Durability of Eco-concrete

The consistently high demand for concrete and the associated global CO2 emissions of 7-9% have motivated the concrete and cement industry to develop innovative solutions to reduce greenhouse gas emissions. One approach is the use of alternative binders and the reduction of clinker content in cement, which form the basis for eco-concrete. Eco-concrete also includes recycled concrete produced in the spirit of the circular economy, which not only saves primary raw materials (aggregate) but can also be produced using clinker-reduced cement types such as CEM II/C cements.

Figure 1: Microstructure of recycled concrete under fluorescent lighting

To ensure structural safety, assess the expected durability of concrete (with regard to life cycle analyses), and verify the performance of new eco-concretes, an in-situ material-technological evaluation of the concrete structure is essential. Betonmikroskopie e.U. specializes in forensic microscopy of building products and concrete damage, with a focus on eco-concrete.

In many cases, damage to concrete structures is initially assessed using traditional methods such as strength tests, porosity measurements, and chemical analyses (Broekmans, MATM, 2017). While these methods can provide valuable insights for solving problems, they are insufficient and unsuitable for new building products like eco-concretes, particularly in recognizing and evaluating potential damage early on.

Figure 1 shows a detailed image of the microstructure of recycled concrete from Germany under fluorescent light, which was produced using CEM II/C cement. Noteworthy in the microstructure are the significant differences in the water-cement ratio (W/C) between the recycled fragments (W/C=0.38) and the newly produced recycled concrete (W/C=0.5). In this example, surface network cracking occurred in the concrete structure, which can result from a variety of causes.

Petrographic analysis of the sample revealed that increased primary plastic microcracking occurred in the contact zone with the coarse recycled fragments, reflecting the processing issues reported by the client. Additionally, the concrete surface showed an above-average carbonation depth (1.5 mm after 1 year) and insufficient resistance to frost (local pop-outs). After just one season, numerous macroscopically visible network cracks appeared. This example illustrates the urgent need for precise examination of the microstructure of eco-concretes in order to identify, understand, and avoid damage causes.