Joints are formed in concrete slabs as part of the process of constructing rigid paving for roads. Joints are discontinuities in the pavement slab that are necessary to allow for expansion, contraction and warping. Rigid paving consists of a reinforced or unreinforced insitu concrete slab laid over a thin granular base course. The rigidity and strength of the pavement enables the loads and stresses to be distributed over a wide area of the subgrade.
Joints are spaced depending on a range of factors:
- The amount of reinforcement used.
- The proposed traffic intensity.
- The slab thickness.
- The frictional restraint of the subgrade.
- The temperature at which the concrete is laid.
Joints comprise a filler which separates the slabs, and a sealing compound which is used to fill the top 25 mm of the joint to prevent the entry of water and grit. Suitable jointing materials include impregnated fibre board, cork, sheet bitumen, and rubber. The joint sealing compound must have good adhesion to concrete, extensibility without fracture, resistance to flow in hot weather, and durability.
A system of dowel bars in introduced between slabs to prevent slab movement and ensure load-transfer. Dowel bars are positioned at mid-depth of the slab at centres of 300 mm. The diameter of the bar usually ranges from 20-30 mm but varies with the slab thickness. A plastic sleeve 100 mm-long is inserted on one end of the dowel to allow free movement of the slab. The sleeve should contain a pad of compressible material at the end.
There are a number of different types of joints:
These are provided along the transverse direction to allow the expansion and contraction of a concrete slab due to temperature and subgrade moisture variation. They are intended to prevent potentially damaging forces accumulating within the slab itself or surrounding structures. Maximum spacing of expansion joints range from 25-27 m in jointed reinforced concrete slabs, and from 40 m (for slabs <230 mm thick) to 60 m (for slabs >230 mm thick) in unreinforced concrete.
These are also known as ‘shrinkage’ joints and are provided along the transverse direction to allow for contraction or shrinkage of the slab during the curing process. Maximum spacing of contraction joints ranges from 12-24 m in reinforced slabs, and from 4-5 m in unreinforced slabs.
Construction joints are provided whenever the construction work stops temporarily. They can be either along the transverse or longitudinal direction.
 Related articles on Designing Buildings Wiki:
- Bituminous mixing and laying plant.
- Code of Practice for Ironwork Systems Installation and Refurbishment.
- Ground conditions.
- Highway drainage.
- Infrastructure and Projects Authority.
- Insitu testing of soils.
- Overview of the road development process.
- Road construction.
- Road improvement scheme consultation.
- Runway construction.
- Types of road and street.
 External references
- ‘Introduction to civil engineering construction’ (3rd ed.), HOLMES, R., The College of Estate Management (1995)
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