Gulf Construction Online - Adding durability to concrete

Concrete Structural Repairs

Corroded structure in need of repair.

Adding durability to concrete

Introduced a decade ago, controlled permeability formwork (CPF) has now become the common and recognised way of achieving improved concrete durability, and has gained acceptance as an effective barrier both on its own and as a part of a barrier system, says Morten Gantriis Sorensen, of Fibertex.

01 April 2004

Corrosion of steel reinforcement bars as opposed to weakening of the actual concrete has long been identified as the reason behind the deterioration of concrete structures.

Although steel reinforcement bars in concrete structures are expected to enable them to endure tensile stresses to which they may be exposed, corrosion of the rebar affects the performance of the entire concrete structure.
Being an expansive process, the product of corrosion – rust – occupies more volume than the steel, resulting in spalling of the concrete cover. New concrete structures remain unaffected by corrosion as long as the ‘film’ protecting the structure - which is created during hydration – is maintained by the alkalinity (freshly poured concrete is normally more than pH12).
However, substances that are able to travel through the concrete – mainly carbon dioxide (carbonation) and chlorides – neutralise the environment, making the protective film unstable and initialising corrosion.
It is the progress of these substances that must be stopped or delayed. This brings the attention to the quality of the concrete cover between the surface of the concrete structure where the corrosion-initiating substances could potentially enter and affect the reinforcement.
It is well known that the denseness, and thereby the durability and strength of the concrete, depends largely on the water/cement ratio (weight of water in the mix divided by the weight of cement). Although less water results in better durability, a certain amount of water is needed to allow concrete to be placed properly. Once placed, the excess water is unnecessary.

Controlled permeability formwork
Because of its ability to drain the outer 20 to 30 mm of concrete, CPF leaves behind a dense, strong and durable concrete cover, which will thereby protect the reinforcement. In short, instead of dealing with the symptoms, CPF helps to improve the quality of the concrete cover. This property of CPF has been proven by bodies such as CIRIA (Construction Industry Research and Information Association) and USACE (US Army Corps of Engineers) and, additionally, by reports from several universities and consulting engineers.
A CPF liner plays a major role in the process. A CPF liner consists of a filter side (to hold back cement particles while letting water to pass) and a drainage layer (to let water away under the formwork, leaving room for new water to be drained). These two basic mechanisms are crucial, but the opening size of the filter, drainage capacity and water retention capacity are also important in achieving maximum durability enhancement. All these values should be included in a specification.
Sometimes CPF liners are regarded – mistakenly – as of use mainly for better aesthetics. Although certainly, CPF improves appearance owing to the elimination of blow holes, this is a beneficial side effect – the air from the surface is removed along with the water, it is important to point out that CPF is mainly recommended, not for aesthetics, but for enhanced durability.

Arabian example
A new pier for an oil refinery in Kuwait – which will operate as a docking space for oil tankers – has been recently completed after almost a year. All casting using CPF has been successfully completed. The CPF liner chosen by the contractor was Formtex, due to its comparatively low price and flexibility of use when considered in relation to conventional straight and flat formwork and was approved by the consultant and client as it has a proven track record and is covered by European Agrément certificates.
A total area of 60,000 sq m is now protected by a dense concrete surface. As CPF was used twice in this project, 30,000 sq m was used. This is one of the largest projects in the world to use CPF.

Cost
Using CPF increases the immediate construction costs. However, a product such as CPF should always be viewed as a long-term investment. The cost advantage will become apparent if the whole-life cost analysis is undertaken – as fewer repairs will be needed and the lifespan is increased. In addition, there are minor savings on other construction materials such as release agents, formwork (both the quality and the damage), and cleaning.

Usage
CPF is applicable to different areas, but is mostly used in aggressive environments where ingress of aggressive elements would be a major problem. The main application areas are: marine structures, bridges and bridge structures, water reservoirs, sewage plants, and underground construction.
The use of CPF has increased dramatically over the past five years. It is now a global technique, accepted as an effective barrier both on its own and as a part of a barrier system. There is now great potential for expanded use of the technique.