Concrete Properties

Lightweight Concrete

Concrete type can be normal weight, sand lightweight, or all lightweight (in CHBDC, these categories are referred to as normal-density, semi-low density and low density). The LRFD Specs define lightweight concrete as having an air-dry density not exceeding 0.120 kcf. However, it is up to the user to explicitly designate a concrete type as lightweight in the Materials Properties page of the Culvert Properties dialog box.  Note that this has changed in the 7^th Edition.  Now the program will select either Normal Weight or Lightweight depending on the assigned unit weight (the user may not assign concrete types).  For unit weights greater than or equal to 135 pcf, the program will treat the concrete as Normal Weight.  For unit weights less than 135 pcf, the program will treat the concrete as Lightweight.  In addition to weight calculations, designating a concrete type as lightweight will influence three other areas of the calculations:

1. Modulus of rupture                                                                                  (LRFD 5.4.2.6)

2. Resistance factor for shear                                                                       (LRFD 5.5.4.2)

3. Tensile and shear capacity of concrete                                                    (LRFD 5.8.2.2/5.4.2.8)

The following moduli of rupture are computed by Eriksson Culvert:

      Condition                                                                                        f_r (ksi)

      Normal wt. concrete, minimum reinforcement                      0.37(f’_c)^1/2 (5^th Edition)

      Normal wt. concrete, minimum reinforcement                      0.24(f’_c)^1/2

      Sand-lightweight concrete                                                     0.20(f’_c)^1/2

      All-lightweight concrete                                                         0.17(f’_c)^1/2

      All concrete types                                                                   0.24(f’_c)^1/2 (7^th Edition)

      Normal-density concrete                                                        0.40(f’_c)^1/2 (CHBDC)

      Semi-low-density concrete                                                     0.34(f’_c)^1/2 (CHBDC)

      Low-density concrete                                                              0.30(f’_c)^1/2 (CHBDC)

For normal weight concrete, the resistance factor for shear, f_v, is 0.90. For both types of lightweight concrete f_v is set to 0.70, but has been increased to 0.80 in the 6^th Edition, and to 0.90 in the 7^th Edition.  For CHBDC, f_s (steel) is 0.90, and f_c (concrete) is 0.80.

In accordance with LRFD Article 5.8.2.2/5.4.2.8, if lightweight concrete is specified, all occurrences of the term “(f’_c)^1/2” within LRFD Articles 5.8.2/5.7.2 and 5.8.3/5.7.3 are modified by the following factors:

      Concrete Type                                           Factor

      Normal weight                                           1.00

      Sand-lightweight                                        0.85

      All-lightweight                                            0.75

Modulus of Elasticity

The modulus of elasticity of the culvert concrete are computed by default using the following equations:

  LRFD 5.4.2.4-1:

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LRFD 5.4.2.4-1, 7^th Edition:

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CHBDC 8.4.1.7:

where,   w_c, γ_c = unit weight (i.e., density) of concrete and f’_c = specified concrete strength

The user can of course override the program calculation of this value through input on the material properties section of the ‘Culvert Properties’ window.