The large amount of ettringite detected in concrete affected by different failure mechanisms (alkali -silica reaction [ASR], freezing and thawing [FT], and wetting and drying [WD]) in sulfate -free environments has been a matter of concern for many researchers over the past years. Two major lines to approach this issue have been defined. One of them considers ettringite as the main cause of damage and the other assumes that it appears after concrete distress. In order to contribute to the clarification of this phenomenon, research was conducted to cover the above issues. This paper reports on the conclusions arrived at up to date, from studies conducted on concrete affected by different damaging mechanisms: FT, WD, and precracking induced by loading and ASR. Changes in length and mechanical strength of test concrete are reported, as well as the evaluations made by optical microscopy (OM), scanning electron microscopy (SEM) and energy dispersive X -ray analysis (EDAX). Secondary ettringite formation (SEF) is closely linked to pore fluid transport in the mass of concrete. Although it was not possible to define clearly its participation in the damaging processes related to ASR or to FT, secondary ettringite crystallization in concretes subjected to WD cycles seems to be one of the most likely mechanisms to account for the volumetric instability observed in these concretes. D

Three dolomitic rocks from Argentina were studied under different test conditions (ASTM C 586, ASTM C 227, CSA A23.2-14A, powdered rock samples immersed in strong alkali Solutions). X-ray diffraction (XRD), optical microscopy (OM) and scanning electrón microscopy (SEM) analyses of the various reaction producís detected under these conditions are presented. Results are compared with the alkali expansivity of the dolomitic rocks and are also used to discuss the scope and applicability of the different reaction mechanisms proposed in the literature. It has been concluded ffom this research that the mechanism originally proposed by Hadley is adequate to explain the reaction mechanisms that opérate in concrete structures under natural Service conditions.

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This paper presents an experimental study on the effects of alkali-silica reaction (ASR) in concrete blocks placed outdoors, exposed to weather conditions. To promote different reaction kinetics and damage levels, the size of the reactive aggregates, the alkali content and incorporation of different fibre types (steel, polymer) were the variables considered. Expansions, crack patterns and air permeability were monitored for more than 3 years. In addition, standard expansion, compression and flexion tests were performed. The volume of concrete involved in ASR enhances the development of expansions and cracking; in the blocks different expansions were measured in vertical or horizontal directions and important cracks were observed. While the prisms expansions were attenuated after the first 12 months, in the blocks the damage by ASR continues growing for more than 3 years. Although the incorporation of macrofibers does not inhibit the development of ASR, it led to benefits such as reduction in cracking and deformations. Keywords Alkali-aggregate reaction Á Concrete Á Degradation Á Fibre reinforcement Á Mechanical properties Á Microcracking Abbreviations ASR Alkali-silica reaction CMOD Crack mouth opening displacement E Modulus of elasticity FRC Fibre reinforced concrete f c Cylinder compressive concrete strength f L Limit of proportionality f R1 Residual flexural tensile strength corresponding to CMOD = 0.5 mm f R3 Residual flexural tensile strength corresponding to CMOD = 2.5 mm 1 Introduction The study of alkali-silica reaction (ASR) has great interest around the world as it is one of the important