SELECTION AND INSTALLATION OF BRIDGE EXPANSION JOINTS

Why do we need joints? Bridge deck joints allow a bridge to expand and contract due to a number of factors such as: temperature changes, deflections caused by live loads, creep and shrinkage of concrete etc. Bridge deck joints are a necessary component of a properly designed and functioning structure.

FACTORS EFFECTING THE SELECTION OF DECK JOINT Design movement of deck joint. Mean temperature. Temperature range of climate based on elevation above sea level. The setting temperature of the joint. Design rotation of deck joint. Effects of bridge skew, curvature, and neutral axis location. Anticipated settlement and rotation due to live and dead loads.

SELECTION OF DECK JOINT TYPE One of the most important decisions a bridge deck joint designer needs to make is what type of bridge deck joint to use. There are numerous types of bridge deck joints available on the market made by a variety of manufacturers. Poured Seal Joints : Suitable for small design movement. Very inexpensive relative to the other joint types. The most widely used material in poured bridge deck joints is silicone.

Poured Seal Joints : The silicone sealant is poured over a backer rod (expanded closed-cell polyethylene foam) that is placed in the expansion gap. The joint is typically armored with angle iron to protect the headers from normally anticipated edge spalling.

Advantages of poured seal joint : All-temperature gunnability (characteristics relatively unchanged over normal installation temperature range). Easy to use (self-leveling, no tooling required). Seals irregular surfaces (can be used to seal joints where spalls have occurred without any forming). Good weatherability (virtually unaffected by sunlight, rain, snow, or extreme temperatures, and stays rubbery from -49° to 300° F without tearing, cracking or becoming brittle). Limitations of poured seal joint : Not recommended for conditions where continuous water moisture is expected. Not to be used in totally confined spaces where sealant is not exposed to atmospheric moisture. May not fully bond to angle iron made of weathering steel.

Compression Seal : Compression seals are seals that are compressed when inserted into the joint opening and remain in the state of compression during all movement phases of the joint.

Compression Seal : Allow movement up to 2.5 inches. Not suited for large skews (> 30°). The seal must have a nominal width that is greater than the largest expected gap opening. They maintain water tightness and seal the joint by maintaining sidewall pressure on the joint interface (joint armoring). The most important factor that will ultimately determine the success of a compression seal expansion joint is proper installation of the seal during construction. If the seal is not correctly installed at the time of construction, the life of the joint is greatly reduced.

Strip Seals : Strip seals expansion joints consist of two extruded steel channels that extend across the full width of the bridge roadway and support neoprene seal.

Strip Seals : Strip seals achieve movement by the gland folding up below the surface of the deck. As with compression seals, debris will fill the opening soon after placed in service. The preferred maximum allowable opening, measured along the centerline of the bridge is 4 inches. Strip seal installations offer greater construction tolerance than compression seals with regards to setting conditions since the seal does not need to be compressed to be placed.

Modular Joints : A modular joint system consists of two or more preformed compression seals or neoprene glands fixed between transverse load distribution members. The transverse load distribution members rest on support bars that allow for sliding movements.

Modular Joints : Modular joint is designed for large movements and use in heavy traffic bridge structures. Modular joints can accommodate movement ranges from 4 to 30 inches. This joint system allows the bridge deck to move while maintaining a smooth watertight riding surface. They are sized according to the movement rating offered and are manufactured in increments of 3 inches beginning with a 6-inch modular system. Modular joints should be avoided whenever possible due to the complexity and high costs of the joint.

Recommendations for Selection of Deck Joint Type To eliminate bridge deck joint problems altogether, consider eliminating bridge deck joints when possible. Discontinue the use of compression seals in new construction, replacement, and rehabilitation due to: Tighter installation tolerance that causes the joint to fall out or become damaged by traffic. Installation and setting problems during construction. Use strip seals with rating of 4 inches for anticipated movements of 2 to 4 inches. To aid the inexperienced bridge deck joint designer, develop a design procedure and standard drawing for modular deck joints.