A Replacement Bound for Crushed Concrete as Aggregate: Adhered Mortar, Water Absorption, Strength, and the Price of Washing

Demolition concrete is routinely crushed and offered as recycled concrete aggregate (RCA) for new concrete. The material differs from natural aggregate in one controlling respect: a fraction of old mortar remains adhered to each particle. That mortar is porous, light, and weak, and it sets both the water absorption and the strength penalty of any concrete made with the aggregate. This paper models that single mechanism and derives, from it, the maximum replacement fraction for three grades of use. SIMULATED, NOT MEASURED: all results are computed from stated input properties, not obtained from laboratory specimens. As-crushed RCA carrying an adhered-mortar fraction of 0.45 has a modelled water absorption of 5.62%, against 0.8% for natural aggregate, and a density of 2,371 kg/m³. Full replacement in a 42.0 MPa reference concrete reduces modelled compressive strength to 24.4 MPa, a loss of 42.0%, halves the elastic modulus ratio to 0.547, and doubles drying shrinkage. The resulting replacement bounds are 28.3% for structural concrete designed at 37 MPa, 87.2% for non-structural concrete designed at 25 MPa, and 100% for road base. A washing and attrition step that reduces the mortar fraction to 0.28 raises the structural bound to 37.4% and the non-structural bound to 100%, at an extra cost of $4.20 per tonne, an effective washed cost of $12.16 per usable tonne after a 12% fines loss, and a saving of only $1.84 per tonne against delivered natural aggregate at $14.00 per tonne. Washing therefore moves a grade boundary, but by less than is commonly hoped, and it does so for strength reasons, not absorption reasons, at the structural grade. High-strength concrete at 50 MPa, prestressed elements, and several other uses listed in the conclusions are ruled out for this material under the stated reference: no replacement fraction above zero satisfies the criteria, because the reference concrete itself does not reach the target.