Memorandum

The unit weight of reinforced concrete for both dead load and modulus of elasticity shall be taken as 160 pcf for all structural analysis, except as approved by the Bridge Design Engineer for unique designs (like floating bridges). See the Bridge Design Manual, Section 4.1.1 and 6.1.2B. The resulting modulus of elasticity may be taken as 4225 ksi and 4725 ksi, for Class 4000 and Class 5000 concrete respectively for all reinforced concrete bridge analysis. For construction camber calculations, 160 pcf would still be used for the unit weight of concrete, but the modulus of elasticity used should be representative of the concrete strength anticipated for the construction stage being analyzed.

Background:

The unit weight of unreinforced concrete measured by the WSDOT materials Lab shows that 212 sets out of 10561 sets of cylinders tested exceeded 155 pcf. Considering the additional weight of reinforcement steel and increased use of High Performance Concrete (HPC) with higher density, it is appropriate to use 160 pcf for the unit weight of concrete.

The range of variability of the modulus of elasticity of concrete in accordance with ACI 318-95-R8.5 is about 20%. Using 160 pcf instead of 155 pcf for modulus of elasticity calculation results to an increase of 4.88% in modulus of elasticity which is within the range of variability.

CCR/bk

cc:     J. A. Weigel - 47340

         E. H. Henley - 47340

         H. L. Coffman - 47341

         F. Higgins - 47340

         R. Lewis - 47354

         Jim Walter - 47365

         Z. Fahoum - 47340