Joints in Concrete

JOINTS IN CONCRETE

Joints in concrete are normally used to prevent cracks when the concrete shrinks by creating forming, tooling, sawing, and placing joint formers. Concrete joints are used to compensate when concrete expands or shrinks with changes in temperature. They prevent cracking of concrete. Types of joints in concrete are described below:

TYPES OF JOINTS

Contraction Joints

A contraction joint is a sawed, formed, or tooled groove in a concrete slab that creates a weakened vertical plane. It regulates the location of the cracking caused by dimensional changes in the slab. Unregulated cracks can grow and result in an unacceptably rough surface as well as water infiltration into the base, subbase and subgrade, which can enable other types of pavement distress. Contraction joints are the most common type of joint in concrete pavements, thus the generic term "joint" generally refers to a contraction joint. Contraction joints are chiefly defined by their spacing and their method of load transfer. They are generally between 1/4 - 1/3 the depth of the slab and typically spaced every 3.1- 15 m. Contraction joints are provided to allow for shrinkage movement in the structure. The contraction joint may be

❖ Complete Contraction Joint

❖ Partial Contraction Joint

❖ Dummy Contraction Joint

Construction Joints

Construction joints key the two edges of the slab together either to provide transfer of loads or to help prevent curling or warping of the two adjacent edges. These are needed to accommodate the construction sequence for placing the concrete. The amount of concrete that can be placed at one time is governed by batching and mixing capacity, crew size, and the amount of time available. Correctly located and properly executed construction joints provide limits for successive concrete placements, without adversely affecting the structure. They must be designed in order to allow displacements between both sides of the slab but, at the same time, they have to transfer flexural stresses produced in the slab by external loads. Construction joints must allow horizontal displacement right-angled to the joint surface that is normally caused by thermal and shrinkage movement. At the same time they must not allow vertical or rotational displacements.

Expansion Joint

The expansion joint depends upon the context of the change of temperature. These joints are provided when the length of the building is greater than 30m and temperature changes by 50°C then a 10 mm expansion joint is provided. The extent of the structure depends on the coefficient of linear expansion of the material. An expansion joint should be provided at the point where the structure changes its direction. The materials used in expansion joints are:

❖ Joint Filler: Bitumen containing cellular material, rubber, and thermocouple expanded plastic, mineral fiber, glass wool. The joint Filler should be compressible material tightly fitted in the gap. It should regain 75% of its original thickness when external pressure is removed from it.

❖ Sealing Compound: Its function is to seal, the joint against the passage of moisture and to prevent the ingress of dust grit, or other matter to the joint. Mastic or hot bitumen and silicon are mostly used as sealing compounds.

❖ Water Bar: The function of the water bar is to seal the joint against the passage of water. The water bar may be made up of rubber, GI sheet, Copper, and aluminum.