Abstract
While on site measurement of air permeability provides a useful approach for assessing the likely long term durability of concrete structures, no existing test method is capable of effectively determining the relative permeability of high performance concrete (HPC). Lack of instrument sensitivity and the influence of concrete moisture are proposed as two key reasons for this phenomenon. With limited systematic research carried out in this area to date, the aim if this study was to investigate the influence of instrument sensitivity and moisture condition on air permeability measurements for both normal concrete and HPC.To achieve a range of moisture conditions, samples were dried initially for between one and 5weeks and then sealed in polythene sheeting and stored in an oven at 50°C to internally distribute moisture evenly. Moisture distribution was determined throughout using relative humidity probe and electrical resistance measurements. Concrete air permeability was subsequently measured using standardised air permeability (Autoclam) and water penetration (BS EN: 12390-8) tests to assess differences between the HPCs tested in this study.It was found that for both normal and high performance concrete, the influence of moisture on Autoclam air permeability results could be eliminated by pre-drying (50±1°C, RH 35%) specimens for 3weeks. While drying for 5weeks alone was found not to result in uniform internal moisture distributions, this state was achieved by exposing specimens to a further 3weeks of sealed pre-conditioning at 50±1°C. While the Autoclam test was not able to accurately identify relative HPC quality due to low sensitivity at associated performance levels, an effective preconditioning procedure to obtain reliable air permeability of HPC concretes was identified.
Highlights
It was noted that, after 4 weeks of drying, there was no significant difference in Relative humidity (RH) for the normal concrete, but the moisture distribution was slightly different with each drying period for the high-performance concrete (HPC)
Based on analysis of data obtained, the following conclusions have been drawn: (i) After 28 days of drying, no significant difference in RH values was observed between the three different concretes tested, albeit that initial rates of moisture loss differed
(iii) While the two HPCs examined exhibited different permeability based on the results of the BS-EN water penetration test, the conventional Autoclam air permeability test with a 50 mm diameter base ring was not able to distinguish these differences
Summary
Based on previous experimental work undertaken at Queen’s University [8,13], two HPCs and one normal concrete were considered as part of this study. With a water/binder ratio of 0.68, the reference, normal concrete contained Portland cement as a binder material only. The first HPC considered (labelled HPC1-PC) contained Portland cement only but at a water/binder ratio of 0.30. PCa (kg/m3) Microsilica (kg/m3) PFA (kg/m3) Sand (kg/m3) Coarse aggregate (kg/m3) Water (kg/m3) Superplasticiserb (%) Water/binder ratio. B Polycarboxylic acid based polymer superplasticiser, as percentage of binder content. A polycarboxylic acid-based polymer superplasticiser, commercially available as Chemcrete HP3, was used for the HPC mixes to maintain a constant slump range. The disparity between the normal concrete and HPC is evidenced by the variation in 28-day compressive strength, which was approximately 24 N/mm for mix NC and 80 N/mm for both HPC mixes
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