The effect of chemical modification on mechano-sorptive creep in bending was studied by experimental work. Stakes with 20 × 20 × 400 mm RTL of Portuguese wood species (Pinus pinaster Aiton) modified with 1,3-dimethylol-4,5- dihydroxyethyleneurea (DMDHEU), m-methylated melamine resin (MMF), tetraethoxysilane (TEOS) and amid wax (WA) were measured under asymmetric moistening conditions over a period of 42 days (app. 1000 hours) with stress level (SL) of 12 MPa, according to ENV 1156.
The cell wall treatments (DMDHEU and MMF resins) had shown significant reduction of creep (creep factors, kc) when compared to untreated wood under similar conditions. Both types of resins and levels of treatments (different WPG) did not shown significant effects.
In the lumen fill treatments, deposit material of TEOS did not affected the creep behaviour (kc); Wax treatment was shown a particular compliance of creep due to avoid exchange moisture (by the hydrophobic effect of wax).
The anti-creep efficiency (ACE) correlated better than other mechanical or physical properties imparted by the modification process
Duarte Barroso Lopes, Carsten Mai and Holger Militz
The long-term performance of a structural member is determined by its durability and deformation with time. The bending creep behaviour of modified wood was assessed experimentally over a period of 35 days (840 hours).
Four chemical modification processes were used: 1,3-dimethylol-4,5-dihydroxyethyleneurea (DMDHEU), mmethylated melamine formaldehyde resin (MMF), tetraethoxysilane (TEOS) and amid wax (WA). Wood stakes with 20.10.200 mm RTL dimensions of Portuguese Maritime pine (Pinus pinaster Ait.) from sapwood part of the stem were used for evaluated the primary creep. Experiments were conducted at bending stresses amounting to 0.1, 0.2, 0.4 of the mean immediate wood bending strength obtained at equilibrium moisture content (EMC). Applying the same stress level (SL, 0.2), wood creep was also determined at the constant low and high moisture content. As results: Between low and medium SL (8 and 16 N/mm2), unmodified wood at indoors conditions did not show any effect in the creep factors (kc). However, at high SL (35 N/mm2) a slight increase (not significant) in the kc was found. It seems that the kc was nearly independent of the SL.
In the lumen fill modification (TEOS and wax), the deposited material has not affected the creep behaviour under various SL. The cell wall modification (with DMDHEU and MMF resins) did not show any differences in the kc for low and medium SL (8 and 16 N/mm2). However, resin modification under high SL (35 N/mm2) has shown a significant reduction related to unmodified wood. Between both types of resin (DMDHEU and MMF) and levels of modification (WPG), significant effect was not found. At saturated conditions, lumen fill modification (TEOS and wax) did not show any effect on creep. In the cell wall modification (DMDHEU and MMF resin), significant reduction was recorded due to the embrittlement effect imparted by the modification (deposit of resin in the cell wall).