Title : TEXTURAL AND MICROSTRUCTURAL FEATURES AND ASSESSMENT OF REACTIVE CONCRETE AGGREGATES IN THE INDIAN SUBCONTINENT
Authors : Ajoy K.Mullick
Publication : 5-9 December 2010
Volume :
Pages :
Price : 250
Abstract : Distress to concrete hydraulic structures in service due to alkali silica reaction (ASR) has been reported from India, Pakistan and apprehended in Bhutan. It had taken the engineers by surprise, because the aggregates did not contain metastable silica minerals and had satisfied the conventional ASTM C227 and ASTM C289 tests prevalent at the time of construction. Contemporary description of such aggregates would be ‘slowly reactive’, which owe their reactivity to the defects in the crystal lattice of the quartz grains showing strain effect, and other mineralogical and textural features of crystalline rocks. It was necessary to evolve appropriate test procedure to detect the potential reactivity of ‘slowly reactive’ aggregate. This paper describes the textural and microstructural features of a large number of aggregate samples similar to those used earlier in the constructions exhibiting ASR, or for new constructions, assessed to be reactive by modified test procedures. Modifications in the test methods in Indian Standards (IS: 2386), to assess such ‘slowly reactive’ aggregate are described. The assessment protocol established involves petrographic tests including undulatory extinction (UE) angle, and accelerated mortar bar tests at 80 OC in NaOH solution (ASTM C1260 or NBRI test). This is further followed up by mortar bar tests at 60 OC. If the aggregates are judged to be potentially reactive, appropriate concrete mix design is verified in concrete prism tests. No instances of ASR have been reported in the new constructions following this procedure. Results of petrographic tests and various mortar bar tests presented in the paper will provide guidance for new constructions.