Fibre-reinforced-polymers versus steel in concrete bridges: Structural design and economic viability

by I Balafas

Steel corrosion in concrete bridges is currently the biggest threat to their durability.  Globally, vast governmental budgets are spent so that bridge stocks remain in an acceptable condition.  To solve the growing problem Fibre Reinforced Polymer (FRP) materials have been proposed as an alternative concrete reinforcement.  Even though extensive research has taken place in the last 20 years, their use has been limited to prototype structures.  The construction industry hesitates to invest in these materials due to their high initial cost.  This thesis looks at their viability from the economic point of view.

A design method was developed which can optimise the use of FRPs in concrete in reinforced and prestressed applications.  The structure was solved for design limits under working and ultimate loads, and the results were plotted on a section depth versus bar area diagram.  A feasible zone was then formed and the optimum solution found by applying well established non-linear optimisation algorithms.  The structure was optimised in terms of flexural as well as shear reinforcement.  The method can be applied for any reinforcing material.

The use of the method revealed that reinforced applications with FRPs were normally governed by deflection and FRP-snapping design constraints.  The prestressed applications on the other hand were governed by tensile stress in the concrete at the working load and FRP-snapping constraints.  The optimum steel initial costs were cheaper than FRPs solutions.  Using FRPs as shear reinforcements in the form of stirrups is not effective and novel forms, which use the FRP properties more efficiently, need to be found.

Life-cycle costs for the structure were also determined.  The bridge lifetime was assumed to consist of two time periods.  The corrosion-initiation period which is the time at which steel starts to corrode, due to its exposure to de-icing salts, and the time-to-cover-cracking period during which active corrosion takes place, with the rust products causing excessive pressure on the cover which finally spalls.

Models were developed to determine those time periods.  In the time-to-corrosion-initiation model chlorides penetrate by diffusion and convection. Chloride binding was also included.  Coupling of humidity and temperature was assumed.  The environment was modelled and introduced into the problem through the boundary conditions.  The problem ends up as a system of differential equations and finite difference methods were employed for their solution.

In the time-to-cover-cracking model the active corrosion and rust production was described as a function of the environmental conditions.  Some of the rust volume fills the concrete pores around the bars, and the rest caused pressure on the cover.  A ring model was used to idealise the structure.  When the rust products pressure was excessive the cracks grew outwards from the bar and eventually reach the surface.

The models show that the corrosion-initiation period was short for high annual average humidity and wide temperature fluctuation between winter and summer.  On the other hand time-to-cover-cracking was short for average annual humidity and high temperature during the year.  Several gaps in the literature were identified and a sensitivity analysis was performed to determine the importance of the various parameters and to guide the future research.

When the bridge is under repair and lane closure conditions take place, user delay costs arise.  Delay costs consist of queue-delay, speed-delay, vehicle-operating and accident costs.  A model was developed to evaluate these and theories from transportation engineering were used.  It was found that user delay costs do not depend on the road size and can be very high for all road types.  Official published figures in the UK are used to determine bridge repair costs.

The results show that the life-cycle costs and in particular user delay costs can be enormous.  Repair costs were normally multiple times lower than the user delay costs.  Thus if a bridge is to be repaired the speed rather than the material cost is of economic importance.  The user delay costs can be so high that even if they occur at a relatively long time from bridge construction, they can justify the much higher initial costs of an FRP-reinforced bridge.

The thesis concludes with the analysis of available USA bridge stock condition data.  Theories on predicting product lifetime were applied to the data to predict the condition of the stock in the future.  The model predicts that a considerable number of bridges have to be repaired in the future and the net present value of those repairs reach very high values.  Important decisions have to be made now to face up to the problem.