Preparing for the Replacement Crowns

Each of the deteriorated log crowns was carefully photographed and documented. Diameter, length from the wall and surface configuration of the cut end of the log were recorded. Then the crowns were cut off, at an angle towards the notch (see figure 2). Next, the original logs were drilled back to the center line of the intersecting walls, using a plumber's bit (see figure. 3). This gave a flat surface to begin the new crowns. The recessing with a flat surface to the center line of the wall was more difficult than first anticipated because the barn builder had anchored each log with four galvanized square spikes 8" x 3/8". These spikes had to be chiseled and sawn back to a flat surface so the new crowns could be properly anchored. 

Figure 2. Small chainsaws were used to cut off deteriorated log crowns. The darker portions of wood at the log ends are the soft punky deteriorated fibers. Photo: Harrison Goodall.	Figure 3. A plumber's bit was used to recess the crown back to the center line to create a flat surface for attaching the replacement crowns. Photo: Harrison Goodall.

In a few cases, deterioration penetrated beyond the center line of the wall, and more material had to be cut back to reach sound wood. A rotary rasp, not unlike a king-size dental drill, was used to remove remaining decay at the center line. The sound wood surface was treated with an epoxy consolidant to encapsulate remaining soft absorbent wood, thereby retarding future decay. Where the decay had traveled into the log walls, sufficient wood was removed and the surface flattened to permit new faces to be attached to the logs (see figure 4). These new faces were notched to match the log surfaces removed.

Figure 4. In a few cases, the deterioration had penetrated beyond the center line of the log corners and portions of the wall face were removed. Photo: Harrison Goodall

Due to a short work schedule, it was not feasible to treat the new wood with a wood preservative such as CCA (chromated copper arsenate). Such a treatment, however, would have been desirable. Pressure-treating with CCA in a salt solution would have left the wood tinted a light green, but with aging and an applied stain, it would have been unnoticeable. A surface- applied preservative was not considered, because it would not penetrate to the center of the 9"- to 12"-diameter logs. This is the area where deterioration is likely to occur first. The logs were, therefore, left untreated.

Replacing the Log Crowns

The photographs of the deteriorated crowns were used to reconstruct the replacements. Each replacement log crown was cut and fitted until it matched the notch of the original. A wood framing not unlike cribbing was constructed at each corner to hold the replacement log crowns as they were marked, cut, and fitted in place from the bottom level up (see figure 5). As much as possible, checks in the wood were placed in a downward position to prevent water from collecting in the wood. As the crowns were extensions of the logs and not supported by the ground, it was necessary to provide temporary support until the replacement crowns were epoxied in place. The log crowns were not anchored until all had been stacked in place with all notching complete. They were then marked to ensure proper alignment when anchored.

Figure 5. A framework or cribbing held the replacement crowns as they were fabricated. All the crowns had to be temporarily put in place to ensure a proper fit. Once stacked up, they were numbered to make sure they would be properly aligned at the time of anchorage. Photo: Harrison Goodall.

The new crowns were anchored to the original logs by fiberglass reinforcing rebars (fiberglass rods that have spiral ridges to enhance bonding) embedded in epoxy bonded to the wood. Three fiberglass rebars were used for most of the connections, one long fiberglass rebar at the top of each new crown to resist the tension forces (from the weight), and two small fiberglass rebars used at the base to counteract any lateral movement. This made a three-point connection (see sidebar). Because buffalo sometimes rub against the building corners, an additional top tension rebar was used on the lower crowns.

The three materials making up the connection were the fiberglass rebars, the structural grade epoxy, and the pine wood of both the original wall logs and the new replacement crowns. The tensile strength of the fiberglass rebar is 80,000 psi (pounds per square. inch), with the bonding shear strength of the rebar at 60,000 psi. The shear strength of the epoxy is 3000 psi; thus all the new synthetic materials are substantially higher in shear strength than the pine, which has a shear strength of only 60 to 90 psi. The wood was the weakest element of the connection. Since the crowns were in effect to be cantilevered from the main wall section, it was necessary to calculate the diameter and length of the reinforcing fiberglass rebars necessary to hold the ends in place and to keep the wood from shearing away from the epoxy.

The reinforcing rebars were intended to counteract the natural force of gravity pulling the crowns away from the wall, due to the weight of the crown itself, from the accumulation of snow and ice, or from lateral loads. With a 12"-28" long log crown 9" to 12" in diameter, it was determined that a 1/2" diameter reinforcing rebar set 12" into sound wood of the upper section of the wall log and 12" into the attached crown was necessary to combat the combined forces of shear, bending moment and vertical loading. (The drilled holes were slightly longer and wider to accommodate the reinforcing fiberglass rebar and the epoxy.) In addition, for horizontal or lateral movement, two 1/2" diameter fiberglass rebars were imbedded 4" into the lower sections of the log walls and a matching 4” into the crowns. The length, diameter, and the placement of the rebars were based on calculations intended to keep the wood fibers from shearing away from the imbedded rebars. Another situation might require different dimensions.
Thus at the Lamar Barn, a 1/2" fiberglass rebar 12" deep on either side of the connecting wood was used in the tension zone (total of 24") and located within an inch of the top of the log. Two short fiberglass rebars 4" long on each side of the connection (total of 8") were used at the bottom of each log crown for lateral strength. The holes for the rebars and epoxy were 3/8" larger than the rebars, for a total of a 7/8" hole.

Installation

The work area was protected with tarpaulins to keep the wood dry. The wood was allowed to dry out to less than 20% moisture content before the new crowns were installed.

The holes (7/8" diameter) for the 1/2" reinforcing fiberglass rebars were drilled into the original logs. Corresponding holes were drilled into the crowns. An 8" piece of flexible tubing was stapled onto the nozzle of a caulking tube in order to reach to the back of the long drilled hole. The caulking gun was filled with a high-strength structural two-part epoxy in a paste-like consistency, which was then injected into the original log. The cavity was filled two-thirds. The paste-like consistency of the epoxy kept it from draining out of the drilled hole. The precut rebar was then inserted using a twisting motion (see figure 6). The replacement crown was also filled two-thirds with epoxy, then aligned, inserted and left to cure (see figure 7).

Figure 6. A flexible tube was stapled onto a caulking gun in order to fill the 12"-12 1/2" deep holes that had been drilled into the sound end of the historic log. The individual hole was filled two-thirds with epoxy adhesive and a 1/2" diameter fiberglass reinforcing rebar was then inserted. Photo: Harrison Goodall.	Figure 7. A 12 1/2" long hole was drilled into the new crowns. This hole was filled in the same manner as were the old logs. The crown was then immediately lifted into position and supported by the cribbing framework for several days until the epoxy cured. Photo: Harrison Goodall.

Working with epoxy is difficult. Setting times depend on the type of formula and the ambient temperature. Some epoxies set up in less than 15 minutes; others can be formulated to dry over a three-day period. The epoxy for this job set in less than 45 minutes, and for that reason, only small amounts were mixed at a time, and work was not undertaken in the direct sun, as this further reduced the effective working time. Curing took place in about 24 hours at temperatures between 60 to 80°F. The cribbing construction supporting the crown ends was kept in place until all the crowns were fully cured.

Finishing

Once the epoxy cured and cribbing was removed, the log crowns were stained. A first coat of oil-based stain was used on the log crowns and eventually the entire building was stained. Had the new wood been treated with a salt-impregnated preservative solution, a full year of aging would have been required prior to staining. It is anticipated that every 3-6 years the building will need re-staining in addition to an annual coating of the new log crowns with a water repellant.