Mechanical Properties of Low-Stiffness Out-of-Grade Hybrid Pine—Effects of Knots, Resin and Pith

Round 1

Reviewer 1 Report

The paper deals with an interesting issue regards to out-of-grade timber. This material in some cases (as this species) suppose a high percentage and some manufacturers wonder if really they are not suitable for other structural uses, considering this destiny as the best value adding. Therefore, it is very relevant to this study. Background and introduction collect properly this justification for the hybrid pine.

The scientific design of experiments, sampling and testing are well raised and very very exposed. All tables and figures are necessary and very useful for the understanding of the study. However, the discussion chapter is long and tough of reading because there are many subchapters and data. It is difficult to reduce them because they are well structured: analyzing data from a different way than that presented in the result chapter (percentages, significance), comparing to other studies and supporting the result obtain with reasons.

Conclusions are in line with the text, much information is exposed but right from the scientific point of view. However, it is suggested to make conclusions regarding that settled after discussion, i.e. and for example: the resin effect has shown no influence except for CPerpG MOE and strength for out-of-grade PEEXPCH-F2 timber although there is a lack of relationship between RAR and CPerpG. Influence of resin content in cell walls but not in all lumen cell, could be the reason; or, shape, position and grain deviation of knots near to pith usual in out-of-grade, does affect bending, CParG, CPerG, tension and shear performances, showing specially higher CPerG; or that related to decrease mechanical properties due to juvenile wood presence, the influence of pith, etc. Thus, readers obtain conclusions clearer from the mechanical behavior of this kind of material (out-of-grade) and if it is suitable or not for structural purposes, because the data themselves are already included in the text.

Several comments and suggestions are included in the document attached.

Comments for author File: Comments.pdf

Author Response

The paper deals with an interesting issue regards to out-of-grade timber. This material in some cases (as this species) suppose a high percentage and some manufacturers wonder if really they are not suitable for other structural uses, considering this destiny as the best value adding. Therefore, it is very relevant to this study. Background and introduction collect properly this justification for the hybrid pine.

The scientific design of experiments, sampling and testing are well raised and very very exposed. All tables and figures are necessary and very useful for the understanding of the study. However, the discussion chapter is long and tough of reading because there are many subchapters and data. It is difficult to reduce them because they are well structured: analyzing data from a different way than that presented in the result chapter (percentages, significance), comparing to other studies and supporting the result obtain with reasons.

Conclusions are in line with the text, much information is exposed but right from the scientific point of view. However, it is suggested to make conclusions regarding that settled after discussion, i.e. and for example: the resin effect has shown no influence except for CPerpG MOE and strength for out-of-grade PEEXPCH-F2 timber although there is a lack of relationship between RAR and CPerpG. Influence of resin content in cell walls but not in all lumen cell, could be the reason; or, shape, position and grain deviation of knots near to pith usual in out-ofgrade, does affect bending, CParG, CPerG, tension and shear performances, showing specially higher CPerG; or that related to decrease mechanical properties due to juvenile wood presence, the influence of pith, etc. Thus, readers obtain conclusions clearer from the mechanical behavior of this kind of material (out-of-grade) and if it is suitable or not for structural purposes, because the data themselves are already included in the text.

Authors’ Response

The authors would like to thank the reviewer for their effort and time in reviewing the paper. 

The below items address the 34 feedback notes within the paper:

1. PEExPCH is the general name and PEExPCH-F2 is a specific hybrid? If does, please explain it in order to differentiate them. Seems they are used interchangeably.
   2. The “F2” refers to the generation of the PEExPCH hybrid. The timber tested for this study is PEExPCH-F2.  Some, but not all researchers specify the generations they are studying.  The literature shows that F1 and F2 have very similar wood properties so using the added detail of F2 may be considered unnecessary.  In some instances, this paper uses PEExPCH to refer to both F1 & F2 together.  But in other instances we don’t know what generation it is.  Given they are very similar and to avoid confusion we have removed the references to “PEExPCH-F2” and used only “PEExPCH” throughout the article.
2. What is the grading criterion used? Stiffness, density? It is supposed that is stiffness because later on say low stiffness is the cause of out-of-grade.
   1720. We have extended a sentence on line 48 to include more detail. “Out-of-grade in this study refers to sawn timber that has failed to meet the MGP structural framing requirements of AS1748 [11] and AS1720.1 [12] which specify minimum characteristic values of stiffness, strength and place limits on defects including as resin shake, wane and distortion.”
3. From several work shifts? species variability considerations?
   143. We have included the number of shifts and also explanation on limits of sample extraction period and justification of same. Sentences added - See lines 136 to 143.
   144. “The samples were collected over three shifts of a mill run of 31-year-old PEExPCH hybrid pine harvested from Cowra, Queensland, Australia. Ideally, sample extraction would occur over many months of production to capture the variation seen in trees.  However, under normal milling processes, identification and segregation of PEExPCH from other Southern Yellow pines was not possible.  This mill run was set up for research purposes and ensured only PEExPCH was being processed and graded during the run.  While the sample extraction occurred over a limited time, the mill data collected for the entire run showed it had a population average MOE and volume percentage of out-of-grade only slightly higher than that in previous mill runs of mixed Southern Yellow pines which included PEEXPCH.  Therefore, it is reasonable to consider the sample representative of an average population.”
4. But are these specific characteristics obtained from a previous analysis of the main and more often out-of-grade characteristics which lead to be out-of-grade?
   1. The characteristics were targeted as they were the most frequent in out-of-grade but not necessarily the cause of them being out-of-grade.
   2. Sentence on line 153 change slightly - Thus, with a focus to provide information for development of grading rules and to understand the impact of specific features that could be removed and the piece finger jointed back together, rather than carry out an in-grade study which would capture a population with random inclusion of a wide range of characteristics present in out-of-grade timber, small sample sizes containing specific commonly occurring characteristics previously discussed were adopted.
5. Then, all samples comes from core wood. So study is for out-of-grade of corewood (probably due to young log diameter) not for out-of-grade of the log
   1. The entire log was processed through the mill run. The log was 31 years old and contained both corewood and outerwood but due to the high performance and value of this outerwood, defects were cut out and shorter lengths optimised to capture as much of this high value outerwood as possible in MGP grades but of shorter lengths.  This left the lower performing corewood to make up the vast majority of the out-of-grade population. 
   2. Simple definition of corewood location added on line 96 – “Corewood in pines is often considered to be the first 10 to 12 growth rings from the pith[20-21]”
   3. Sentence on line 162 modified - The majority of out-of-grade was found to be from the corewood zone off the tree and as pith is an inherent characteristic of corewood of pine, no separate sample set was obtained for pith but quantities occurring within each sample type are included in Table 1.
6. For all the text, why MOE and strength? Would be more usual MOE and MOR or stiffness and strength.
   1. In major timber publications (eg Wood handbook) MOR is used for bending strength, whereas all other strength properties are referred to as “strength”. Modulus of elasticity is used for all properties except shear which is referred to as modulus of rigidity or shear modulus.  In the Australian standards documents modulus of elasticity and modulus of rigidity are used while strength properties of bending, shear, tension and compression are all referred to as “strength”.  Looking at some journal publications a mix of shear stiffness and shear modulus are used.  We have now changed shear MOE to shear moduli for correctness and to align. 
7. Static MOE? What is considered “true MOE”?
   1. I could not find a consistent name for this property throughout the literature. “True MOE” is a term used by Doyle and Markwardt who describe it as the axial MOE of timber.  It excludes the shear deformation that occurs in a bending test.  Bano refers to it as “E_m” or “apparent MOE” and state it is a combination of compressive and tensile MOE as these are not always the same.  The wood handbook refers to it as “adjusted bending E_L”. 
8. It is really not well understood why decide to use on-flat bending instead of on-edge bending.
   1. Sentence added on line 187 “On-flat rather than on-edge bending has been used because it is closer aligned with the orientation most often used in engineered wood products such as glulam and CLT. Additionally, the smaller sample size enabled isolation of the characteristics in the different sample types of clear, resin and knots.”
9. Thick is 35 but in picture is 33mm
   1. The samples had to be planed prior to gluing to achieve sufficient bond quality for the test. They were planed to 33mm thickness.
   2. Sentence added to line 242 – “The wide faces of the samples were planed prior to gluing to the steel plates to achieve sufficient bond quality for the test.”
10. Manage headings: “Average MOE”
    1. Amended
11. A general comment: it is noticed many disagreements con other studies. That, generates doubts about which results believe, or depressing to people that use wood
    1. Yes, timber is well known to be a highly variable material. There is very limited publications on many of these properties and they do not all agree.  We consider it important to include the information from the available studies even though they do not align with ours or others findings. 
12. PEExPCH-F2
    1. Changed to PEExPCH
13. This is supposed to be one of the aim of this study, what kind of additional research means?
    1. We have modified two sentences to make it clearer that a more in-depth understanding is needed. The geometry, grain deviation, soundness and knot angle are all variations considered in studies looking at outerwood knots but no information could be found on corewood knots.
    2. Change to sentence line 372 – “They are more likely to be in combination with grain deviating throughout multiple smaller conical shaped and high angled knots connecting with or near the pith rather than large grain deviations around large cylindrical shaped single knots found in outerwood. Additional research is needed to get a more in-depth understanding of these unique characteristics and geometry of corewood knots, and to understand their impact on bending in more detail .
14. On average
    1. Amended
15. On average
    1. Amended
16. Then, resin influences more on MOE than knots and the contrary on MOR, and pith has not big influence. These kind of conclusion could be incorporated in conclusion chapter.
    1. Incorporated into conclusions
17. 71%, as previous
    1. Amended
18. On average
    1. Amended
19. On average
    1. Amended
20. Further studies required
    1. Added note on line 571 “however, further studies are required.”
21. It is already in Structural performance. That happens in other chapters also because it talk issues then is analysed in resin, knots, pith effect.
    1. Deleted in to avoid repetition
22. On average
    1. Amended
23. On average
    1. Amended
24. But also growth rings in the pith are less denser due to higher cell lumen surface.
    1. Changed the wording to make clearer on line 593 –
    2. “There is much less latewood than earlywood in corewood, but with its smaller cell lumen and thicker cell walls, latewood resists buckling and crushing failure more than early wood [65]. The tighter radius growth rings in the pith samples would contribute to the higher strength in clear and resin samples because the latewood acts as a pillar and protects the weaker earlywood from crushing [66] and from dependence on rolling shear properties which are weaker than CPerpG properties [35]”
25. On average
    1. Amended
26. On average
    1. Amended
27. Shear
    1. Changed to “shear modulus and shear strength”
28. Shear
    1. Changed to “shear modulus and shear strength”
29. Shear
    1. Changed to “shear modulus and shear strength”
30. Shear
    1. Changed to “shear modulus and shear strength”
31. Grade
    1. Amended
32. This is good, give examples
    1. . Example added on line 854
    2. “For example, inner layers in a 5 layer CLT panel that experience low bending stresses.”
33. Of the resin samples?
    1. Modified wording to make clearer
    2. With an overall average RAR of 0.58, resin was similar to clear…….
    3.  
34. See comments for authors
    1. Additions have been made in the conclusions section to provide clearer conclusions to the reader as suggested.

Reviewer 2 Report

  I had the opportunity to review the paper proposed for Forests with the name "Mechanical properties of low stiffness out-of-grade hybrid pine - effects of knots, resin and pith “. 

The authors cared about the detailed elaboration of the current state and results. The manuscript has provided compression parallel to grain, compression perpendicular to grain, shear and strength in tension together with the effect of resin, knots and pith on these mechanical properties and density.

 After reading the article, I have to evaluate it positively, but I have a few formal mistakes such as:

1. the Latin name of the tree species should be written in italics (Line 11 and 35, References, such as 2,3,5 and so on. 

2. abbreviations of journals must be used in journal references

3. in the introduction, “juvenile and core wood can have some good properties and attributes including increased durability and transverse mechanical properties“  Please specify the durability and whether this research was focused on fungal resistance.  Generally, juvenile wood has a correspondingly lower resistance to decay by brown rot and white rot fungi than mature heartwood.

Author Response

After reading the article, I have to evaluate it positively, but I have a few formal mistakes such as:

1. the Latin name of the tree species should be written in italics(Line 11 and 35, References, such as 2,3,5 and so on.
2. abbreviations of journals must be used in journal references
3. In the introduction, “juvenile and core wood can have some good properties and attributes including increased durability and transverse mechanical properties“ Please specify the durability and whether this research was focused on fungal resistance. Generally, juvenile wood has a correspondingly lower resistance to decay by brown rot and white rot fungi than mature heartwood.

Authors’ Response

The authors would like to thank the reviewer for their effort and time in reviewing the paper.  Amendments have now been made in response to all feedback as explained below.

1. The Latin name of the tree species should be written in italics(Line 11 and 35, References, such as 2,3,5 and so on.
   1. Amended in lines 11, 35 and throughout references
2. Abbreviations of journals must be used in journal references
   1. Amended
3. In the introduction, “juvenile and core wood can have some good properties and attributes including increased durability and transverse mechanical properties“ Please specify the durability and whether this research was focused on fungal resistance. Generally, juvenile wood has a correspondingly lower resistance to decay by brown rot and white rot fungi than mature heartwood.
   1. Amended line 62 to make clear it is durability for mechanical degradation, not natural durability.
   2. Nonetheless, despite the low bending MOE, some studies have found that juvenile and corewood can have some good properties and attributes including increased durability with respect to mechanical degradation, and transverse mechanical properties