Old Columns, New Life: Recycled-Component Concrete Columns Pass Their Toughest Tests Yet

Every year, the world’s cities chew through staggering quantities of concrete, and every year they spit out a mountain of rubble in return. In China alone, ready-mixed concrete production hit 2.49 billion cubic meters in 2024, while construction and demolition waste soared from about 30 million tons in 2000 to 292 million tons in 2022, growing at an average of 10.8 percent per year. Cement clinker production for all that fresh concrete released roughly 1.6 billion tons of carbon dioxide in China in 2024, and the voracious extraction of natural aggregates has scarred landscapes and ecosystems. Now, a team of researchers led by Yan Li and Bo Wu of South China University of Technology has reported the first experimental evidence that a bold new recycling strategy, called recycled-component concrete, can carry the load in one of the most demanding structural roles imaginable: the seismic-force-resisting columns of buildings in earthquake country.

The idea behind recycled-component concrete is deceptively simple. Instead of crushing waste concrete down into small recycled aggregates, or even into mid-sized concrete lumps of 60 to 300 millimeters, the new approach salvages entire concrete members, such as columns recovered through selective dismantling, and gives them only minimal processing: stripping off the rendering layer, cutting to length, and roughening the surfaces. These chunky recycled components are then placed directly inside the formwork of a new member, and fresh self-compacting concrete is cast around them. The result is a hybrid column in which nearly half the cross-section is made of old, already-cured concrete, complete with its original reinforcing bars and stirrups still intact inside.

The advantages over conventional recycling are substantial. Crushing waste into recycled aggregates still demands all the cement of ordinary concrete, so the carbon penalty of clinker production remains untouched. Recycled lump concrete improves on that by replacing some of the new mix, but casting quality limits lump replacement to about 35 percent, and the crushing process generates fine powder that is difficult to use. Recycled-component concrete, by contrast, can replace more than 50 percent of the new concrete, produces far less waste powder, and preserves the old reinforcement in usable condition, potentially trimming the amount of new steel required. Earlier studies in Europe had reused cut concrete slabs directly in new buildings, with one prototype floor system showing embodied carbon 80 to 94 percent lower than conventional concrete floors. But direct reuse runs into a stubborn problem: salvaged members rarely match the dimensions or strength grades their new positions demand. Casting new concrete around a recycled component elegantly dissolves that mismatch, because the surrounding mix absorbs any dimensional discrepancy and a stronger new mix can compensate for weaker old concrete.

What had never been tested, until now, was whether this hybrid configuration could survive the two brutal trials that matter most for columns: crushing axial compression and repeated cyclic earthquake loading. The concern was real. In a recycled-component concrete column, the old concrete sits as a concentrated block in the center of the section, surrounded by new concrete, creating a large, regularly shaped interface between the two generations of material. If that interface were to delaminate or slip under load, the whole concept would collapse, figuratively and literally. There was also a puzzle for the engineers’ calculators: the column contains two overlapping systems of stirrups, the new outer hoops and the old hoops locked inside the recycled component, and existing strength-prediction models, built for a single confinement system, simply did not apply.

To find out, the team fabricated nine square column specimens, 400 millimeters on a side and 1200 millimeters tall, for axial compression testing, seven of them recycled-component columns with a replacement ratio of 46 percent and two conventional references. Each recycled component was a 270 by 270 millimeter column, cut and roughened from a prefabricated 300 by 300 millimeter old column, its eight 12-millimeter longitudinal bars and 6-millimeter stirrups retained. The specimens were squeezed in a 15,000-kilonewton machine under displacement control, with strain gauges glued to both old and new stirrups to reveal how the two confinement systems shared the work. Seven more specimens, cantilever columns with a shear-span ratio of 3.375, were subjected to low-cycle reversed lateral loading under a constant axial load ratio of 0.4, mimicking the demands of a major earthquake.

The axial results were encouraging. During testing, no large-area delamination or spalling of new concrete from the recycled components occurred, confirming that the bond between old and new material held firm all the way to failure. The recycled-component columns carried somewhat lower peak loads than the references, from 7571 to 8339 kilonewtons against 8445 to 8648 kilonewtons, but the shortfall traced cleanly to the weaker old concrete, not to any interfacial weakness. Crucially, when the team raised the strength of the new concrete, specimen CR6 reached 8339 kilonewtons, nearly matching the 8445-kilonewton conventional reference, proving that a stronger surrounding mix can compensate for weaker salvaged material. Inclined-crossed stirrups, arranged in an X-pattern instead of conventional rectangular hoops, slightly increased capacity at the same steel quantity, and strain measurements showed they mobilized their confinement more effectively.

The seismic tests delivered the headline finding. Under reversed cyclic loading, the recycled-component columns and the conventional reference failed in the same characteristic way, with flexural cracks forming at the base, evolving into diagonal flexural-shear cracks, and culminating in concrete crushing within the plastic hinge region. Their hysteretic loops were similarly full, indicating strong energy dissipation, and their skeleton curves were nearly indistinguishable. Ductility coefficients ranged from 3.43 to 3.80, comfortably above the minimum of 3 required for reinforced concrete columns, and all but one specimen exceeded the 2 percent ultimate drift ratio demanded by Chinese seismic design codes. Remarkably, the recycled-component columns actually developed higher peak lateral loads than the conventional reference, reaching up to 370.2 kilonewtons against 343.9 kilonewtons, thanks to the stronger old concrete and the higher yield strength of the retained reinforcement. Incorporating a recycled component while slimming the new longitudinal bars from 18 and 16 millimeters down to 12 millimeters cost only 4 percent of ductility.

The experiments also delivered practical verdicts on construction details. Drilling post-installed rebars into the recycled components proved counterproductive under axial load, reducing peak capacity by 4.2 percent, apparently because the drilling introduced internal defects, and offering no seismic benefit either. Welded connectors between new and old stirrups improved ductility by only 4.7 percent, a gain judged too small to justify the cumbersome installation. The team’s recommendation is refreshingly economical: skip both. On the other hand, widening the spacing of the new stirrups cut the equivalent viscous damping coefficient by 17.6 percent and dropped one specimen below the drift-ratio requirement, so the confinement steel in the plastic hinge zone must not be skimped. Inclined-crossed stirrups emerged as the preferred arrangement, boosting energy dissipation by up to 8.6 percent, and their orientation relative to the loading direction made virtually no difference, freeing contractors from fussy placement.

Finally, the researchers closed the analytical gap. They divided the hybrid cross-section into three regions: unconfined cover concrete, concrete confined by the new stirrups alone, and the central old concrete squeezed simultaneously by both stirrup systems, then summed the regional contributions along with the old and new longitudinal steel. The resulting formula predicted the measured axial capacities of all nine specimens with a maximum error of just 8 percent, and only 5 percent for the conventional columns, showing that the dual-confinement model works across the spectrum. Taken together, the study offers the first technical foundation for columns that swallow nearly half their volume in salvaged concrete and part of their steel in salvaged reinforcement, while standing up to earthquakes as reliably as their all-new counterparts. In a construction industry desperate to cut its carbon ledger, that is a result worth building on.

Subject of Research: Axial compressive and seismic performance of recycled-component concrete columns made from reused waste concrete members

Article Title: Axial compressive behavior and seismic performance of recycled-component concrete columns

Article References: Li, Y., Wu, B., Zhang, H., & Wang, F. (2026). Axial compressive behavior and seismic performance of recycled-component concrete columns. Case Studies in Construction Materials, 25, Article e06453. https://doi.org/10.1016/j.cscm.2026.e06453

Image Credits: AI Generated

DOI: 10.1016/j.cscm.2026.e06453

Keywords: recycled-component concrete, recycled concrete, construction and demolition waste, seismic performance, axial compression, confinement, stirrups, carbon reduction, sustainable construction, cyclic loading, structural engineering, circular economy