Adhesion strength and shrinkage of shotcrete

Shotcrete is widely used as rock support in mines and in civil engineering projects. In many cases the adhesion between the shotcrete and the rock is one of the most important properties.The objective of this study was to: (i) identify typical failures of shotcrete in a mining environment, (ii) measure the growth of the adhesion strength of shotcrete with respect to age, (iii) investigate the influence of surface treatment (scaling and cleaning) on the adhesion strength of shotcrete and (iv) investigate the occurrence of shrinkage cracks in the shotcrete and failures in the interface between the shotcrete and the rock.The failure mapping showed that most of the observed failures of shotcrete are in areas with a thin shotcrete layer (<20 mm) together with a low adhesion strength. Furthermore, the measured average thickness of shotcrete (40 mm) was almost the same as the ordered thickness, the problem is the wide scatter. In 18% of the thickness measurements, the shotcrete thickness was less than or equal to 20 mm. If the shotcrete lining in the mine has the same thickness distribution as during the tests, the supporting ability of as much as 18% of the shotcrete lining can be considered as more or less negligible.No differences in free shrinkage were observed between plain and steel fibre reinforced shotcrete. Relatively large areas with lack of contact between shotcrete and wall were observed in the shrinkage tests. This indicates that restrained shrinkage can destroy the bond between the shotcrete and rock. The results from the restrained shrinkage tests show the importance of the bond at the interface. If bond failure occurs, the ability to distribute and control the crack width ceases.The growth of the adhesion strength on a sandblasted concrete wall and the growth of the compressive strength showed a correlation. However, the scatter in the results indicates that further investigations have to be done in order to establish a reliable relation between the growth in adhesion and compressive strength with age. The results from the field tests showed that the adhesion strength was significantly higher on rock surfaces that had been water jet-scaled (with 22 MPa water pressure) than those treated by mechanical scaling followed by cleaning of the rock surface (water pressure 0.7 MPa).