Testing of Concrete in Structures: Fourth Edition

Chemical testing and allied techniques 2899.11.1.3 Reliability and applicationsIt is claimed that this method can measure the cement content to ±10%, andthe total aggregate content and coarse/fine aggregate ratio can be assessedsimilarly. However, the total water content cannot be assessed, since waterand air voids cannot be distinguished, except for entrained air voids whichwill be identifiable by their spherical nature and uniformity of size. In situationswhere cement content cannot be determined chemically this approachmay be valuable. Details of precision experiments are given by the ConcreteSociety (372).This method has also become accepted for measurement of air entrainment.The choice between point count or linear traverse techniques willdepend on circumstances, but Markestad (400) has indicated problemsof component identification leading to uncertainties with the point countapproach. Whichever method is used, the procedures are tedious andrequire specialized equipment and skill in both sample preparation andmeasurement. Modern electronic aids have eased the burden of data collation,and automatic image-analysing microscopes will probably be morewidely used.9.11.2 Thin-section methodsApplications of microscopic examination of thin sections have been outlinedin earlier sections of this chapter, and include identification of mix components(372), carbonation and causes of deterioration. Considerable growthin usage of this approach has been experienced in connection with alkali–silica reaction where the method is invaluable in confirming that reactionhas occurred, examining the size and extent of cracks, and identifying reactiveaggregate particles as illustrated in Figure 9.7. When diagnosing ASR asa contributory or sole cause of damage, it is essential that cracks radiatingfrom a reactive particle, with gel present, are observed (the so-called ‘sitesof expansive reaction’). The mere presence of gel is not conclusive evidenceof expansive ASR.Sample preparation involves cutting a slice of concrete from a core bydiamond saw (preceded if necessary by vacuum resin impregnation), dryingand impregnation by low viscosity epoxy resin. This will then be cut andground using standard petrographic procedures to a 30 m thickness usingoil lubrication to avoid the dissolution of water-soluble materials. Detailedprocedures have been described by Poulsen (401), and are summarized byPalmer (33).Samples will typically be examined with a petrographic microscope underordinary and polarized light. Micrographs may be produced for recordpurposes and to illustrate interpretation, which is highly specialized.