WOOD WISE
CCA or boron can form excellent bonds
By Qiniso Mbhamali , Zahra Naghizadeh and Brand Wessels at the Department of Forest and Wood Science , Stellenbosch University
South African Pine wood that has been treated with CCA or boron can form excellent bonds when using PUR adhesive .
With the increasing movement towards environmentally friendly and sustainable building materials in the construction industry , engineered wood products have become a preferred structural material . wood surface properties and interfere with bond formation and lead to poor bond quality . Furthermore , several studies have reported that increasing concentration levels of preservative drastically affects bond strength and durability .
Glued laminated timber ( glulam or GLT ), laminated veneer lumber ( LVL ) and cross-laminated lumber ( CLT ) have become popular options , as they have lower environmental impacts when compared to mineral based building products such as concrete and steel . However , mass timber structures are susceptible to bio-deterioration especially in tropical and subtropical regions resulting in reduced service life of these structures . Preservative treatment before gluing is the most effective way to preserve timber structure against biological agents .
Post-treatment is not possible as very large laminates cannot be treated after production due to dimensional constraints of treatment facilities . However , pre-treatment also presents complications as some wood preservative deposits have been reported to change
In a study at Stellenbosch University the effect of CCA and Boron treatment on the bond strength and durability of polyurethane ( PUR ) joints was evaluated . Different treatment concentrations were evaluated for both CCA ( 2 % and 4 %) and the boron-based disodiumoctaborate tetrahydrate ( DOT ) treatment ( 1.67 % and 3.3 %).
SHEAR STRENGTH RESULTS
All groups in the experiment met the average requirements of the European standard EN 14080 ( 2013 ) for shear strength ( Figure 1 ) and wood failure percentage . Interestingly , the 4 % CCA treated specimens displayed superior shear strength in comparison to the 2 % CCA specimens . This was contrary to what has been reported , that with increasing CCA concentration levels , the adhesion between the adhesive and wood laminates often decreases .
Figure 1 : Shear strength of CCA and boron ( DOT ) treated blocks . The shear strength of all the treatments and concentration levels that was evaluated exceeded the European requirements according to EN 14080 .
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