The intrinsic gas permeability coefficients of different plain and slag blended mortar systems using fine sand (≤ 2mm), have been found to be markedly higher than some data obtained by others for concrete systems. I believe this is due to the higher surface area of the fine aggregate and higher binder:aggregate ratio in the mortar compared with concrete, as permeability would mostly occur through the continuous pores in the paste, and possibly, the paste-aggregate interfacial zone. These 2 mechanisms are more in the mortar matrix compared with concrete and therefore may be responsible for the higher permeability observed in the mortar systems.
I will appreciate expert views about this please.
Dear Joseph,
in principle pastes and mortars should be more compact respect to concrete due to the formation of the Interfacial Transition Zone (ITZ) at the interface with the larger aggregate particles of concrete and so the permeability of concrete should consquentely be higher. Anyway, it is difficult and often misleading to compare this kind of data with literature data, as too many parameters (mixing, compacting, type of cement etc) can vary and give rise to different permeability values. Hope these considerations can be useful.
It depends on mix composition and capillary porosity. For the same-size specimens if concrete has more capillary porosity than mortar, then it will have more gas permeability than mortar.
Thanks Md, the issue is which of the 2 systems (concrete and mortar) would one expect to have more capillary porosity?
Dear Joseph,
Porosity of concrete or mortar depends in the first place on the capillary porosity of the paste itself, which for well-hardened material could be predicted by old Powers' Equation (as a function of w/c-ratio and degree of reaction /hydration and the volume fraction of your paste). When comparing 2 systems with the same paste volume, cast in imaginarily ideal conditions, actual capillary porosity will not be the same due to the abovementioned ITZ. In theory, higher surface of the aggregate gives higher ITZ, hence higher permeability. For HP concretes negative effect of ITZ could be eaten up by the efficient SCMs like silica fume and superplasticizers. Especially for the lean mixes, packing of the inert materials plays a significant role. There the permeability, I think, would be dependent on the way the sieve curves match, providing the best possible aggregate percolation. There may be a multiple choice answer here, I am afraid. Hope I could help somewhat.
If you can keep the mortar composition same in both mortar and concrete specimens, the concrete specimen would provide more gas permeability due to the increased degree of porosity in the transition zone between mortar and coarse aggregate.
Permeability occurs through the paste or mortar, not aggregates. So, the porosity of paste or mortar has the main role in the permeability coefficient. If you want to compare different concrete samples, it is better to have an estimation of the volume of paste or mortar
Bond strength depends on
- Surface structure of the aggregate
- Type of aggregate used like Natural Aggregate, Artificial Aggregate etc.,
- Mortar used i.e CM 1:6 ,1:3 1:2 1:1 etc.,.
- Curing Periods
- Addition of Admixtures and Additives
- size , shape, Type of materials,Aspect ratio etc.
- Type of compaction i.e Hand,Machine etc.,
Dear Joseph
Please check this document.
http://www.rilem.org/global/gene/link.php?doc_id=4839&fg=1
Ammar
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