|
Strength
|
|
Class
|
C12
|
C16
|
C20
|
C25
|
C30
|
C35
|
C40
|
C45
|
C50
|
C55
|
C60
|
C70
|
C80
|
C90
|
|
fck (MPa)
|
12
|
16
|
20
|
25
|
30
|
35
|
40
|
45
|
50
|
55
|
60
|
70
|
80
|
90
|
|
fck,cube (MPa)
|
15
|
20
|
25
|
30
|
37
|
45
|
50
|
55
|
60
|
67
|
75
|
85
|
95
|
105
|
|
fcm (MPa)
|
20
|
24
|
28
|
33
|
38
|
43
|
48
|
53
|
58
|
63
|
68
|
78
|
88
|
98
|
|
fctm (MPa)
|
1,6
|
1,9
|
2,2
|
2,6
|
2,9
|
3,2
|
3,5
|
3,8
|
4,1
|
4,2
|
4,4
|
4,6
|
4,8
|
5,0
|
|
fctk,0.05 (MPa)
|
1,1
|
1,3
|
1,5
|
1,8
|
2,0
|
2,2
|
2,5
|
2,7
|
2,9
|
3,0
|
3,1
|
3,2
|
3,4
|
3,5
|
|
fctk,0.95 (MPa)
|
2,0
|
2,5
|
2,9
|
3,3
|
3,8
|
4,2
|
4,6
|
4,9
|
5,3
|
5,5
|
5,7
|
6,0
|
6,3
|
6,6
|
|
Ecm (GPa)
|
27
|
29
|
30
|
31
|
33
|
34
|
35
|
36
|
37
|
38
|
39
|
41
|
42
|
44
|
|
εc2 (‰)
|
|
2,0
|
|
|
|
|
|
|
|
2,2
|
2,3
|
2,4
|
2, 5
|
2,6
|
|
εcu2 (‰)
|
|
3,5
|
|
|
|
|
|
|
|
3,1
|
2,9
|
2,7
|
2,6
|
2,6
|
|
n
|
|
2,0
|
|
|
|
|
|
|
|
1,75
|
1,6
|
1,45
|
1,4
|
1,4
|
Concrete strength classes are determined by the Characteristic Compressive Cylinder Strength fck of concrete at 28 days , with a maximum permissible value of Cmax=90 [EC2, 3.1.2(2)A].
The characteristic compressive strengths fck and the corresponding mechanical properties required by design are presented in the following table 3.1 [EC2, 3.1.2(3)] & [EN 206-1].
The strength parameters for each concrete class are:
fck Characteristic compressive cylinder strength of concrete at 28
fck,cube Characteristic compressive cube strength of concrete at 28 days, according to EN 206-1
fcm Mean value of concrete cylinder compressive , fcm = fck + 8 (MPa)
e.g. for C20, fcm = 20+8=28 MPa
fctm Mean value of axial tensile strength of concrete,
fctm = 0,30×fck2/3 ≤ C50/60 andι fctm=2,12×ln(1+fcm/10) > C50/60
e.g. for C20, fctm=0.30×202/3=2.21≈2.2 MPa
and for C70, fctm=2.12×ln[1+78/10]=2.12×ln(9.8)=4.84≈4.8MPa
fctk,0.05 5% fractile value of axial tensile strength of concrete
fctk,0,05 = 0,7×fctm 5%
π.χ. για C20, fctk,0,05 = 0.7×2.2=1.54≈1.5MPa
fctk, 0.95 fctk,0,95 = 1,3×fctm
e.g. for C20, fctk,0,95 = 1,3×2.2=2.86≈2.9MPa
Ecm Secant modulus of elasticity of concrete
Ecm = 22(fcm/10)0,3×103
e.g. for C20, Ecm=22×[28/10]0.3×103=29.96×103≈30GPa
εc2 (‰) Compressive strain limit in concrete for concrete in pure axial compression or strain in concrete at reaching maximum strength fck , assuming use of the bilinear stress-strain relationship. For fck ≥ 50 Mpa à εc2(‰)=2,0+0,085(fck-50)0,53
e.g. for C70, εc2=2,0+0,085(70-50)0,53=2.416≈2.4‰
εcu2 (‰) Ultimate compressive strain limit in concrete which is not fully in pure axial compression assuming use of the parabolic-rectangular stress-strain relationship.
For fck ≥ 50 Mpa à εcu2 (‰)=2,6+35[(90-fck)/100]4
n exponent for fck ≥ 50 MΡa à n=1,4+23,4[(90- fck)/100]4
e.g. for C70, n=1,4+23,4[(90-70)/100]4=1.4+0.04=1.44≈1.4