Joints in external paving
Tensile and compressive stresses will be induced in concrete by restrained thermal and moisture-related contraction and expansion effects. Thermal changes can result from the heat of hydration of the concrete or from exposure to the environment. Changes in moisture content of the concrete may result in shrinkage or expansion, but shrinkage is predominant. Joints are provided in pavements to reduce the stresses generated by thermal and moisture changes and to allow horizontal movement. As the heat of hydration movements occur in the first few days after construction, joints need to be provided at an early stage – usually within 24 hours of casting.
Joints may be formed during the casting process, or induced by saw cutting green or hardened concrete. Joints are formed in wet concrete at the edges and ends of strips. Occasionally extra joints are required, such as day joints (at the end of a day’s work) and sometimes when work has to be stopped because of bad weather or a breakdown in concrete supply or concreting plant.
Correctly located and detailed joints reduce tensile and compressive stresses, reduce the risk of cracking within bays, accommodate thermal and shrinkage movements, provide load transfer from bay to bay, reduce the effect of the joints on ride quality by minimising the occurrence of stepping and reduce damage to arises/joint edges.
Various types of joints are required, depending on their required function, e.g. whether they are required to accommodate expansion or contraction. Details of the various types are given in Concrete Society Technical Report 66, External in-situ concrete paving, along with guidance on the required spacing, which is a function of the amount of reinforcement provided.