domingo, 14 de agosto de 2011

SERVICE LIFE PREDICTION FOR BUILDING JOINT SEALANTS

SUMARY :

This research will deliver critical measurement science for predicting sealant service life (a significant part of the US building industry) and continue the global leadership in service life prediction performance-based standards for sealant materials. In this project, high precision sealant mechanical property data will be generated from laboratory and field exposures as a function of ultraviolet radiation (UV), temperature, moisture and mechanical loading for use in a reliability-based methodology for service life prediction.  

DESCRIPTION :

Objective:  Develop, implement, and transfer reliability-based service life prediction tools for sealants capable of relating field and laboratory exposure results to the sealants industry, by 2013.
What is the problem? Nationally, homeowners spend >$60B/yr repairing their homes.[1]  Insurance studies show that water damage is 10 times more likely than fire, with 28% of all property claims related to water-related damage.[2] Field studies indicate that 50% of sealants fail within 10 years, and 95% fail within 20 years. Currently, no methodology exists to predict failure of sealants.  A critical attribute of an effective sealant is the ability to span and seal gaps between dissimilar building materials.  These polymeric materials experience daily (~ ±7%) and yearly cycles (~ ±25%) of strain deformation.  As these materials are exposed to the weather, molecular changes occur that eventually prevent the sealant from responding to imposed strains, leading to failure of the sealant.  When undetected, this bulk water intrusion and air infiltration leads to significant preventable repair cost and energy loss.  Characterizing these molecular changes and attributing them to specific exposure conditions enables the development of models to predict in-service performance. Resulting improvements in sealant performance, known durability result in decreased maintenance and repair costs, and ultimately increased sustainability.

Why is it hard to solve? Dissimilar building materials expand and contract at different rates when exposed to thermal or humidity cycling.  Polymeric sealants are used to seal these ever changing gaps.  Sealant materials respond to the imposed changes in gap dimension (strain) through a variety of mechanisms.  Some sealants adopt a new equilibrium dimension (decreasing the stress) through viscous dissipation of the imposed strain.   Other sealants elastically deform and keep a high stress level. These are two of the many mechanisms observed for sealants responding to changing gap conditions.  The sealant will fail when the molecular changes no longer enable the sealant to respond to the imposed deformation.  The short range modulus (stress/strain) of a sealant changes with time, imposed stress level and history of strain.  As these sealant materials are exposed to a suite of environmental variables (temperature cycles, moisture changes, UV radiation, and mechanical deformation) the molecular chemistry will change and thus the sealants ability to respond to imposed strain will change.  Predicting when those molecular chemistry changes will result in macroscopically observable tearing, delamination, or loss of adhesion requires knowledge of how the components of the environment change the molecular chemistry. The macroscopic failure may take years to observe, but the underlying molecular changes occur on a much shorter time scale. Quantifying these chemical changes and then attributing them to specific environmental factors and then relating that to macroscopic failure presents a series of significant technical challenges. 

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