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A warning is given that the descending branch highly depends on the testing procedure and its formulation should be used with caution.

Using the relations (1,2,3,5,6,7) the diagram shown in Fig. 3.3 is obtained. Since the descending branch for HSC is not very reliable, a simplified formulation is preferred, in that the lines according to Eq. (1) are continued beyond the top, Eq. 6, until a defined value £cu is reached, according to

£cu ( %%) = 2.8 + 27 [(98 - fcm)/100]4 for fck > 55 Mpa (8)

In this way the simplified curves shown in Fig. 3.4 may be obtained.

£cu ( %%) = 2.8 + 27 [(98 - fcm)/100]4 for fck > 55 Mpa (8)

In this way the simplified curves shown in Fig. 3.4 may be obtained.

Figure 3.3. Mean stress-strain relations, obtained by combining Eq. (1-3) and (5-7).

a [MPaJ

a [MPaJ

Figure 3.4. Simplified mean stress-strain relations, according to the new formulation, combining Eq. (5,6,1.

3.1.6 Design compressive and tensile strengths

C3.1.6 Design compressive and tensile strengths

The value of the design compressive strength fcd is defined as fcd = acc fck /yo

where acc is the coefficient taking account of long term effects on the compressive strength and of unfavourable effects resulting from the way the load is applied; Yo is the partial safety factor for concrete, which is 1,50 [Table 2.1N-EC2]. A well known research program focussing on the effects of long term loading was the one carried out by Rusch [Rusch, 1960]. He carried out tests on concrete prisms, which he loaded to a certain fraction of the short-term compressive strength: subsequently the load was kept constant for a long period. If the long-term loads were higher than about 80% of the short-term bearing capacity, failure occurred after a certain period. Fig. 3.5 reproduces Rusch's diagrammatic representation of concrete strains as a function of the applied stresses for several loading times.

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