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As of April 24, 2026 — A Swiss highway experiment that started more than 30 years ago is now among the strongest real-world evidence that recycled concrete can be successfully used in new pavements. Tests on the A13 between Oberriet and Haag in the St. Gallen Rhine Valley showed that concrete sections made with 66–100 percent recycled aggregate by mass remained in “good or very good” condition after 20 years of service, according to a report prepared for the Swiss Federal Roads Office (FEDRO).
The story began in 1990/91, when the A13 was turned from a motorway into an autobahn. The original pavement, built in 1967, was completely renewed. Instead of discarding the old slabs, the project crushed and processed them into aggregate. Part of that aggregate was used for cement stabilization of the foundation, while another part was put into a large-scale experiment to produce new concrete pavement. At the time, engineers had little reliable data on how recycled concrete would survive the combination of freezing winters, de-icing salts and heavy traffic that characterize Alpine highways.
The experiment was designed to answer a practical question: can high proportions of recycled aggregate perform like natural aggregate in a rigid road surface? To test this, the project built several types of concrete with different amounts of recycled aggregate. In the recycled mixes, the recycled material made up between 66 and 100 percent of total aggregate by mass. These were compared with a reference concrete made exclusively from natural aggregates. Two ways of preparing the recycled aggregate were included. The first, designated 1a/1b, used a single graded all-in aggregate without separating it into size classes. The second method separated the recycled material into standard fractions: 0/4, 4/8, 8/16, and 16/32 millimeters. The experiment also tested whether the fine 0/4 fraction should be recycled or natural, to see if this affected performance.
After two decades, researchers returned to those test sections for a full forensic evaluation. The investigation covered visual inspection, microscopy, air permeability, carbonation depth, chloride content, water conductivity, resistance to chloride migration, and compressive strength. These methods were chosen to expose any weaknesses in the concrete caused by freeze-thaw cycles, salt intrusion, and abrasion from traffic. The report, titled “Dauerhaftigkeit von Betonfahrbahnen aus Betongranulat” (Durability of concrete pavements made of recycled concrete) and published on ResearchGate, concluded that all recycled concretes were still in a good or very good state.
Differences were minor but instructive. The natural-aggregate reference concrete performed slightly better than all recycled concretes. Among the recycled mixes, the all-in aggregate 1a/1b was somewhat less durable. The report attributes this not to the recycled material itself but to practical difficulties during aggregate preparation, placing, and compaction. This detail matters: it indicates that the quality of the end product depends on process control, not just on the choice of recycled versus natural material.
All test sections, including the reference, showed the beginning of an alkali-silica reaction. This is a chemical reaction between certain siliceous components in the aggregate and the alkaline pore solution in the cement paste, which can lead to expansive cracking over time. The early stage observed after 20 years is not necessarily fatal. The authors estimate that the recycled concrete pavements can continue to serve and reach a service life of 40 to 50 years. They also found no meaningful difference in performance between mixes with recycled fines and those with natural sand in the 0/4 fraction.
The results come with a clear caveat. They apply only to the aggregate used in this trial, which came from an old Swiss concrete motorway that had a uniformly high concrete quality and very little contamination. If concrete is to be recycled into new pavement aggregate, it must be tested for alkali reactivity, chloride content, and other harmful substances before reuse. Not every demolished pavement will meet that standard. Roads and airport traffic areas, however, are especially good candidates because their concrete is dense, homogeneous, and generally produced under strict quality control.
Looking ahead, the study recommends building concrete pavements in two layers. The lower layer can use recycled concrete aggregate, while the upper layer uses natural aggregate. This design separates structural performance from surface performance and allows engineers to tune the top layer for noise reduction and skid resistance. The lower layer becomes a legitimate outlet for large volumes of recycled material, reducing both disposal and demand for virgin stone.
For United States agencies, the Swiss evidence is a timely counterpoint to the common caution that recycled concrete should only be used below the surface. Many state DOTs already use recycled concrete aggregate for base courses and backfill, but do not yet allow high percentages in new rigid pavement. The main barrier has always been long-term performance data under harsh field conditions. The A13 test gives designers a set of 20-year observations from a climate that is comparable to much of the northern US, with heavy snow, freeze-thaw cycles, and widespread use of road salt.
There is also an environmental reason to pay attention. Cement production is responsible for roughly 7 to 8 percent of global carbon dioxide emissions, and concrete pavements are one of the largest construction end uses. Extending the useful life of existing concrete instead of mining and crushing new aggregates reduces both CO2 emissions and construction waste. The A13 project demonstrates that such a circular approach is not only environmentally logical but technically feasible at scale.
As American highways built decades ago reach the end of their intended service lives, large volumes of old concrete pavement will become available. The Swiss example shows how that material can be turned into high-quality aggregate and placed back into structural layers. The FEDRO report, based on 20 years of monitoring, adds the kind of evidence needed to move recycled concrete from an experimental curiosity to a routine construction material. For pavement engineers and sustainability officers, the message is simple: with careful testing, process control, and a two-layer design, 100 percent recycled concrete aggregate is a durable choice.
