Joints in concrete

Joints are easy to maintain and are less detrimental than uncontrolled or uneven cracks. Concrete expands & shrinks with variations in moisture and temp. The overall affinity is to shrink and this can cause cracking at an early age. Uneven cracks are unpleasant and difficult to maintain but usually do not affect the integrity of concrete.
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1. Joints in concrete structures PRESENTED BY AGLAIA

2. Introduction Joints in concrete construction are construction, expansion, contraction and isolation joints. These joints are placed in concrete slabs and pavements at regular intervals to prevent development of cracks in concrete. Types of joints in concrete constructions are: Construction Joints Expansion Joints Contraction Joints Isolation Joints

3. Construction Joints They are placed in a concrete slab to define the extent of the individual placements, generally in conformity with a pre determined joint layout. They must be designed in order to allow displacements between both sides of the slab. They have to transfer flexural stresses produced in the slab by external loads. They 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.

4. Types of Construction Joints in Concrete Structures

5.  Expansion joints They are placed in concrete to prevent expansive cracks formed due to temperature change. Concrete undergoes expansion due to high temperature when in a confined boundary which leads to cracks. Expansion joints are provided in slabs, pavements, buildings, bridges, sidewalks, railway tracks, piping systems, ships, and other structures. This article emphases on need of expansion joint in concrete, characteristics of expansion joints, types of expansion joint and installation of expansion joints.

6. Cracks developed due to expansion of concrete.

7. Need of Expansion Joint in Concrete C oncrete moves during expansion and shrinkage, due to which the structural elements shift slightly. To prevent harmful effects due to concrete movement, several expansion joints are incorporated in concrete construction, including foundations, walls, roof expansion joints, and paving slabs. These joints need to be carefully designed, located, and installed . If a slab is positioned continuously on surfaces exceeding one face, an expansion joint will be necessary to reduce stresses. Concrete sealer may be used for the filling of gaps produced by cracks.

8. Characteristics of Expansion Joints They permits thermal contraction and expansion without inducing stresses into the elements. It is designed to absorb safely the expansion and contraction of several construction materials, absorb vibrations, and permit soil movements due to earthquakes or ground settlement. They are normally located between sections of bridges, paving slabs, railway tracks, and piping systems. They are incorporated to endure the stresses. It is simply a disconnection between segments of the same materials. In the concrete block construction, they are expressed as control joints.

9. Types of Expansion Joint Bridge Expansion Joint Masonry Expansion Joint Railway Expansion Joint Pipe Expansion Joints

10. Bridge Expansion Joint They are designed to allow for continuous traffic between structures while accommodating movement, shrinkage, and temperature variations on reinforced and prestressed concrete, composite, and steel structures. Expansion joint in bridges

11. Masonry Expansion Joint Clay bricks expand as they absorb heat and moisture. This places compression stress on the bricks and mortar, encouraging bulging or flaking . A joint replacing mortar with elastomeric sealant will absorb the compressive forces without damage.

12. Railway Expansion Joints if the track is laid on a bridge having expansion joint, providing a expansion joint in the track becomes mandatory to mitigate the expansion in base concrete structure. Fig 3: Expansion joint in railway tracks.

13. Pipe Expansion Joints Pipe expansion joints are necessary in systems that convey high temperature substances such as steam or exhaust gases, or to absorb movement and vibration.

14. Based on the type of material used in making of joint, expansion joints are further classified into following types,  Rubber expansion joint Fabric expansion joint Metal expansion joint Toroidal expansion joint Gimbal expansion joint Universal expansion joint In-line expansion joint Refractory lined expansion joint

15. Use of filler material in expansion joint

16. Installation of Expansion Joints The depth of an expansion joint is usually one fourth of the slab thickness, or more if necessary. The expansion joint gap depends on the type of slab, like floating slab floor, vehicle pavement, sidewalk, or monolithic slab foundation. It is also influenced by the slab dimensions, type of concrete, and the reinforcing materials being used. Cracks in concrete may occur at the expansion joints due to improper concrete mix or curing. These conditions cause shrinkage between the expansion joints and cracks can be formed.

17. Pre-Concrete Installation When the site is prepared for the concrete pouring and the provisioning of the expansion joints in slabs are made prior to the placing of concrete. An individual expansion joint is created by the insertion of a flexible material that runs along the joint length. After Concrete Installation Once the concrete is set, suitable tools are used for making grooves in the poured concrete for placing of the joint materials.

18. Contraction Joints Contraction joints in concrete are provided at regular interval to from a weak plane, so that cracks are formed at the joints but not in undesired places. Contraction joints are provided in concrete pavements, slabs, walls, floors, dams, canal linings, bridge, retaining walls etc. When concrete is placed, due to shrinkage, creep and thermal movement concrete tends to reduce in size due to which small cracks are formed in the concrete at weak zone.

19. Cracks formed due to shrinkage of concrete.

20. Need of Contraction joint in Concrete Concrete tends to shrink or reduce in size when it starts hardening. This shrinkage of concrete creates tensile stresses in the concrete which develops the minute cracks at the weak plane. These cracks are restricted and prevent the formation of large cracks due to the presence of reinforcement in the concrete. But if its unreinforced concrete, the small cracks tends to develop into a large cracks at irregular interval To prevent such cracks, contraction joints must be installed at appropriate intervals. It is also recommended to install these joints in reinforced concrete too.

21. Fig 2 : Forming of vertical contraction joint Fig 3: Sawed contraction joint.

22. Fig 4: Finishing of contraction joint.

23. Isolation Joints The joints provided to accommodate the expansion of adjacent parts in a building are known as expansion joints. They allow movement to occur between a concrete slab and adjoining columns and walls of a building. They are provided to separate new concrete from existing or adjacent construction, which might expand and contract differently or experience different soil settlement or other movement. If the fresh concrete were not separated from these elements by an isolation joint, a crack could form where the two meet. The should be 1/4 in. to 1/2 in. wide, and filled with a molded fiber, cork, or rubber strip that is set 1/4 in. below the surface.

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