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Sample test walls constructed of traditional and non-traditional masonry units.
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Research shows that solid wood fiber-ash masonry units have many benefits.

We also investigated the effect of adding a wood preservative on compressive strength development. Freeze-thaw resistance, shrinkage behavior, and the load-deformation behavior of the material were studied to determine the durability and ductility of the units. The study evaluates the viability of fly ash as a suitable material for masonry units through its physical, chemical, mechanical, and durability characteristics at fresh and hardened states.

But WFFA has some potential shortcomings. Since this material is made of wood fibers, its long-term durability performance requires further investigation, although freeze-thaw resistance appears to be excellent.

Strength development is slower compared to concrete because of the slow reaction capability of high calcium fly ash. As wood fibers are prone to decay, special care may be needed at the jobsite or in producing the units. Aesthetic appearance of this material currently may not be viewed as flexible as CMU or clay brick. The interaction of this material with other additives, and long-term performance still require investigation.

Experimental program

The materials used in preparing WFFA masonry units included wood fibers from white oak, an ASTM Class C fly ash from a nearby power plant, Type I portland cement, chemical admixture(s), wood preservation material, and potable water. The wood fibers used in the study were obtained from treetops (upper and lower limbs) and branches from locally grown white oak. The moisture content of the obtained wood fibers was 2%, in accordance with ASTM C 566-04.

Our study identified an optimum amount of wood fiber and fly ash content to obtain a desired target compressive strength of 1000 psi. This target was selected to compete with low-end masonry products and aerated autoclave concrete. We investigated the change in the strength level due to adding cement to obtain a comparison value. This allowed us to predict the strength of units, with and without cement content, to determine a feasible strength range level.

Thirty-five mixture proportions were selected based on discussions with members from industry, a literature search, and past experiences of research team members. From this phase, a general mixture was finalized. Nine test specimens per mixture were produced; three specimens per test would be tested.

Here is what we have concluded:
  • Compressive strength levels exceeding autoclave aerated concrete strength class AAC 2.0 to 6.0 (290 to 870 psi) and comparable to low-grade clay masonry brick (2500 psi) can be produced using a WFFA-based material.
  • The WFFA material provides an alternative green building material to traditional clay or concrete masonry for infill wall systems. However, appropriate long-term studies should be performed to validate its long-term behavior in an in-situ building construction application.
  • Based on the compressive test results between different curing regimes, three-day moisture curing exhibited 26% higher strength development than air curing.
  • Wood fiber lengths in the 0 to ½-inch range exhibited 11% and 23% strength gain over ½ to 1 inch, and 1 to 2-inch fiber lengths, respectively. These results were based on 2-inch cubic specimens. Further investigation examining the effects of size is recommended.
  • Bark from lower limbs and branches helps in gaining compressive strength. But further research is needed on bark content and bark from different sources.
  • Monitoring the moisture content of the fibers is recommended before mixing. The presence of moisture was detrimental to the compressive strength development. As moisture increases, compressive strength decreases.
  • Adding a wood preservative to prevent mold development did not significantly affect the compressive strength development of the units.
  • The wood fiber-fly ash material exhibited excellent resistance to freeze-thaw cycles compared to other cementitious materials. No mass loss was observed for 300 freeze-thaw cycles. This is attributed to the insulating properties of wood and ability of the wood fibers to uptake free moisture.
  • Based upon the load-deformation development curve of the material, it appears that the material has good damping and energy absorbing characteristics.
  • There was no dramatic change in the shrinkage level using high volumes of fly ash with wood fibers, compared to other cementitious materials such as concrete.

This paper was presented at the North American Masonry Conference in St. Louis in June. For the entire version, e-mail the author atjmyers@umr.edu.

The author is an associate professor of Civil, Architectural and Environmental Engineering at the University of Missouri-Rolla. Nidhi Joshi, design engineer, Kirkpatrick Forest Curtis, Oklahoma City; and Robert Sinclair, president, Encore Building Solutions, St. Louis, contributed to this paper.