A modification factor, λ, has been added to all strength equations within ACI 318 that contain the term $\sqrt{{f^{\prime }}_c}$. This parameter is discussed in detail in Section 8.6.1 of ACI 318-08. Section 8.6.1 replaces Section 11.2 in the 2005 edition of the code. As a result, all sections after 11.2 in ACI 318-08 are now decremented by 1 (e.g., 11.4 in '05 is 11.3 in '08).

The parameter λ accounts for the lower tensile strength of lightweight concrete.

• λ = 1.0 for normal weight concrete
• λ = 0.85 for sand-lightweight concrete
• λ = 0.75 for all-lightweight concrete

Linear interpolation is allowed between these values as discussed in Section 8.6.1.

As a result of the addition of the λ parameter, the form of several concrete shear strength equations has changed in ACI 318-08. Changes to the most basic equations are shown below.

becomes

becomes

becomes