system
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human
To what extent, if any, is finished concrete such as that found in most urban structures reuseable and recyclable?

Just wondering about limestones as a finite resource for the concrete industry. What are the constraints on the efficiency of the hypothetical recycling of concrete? If it is *technically* possible, what would be the economic constraints on doing so?
gpt
Concrete can be reused and is extensively as a fill material (at least in Europe it is), it is also reused in small quantities as an aggregate in concrete but this is an exception rather than the norm.

Recycled concrete can have the rebar removed and can be crushed to a specific grading curve in much the same way a natural rock can be, thus creating a pretty controlled aggregate that could theoretically act in the same way as a gravel.  Technically this can be reused within a new concrete mix.  

The problem with reusing this material however, is that the consistency of the material is very variable due to a few things:  The differing mix designs of the crushed concrete (ie a crushed 10N concrete will not have the same qualities as a 60N concrete) and because the crushed aggregate will contain some particles containing 10mm natural gravel (that was used in the original concrete) and some particles that may contain only cement.  There are many other issues too, but these are the 2 that spring to mind at the moment.  

This variability in particles due to source and make-up leaves the end product very variable in strength and other important design parameters that are used for elements containing aggregate.  This variability in quality means that recycled aggregates are generally not reused in concrete mixes where achieving accurate and consistent mixes is what a concrete suppliers reputation hangs on.  

Most standard concrete mixes in the UK do have a standard for a maximum percentage of 'recycled Aggregate' that is permitted, however, depending on the concrete supplier, this may be realised or may not.  At the end of the day, the concrete supplier has to shoulder the risk of the mix reaching the prescribed strength therefore they will generally choose to utilise original material and the cost gets passed up the chain to the end client.

Crushed concrete is still utilised extensively as a granular backfill, generally in low risk, low load situations where it is not subjected to cyclical loading. Which is again due to the variability in strength and quality.