Title:
Use of Composite Testing for Evaluating of Keyway Grout for Precast Prestressed Bridge Beams
Author(s):
Robert J. Gulyas and Jeffery T. Champa
Publication:
Materials Journal
Volume:
94
Issue:
3
Appears on pages(s):
244-250
Keywords:
bond strength; component testing; composite assembly testing; drying shrinkage; expansion; keyway; keyway grout; length change; longitudinal shear bond; magnesium ammonium phosphate (mgNH4PO4) mortar
DOI:
10.14359/305
Date:
5/1/1997
Abstract:
Precast, prestressed concrete voided members are used in bridge decks. All members have keyway joints which are later grouted to provide shear capacity across adjacent members. Typically a non-shrink grout is used to fill the keyway joint formed by two female-to-female shapes in the precast members. Even though these decks can be overlaid with portland cement or asphaltic concrete, these types of keyway joints often develop leaks on the underside of the bridge deck, wicking chloride laden salts into the precast concrete. Corrosion has been noted in beams adjacent to the keyway joints. Cracking in the overlay above the joint is seen in the field providing access of the chloride laden deicing chemicals or sea spray. Non-shrink grout is a poor choice for the grouting of these keyways. Designed for correction of vertical settlement shrinkage due to bleeding, these types of grouts generally develop long-term drying shrinkage upon exposure to air. Loss of interface bond between the grout and the member occurs, and shrinkage adjacent to the precast member is noticed in the field. Vertical faulting has been observed indicating a poor of load transfer across adjacent members. A lab study compared non-shrink grout with magnesium ammonium phosphate (MgNH4PO4) mortar in keyway-shaped composite specimens comprising both the grout and the keyway assembly. These specimens were initially tested in direct tension, vertical shear, and longitudinal shear. A minimum 250 percent increase in performance was observed with the MgNH4PO4 grout compared to non-shrink grout in initial tests. Longer term testing indicates virtually no loss in performance with the MgNH4PO4 keyway mortar while the non-shrink grout lost 36 percent of its initial capacity after longer drying shrinkage. Differences may be due to the better bond strength and lower drying shrinkage of these types of mortars compared to the non-shrink grouts which are in use by most DOTs in the U.S.