Plenty of Rural Roads are concrete surfaced in Sri Lanka, during the past 12 years. They play a vital role in accessibility of rural community for their economic and social needs. The technical officials responsible for construction and supervision are required to be knowledgeable in, why expansion joints are placed, how they are placed and sealing of those for better performing and long lasting Road pavements.
Those referred to as contraction/control joints are placed in concrete slabs to control cracking of concrete slab randomly. Wet concrete can be molded in to any shape, but when material hardens there is reduction in volume, technically referred to as shrinkage. When shrinkage is restrained by contact with supporting surface, granular fill, adjoining structures or reinforcements the concrete will cracks.While concrete is very strong in compression but weak in tensile strength.Tensile strength is only 8-12% of compressive strength,therefore due to contraction tensile stress develops and cracking of concrete occur.
To control random unplanned cracking of concrete slabs, cracks are induced at the surface at predetermined locations, where concrete can cracks in straight line. This produces aesthetically pleasing appearance since cracks takes place below the concrete surface at pre determined locations. Proper Joint system is to ensure;
- Control cracking
- Divide the pavement into practical construction size
- Accommodate slab movements
- Provide load transfer
The science of determining concrete pavement has been based on theoretical, laboratory, experimental and performance evaluations of pavement already in service. Lack or inadequate joint formation may cause cracks to develop at locations other than those intended.
Formation of Contraction Joints
Sawing is the most common method of creating transvers construction joints. The initial saw cut provides a plain of weakness where cracking will begin. The initial saw cut in hardened concrete should be at least one fourth of the thickness of slab (D/4) and have a min width of 3mm (1/8inch). At times depth of cut is D/3 where hard base is used such as stabilized bases or roller compacted concrete.
The joint must be flushed or blown clean immediately after sawing to keep residue from settling up. There are two types of joints, doweled and un doweled. If dowels are used care must be taken to ensure saw cut is centered over the dowel.
Un doweled contraction joint
Doweled contraction joint
Types of Joints
The four general types of joints are;
- Transvers construction joints
- Transvers contraction joints
- Longitudinal joints
- Isolated and Expansion joints
Transvers construction joints
Transvers construction joints are installed at end of a day long operation or any other interruption.These is usually placedat locations of a plannedjoint.
Transvers contraction joints
Transvers contraction joints are that constructed to transvers direction to the Road centerline and in intervals predetermined to control transverse slab cracking.
Longitudinal Joints are constructed parallel to Road centerline that control cracking and deleanate lane of traffic.
Isolated and expansion joints
They are placed to allow movement of pavement without damaging adjoining pavement
PURPOSES OF THE JOINT SEALING
The main purpose of the joint sealing is to minimize infiltration of water through the surface and incompressible material getting into the pavement joint and restrain freedom to move.
Selection of joint sealant is dependent on expected movement. Generally short joint spacing such as less than 4.5m (15ft) amount of joint opening and closing is small.
There are many acceptable materials for sealing joints in concrete pavements. Sealants are mostly classified as Liquids (field molded) and preformed (Compression).
– Liquid sealant may be hot or cold poured, single or two components, and self leveling or too label. They assume the shape of the sealant reservoir and depend on long term adhesion to joint face for successful sealing.
– Preformed sealants are shaped during manufacture and depend on long term elastic properties.
Prior to sealing the joint opening should be thoroughly cleaned of foreign material. Joint face cleanliness directly affects the adherence of sealant to the concrete. Therefore, proper cleaning is essential in order to obtain a joint surface that will not impair bond or adhesion with sealant. Cleaning can be done by sand blasting, water, compressed air, wire brushing or number of other ways.
For liquid sealants the surface should be dry and sealant should not be placed during cold weather. Workman ship should ensure to avoid spilling to top exposed surface of concrete. There are hot pouring sealant as well as cold pour sealants. The full description of sealants is given in following table.The selection of material will depend on scale of work and design. For Small roads constructed in Sri Lanka the first category is suitable.