Construction Issues Associated With Modular Bridge Systems

Given the time constraints during a bridge rehabilitation/replacement project or in cases of an emergency, it is necessary for the bridge to be constructed as fast as possible.

Safety of the construction workers and the public is generally enhanced with the shorter construction time. In some instances, the use of modular units have can reduce the construction time significantly which is imperative when the user cost of a facility is high.

The ease and speed of construction of a prefabricated bridge system is paramount to its

acceptance as a viable system for general use. Therefore, the criteria used in this study to

formulate the process of selecting systems for further evaluation place a high emphasis

on these factors.

Connections of the modular units are important elements for accelerated bridge construction, since it is envisioned that the new system will consist of either an entirely prefabricated bridge system manufactured off site, transported and erected in place or modular bridge units which can be easily assembled and connected together to form the bridge system.

Number of joints (longitudinal vs. transverse)

A major design and detailing factor in the modular units system is the development of proper joint details that require minimum time for installation and are capable of providing the desired long term durability. There are various jointing techniques that have been used; however, talking to a bridge engineer one most often hears “The ideal joint is no joint”. Joints are prone to deterioration and are considered the weakest link in any structure, thereby reducing their effectiveness and long term performance. Freeze and thaw along with deicing salt are some of the parameters affecting joint performance.

Therefore, minimizing the number of joints in a modular bridge system and using appropriate joint material are important factors that require careful examination.

The number of longitudinal joints versus transverse joints In a particular span is a function of the configuration of the main components of a particular modular system and the span length. Typically, systems that have short span lengths have no transverse joints and no or a few number of longitudinal joints depending on the width of the span. In contrast, systems that have greater span lengths typically have few or no longitudinal joints and more transverse joints.

The number of joints and the type of joint detail is crucial to both the speed of construction and to the overall durability of the final structure. Therefore, of the various criteria used for selecting the systems to be developed further, a total of 20% of the criteria importance is place on joint quantity, type and details.

Joint Details

As noted previously, minimizing the number of joints is desirable. However, for a

construct the bridge will be greater than that in conventional cast-in-place concrete deck slab construction.

The ideal joint detail for a prefabricated modular system is that of a simple “shear key”

configuration with the use of an epoxy, non-shrink grout to fill the joints. The joints also may have a bolted connection or transverse post-tensioning. For systems such as the Railroad Flatbed modular system, the joint is of a width that requires the use of cast-in-place concrete due to the large volume required.

A typical deck slab longitudinal joint that has been used successfully is shown in Figure 3.4.

Figure 3.4 A Typical Joint

The performance of all joints in the final bridge structure is crucial to durability and long term maintenance.

Continuity Details

Provision for using prefabricated steel modular systems as continuous bridges with

superimposed dead loads and live load are possible, but it may not be cost-effective in

regard to the advantages gained. Providing longitudinal continuity has many beneficial

effects such as: i) increased structural stiffness, ii) slight reduction in structure depth, iii)

reduction in transverse cracking in the deck and iv) reduction in the number of transverse

joints over the supports.

However, the use of prefabricated deck systems inhibits the placement of the significant amount of reinforcing steel or post-tensioning necessary to resist negative moments from longitudinal continuity. Post-tensioning bars or tendons can be installed external to the girders and deck, similar to the new St. Simons Bridge approach spans in Brunswick, Georgia.

Providing full system continuity under dead load and live load could prove difficult and time consuming for the objective of accelerated construction. Providing continuity for live loads only has been used successfully for many years and therefore is recommended for the new system.

The issues of durability and maintenance are most affected by the addition of continuity details. The modular systems selected for further study will be fully evaluated for the use of continuity through the bridge deck to resist applied live loads.

Closure Joints Materials

As noted previously, the use of cast-in-place concrete should be kept to a minimum for accelerated construction methods due to placement, finishing and curing time. The ideal material for any closure joints is a non-shrink grout, possibly with the addition of a fine aggregate to increase volume.

Another option for large volume joints is the use of fast-setting magnesium-based proprietary concrete mixes. Polymer materials are also gaining popularity and could prove effective as the material of choice for joints.

The closure joint design and material are important issues that are currently being researched under NCHRP project 10-71 “Cast-in-Place Reinforced Concrete Connections for Precast Deck Systems”. The knowledge gained from this recent research and past experience could be easily utilized to develop and test the joint details for the proposed bridge systems.

Riding Surface Preparation

To the traveling public, a bridge is only as good as the smoothness of the riding surface.

Due to irregularities in the riding surface that can occur at longitudinal and transverse joint locations between modular components, it may be necessary to profilograph, or

“plane”, the deck surface after construction is complete. In recent years, many states have taken to using planed deck through diamond grinding the surfaces, and the availability of equipment and the cost is reasonable.

Utilizing Cast-In-Place Concrete

Cast-in-place concrete could be used efficiently providing that the new system is self

contained, thereby eliminating the need for forming and shoring. There are many cases

where a large portion of the concrete deck was replaced and the bridge opened for traffic

within 48 hours. For this type of bridge system, polymer concrete can be used or the

specified concrete must have a low water-to-cement ratio with as much rapid setting

characteristic as possible. The most rapid setting mix would utilize Type III cement with

rapid setting admixtures.