would only increase the strength of the composite by 1% over that of
the wood alone.
Conversely, the more severely deteriorated the wood, the greater
the strengthening effect of a given consolidation treatment. This idea is
illustrated in Figure 1, which shows improvement factors for different lev-
els ofdeterioration (Schniewind 1990a). In this example the strength of
the most severely deteriorated wood was increased by 47%, while that of
the least damaged wood improved by only 10%.
Epoxy resins can be formulated with excellent strength proper-
ties, which is an important justification for their potential application in
the repair and consolidation ofengineered structures. Accordingly, epoxy
resins will result in probably the best possible strengthening in the con-
solidation of deteriorated wood if their use can be justified. Similarly,
vinyl monomers polymerized in situ in normal wood at a loading of
approximately 50% produced increases in bending strength on the order
of 70–80% (Siau et al. 1968) and from 100% to more than 600% increases
in compression strength perpendicular to grain (Meyer 1989). Consistent
with Equation 5b, the greater increases in compression strength perpen-
dicular to grain are possible because wood is weaker perpendicular to
grain than parallel to grain, and strength parallel to grain is the determin-
ing factor in bending strength.
The strengthening effect ofsoluble thermoplastic polymers tends
to be significantly less, because it is rarely possible to achieve levels of
loading as high as 50% and because of the lower strength of the resins
themselves. Physical data for several commonly used soluble resin consoli-
dants and their improvement factors are shown in Table 2. Deteriorated
Douglas-fir was used, with the loading between 20% and 23% (Schniewind
and Kronkright 1984; Wang and Schniewind 1985). Butvar B98 is seen to
give the greatest strengthening, followed by Acryloid B72. All but the
three PVA resins with the lowest molecular weights gave statistically
significant levels of improvement. Considering that the tensile strength of
normal Douglas-fir is on the order of125 MPa, it can be seen that the ten-
sile strengths of all resins for which data are available are less than that of
the wood. This is particularly true for the PVA resins, which also have
glass-transition temperatures, Tg,either below or not much above room
temperature—bringing them close to or into a rubbery, rather than glassy,
rigid state. Of course, in some circumstances such flexibility may beC W P 97Schniewind and Kronkright 1984
Wang and Schniewind 1985
Carlson and Schniewind 1988Bending strength of untreated controls (MPa)Improvementfactor40 50 60 701.51.41.31.21.11.0Figure 1
Improvement factor: ratio of bending-
strength values ofwood treated with Butvar
B98 to bending-strength values ofuntreated
wood, for deteriorated Douglas-fir atvarious
levels of residual bending strength (degrees of
deterioration).