Hardwood Floors December 2018/January 2019

AT THE SITE

Expansion of Solid and Engineered Rift White Oak Flooring with Increase in Moisture Content

marked locations. Marking the spot for measurement insured that the same spot was measured each time. The moisture content samples were only an inch along the grain whereas the samples used for width, thickness, and length measurements were 33.5 inches long. Because of their short length, it is possible that the moisture content samples responded more quickly to environmental changes than the larger samples. If this did happen, it would make the dimensional changes given in Table 1 somewhat conservative. That is if the large samples had been given more time, the dimensional change would have been somewhat larger. Table 1 presents the results for both the solid and engineered wood flooring specimens. The specimens were measured May 1, 2016, while they were in the conditioning room. The specimens were then moved to outside, but covered, storage. Four additional sets of measurements were taken through Aug. 25. Note that the initial moisture content of the solid flooring was 7.4 percent and the engineered flooring was 7.0 percent. In just eight days of outside, but covered exposure, the moisture content of both samples jumped to more

than 12 percent. FromMay 9 through June 10, the moisture content actually decreased. Little rain was received during this period, so in spite of it being spring and early summer in Indiana, we had low relative humidity resulting in a declining moisture content for the samples. The last set of measurements were taken Aug. 25. Beginning in July, rainfall and relative humidity increased, and the moisture content of the samples went to more than 14 percent. The manufacturing histories for the engineered and solid flooring are unknown, so it is difficult to make definitive statements. The somewhat consistently lower moisture content for the engineered flooring as compared to the solid flooring is probably the result of a reduction in hygroscopicity. Shmulsky and Jones (2016) indicate that “wood that has been subjected to temperatures in excess of 100 degrees centigrade for long periods becomes less hygroscopic; i.e., it equalizes at a lower moisture content than normal wood.”These authors indicate that this phenomenon is more important in fiberboard and particleboard products because they are typically subjected to higher temperatures during manufacturing than plywood.

This small experiment included two separate tests. Both tests included 4-inch solid rift cut white oak flooring and 4-inch rift cut engineered wood flooring with a 4-mil sawn wear layer and an 11-ply platform. The material was part of both flooring experiments being conducted at Purdue University. The first test consisted of one piece of each type of flooring. Both test pieces measured 4 inches wide on the face and about 33.5 inches along the length. The material was conditioned at 75 degrees Fahrenheit and 35 percent relative humidity. These conditions should result in an equilibriummoisture content of about 6.8 percent for solid wood. An inch-long piece from each sample was weighed and set aside to determine moisture content on an oven dry basis. The two samples were weighed as each set of dimensional change measurements were taken and then oven-dried and weighed at the end of the experiment. The length of each piece was measured to 1/1000 of an inch in one marked location with a digital caliper. Likewise, the width was measured in three marked locations and the thickness in three

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