Diagnoses for Early Deteriorations of Concrete
Hidenobu TATEMATSU, Chief, Inorganic Materials Lab.
Jun TAKATA, Deputy Chief Researcher, Inorganic Materials Lab.
Takahiko SASAKI, Researcher, Inorganic Materials Lab.
Alakali-aggregate reaction and salt injury are principal phenomena in concrete deteriorations which occur during early period of service life of concrete structures. Alkali-aggregate reaction yields silica gel which swells by absorbing water and develops cracks. Salt injury is caused by sea water penetrating into concrete, salt particles sprayed from the sea and by the use of sea sands not sufficiently washed, which results in the corrosion of reinforcement. Carbonation also occurs with the early deteriorations. Diagnosis of the early deterioration is done in two ways, that is, the corrective diagnosis for the damaged concrete and the preventive diagnosis for the concrete which is not damaged yet. In the following are illustrated some diagnoses methods which have been developed at RTRI.

Firstly, a detection method for alkali-aggregate reaction was developed. During investigations on actual concrete structures, it was revealed that alkali-aggregate reaction could be detected through direct observation of the swollen gel. Based on the observation, we designed a procedure to identify the alkali-aggregate reaction product(Fig.1: typical exampel) easily, using a scanning electron microscope and micro area X-ray analysis at the same time.

Secondly, a correction method was designed to be applied to the results of rest potential measurement. Usually, corrosion of reinforcement caused by salt injury is inspected by the rest potential method which evalutes the corrosion state using the electric potential between reinforcement and a reference electrode (Fig, 2). In order to compensate for the influence of the quality of cover concrete, we proposed a correction method considering the water contents, carbonation depth, and distribution of electrolytes in the cover concrete. This method could improve accuracy of diagnoses when it was applied to the actual concrete structures.

Additionally, we invented a new indicator to evaluate the quality of concrete. After the relationship between early deterioration and carbonation was established, we thought, the carbonation rate could be used as an indicator which would show whether the concrete could be damaged easily by early deteriorations. According to this idea, an accelerated carbonation test was carried out with some test pieces prepared in different conditions. It was shown by the test that the pieces prepared under worse conditions were carbonated more rapidly. Moreover, it was found that, of the three types of calcium carbonate products (calcite, aragonite, vaterite), the occurrence of metastable phase vaterite was strongly related to the lowering of quality of the test pieces (Fig. 3). We proposed this method as an "accelerated carbonation method", and confirmed its effectiveness with actual concrete structures.

RTRI is recently developing new repair methods as well as improving diagnosis procedures aiming at practical use and higher accuracy.