ABSTRACT

This study addresses the resistance to chloride ion penetration in the prestressed concrete large diameter cylinder piles exposed to the severe marine splash/tidal zone. Time to initiation of corrosion was predicted by using the Fick's 2 "d law for chloride penetration. The effects of water to binder ratio (w/b), mineral admixtures, and cover thickness were evaluated by the comparative studies, based on one-dimensional direct calculation, two-dimensional analysis using a computer software application, and case studies. The influences of environmental characteristics, manufacturing process, and local experiences were taken into account. The findings include i) tile remarkable durability enhancement for the utilization of low w/b high strength concrete mixture, compared with the increase of cover thickness, ii) the ineffectiveness of fly ash inclusion; and iii) the importance of selection of manufacturing process, structural design, and construction procedure.

INTRODUCTION

Prestressed concrete large diameter cylinder piles have been widely used to support many different types of marine concrete structures .(piers, port terminals, wharves, and bridges etc.) throughout the world with substantial savings in cost and construction time. The piles are capable of providing the large lateral and vertical strengths to resist the seismic loads even in the soft deep marine clay zone. Utilization of the large diameter cylinder piles eliminates the need for any batter piles, thus resulting in a structure that is both simple to build and cost effective.

In many cases, the piles are exposed to the severe marine splash/tidal zone and should be durable against corrosion and deterioration. Utilization of low w/b high strength concrete mixture is common for the marine piles. Compressive strength is generally higher than 48 MPa (7,000 psi) with low water w/b (sometimes less than 0.3) accompanying low chloride permeability. However, experience has shown poor performance of the piles in several different n larine environments. The lack of durability can ~very often be attributed to the factors such as mixture design, cover depth, exposure conditions, manufacturing process, driving condition, and structural design details etc.

Chloride-induced corrosion damage is the most common durability problem associated with the marine piles. Time to corrosion initiation is determined by how rapidly the reinforcement is depassivated as a result of the fact that chloride ions penetrate to the concrete cover, and by the concentration which is required for the start of the corrosion process. The penetration sequence is normally described as a diffusion process. In practice, the transport is not always quite so clear-cut but is rather combination of capillary suction and diffusion. One example of this is the fact that partly dried-out concrete absorbs a chloride solution through capillary action. On the other hand such rapid chloride penetration is not acceptable if we are expecting a service life longer than 30 years, [1]. Therefore, the time dependent diffusion model is applicable for high quality marine concrete piles.

While the chloride ions are considered to be a major cause of corrosion, other causes include carbonation of concrete cover, erroneous specification, and improper construction practices. Premature crack initiation can lead to severe corrosion damage allowing rapid chloride penetration and reduce the service life of the pile supported structure. Therefore, it is essential to get rid of possible causes of the premature cracking during manufacturing, transporting, and driving. Manufacturing processes of the cylinder piles are quite different from the simple process of typical solid one.

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