Page 300 - Forensic Structural Engineering Handbook

DEFECTS, DETERIORATION, AND DURABILITY 9.11

extreme conditions, these deterioration mechanisms working in concert result in failure of
the wall and tremendous economic loss and hazards.
Some mortar deterioration is expected. Mortar may be considered a maintainable com-
ponent with a design life of approximately 30 years, and it is expected to require condition-
based maintenance such as repointing. Mortar acting as a sacrificial element accommodates
the majority of the structural movement and absorbs the majority of the water. Eventually,
the binders within the mortar erode or break down. Mortar deterioration is readily verifi-
able visually, or by using a probe. Generally, loose, friable mortar or mortar that is crack-
ing or separated from the surrounding masonry has reached the end of its service life, but
the same conditions can also result from defective original construction.

DURABILITY

Durability has always been a major issue in failure analysis, but it has become more impor-
tant in the current era because of the design emphasis on sustainability. Obviously, maxi-
mizing durability is crucial to sustainability for any material.
Durability, from the same root word as duration, is the relationship between design life
and service life; it encompasses both the expectation of how long a component or system
should last (design life) and its actual performance (service life) compared to its design life.
Thus, one can say steel is a durable material (it has a long design life), while its durability
in a particular application may be compromised by corrosion.
To evaluate the design life of a material or system, it is useful to consider the design life
of whole buildings. Table 9.1 describes five building categories based on design life. A
selected design life for a building may be self-fulfilling, because its materials and details
are chosen to have commensurate durability. For example, designers select very different
products for a retail building with a design life of 30 years than for an institutional facility
with a design life of over 100 years.

Design Life
ASTM E2018, Standard Guide for Property Condition Assessments: Baseline Property
Condition Assessment Process, 13 defines the term “expected useful life” (equivalent to
design life used here) as the average amount of time that an item is estimated to function
when installed new and assuming that routine maintenance is practiced.

TABLE 9.1 Categories of Building Design Life 18

Building design life
Category description (in years) Examples
1. Temporary accommodation Up to 5 Facilities used during construction;
temporary exhibition buildings
2. Short life building 5–30 Temporary classrooms; buildings for
short-life industrial processes; modular
buildings
3. Medium life building 30–60 Most industrial buildings; retail and
warehouse buildings
4. Normal life building At least 60 New health and educational buildings;
new residential structures
5. Long life building 60–120 Major theaters; courthouses; government
and institutional high-quality buildings