Use of recycled demolition aggregate in precast products, phase II: Concrete paving blocks

A study undertaken at the University of Liverpool has investigated the potential for using construction and demolition waste (C&DW) as aggregate in the manufacture of a range of precast concrete products, i.e. building and paving blocks and pavement flags. Phase II, which is reported here, investigated concrete paving blocks. Recycled demolition aggregate can be used to replace newly quarried limestone aggregate, usually used in coarse (6 mm) and fine (4 mm-to-dust) gradings. The first objective, as was the case with concrete building blocks, was to replicate the process used by industry in fabricating concrete paving blocks in the laboratory. The compaction technique used involved vibration and pressure at the same time, i.e. a vibro-compaction technique. An electric hammer used previously for building blocks was not sufficient for adequate compaction of paving blocks. Adequate compaction could only be achieved by using the electric hammer while the specimens were on a vibrating table. The experimental work involved two main series of tests, i.e. paving blocks made with concrete- and masonry-derived aggregate. Variables that were investigated were level of replacement of (a) coarse aggregate only, (b) fine aggregate only, and (c) both coarse and fine aggregate. Investigation of mechanical properties, i.e. compressive and tensile splitting strength, of paving blocks made with recycled demolition aggregate determined levels of replacement which produced similar mechanical properties to paving blocks made with newly quarried aggregates. This had to be achieved without an increase in the cement content. The results from this research programme indicate that recycled demolition aggregate can be used for this new higher value market and therefore may encourage demolition contractors to develop crushing and screening facilities for this.

Research highlights
► The factory technique for casting concrete paving blocks was successfully replicated in the laboratory and therefore the effect of replacing newly quarried aggregate with recycled demolition aggregate could be investigated.
► The physical characteristics of C&DW aggregates may adversely affect the mechanical properties of the flags. However, levels of replacement of quarried limestone aggregates with C&DW derived aggregates have been determined that will not have significant detrimental effect on the compressive strength.
► The levels for masonry derived aggregate were determined to be 60% for the coarse fraction, i.e. 6mm, and 20% for the fine fraction, i.e. 5mm-to-dust.
► The levels for masonry derived aggregate were determined to be 60/20% for both coarse and fine fraction replacement.
► The high water absorption of recycled aggregate appears to influence adversely the concrete water absorption. The critical or deciding factor for the level of replacement of newly quarried limestone aggregates with recycled demolition aggregate may have to be the water absorption rather the strength. Recommended replacement percentages of 60% for coarse masonry-derived aggregates may have to be reduced to 30% to maintain water absorptions below 6%.