Page 420 - Forensic Structural Engineering Handbook

CONCRETE STRUCTURES 12.11

used on two-way slabs. Consequently, it is not uncommon to find large deflections at the
centers of cast-in-place concrete slabs.
Removal of formwork prematurely can cause a safety problem as well as a deflection
problem. If concrete strengths are not high enough to carry dead load shears, collapse of a
structure may result, as shown in Fig. 12.8 and described in Ref. 12.

FIGURE 12.8 Bailey’s Crossroads. (Reproduced from National Bureau of
Standards Report, February 1997.)

Early removal of forms and reshoring can also cause serviceability problems by increas-
ing cracking and creep-related deflections. In general, the earlier concrete receives load, the
greater the creep will be per unit stress. Similarly, the modulus of elasticity is much lower
at an early age than it is at a later age. For these reasons, cracking and deflections are related
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to the properties of the concrete at the time formwork is removed. Reshoring after removal
of forms generally has little effect on reducing deflections started by early removal of
forms. However, stay-in-place shores are effective in controlling sag.

METHODS OF ANALYSIS

Instantaneous Properties

In doing forensic work on an existing reinforced-concrete structure, it is usually necessary
to determine the physical properties of the concrete and reinforcement. In relatively new
structures, construction records may provide adequate data. Quality control tests of con-
crete can often be used to estimate concrete strength. When the increase in strength due to
time is taken into account, construction records provide a good starting point.