Predicting bending strength of fire-retardant-treated plywood from screw-withdrawal tests
This report describes the development of a test method and predictive model to estimate the residual bending strength of fire-retardant-treated plywood roof sheathing from measurement of screw-withdrawal force. The preferred test methodology is described in detail. Models were developed to predict loss in mean and lower prediction bounds for plywood bending strength as a function of a screw-withdrawal force. Our analysis of fire-retardant-treated plywood from three different studies, each with various fire-retardant-treatment, processing, plywood thickness, and exposure temperature groupings, clearly indicated that different fire-retardant-treatments and plywood thicknesses could not be grouped into a single “universal” model. Nevertheless, some grouping was possible; parameter estimates for several grouped fire-retardant formulations and plywood thicknesses are reported for mean trends and lower prediction boundaries. Although the models were shown to acceptably predict plywood bending strength, additional work is needed to expand these models to address the effects of plywood quality, wood temperature, and moisture content at time of test.
MECHANICAL CHARACTERISTICS OF CHEMICALLY DEGRADED SURFACE LAYERS OF WOOD
Institute of Theoretical and Applied Mechanics, Academy of Sciences of the Czech Republic, v. v. i. Abstract. The aim of this research was to find out whether the effect of chemical corrosion changes mechanical characteristics of surface layers of wooden construction elements. Degradation of the surface layers of wood was caused by chemical reactions of the basic substances of wood mass with compounds contained in antifire coatings. Fire retardants containing corrosive substances were often and repeatedly used in the Czech Republic on many wooden building constructions. This process of chemical corrosion is in practise called as "surface defibering of wood". This contribution presents standard and special experimental methods used for measuring the selected mechanical characteristics (compression strength, tension strength, bending strength, hardness and impact resistance) in the damaged surface layer of wooden construction elements. The material for experimental measuring was a construction element removed from a historical roof (ca 150 years old). Mechanical characteristics of the surface layer of the defibered element were compared with the values measured in the deeper subsurface layer of non-damaged wood. The results of the experiments proved loss of cohesive strength and decrease of mechanical characteristics of wood only in a thin surface layer.
Mechanical behaviour and durability of high volume fly ash cementitious composites
The purpose of this research is to separate different morphological particles of ASTM class F fly ash, and study their effect on mechanical behaviour and durability of high volume cementitious mixtures. In this research wet separation of raw fly ash is carried out, which resulted in three layers of different morphological particles. The first layer of particles float, comprise of about 1-5% of fly ash, is identified as cenospheres or hollow spheres. The second layer of particles is measured to be 55-60% of raw fly ash and consisting of porous spherical and rounded particles rich in Si and Al. The third layer particles is measured to be about 35-40% of raw fly ash. High volume fly ash cementitious composites containing second or third layer particles are tested under compression and bending, highlighting a higher strength and ductility in comparison to cementitious ones containing raw fly ash particles. KEYWORDS. Cementitious matrix; Raw fly ash; Second layer particles; Third layer particles.