JOINTS IN CONCRETE STRUCTURES

INTRODUCTION

In nowadays, reinforced concrete is the most popular construction material, we have  larger reinforced concrete structures spanning over a larger area such as Dams, retaining walls, rigid pavements, concrete channels and buildings; their construction methods involves the use of different kinds of joints so they can function effectively over its designed life.

Concrete joints, these are discontinuities that are made within concrete structures or between consecutive concrete structures or structural members so as to allow horizontal movement.

THE NEED OF JOINTS

Concrete material has one great weakness that is its low tensile capacity, due to its weakness concrete is subjected to cracking where by some of them are unavoidable, so crack control in reinforced concrete structures is needed for two reasons.

1. Aesthetic of concrete structure

Structures such as building their aesthetic value is the one of the important aspect during its design and construction, cracks formed on major framing elements such as columns and walls leads to rise of questions on structural adequacy of structure.

The formed cracks may have affection to structural performance of structure or not, but people with no knowledge concerning structural design will only think that the building is inadequate for use.

2. Air and moisture ingress

When cracks are formed on concrete surfaces with substantial width invites the ingress of air and water into framework of the structure which have deleterious effects to the structure.

TYPES OF JOINTS

During construction of larger reinforced concrete different types of joints are provided at adequate spacing and locations; so as to make structure function as intended, the following are most popular types of joints which can be seen in different structures.  

1. Construction Joint

With exception to very small concrete structure, it is impractical to cast concrete continuously on consecutive concrete panels or masses, construction joint is required for sequential placement of concrete  during construction.

These are provided when a new section of concrete is poured adjacent to another concrete section that has already set; the purpose of a construction joint is to allow some horizontal displacements that are caused by thermal and shrinkage, while against rotational and vertical movement.

Characteristics of good construction joint

a). Provide a water tight surface which does not allow ingress of water through the joint.

b). Allows for flexural and shear continuity through the joint (without continuity weak region result).

2. Expansion Joint

In our environment always temperature changes with time and seasons, changes in temperature lead to volume changes of different materials with accordance to their coefficient of expansivity.

Volume change of concrete induces stresses in reinforced concrete structures that are restrained, no stresses are developed in unrestrained structure, and the induced stresses vary with the magnitude of temperature change.

Larger temperature change can result to substantial stresses that must be counted for in design, while low temperature changes may result in negligible stress.

Characteristics of good expansion joint

a). Should be wider enough to prevent two potions at joint to come in contact, when they subjected to expected maximum temperature.

b). Should permit expansion and contraction of two separate segments of concrete structure without affecting structure integrity and serviceability.

3. Contraction Joint

Drying, shrinkage and contraction (due to decrease in temperature) induces tensile stresses in restrained concrete structure, crack will be formed when tensile capacity of concrete is exceeded.

A contraction joint is the sawed or tooled groove in a concrete slab that creates a weakened region; the weakened region is created by reducing concrete cross sectional area and reinforcements.

The concrete cross section should be reduced to a minimum of 25%, to ensure the section is weak enough to form a crack, for reinforcement thy can be provided for about 50% or 0%.

Contraction joint regulates the location of cracks; unregulated cracks can grow and result to unaccepted rough surface as well as water infiltration into the base, sub-base or sub-grade which lead to other types of distresses.

 Characteristics of good expansion joint

a). Have lower strength when compared to other sections.

b). Have reduced cross section area and reinforcement.

CONCLUSION

For large concrete structures to perform better their functions per designed life, different kind of joints which are construction joint, expansion joints and contraction joint should be provided at adequate spacing and location.