Results and Discussion

Tilt-Up Warehouse Exposures

Early Observations-The applicator found that the acrylic sealant needed to be handled somewhat differently than the polyurethane sealants that he was used to working with. As mentioned earlier, the acrylic sealant sent to the job site was not optimized for application properties. The applicator, in fact, found that the acrylic sealant was lower in viscosity than the polyurethane sealant and had a shorter open time. The lower viscosity made the acrylic sealant easier to gun but more difficult to tool since it offered less resistance to the tooling imple­ments. The shorter open time meant that the acrylic sealant needed be tooled more frequently and after shorter application lengths. Since the applicator was aware that the acrylic sealant would shrink more than the polyurethane seal­ant, there was some attempt to apply the acrylic sealant at slightly greater application depths. However, this process was not optimized or quantified in this exposure series. At the end of each day, the applicator of the acrylic sealant used soap and water to remove residual sealant from his tools. The applicator of the polyurethane sealant used toluene.

After 24 h the polyurethane sealant was still tacky and glossy and had picked up a fair amount of dirt. The tack and gloss, however, decreased over the succeeding days. The acrylic sealant skinned to a smooth, nontacky surface in less than 1 h and picked up much less dirt than the polyurethane sealant in the first 24 h. From the pieces of sealant pulled from the joints, it was determined that the polyurethane sealant cured through in 5-7 days and that the acrylic sealant took closer to 30 days. From these pulled pieces it was also found that the cured acrylic sealant is slightly harder and stiffer to the touch than the cured polyurethane sealant.

Longer Term Observations—As determined visually and in comparative pho­tographs, the dirt pickup of the two sealants after 1, 2, and 3 years is compa­rable. Dirt pickup, as might be expected, varies from elevation to elevation. However, from a distance, and from any elevation, the acrylic and polyurethane sealants look similar. After 3 years and at a distance of 20 ft (6 m), the acrylic and polyurethane sealants on the east side of the building (Fig. 3) are indistin­guishable. Closer views of the two sealants on the north side of the building (Fig. 4) also reveal a similarity in overall appearance. On more detailed inspec­tion, however, differences between the two sealants become apparent. A close-up comparison of the two sealants on the east elevation of the warehouse (Fig. 5) reveals considerable crazing of the surface of the polyurethane sealant but no apparent crazing of the acrylic sealant. This is a trend seen throughout the warehouse exposure—after 3 years of exposure, the polyurethane sealant exhibits considerable crazing in all vertical joints in all elevations. The acrylic sealant, in contrast, exhibits no visible crazing on any of the vertical joints.

The polyurethane sealant also exhibits severe crazing in all vertical and horizontal parapet joints (Fig. 6). This crazing was readily apparent after 2 years of exposure and has become significantly more pronounced during the third year of exposure. In addition, the polyurethane sealant in one of the corner parapet joints has developed crazes that extend several millimeters into

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FIG. 3—View of the east elevation of the El Paso warehouse with an acrylic sealant joint on the left and a polyurethane sealant joint on the right.

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FIG. 4—Closer views of the sealants on the north elevation of the El Paso warehouse. The acrylic sealant is on the left; the polyurethane sealant is on the right.

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FIG. 5—Detailed views of the sealants on the east elevation of the El Paso warehouse. The acrylic sealant, on the left, exhibits no surface crazing. The polyurethane sealant, on the right, exhibits fine surface crazing.

the sealant bead and which may be compromising the functional performance of the sealant. The acrylic sealant, after 3 years of exposure, exhibits slight surface crazing or wrinkling in the horizontal joints at the top of the parapet (Fig. 7) but no visible degradation in any of the vertical parapet joints. This crazing or wrinkling was not present after 2 years of exposure. The greater degradation of the sealants in the parapet joints is due to the fact that these sealants are subjected to harsher exposure conditions than those installed at ground level. Sealants installed vertically around the interior of the parapet see reflected UV from the light colored roofing material; those installed in the hori­zontal joint at the top of the parapet see direct and continuous exposure to the sun.

The polyurethane sealant chalks markedly after 2 and 3 years of exterior exposure, as measured by transfer of white residue from the sealant surface to the inspector’s index finger. Chalking is particularly severe in the parapet joints. The acrylic sealant, in contrast, chalks little in either the vertical or the parapet joints. The polyurethane sealant also softens noticeably during exposure, par­ticularly in the parapet joints where the exposure conditions are most severe. The acrylic sealant does not change noticeably in hardness during exposure.

When sealant samples were pulled from representative joints to test for adhesion, both sealants pulled out easily due to substrate failure within the friable concrete mortar in the decorative aggregate surface layer. However, the mortar is holding up adequately for the joints experiencing movement, and no joint failures have been noted to date. Functional performance of both of the applied sealants appears to be intact, with the possible exception of the poly­urethane sealant in the parapet corner joint noted above.

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FIG. 6—Polyurethane sealant in a vertical joint in the warehouse parapet {top) showing surface crazing and chalking. Polyurethane sealant in a horizontal corner parapet joint {bottom) showing severe and deep crazing.