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CARBONATION & CHLORIDES IN CONCRETE

1  Introduction Concrete structures are designed for both strength and durability, with modern day designs it is expected that the structure should last for of 50 years of service life. In the past several years the durability of concrete structures has been affected by corrosion of reinforcement steel inside it. Carbonation and chlorides are the two main reasons for low quality durability in concrete. The combination of both carbonation and chlorides has led to expensive repairs as well as shorter life of structures. Major volume of concrete produced is used in commercial, residential and industrial structures, given the type of environmental conditions they are exposed to; these structures are exposed to high risk of carbonation and chloride induced reinforcement corrosion. It should therefore be taken into account that the potential service life of many structures is governed by carbonation-induced corrosion

In United Kingdom the common causes of corrosion in concrete are de-icing salts and corrosion due to insufficient cover at the covercrete zone. Carbon dioxide ingress in to the cracks of concrete and in the presence of moisture reacts with calcium hydroxide and neutralizes it. The loss of passivation layer over reinforcements leads to corrosion of steel. Unlike carbonation in case of chlorides since the aggregates are dense with no interconnected pores it is always assumed that ingress of external chloride iron is depended on quality of cement paste fraction of concrete. 2  Research on Carbonation and Chlorides Carbon dioxide is among the major gases present in the atmosphere and it is present in the range of 0.03% (BRE, 1995). With globalization and mass industrialization the level of CO2 is expected in the range of 600 to 1000 ppm in the next 100 years. Although construction industry is responsible for less than 5% of total CO2 emissions, the major part of emission in  CO2 in construction industry is through production of cement. Throughout quarrying, production, packing and transport of cement accounts for a major part of it. The CO2 in atmosphere is re absorbed during its life cycle by concrete and this leads to the problem of corrosion (Claus, 2007). In chloride research a study by Powers et al(1954) has found that water permeability of aggregate can vary widely with water permeability of mature paste with w/c ratio 0.48 can be between 10 to 0.001 times that of aggregates used in concrete. The volumes of aggregate in normal concretes are in the range of 65 to 75%. It is assumed that the variation in aggregate quality can influence the ingress of chlorides into concrete. In construction industry, sustainability in use of raw materials and reducing environmental impact of concrete are of significial importance. The use of supplementary materials like fly ash, ground granulated blast furnace slag, silica fumes and metakaolin are rapidly increasing. In this project an accelerated test method development for checking the durability properties of concrete in carbonation and chloride exposures are studied.

3  Aim and Objectives To determine whether it is possible to rapidly assess undone or unfamiliar concrete constituent materials in terms of their performance in XD3/XD4 and XS3/XS4 as defined in BN EN 8500-2. The framework by which this aim is to be considered is that the emerging equivalent performance method specification. The scope therefore of the work is confined to these materials defined EN 197 part I and part II and make it necessary to identify the benchmark concrete. The additional premise of the work is that it is possible to group together material of similar characteristics i.e. concrete families. The objective is to analyse the extensive literature arriving from major 6& 7 that has been formed the development of probabilistic for durability and in competition the equivalent probability method. •	To analyse EN 206/8500 and its long time performance data. •	To analyse the historic development of accelerated /rapid durability tests. •	To define an experimental program to define and demonstrate the required matrix which allows families to be assembled. •	To produce a framework of guidelines suitable for specifiers, client engineers and distinguish with which concrete can be selected economically and sustainably in the intended working life design as per BS EN 206.

4  Methodology In general the carbonation penetrates from the exposed surface of the concrete(BRE, 1995) and the process takes place in many years. It generally takes 25 years for carbonated structures to corrode due to carbonation induced corrosion (Parrott, 1987). The long term durability testing of concrete in laboratory can take many years and it delays the durability study on casted concretes. Researchers over the year from different parts of the world have come up with their acceleration carbonation chambers. In this chamber CO2 is passed in a higher concentration to carbonate the test samples at a faster rate. The temperature and relative humidity are the other two factors which play key roles in process of carbonation. Dhir et al, (1995) developed a acceleration tank in Dundee at an CO2 concentration of 4%, RH 50±5%, and 20°C room temperature to study the samples. In the current study mixing of concrete designs at two different water cement ratio for different aggregates and proportions will be carried out

The following tests will be carried out on the aggregate and carbonated concrete samples a)	XRD- Method used to study collaborative with the results obtained from the phenolphthalein test. b)	Infrared Spectroscopy- Method based on studying infra-red spectroscopy to study the bi-products of carbonation. c)	Thermal Analysis- To distinguish between different type of carbonates, calcite and verities. d)	Microscopy- Direct observation of the pronounced double refraction in polarized light of calcium carbonate with microscope.

And finally the objectives of the tests are to determine. •	Testing of concretes for different exposure condition in the acceleration curing tank •	Measuring the depth of carbonation for regular interval of time. •	Life Cycle analysis (LCA) for studying the embodied energy levels in concrete. •	As per BS 8500 arriving on a group of concrete mix intended to meet the durability standards of carbonation. •	Test results will be analysed for recommending or further addition on current method of accelerated carbonation tank.

5  Summary The study is conducted to develop a rapidly assess undone and unfamiliar concretes for exposure classes in XD3/XD4 and XS3/XS4 as per BS EN 206. The test results of trials conducted at laboratory can be used for arriving at a benchmark concrete which exhibits the property to resist both carbonation as well as chlorides. It is also important to identify and group the family of concrete which exhibits similar characteristics. With the test method development it will be easy to design or specify concrete for intended working life of 50 years with the data of concrete families. Hence in future it will be easy for specifier, client engineers to distinguish which concrete can be selected for a given class of exposure.

BUILDING RESEARCH ESTABLISHMENT, May 1995. BRE Digest, Carbonation of Concrete and its effect on durability, Digest 405

DHIR R K, JONES, M R and MCCARTHY, M J, 1992. Pulverized-fuel ash concrete: Carbonation induced reinforcement corrosion rates, Proceedings of the Institute of Civil Engineers Structures and Building, Volume 94, pp 335-342

PARROTT L J, July 1987. A review of carbonation in reinforced concrete. C & CA/BRE  Report C/1-0987. British Cement Association

BS 8500-1: 2006. Concrete – Complimentary British standard to BS EN 206-1

Hobbs, D.W., Aggregate influence on chloride ions diffusion into concrete. Cement and Concrete Research, Vol 29, pp 1995-1998,1999 Powers, T.C., Copeland J.C and Mann., M.N., Permeability of Portland cement paste, Proceeding ACI 51, pp. 285-298, 1954 /ref>