Durability of Concrete Bridges and Roadways
Dr Anil K Kar, Chairman, Engineering Services International, Kolkata
Introduction
Much of the bridges and roadways construction is with concrete: reinforced and prestressed. This preference for concrete as a construction material stems from its strength, versatility, easy formability as well as the easy availability of its constituent materials.
Concrete does have its weakness in resisting tensile forces, flexural tension, shear and torque. Strengthening elements, viz., steel reinforcing bars (rebars), prestressing elements, fibres, etc. are used to overcome the weakness.
Figure 1: Distress in underside of RCC deck slab of bridge (Bankim Setu, an R.O.B. over Howrah Station platforms)Concrete structures, built until the 1960’s, proved to be durable. But this happiness with the performance of concrete bridges, roadways and other structures, in terms of durability is over, as concrete structures, particularly those constructed during the last three or four decades, have been characterized by early decay and distress. In most cases, this distress is caused by corrosion in reinforcing bars (rebars) in the case of reinforced concrete structures (Figure. 1) and corrosion in prestressing elements in the case of priestesses concrete structures.
A survey1-4 by Engineering Services International on concrete bridges and buildings in Kolkata has clearly revealed that the widespread existence of good old structures and neighboring new structures in distressed conditions cannot justify any suggestion that the greater corrosiveness of the environment of recent times is primarily responsible for early distress in recently constructed concrete structures.
Furthermore, the widespread and universal distribution of distressed concrete structures of recent vintage discounts poor workmanship as being the main cause of early distress in such structures.
It was borne out in the survey of public domain structures in Calcutta1-4 that high strength rebars, with surface deformations and limited ductility, could be largely to blame for the early decay and distress in reinforced concrete structures of recent decades. These high strength rebars were introduced about four decades ago. Today virtually all reinforced concrete construction is with such rebars.
And yet, concrete codes in India and elsewhere permit, or even prefer the use of such high strength rebars with surface deformations in the construction of concrete structures.
Over the years, starting about four decades ago, the composition of cement has changed and the period of wet curing of concrete has also gone down from 28 days to 3-14 days or even less. These two features of materials and construction practices also had deleterious effects on the durability of concrete structures of recent construction5,6.
Today’s concrete structures are thus inherently condemned to be much less durable (Figure. 1) than concrete structures of the past. Like the recently constructed Jogeshwari flyover in Mumabi (Figure. 2), the flyover at Thane also had to be shut down for a week for repairs.
