Alexander K. Bardow, Massachusetts Highway Department
Experience has shown that bridges with continuous beams and rigid frame abutments exhibit better long-term durability than bridges with leakage-prone roadway joints. Thus, when the existing Great Road Bridge (State Route 119) over the Boston & Maine Railroad in Littleton, MA, required replacement, every attempt was made to eliminate these joints.
Building a cast-in-place structure was not feasible since the falsework would reduce the existing railroad clearances. The Massachusetts Highway Department (MassHighway) decided to use “emulation design” to replicate cast-in-place concrete construction with precast, prestressed concrete beams and precast concrete abutment panels.
A three-span structure, comprised of two 46 ft (14 m) end spans and a 54 ft (16 m) central span, proved to be the most economical. The piers were designed as conventional, cast-in-place concrete elements. However, using the emulation design technique, the pier components could have been precast concrete and connected in the field using reinforcing bar couplers—an option that MassHighway may consider for future projects.
The superstructure consists of adjacent, 21-in. (530-mm) deep prestressed concrete deck beams. The beams are rigidly connected to the precast abutment panels using reinforcing bar couplers. This eliminates the deck joint and provides an efficient means of transferring longitudinal loads to the approach embankment fill.
At the piers, the beams are made continuous for live load by using grouted sleeves to connect the reinforcing bars that project horizontally from the tops of adjacent beams. The continuity closure pour of high performance concrete (HPC) is reinforced and extends into a closed-cell, foam-lined key in the top of the pier cap. This detail provides longitudinal and transverse restraint for seismic forces but allows beam rotation under load.
High Performance Concrete
To enhance durability, a high performance silica fume modified concrete was required for all cast-in-place superstructure components, including the continuity closure pour, sidewalk slab, and barriers. The concrete was specified to have a maximum water-cement ratio of 0.40, a minimum compressive strength of 5000 psi (35 MPa), and a maximum permeability of 1000 coulombs at 90 days. The HPC contained 635 lb/cu yd (377 kg/cu m) of cement and 50 lb/cu yd (30 kg/cu m) of silica fume. A high-range water-reducer conforming to ASTM C 494 Type F or G was specified to help achieve the specified maximum slump of 6 in. (15 mm) and to improve workability. Finally, the specifications called for a fog mist during the placement and finishing operations to prevent surface drying followed by a seven-day minimum wet burlap cure. No problems were reported during either the placement and curing operations or in the final product.