Page 423 - Forensic Structural Engineering Handbook

12.14 MATERIAL-SPECIFIC FORENSIC ANALYSES

TABLE 12.2 Expected Structural Concrete Strengths
Year of construction Low Usual High
Prior to 1918 800 2,000 2,000
1918–1930 1,000 3,000 4,000
1930–1950 2,000 3,000 5,000
1950–1960 2,000 4,000 6,000
1960–1970 2,000 4,000 8,000*
1970–1980 3,000 4,000 9,000*
1980–1990 3,000 4,000 11,000*
After 1990 3,000 4,000 18,000*
*Strengths over 6000 psi are not common for cast-in-place concrete outside
of large metropolitan areas in the eastern United States.

resolved or if dimensions and material properties cannot be established by measurement. 2
Load tests also can be used to rate old structures that have no construction records and
where determining material strengths and reinforcement locations is not cost-effective.
Historically, load tests were intended to be used only where flexural strength was in ques-
tion. However, in-place load testing has been permitted by chapter 20 of ACI 318 since 1995.
In planning a load test, safety is the most important consideration. Any portion of the
structure that has heavy loads, or might collapse in the event that the structure cannot carry
the applied load, should be safely shored. The shoring should be provided in such a way
that it does not interfere with the load test.
Following the procedures of ACI 318, loads should be arranged to create maximum
deflection and stresses in the critical regions of the structure in doubt. In some cases,
more than one loading application may be required. Intensity of the load applied should
not be less than 85 percent of the quantity 1.4D + 1.7L, where D is dead load and L is
live load.
Prior to starting a load test, the owner, engineer, and building official should all agree
on a protocol for the load test. All parties should agree to such items as whether to load net
area or gross area of a floor and how the load should be arranged. Acceptance criteria are
spelled out in ACI 318, chapter 20. It is noted that all structures, both reinforced concrete
and prestressed concrete, can be reloaded if deflections under the first loading exceed lim-
iting values.
In some cases, loading to destruction may be desirable. This is particularly true when a
question arises concerning bond of pretensioned strand. Under most conditions, field load
testing to destruction is done with applied deadweight, such as stacks of concrete block.
Because inability to quickly remove load creates safety hazards, this should be done with
great caution (Fig. 12.10). A preferred alternative is to transport the structural element to a
laboratory and perform the test to destruction with hydraulic equipment that can safely be
controlled.

Laboratory Tests of Components
Laboratory testing of components provides a method for determining the causes of distress
in the field. In cases where field load testing is impractical (e.g., failure has already
occurred), construction and testing of small-scale models in the laboratory provide an alter-
native strategy for determining field performance. 18
Where precast elements are involved, particularly when there is a question concerning
bond of pretensioned strand, full-size component tests to destruction may be conducted