In 2025, California ran six 100%-recycled-asphalt road sections under a heavy-vehicle simulator; rain-triggered base shear failures and poor moisture control caused sections to fail

Recycling an old road into a new one can cut waste, reduce the need for fresh aggregates and extend the life of asphalt materials. But a large-scale California experiment in 2025 produced a more complicated result. When pavement made with 100% reclaimed asphalt pavement was subjected to simulated heavy traffic, some sections failed sooner than expected, highlighting the importance of water, construction quality and the strength of the layers underneath.According to the University of California Pavement Research Centre, a Heavy Vehicle Simulator (HVS) was used to repeatedly load pavement sections with heavy wheels during the experiment. Six tests were carried out on a specially constructed track to examine cold recycled pavement made with 100% recycled asphalt pavement. The results did not show that the recycled asphalt itself caused the failures. Instead, investigators linked the most serious problems to localised shear failures in the aggregate base after heavy rainfall and to incorrect water contents in part of one recycled layer.Putting recycled asphalt under extreme trafficThe test track was designed to reproduce the stresses roads experience from repeated heavy vehicles. The pavement structure included a 225 mm aggregate subbase, a 180 mm aggregate base, a 120 mm cold recycled layer and a 60 mm rubberised hot-mix asphalt surface. The recycled layer used reclaimed asphalt pavement as its material and was produced using different recycling agents, including emulsified and foamed asphalt.The researchers tested the sections at two pavement temperatures, about 30°C and 50°C, and under two moisture conditions. Some sections were tested as compacted, while others were exposed to a constant flow of water. Traffic loading continued until a section reached either 12.5 mm of terminal rutting or a specified level of cracking. The results varied widely. The number of equivalent single-axle loads needed to reach terminal rutting ranged from about 200,000 to more than 15 million. The section that failed at the lowest traffic level was tested at high temperature and already had construction problems with foaming water and mixing moisture.Rain exposed a weakness below the recycled layerThe investigation provided one of the clearest lessons from the experiment. Heavy rainfall caused water to enter the pavement structure, and localised shear failures developed in the aggregate base. Once that underlying layer weakened, repeated heavy loading could cause substantial deformation even if the recycled asphalt layer itself was not the source of the problem.This matters because cold recycled asphalt mixtures can have relatively high void content and need protection from water. Earlier field and laboratory work has shown that pavement design, climate and the thickness of adjacent layers can strongly influence the performance of cold recycled pavements. A study published in the Journal of Cleaner Production has also found that climate and pavement structure are important factors in the performance of cold central-plant and cold in-place recycled pavements, while these recycling approaches have offered substantial reductions in energy use and greenhouse-gas emissions.Moisture control became just as important as recyclingAnother major problem emerged during production of the cold central-plant recycled material. Investigators found that the foaming-water and mixing-moisture contents used in part of one test lane were incorrect. Too much or too little water can affect how recycled material is mixed, compacted and performs under traffic.According to the California report, foamed-asphalt recycling depends on carefully controlled water content, as water is used to create the foam that helps distribute the asphalt binder through reclaimed pavement. The study found that the optimum foaming-water and mixing-moisture levels established during mix design had to be reproduced during construction. Departures from those levels in either direction could affect the performance of the recycled layer.The California experiment ultimately delivered a more useful message than a simple success-or-failure story. According to the researchers, properly designed and constructed cold recycled layers can perform well. But pavements with marginal aggregate bases that are vulnerable to shear failure when wet need extra attention, especially where heavy axle loads and extreme weather are expected.You use AI every day. Now get your AI Quotient. Take the AIQ test.