010

8.8.7 Long-Term Effects on Crack-Width Development

Limited studies on crack-width development and increase with time show that both sustained and cyclic loadings increase the amount of microcracking in the concrete. Also, microcracks formed at service-load levels in partially prestressed beams do not seem to have a recognizable effect on the strength or serviceability of the concrete element. Macroscopic cracks, however, do have a detrimental effect, particularly in terms of corrosion of the reinforcement and appearance. Hence, an increase of crack width due to sustained loading significantly affects the durability of the prestressed member regardless of whether prestressing is circular, such as in tanks, or linear, such as in beams. Information obtained from sustained load tests of up to 2 years and fatigue tests of up to one million cycles indicates that a doubling of crack width with time can be expected. Therefore, engineering judgment has to be exercised as regards the extent of tolerable crack width under long-term loading conditions.

8.8.8 Tolerable Crack Widths

The maximum crack width that a structural element should tolerate depends on the particular function of the element and the environmental conditions to which the structure is liable to be subjected. Table 8.14, from the ACI Committee 224 report on cracking, serves as a reasonable guide on the acceptable crack widths in concrete structures under the various environmental conditions encountered.

8.8.9 Example 9: Crack Control Check

A pretensioned prestressed concrete beam has a T-section as shown in Figure 8.14. It is prestressed with fifteen ig-in. diameter seven-wire strand 270-K grade. The locations of the neutral axis and center of

PHOTO 8.8 West Kowloon Expressway Viaduct, Hong Kong, during construction, comprising 4.2 km dual three-lane causeway connecting Western Harbor Crossing to new airport (courtesy Institution of Civil Engineers, London, and [5]).

Seven-wire strand

Seven-wire strand

FIGURE 8.14 Beam cross-section in Example 9 [5].

gravity of steel are shown in the figure. f' = 5,000 psi, Ec = 57,000yf', and Es = 28 x 106psi. Find the mean stabilized crack spacing and the crack widths at the steel level as well as at the tensile face of the beam at Df = 30 x 103 psi. Assume that no failure in shear or bond takes place.

Solution

Mean stabilized crack spacing

20 62 in acs =1. 2\ ^ ) = 1. 2() = 5 . 7 in. (145 mm)

Maximum crack width at steel level

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