Control of cracking - Concrete Structures: Stresses and Deformations - page 401

11.9 Cracking of high-strength concrete

The e ects of use of high-strength concrete (HSC) on cracking of reinforced

concrete structures with or without prestressing are brie

ff

y discussed here.
For this purpose, consider a reinforced concrete member subjected to an axial
force, N , as shown in Fig. 11.1(a). If concrete of higher strength is used for
the member, cracks will occur at higher N values, because of the increase of
the tensile strength (see the dashed lines in Fig. 11.1(a) ). Also, because of the
improved bond, slip between the concrete and the reinforcing bars occurs at
higher bond stress; this decreases the crack spacing (see Appendix E) and also
increases the e

fl

ening, thus reducing the width of
cracks. Therefore, use of HSC may prevent the cracking, or when N is greater
than the cracking value the crack width will be smaller compared with an
identical member with the same reinforcement but lower concrete strength.
On the other hand, when N is caused by an imposed displacement (Fig.
11.1(b) ), the

ff ect of the tension sti ff

rst crack will be formed at a higher N value and higher steel
stress will occur at the crack. This means that a larger steel ratio is necessary
to avoid yielding of steel at cracking (see Equation (11.20) ). In spite of the
higher steel stress at the crack, the crack width will increase only slightly,
because of the increase of tension sti

fi

ff ening e ff ect due to improved bond.

The e ff ects of use of HSC on cracking of members subjected to bending

is not di erent from what is discussed above. This is evident in Fig. 11.8

which summarizes the results of long-term tests 8 on simply supported slabs

ff

Figure 11.8 Mean crack width w m and mean crack spacing s rm observed in tests on
reinforced concrete slabs of varying concrete strength.