The replacement of a small two-span bridge was recently completed in North Haven, ME using Northeast Extreme Tee (NEXT) beams constructed with lightweight concrete. This is the first time that NEXT beams have been designed with lightweight concrete. For this bridge, lightweight concrete was used to allow the reuse of the existing pier, which helped speed the construction and to reduce the girder weight for shipping and handling. The bridge site is located on an island, so all materials used to build the bridge had to be transported 12 miles on a barge. Using the NEXT beams simplified construction and reduced the requirement for field-placed concrete.
The prestressed concrete fabricator, J.P. Carrara & Sons, Inc., of Middlebury, VT, chose Norlite lightweight coarse aggregate manufactured in Cohoes, NY to make the lightweight concrete for the beams. The design compressive strength for the self-consolidating lightweight concrete was 6,000 psi, with a density of 120 pcf.
The beam cross-section used for the bridge was the full-depth deck section, or the NEXT D section, which was developed by the PCI Northeast Technical Committee. For this bridge, the beam was 28.5 in. deep with an 8’-4” wide top flange that serves as the bridge deck. The two beams that were used to comprise each span were connected by an 8” wide longitudinal closure joint.
Since this beam section includes an 8 in. thick structural deck, the beams can be very heavy. The use of lightweight concrete for NEXT D beams was anticipated by the PCI NE Technical Committee by providing a preliminary design chart for lightweight concrete for the NEXT D beams in their Guidelines for Northeast Extreme Tee Beam (NEXT Beam), as shown below.
While a 12 ft. wide top flange is included in the preliminary design charts for the NEXT F section (which has a 4 inch thick top flange so is significantly lighter than the NEXT D section), the 12 ft. width is not included in the NEXT D section charts because the Technical Committee thought that the beams would be too heavy to be transported economically in the Northeast. The figure below demonstrates that a 36 in. deep lightweight concrete NEXT D beam that has a 12 ft. wide top flange only weighs slightly more than a normal weight concrete NEXT D beam with an 8 ft. wide top flange. Therefore, use of lightweight concrete for the NEXT D sections can facilitate use of wider top flanges which typically provide a more efficient design. Use of lightweight concrete for NEXT beam designs may also allow designers to use shallower sections, addressing the vertical clearance issue below bridges which is often a very important issue in the Northeast.