Initial situation
The findings on chemical concrete attack by carbonic acid are essentially based on studies from the 1980s. There is no empirical background for new, clinker-efficient cements as to how they should be assessed in the event of chemical attack. Also in cases where high performance concretes are to be used in a severe chemical attack (XA3), where it would be possible to abstain from ‘additional protective measures’ required by the standard, there is no closed standardised concept. Reference is made here to an undefined expert solution. A standardised testing and assessment concept is required for both cases, which does not currently exist in Germany. The aim of a joint research project by VDZ and RWTH Aachen University (ibac) was to develop the key points of such a concept.
Trials
The starting point for several series of tests was the determination of the degree of damage caused by the attack of carbonic acid on concretes. The concretes were stored in different concentrations of carbonic acid under both static and flowing conditions. Both destructive (loss of mass by brushing) and non-destructive measurement methods (NMR) were used as test methods. The results were validated by light and scanning electron microscopy. The cements used were primarily those with several main constituents in addition to Portland cement.
Results
The research project was able to show that a distinction must be made between the depth of erosion and the depth of inner damage during attack by carbonic acid. By measuring both the damage depth and the scaling using the non-destructive NMR-MOUSE method, it was possible to develop a promising test methodology. The damage or scaling rate follows a functional relationship, so that long-term damage rates can be extrapolated after a test period of around one year. The results of the NMR investigations were confirmed by microscopic analyses. Such a measuring method for the practical measurement and evaluation of the attack of carbonic acid on concrete did not exist until now.
Concretes with CEM III cements or with the combination of CEM I and fly ash show internal damage of the microstructure, which can be attributed to reduced calcium hydroxide contents. However, this does not have a negative effect on the scaling relevant for a structure.
