An expansion joint is also known as a movement joint, which is an assembly consisting of separation in the wall a flexible material like a bond breaker or sealant. Bond breaker materials may comprise of sprays, liquids, rods, or tapes. They are required for breaking the bond between building sections for ensuring the sections can separate. The material being compressible, it can accommodate the movement of adjacent materials. For closing the joint opening and mitigating moisture from entering the joint, a flexible sealant is applied along with accommodating movement between wall sections. When performing a building envelope survey, all of these factors must be taken into account.
By temperature differentials, stress, and moisture infiltration, adjacent materials in a wall assembly are affected that causes movement between wall sections. Contraction and thermal expansion are the temperature movement of building materials and is very much common in areas that experience seasonal climate changes. When heated or cooled by ambient temperatures, a long masonry wall will contract or expand over its length and height. When heated, the individual masonry units will elongate, and when cooled it will get deformed. The changes in the height and length of the wall will create internal stress within the wall. Cracks will develop if the stress is not relieved. A temporary change in length is elastic deformation, volume, or shape of a material under stress. Vertical loads like dead and live loads will create stress in building materials. The weight of the building or structure on itself. As they are permanent, the materials which comprise the building are considered to be dead weight. Live loads are rather moving or permanent than being permanent or fixed. For example, live loads are materials, people, office equipment and furniture or shelving which is not bolted down. Other loads that cause building materials to get deformed and deflect in volume, shape, and length are seismic activity, wind, and snow. A good example of this is the diving board. For example, let’s just imagine that a person is standing on the edge of the diving board, the board itself is the dead load and the person is the live load. When the person is on the edge of the diving board, you can see the deflection in the board as it dips towards the pool. It gets deformed or bent due to the weight of the person. The deformation will be higher with the weight of the person and vice versa. The diving board will also experience variable deformation as the person walks up and down the board. When the person jumps up and down the deformation will get exaggerated. This will create stress.
By the expansion and contraction of the materials from the increase or decrease in moisture content, moisture movement is caused. Materials like concrete, masonry block and wood get expanded because of saturated water and get the return to the original state after drying out. Think of these building materials as a dry sponge when they are installed first. Through rain events or leaks, when the materials are saturated they will grow and expand in size like a dry sponge that absorbs water. Like the same ways, when the material decreases in size when it dries out. Stress is created within the material by these variations and any abutting material.
The Importance of Expansion Joints on Concrete Bridge Decks
Expansion joints are vital to the life of a bridge itself and must be correctly installed and maintained in order to function properly. The two major causes of joint failure are improper installation and insufficient maintenance of the joint. Oftentimes, when dirt, dust or other types of debris build-up in and around these joints (caused by traffic and environmental conditions), the joints lose their ability to expand and contract with the structure, rendering them less effective. When this happens, the reduced flexibility will result in cracking or distortion/crushing of the joint material, compromising the entire bridge’s structural integrity.
Bridge Deck Expansion Joints MUST:
- protect concrete edges from damage caused by vehicle traffic,
- act as a seal to prevent foreign objects/debris from entering the joint opening.
- allow for expansion and contraction caused by thermal cycles, and
- allow for the natural creep and shrinkage of concrete.
Important Elements to Consider
When using concrete as a building material, there are some very important things to consider. Of course, the strength and weight load of the concrete are important, but another vital component is the affect the environment has on the material. Thermal cycles within the geographical area must be taken into consideration. As seasons and weather change, the structure will expand and contract with the temperature. Sometimes, this is overlooked, and when these structures expand and contract, there can be serious issues with the expansion joint system.
he heating and cooling cycles of concrete bridge decks will cause expansion and contraction, respectively. If the contraction is greater than the tensile strength of the construction materials, cracking will occur. And inversely, when the expansion is restricted, distortion or crushing of the materials can occur. To prevent these types of failures, concrete bridge decks require joints fitted with materials that allow the structure to expand and contract without causing damage to the structural integrity. These are known as expansion joints, and there are many different types.
Left Image -Failed Expansion Joint – Undersized
The 2 Types of Expansion Joints
Open joints – allow water and debris to move freely through the joint but are often subject to debris buildup, which can render them ineffective. These types of joints include:
o   butt joints
o   sliding plane joints
o   finger joints
Closed joints –Â prevent debris and water from penetrating but must be designed specifically for the size and movement properties of the intended joint. Â These include the following types of joints:
o   poured joints
o   asphalt plug joints
o   compression seals
o   strip seals
o   reinforced elastomeric joints
o   modular elastomeric joints
Each of these joint types has a specific application method and set of parameters which must be followed to ensure the joint’s effectiveness. Proper sizing of the joint, along with an accurate assessment of the anticipated movement of the structure, must be considered when selecting the proper joint type for the intended project. Some other things to consider are substrate preparation, climatic factors, lane closure requirements, manufacturer requirements, physical properties and limitations of the joint material.
Many of the roads and bridges in the US have expansion joints which are structurally deficient. The main causes of failure range from tearing, separation of the seal from the substrate, snowplow damage, concrete deterioration, debris accumulation and improper alignment of deck sections. The majority of these failures can be attributed to improper installation or inadequate maintenance practices.
