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Concrete In Australia : December 2008
TECHNICAL Test methods for assessing durability ARRB Conference in Adelaide 30 July-1August 2008 The simple VPV (volume of permeable voids) test method AS 1012.211,2,3,4,5 (previously ASTM C642)4 has demonstrated its ability to rank the durability performance of various concretes, by its sensitivity to such major influencing factors as water/cementitious material (W/C) ratio, water content, type and adequacy of vibration and compaction, cementitious material type and content, concrete grade and strength, curing regimes, evaporative moisture losses, shrinkage effects, on-site temperature conditions, the age of the concrete tested and of course the pore system or microstructure of concrete. The method has proven itself to be a very useful tool for both end-performance assessment as required by specifications and quality control in terms of the manufacture of concrete with particular emphasis on the control of the total water in the actual concrete mix. The paper introduces the principles underpinning the VPV durability test method and highlights its advantages over other durability test methods such as chloride diffusion, permeability testing, rapid chloride permeability etc. The report discusses its repeatability and reproducibility (compared to all the other tests which have no proven record) and presents the benefi ts of its introduction into the VicRoads structural concrete specification. Finally the paper recommends that the VPV test method AS 1012.21 is the most appropriate durability test method available to ensure the long-term performance and durability of concrete structures requiring 100 year service life. Physical structure of cement paste The main factor controlling concrete penetrability by aggressive agents is the pore structure of the concrete and in particular the interconnected capillary pores and other void systems within the hardened cement paste matrix1,2 . An understanding of the of cement particles which were originally too large to hydrate completely. The matrix also consists of a system of capillary pores communicating to the surface of the concrete, which represent the volume formerly occupied in the fresh paste by water-fi lled spaces which have not been fi lled by the products of hydration. The volume of capillary pores depends on the W/C ratio and the degree of hydration, and therefore the higher the W/C ratio the greater is the capillary porosity. Incomplete hydration, usually caused by lack of curing, further increases the volume of capillary pores. The capillary porosity in the paste makes concrete of course permeable to both liquids and gases. Within the structure of the hydrated gel itself there are development of the chemical and physical structure of cement paste matrix is therefore warranted. In simple terms the hardened cement paste matrix consists of a continuous gel of CSH which also contains crystals of Ca (OH)2 , other minor components and unhydrated remnants interconnected interstitial voids known as gel pores, which are considerably smaller than the capillary pores. These may also provide a route by which gases and liquids can permeate through concrete. However, because they are so small, their Fred Andrews-Phaedonos, Principal Engineer – Concrete Technology, VicRoads. This paper was fi rst presented at the 23rd contribution to total penetrability of cement paste is very low. Nevertheless, even if capillary pores become discontinuous with low W/C ratios, ordinary concrete can never be completely impermeable due to the gel porosity. The penetrability of concrete is generally several times greater than that of cement paste owing to a third pore system which is provided by air voids and micro cracks, some of which are due to bleeding, humidity and temperature changes and inadequate compaction. Cracks may also form at the paste-aggregate interface due to the restraining effect of the aggregate on cement paste shrinkage. Although, some of the air voids may be distinct cavities isolated from one another, many others may be connected to one another by the gel pore system. Certain other voids, particularly cracks, may be oriented so as to provide a continuous network through the concrete, thereby significantly increasing penetrability. The above is a very simple account of the chemical and physical structure of the cement paste. However, it is quite clear that the various pore systems present in concrete (ie. capillary pores, gel pores, air voids and micro cracks) if interconnected, may represent the volume of permeable voids (VPV) as measured by the durability test method AS 1012.21. Effectively the normal water immersion associated with AS 1012.21 measures the capillary and gel pore volume and the boiling part of the method measures the volume of the third pore system provided by air voids and micro cracks. Transport mechanisms in concrete Deterioration mechanisms which adversely affect the long term durability of reinforced and prestressed concrete bridge and other structures are generally associated with the ingress of various liquid or gaseous aggressive agents1,2 . It is widely accepted that all deterioration mechanisms affecting concrete are either infl uenced or promoted by the availability and transport of water through the permeable voids (interconnected voids) of the pore structure of concrete. Water provides the medium by which many aggressive agents (ie. chlorides, sulphates etc) are transported into concrete. It also affects the initiation and rate of some deterioration processes such as carbonation. Deterioration processes such as corrosion of the steel reinforcement, chloride ingress, alkali/aggregate reactions, sulphate and chemical attack can be controlled by restricting the movement of moisture in the concrete (ie. by controlling penetrability). For many structures the primary transport mechanism is considered to be absorption of surface water due to capillary suction, particularly in their early life. Current objectives are to achieve low penetrability by modifying/refi ning the pore structure of the concrete (ie. in terms of fi neness, tortuosity, discontinuity etc) and therefore minimising the volume of permeable voids (VPV) 1,2,3,4,5 . This is generally achieved by the use of supplementary cementitious materials (SCMs) and other pore modifying admixtures which are capable of improving the impermeability of concrete. Concrete in Australia Vol 34 No 4 25