Coarse Aggregate change 5

[NADOR123]

Hi everyone,

Is there any problem can be developed, if during concrete tank construction, the coarse aggregate type has been changed in the wall design mix to a diffrent type other than the one used in the tank's slab.

 

[Speedycrete]

I am by no means an expert, but if the coarse aggregate has gotten nominally larger (and is still of the same durability), there should be no adverse affects to the strength density of the concrete. It may however affect the pumpability and workability of the concrete - coming from a former pumper......

 

[civilperson]

Specific gravity and thus unit weight may change if an aggregate is changed. Dolomite can add significant weight to the structure if used in place of other rock.

 

[iandig]

Don't forget that if the aggregate has changed, this may also include its physical properties such as shape and surface texture, which in turn may affect the density and compaction characterisitcs of the mix. Also changes in the aggregate may include different chemical compostion such as silica content - need to consider alkali silica reaction etc... Also there is the issue that with a change in aggregate there may/would be a change in the grading so also need to consider the silt content [fines content/material <75µm/63µm pending whcih standard you are working to]

 

[BigH]

All very good points - if the aggregate changes, there needs to be a new concrete mix design approved by the EOR. The question, I think, is of one of compatability of the two mixes of concrete - especially at the interface of the wall and the slab. This is probably handled by the inclusion of waterstops between the slab and the wall - something that one would do in any event even with the same mix.
Each concrete mix would have its own characteristics. We do not know, from the original post, what the difference of aggregate is - other than it is likely from a different source. The gradation (say nominal or maximum aggregate size) difference between the two is not known. One would presume that each of the aggregates has a history locally of being used successfully in concrete. So, unless the source is a new one with no history, one would presume that considerations of alkali reaction or other detrimental effects would not be of concern.
In a concrete tank one would presume that permeability would be of an utmost concern. You would want a "tighter" mix for a concrete tank holding liquid. This would be a matter to consider in approving the concrete mix design. If the wall has been poured with an unapproved mix design, then I would say you have certain problems (potential) that must be addressed by the design and construction players - both from a technical and contractual point of view.

 

[shin25]

BigH has very good points.

Also, you may need to consider the potential change in other characteristics of the mix as well, like shrinkage, freeze thaw durability, compressive strength, etc. All these properties can be affected by chaging the aggregate type. Remember that the aggragates (coarse and fine) makes the bulk of the mix, so their characteristics must be evaluated carefully.

 

[uwrockguy]

An easy check for quick compatibility (disregarding the asr, shrinkage, pumpability issues, etc.), compare the aggregates relative densities (formerly specific gravities under ASTM) and the dry-rodded unit weight. If similar, some comfort can be given to the switch. Still would want to see the overall particle distribution... If these two values are truly dissimiliar, you have a massive problem if the mix proportions were not reproportioned. Both of these tests should be available from the aggregate producer(s) as it is commonly tested for submittal purposes on a yearly basis.