In my opinion plastic material should not be used within concrete at all -because of the recycling process. If you want to improve the concrete properties use steel fibres - these distribute the cracks optimally, the concrete obtains a denser structure - the brittle (unreinforced) concrete becomes a more ductile steel-fibre-concrete.

Article Project CC Modelling and quasistatic simulation of the stiff...

By definition, concrete is a rock made from a mixture of sand,gravel, cement and water in definite proportions with physical properties rely on coarse aggregates (gravels) for its strength. To replace the aggregates (sand and gravel) by plastic powder is making mortar not a concrete by definition.

Although utilising plastic as an aggregate in concrete might have positive environmental effects, extensive testing is necessary to guarantee that the required performance criteria and desirable qualities are fulfilled.