength of strengthening concrete overlays’, Engineering Structures. Elsevier Ltd, 32(8), pp. 2387–2393. doi: 10.1016/j.engstruct.2010.04.013.

Mattock and Hawkins, N. M., A. H. (1972) ‘Shear Transfer in Reinforced Concrete—Recent Research’, PCI Journal, 17(2), pp. 55–75. Available at: https://www.pci.org/PCI_Docs/Publications/PCI Journal/1972/March-1972/Shear Transfer in Reinforced Concrete - Recent Research.pdf.

Moon, J.-H. and Oh, Y.-H. (2013) ‘Evaluation of Design Method and Shear Transfer Capacity on the Horizontal Interface of PC Composite Beams’, Journal of the Korea Concrete Institute, 25(1), pp. 81–90. doi: 10.4334/jkci.2013.25.1.081.

Randl, N. and Wicke, M. (2000) ‘Schubübertragung zwischen Alt- und Neubeton. Experimentelle Untersuchungen, theoretischer Hintergrund und Bemessungsansatz’, Beton- und Stahlbetonbau, 95(8), pp. 461–473. doi: 10.1002/best.200000870.

Author Response

Your suggestion is right and valuable for our research.

"Please see the attachment"

Author Response File: Author Response.docx

Round 2

Reviewer 2 Report

The reviewer would like to thank for the effort that the Authors put into introducing changes to the manuscript. In its current form, the paper is much more understandable for the reader and presents the results of research conducted by the Authors more clearly.

I suggest including the following changes to eliminate any inaccuracies:

• Sentence in lines 231-232: "This results implicate the composit concrete beams with T-type interface" - I understand the meaning of the paragraph, but this sentence seems unclear
• Sentence in lines 381-383: “Figure 7 and Figure 8 show the distribution of strength ratios from the push-off tests depending on the interface roughness classified as un-cracked, pre-cracked, rough and 382 smooth” - I suggest changing the “interface roughness" to "interface conditions"- it is difficult to consider surface roughness in case of initially uncracked monolithic specimens.
• Sentence in lines 387-388: I am not convinced about statement: “However, as the clamping stress increases, the horizontal shear strength of the interface increases proportionally regardless of the surface roughness” – the increase in shear stress resulting from clamping stress is different depending on the method surface preparation as evidenced by the distribution of points on the graph (different slopes of the regression curves)
• I suggest changing the division of the axis in Fig. 8 - it would be clearer to show the results in the 1: 1 ratio, so that line representing experimental to calculated shear strength = 1 would be inclined at 45°.
• Please explain in the text that strength f'c (appearing in the following figures) always refers to weaker concrete - at the moment such a information appears only in the tables

After introducing changes, the paper can be considered for publication.

Author Response

Your suggestion is valuable and helpful for our research.

Please see the attachment.

Author Response File: Author Response.docx