Mechanical Properties of Sprayed SFRC

4. Durability of SFRC: An illustration in tests of SFRC sprayed 20 years ago

A practical engineering application of sprayed SFRC has been conducted 20 years ago in a conveying tunnel in Garou Mine, Hokkaido, Japan. The spraying construction of SFRC was carried out at the distance of about 460 m from the entrance of the tunnel, with the sprayed length of 21 m, average thickness of 10 cm and the total volume of the sprayed area being about 60 m3. The main proportions of the SFRC were: Water/Cement was 47`50%, steel fiber volume fractions Vf were 0`2%.

According to the recording, the bending strength was 5`6 MPa and the compressive strength was 40`60 MPa when tests were performed at the concrete age of 28 days. And also, a test on SFRC's neutralization carried out one year latter showed the depth of neutralization was 1`2 mm.

For the purpose of verifying the durability of SFRC, six positions within the sprayed section had been chosen randomly for sampling specimens after 20 years. As the observation of specimens, it is found that though the steel fibers on/near the surface touched with air were rusted, those buried in were all sound. Neutralization and pH value testing results (see Table6 and Table7) with those specimen show the depth of neutralization of the SFRC was less than 2 cm. Uniaxial tensile and compressive tests (testing methods are similar to those mentioned above) on those specimens show the result given in Table8, Table9 and Fig.12, Fig13. Though some extent of scatter exists in data, the mechanical properties such as compressive strength and tensile strengths show qualitative agreement with those tested 20 years ago, and the residual strength is considered having been kept in some extent by comparing to the results mentioned above in chapter 2.2 and 3.1.