(See also http://transportation.mst.edu/research/r317/)

Status

 Complete                            View Final Report: PDF

Sequential Number

Identification Number

Matching Research Agency

Precast/Prestressed Concrete Institute (PCI)

Principal Investigator

Student Involvement

One graduate student

Project Objective

The overall goal of the proposed project is to determine the influence of LWC on the direct shear transfer across a plane of concretes cast at different times. Specific project objectives are:

1.      to determine an appropriate modification factor λ for LWC (relative to NWC) for use in current PCI Design Handbook shear friction design provisions for the case of concrete placed against hardened concrete;

2.      to determine and account for precast plant practices typically used to prepare the partially hardened concrete surface of a member immediately prior to casting additional components of the member (e.g. corbels, ledges); and

3.      to propose modifications to the PCI shear friction design method based upon results from this research as well as other test data published in literature.

Project Abstract

This project is aimed at studying the influence of aggregate type on direct shear transfer across an interface of concretes cast at different times. The shear friction design concept is applicable in conditions where direct shear must be transferred across a structural concrete plane or interface, such as an existing crack or an interface between dissimilar materials or concretes cast at different times. Shear friction provisions are commonly used in the design of precast-prestressed concrete elements and connections in building and/or bridge structures including corbels, dapped double tees, beam bearings, and diaphragms. These types of connections are critical because there is little or no redundancy.

Data used to develop shear friction provisions in both the ACI 318 Code and the PCI Design Handbook are predominantly from experiments with specimens constructed of normalweight concrete (NWC). Only a limited number of studies have been performed on lightweight concrete (LWC), and particularly for conditions with concrete surfaces cast at different times. This condition may exist, however, due to precast plant practices and the increasing use of self-consolidating concrete (SCC), and where projecting elements might be cast after the underlying concrete has partially hardened. Alternatively, projecting elements might be cast in advance and inserted into the fresh concrete when the main member is cast, resulting in a similar condition. It should also be noted that the influence of SCC on the interface shear has not been thoroughly studied. In summary, lack of LWC test data and clear and consistent design provisions underscore the need for a systematic approach to isolate and examine the influence of factor λ on the interface friction so that it can be applied clearly and confidently in shear friction design.

Anticipated Benefits

The topic of this study has been identified by PCI and producer members as a research need for the precast/prestressed concrete industry. LWC is commonly used in precast-prestressed concrete members, and certain elements and connections of these members require the use of shear friction design provisions (e.g. corbels, dapped double tees, beam bearings). These elements and connections have little or no redundancy; thus their design is critical. Shear friction design provisions presented in the ACI 318 Code and the PCI Design Handbook are largely empirical and are based on physical test data, yet a paucity of data exist on specimens of LWC, especially for conditions in which concretes are cast at different times. The LWC modification factors set forth by ACI 318 and the PCI Design Handbook have a direct effect on the shear strength and area of shear friction reinforcement required for elements and connections designed by shear friction provisions. Thus findings from this research will lead to shear friction designs of LWC elements and connections that are safe as well as economical.   

Milestones

Project Deliverables

A peer-reviewed journal article and a technical presentation

Technology Transfer Activities

Results of this study will be disseminated through at least one peer-reviewed journal article and one technical presentation.   

Transportation Research Board Keywords

Bridge superstructures, lightweight concrete