Preliminary assessment of the secondary setting of Portland cement in recycled crushed concrete incorporated in cold recycled road base mixes with foamed bitumen

Abstract

The recycled crushed concrete (RCC) unlike the natural aggregates, has a residue of the unhydrated Portland cement (PC) covering the aggregate grains, which may result in a secondary cementation process after its application in a road base. The paper presents the results of exploratory research on the phenomenon of the secondary setting of Portland cement in recycled crushed concrete aggregates in cold-recycled mixes with foamed bitumen (FB). The tests were performed using two mixtures, i.e. Mix-FBREF reference mixture without the addition of RCC, and Mix-FBRCC research mix containing 25% of RCC. The composition of mineral mixtures with foamed bitumen was designed using typical recycled materials, used as a part of the road repair works of pavements, i.e. Reclaimed Asphalt Pavement (RAP), Reclaimed Aggregates (RA) and new aggregates (NA). All mineral materials used in the tests met the requirements in terms of suitability for the foamed bitumen cold-recycled mixtures, intended for basecourse. The total amount of the bituminous binder in the Mix-FB mix was 5.5%. To assess the phenomenon of the secondary setting of Portland cement contained in the recycled concrete, tests were conducted to evaluate the increase of the unconfined compressive strength (UCS) in time of the material. The strength of UCS samples of Mix-FBREF and MixFBRCC mixtures was determined after 3, 4, 7, 14 and 28 days of conditioning. The tests showed that the value of the analysed parameter in the reference mixture increased by about 180 kPa with time, whereas for the mixture containing the concrete recycled aggregate the difference was virtually unnoticeable (only by 6 kPa). The obtained test results were subjected to the statistical analysis, which showed that for each of the considered sample conditioning periods, no statistically significant differences between the strength values of unconfined compression of the tested mixtures were found. The presence of bituminous binder in basecourse cold-recycled mixtures, (old, with RAP and new, in foamed form) yields the bituminous binding dominant over the effects of PC binding, therefore minimizing the effects of the secondary Portland cement setting.