Multi-storey Car Park Repair and


Multi-storey car parks are essential items of infrastructure that are often neglected but continue to be the backbone of everyday life in most towns and cities around the world. In the UK the number of multi-storey car parks increased dramatically after the Second World War, accommodating the growing demand from the public and the need for rejuvenation. Poor construction and the lack of knowledge for corrosion prevention resulted in a stock of structures that have aged poorly. The following looks at a holistic approach for their evaluation and remediation using galvanic corrosion control and waterproofing coatings.


INTRODUCTION


Many of the towns and cities in the United Kingdom (UK) were heavily developed in the 1960’s, following the need for modernisation and the replacement of buildings impacted during the Second World War. Such modernisation and change in times brought with it an increase in demand for shops and other commercial opportunities, as people at all social levels had greater levels of disposable income. Such change coupled with an increase in the use of cars as a preferred mode of transport, led to an increased need for public parking and in particular, multi-storey car parks.


It has been estimated that there are more than 4000 multi-storey car parks in operation in the UK, with many being unappreciated, but essential items of infrastructure, where closure for expensive routine repairs can create chaos in the surrounding areas. Often these structures are located adjacent to, or attached to, shopping and business centres. For this reason, there has been a general increase in the use of often-superficial schemes to increase their attractiveness but which often ignore the underlying issues that can negatively impact structural stability and maintenance.


STRUCTURE CHARACTERISTICS


Many of these structures were originally designed and built to standards that failed to recognise the harsh environmental pressures that would impact upon them over their life. The majority of these structures were made from reinforced concrete, which at the time was generally considered to be a stable medium. This lack of understanding led to common design features (poor quality concrete, poorly designed construction and movement joints and low levels of concrete cover), that have resulted in extensive and repetitive maintenance requirements over their life. (1) The photographs below are common sights of deterioration on multi-storey car parks in the UK today.


Figure 1: Typical examples of Car Park Deterioration


Maintenance A holistic approach for Corrosion Control


Chloride induced corrosion is now considered the most significant cause of reinforced concrete deterioration globally. It is important to recognise that it wasn’t until 1977 (2) that the use of cast-in chloride as an accelerating admixture was restricted in the UK. The principle source of chloride coming into contact with multi-storey car parks however is in the form of de-icing salts and airborne chlorides in and around the marine environment. The commonly encountered design features above, all increase the probability of chloride reaching the reinforcement steel and it is largely the chloride content in the vicinity (2) of the reinforcement that controls the risk of corrosion. In temperature climates where de-icing salts are used, chlorides tend to build along trafficked areas and at wheel positions in parking bays. These areas of high contamination are often most common on the first two levels of a car park, where the entrance is located and through-traffic is continuous.


EVALUATION OF STRUCTURES


Over the past 10 years, testing has become a standard process in the evaluation of all concrete structures. While the need for testing is now better understood, the degree and level of testing is often inadequate to fully determine the extent of deterioration due to chloride-induced corrosion. With more and more engineering degrees removing corrosion modules from their syllabuses, concrete testing from an engineer’s point of view becomes part of the course but is rarely adequate for the determination of corrosion risk.


With the economic downturn in full effect and with ever-greater budget constraints felt in both the public and private sectors, best value for money has never been so poignant a phrase. It is therefore imperative that corrosion and deterioration mechanisms are fully understood and appropriate methods for remediation implemented. Corrosion testing provides important information to allow the owner and/or their consultant to assess the extent and magnitude of existing corrosion, the risk of future corrosion, and design an economical corrosion control approach.


When it comes to corrosion testing for concrete, BRE Digest 444 part 2 is a good summary. General testing reports often denote chloride levels with depth, carbonation depth, concrete cover over the reinforcing, concrete delamination, and half-cell corrosion potential testing. All of these are vital tests and their general use on structures can be implemented more frequently and in depth.


Often, test reports only identify three or four select zones for half-cell corrosion potential mapping over a car park. With a typical active concrete surface area of 8,000-12,000m2, such select test areas represent 1-2% of the total. While advanced corrosion is easily identified and accounted for via the detection of concrete spalling and selamination, other potential corrosion sites remain invisible to the naked eye. It is therefore ‘potluck’ with sample testing to identify hidden vulnerabilities.


While testing is often seen as a ‘black hole’ for money and is usually an upfront cost, its use can be considered an investment to economise the repair and corrosion control design. For example, if a limited testing regime (as defined above - 1-2% of active area) indicates a high corrosion risk, and the same corrosion rick is assumed for the entire structure, then a global remedial measure such as impressed current cathodic protection might be deemed necessary.


However, in practice, structures rarely experience corrosion uniformly (fig 2) and a full-scale impressed current cathodic protection approach could be unnecessary to achieve the owner objectives for durability and service life. Through an enhanced testing programme, the asset owner can significantly reduce the need for these items, saving potentially hundreds of thousands of pounds in future access, closure, treatment and reduced revenue.


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