Question 2: How Does Joint Geometry Impact Bond-Line Stress?

Depth—A thicker sealant bead requires more force in order to be depressed than a thinner one. Therefore, using force control, the depression is reduced when the bead is thick and increased when the bead is thin, per any given main­tained force. Studies by the author using controlled force revealed that the force-to-movement/depression relationship is predictable within the movement capability range of high recovery sealants, consistent with their elastomeric design intent. In fact, during early exhaustive data research for calibration of the force-controlled device, it was discovered that the sealant pigmentation affected movement (as indicated by bead depression readings in two out of three sealants in the study) by a very small but measurable amount [10]. If the force had not been accurately controlled, and if the force-to-movement relation­ship of the sealants were not consistent, this discovery would never have been made.

Changes in the bead depth can be visualized during the procedure via close observation of the roller on the bead from the calibrated starting position when the force is controlled (the roller will ride higher if the bead is thicker and lower if the bead thins), or can be felt when using screen roller “depression inducement."

Width—Determining an appropriate bond-line stress requires an awareness that sealed joint substrates are subject to thermally induced expansion and con­traction [11]. This is why Section 4.3 of ASTM C 1736 mentions the potential influence of temperature during testing, and Section 8.1.4 requires that the tem­perature and other ambient conditions during evaluation be directly connected to the building elevation, floor line, date, and time of day in a comprehensive report. ASTM C 1736 does not dictate specific temperature parameters for test­ing, stating only that the temperature during testing must be recognized as im­portant to consider and accurately report.

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Question 3: What Level of Stress on the Bond-Line Produces Usable Information Without Damaging the Seal?

Usable information can be obtained from surprisingly small force loads able to depress and elongate the bead. For example, the author’s research using force control has determined that on average, approximately 9 to 11 lb (40 to 49 N) is sufficient force when evaluating a sealant of the ±50 % movement capability range. Section 3.1.2 of ASTM C 1736 defines “target depression" as “[t]he amount of depression needed to reveal sealant adhesion anomalies, determined either in field or in laboratory, during or prior to a sealant evaluation." Section 6.1 states, “[T]he amount of elongation should be based on joint design and/or joint width, and should produce the elongation within the limits of the joint design."

Section 7.3 (“Depression Inducement Procedure") provides the option of utilizing skill and expertise with devices such as a screen roller in “attempting to control the depression by manually varying the force on the roller."

In Section 7.4.3 (“Force Control Procedure"), force is stipulated to “produce a controlled strain in the sealant and stress on the bond-line sufficient to reveal adhesion anomalies, but less than the amount that could harm the weath – erseal." Section, under the heading “Calibration of Force," states, “The amount of force applied to the sealant to create an effective bond-line stress will vary, depending on a given sealant’s designed properties in combination with a specific sealant configuration. It is important, when calibrating the device in­laboratory or in-field, to establish a sealant force target(s) for a given evaluation that produces an appropriate bond-line stress."

Information decreases proportionately as the aspect ratio (depth/width) of the sealant bead increases.[36] However, a lack of information is information in itself in the case of an excessively thick sealant bead; the bead should stretch enough under a reasonable force load that a depression in the bead geometry can be measured. If it will not do so, the bead is unlikely to accommodate all anticipated joint movement during its expected service life, and the long-term durability of the seal can be brought into question. ASTM C 1736 does not dic­tate a course of action if this occurs. However, as stated earlier, such an occur­rence might provide the opportunity to implement repairs to a bead section that might otherwise prematurely fail, as the method will reveal this type of anomaly.