Reid W. Castrodale, PhD, PE, Expanded Shale, Clay and Slate Institute (ESCSI)
When the first edition of the AASHTO LRFD Bridge Design Specifications was published in 1994, a special shear resistance factor, φ, for lightweight concrete of 0.70 was introduced. The shear resistance factor for normal weight concrete was 0.90. The lower shear resistance factor for lightweight concrete was introduced because of a lack of available data to evaluate the statistical variability of lightweight concrete (Mertz, 2012). However, a special shear resistance factor for lightweight concrete had never appeared in the AASHTO Standard Specifications or the ACI Building Code Requirements for Structural Concrete (ACI 318). Both of these other documents had used the same resistance factor for normal weight and lightweight concrete since strength design was introduced in the early 1960s. Even the most recent edition of ACI 318, which was released in 2008 and incorporated significant revisions to the treatment of lightweight concrete throughout the code (including shear provisions), did not include a special shear resistance factor for lightweight concrete. Both of these codes did use a factor (λ in ACI 318) to reduce the concrete contribution, Vc, to the shear capacity of lightweight concrete members. This reduction factor is also used in the LRFD Specifications.
The difference in the shear resistance factors for lightweight and normal weight concrete may seem minor. However, it was found that the use of a special shear resistance factor for lightweight concrete combined with the reduction factor applied to the concrete contribution for shear design of lightweight concrete, has resulted in a significant reduction in shear resistance for lightweight concrete members designed using the AASHTO LRFD Specifications when compared to designs performed using the AASHTO Standard Specifications. The reduced shear capacity for lightweight concrete members requires an increased shear width for a cross-section and/or an increased quantity of shear reinforcement. Furthermore, the special shear resistance factor for lightweight concrete not only reduces the shear capacity of the concrete but also reduces the contribution of the shear reinforcement.
For some types of elements, such as prestressed concrete girders, the change was not very significant. However, in other elements, the increase was significant. For example, in one concrete segmental box girder bridge, the use of the special shear reduction factor eliminated lightweight concrete from consideration as a design alternate. In another case, the special shear reduction factor made the use of lightweight concrete uneconomical for precast pier caps for a bridge project in NY. In both projects, the LRFD design eliminated the benefit of the reduced density of the lightweight concrete by requiring an increase in member size or reinforcement to offset the reduced shear capacity.
Since the special shear resistance factor for lightweight concrete was introduced because of insufficient data for a statistical evaluation of the shear resistance factor, the Expanded Shale, Clay and Slate Institute (ESCSI) coordinated the collection of lightweight concrete cylinder compression test results from projects across the US. This data was submitted to Professor Andy Nowak of the University of Nebraska. Combined with a small amount of data from a previous study, a total of 8,889 data points were used by Professor Nowak and his team to perform a statistical analysis of lightweight concrete. The analysis revealed that the statistical parameters for lightweight concrete were similar to, or in some cases better than, those of normal weight concrete (Nowak & Rakoczy, 2010).
As the second step in the evaluation of the shear resistance factor for lightweight concrete, shear test results were compared to shear capacities computed using the AASHTO LRFD Specifications. While a limited quantity of shear test results for lightweight concrete was available, they were sufficient to allow the researchers to conclude that the resistance factor for shear for lightweight concrete could be increased from its current value of 0.7 to a value of 0.8 (Paczkowski & Nowak, 2010).
Based on the work by Professor Nowak, the AASHTO Subcommittee on Bridges and Structures (SCOBS), at their annual meeting in 2011, approved increasing the special shear resistance factor, φ, for lightweight concrete that appears in Section 5.5.4.2.1 of the AASHTO LRFD Specifications from 0.70 to 0.80. The shear resistance factor for normal weight concrete remained unchanged at 0.90. This change has improved the situation for shear design with lightweight concrete, allowing the wider consideration of the material as an option improving the economy of bridges. Shear test results that have recently become available from NCHRP Project 18-15 and an FHWA research project may allow a further reevaluation of the shear resistance factor for lightweight concrete.
References
Nowak, A.S. and Rakoczy, A.M., “Statistical Parameters for Compressive Strength of Lightweight Concrete,” Paper 68, Proceedings, Concrete Bridge Conference, Phoenix, AZ, February 24-25, 2010.
Paczkowski, P. and Nowak, A. S., “Reliability Models for Shear in Lightweight Reinforced Concrete Bridges,” Paper 69, Proceedings, Concrete Bridge Conference, Phoenix, AZ, February 24-25, 2010.
Mertz, D. R., “AASHTO LRFD – 2012 Interim Changes Related to Concrete Structures,” ASPIRE Magazine, Summer 2012, p. 56.