Quality Control and Pre-Construction Testing For Shotcrete

After designing and proportioning, a trial must be made to see how the shotcrete works together with the machine. This is only obtained by using the application equipment on site, not laboratory tools.

1. Concrete Mixes for Spraying

Concrete can be made from a batching plant for wet process. The fresh concrete is transported by mixer trucks and is delivered to spraying machine during shotcreting. Dry mix for dry process can be made using site mixer and delivered to spraying machine using a conveying belt. In some case it is allowed to feed the shotcrete machine manually using shovels.

2. Shotcrete Equipment and Application

Shotcrete machines including pump, dosing unit, air compressor must works in same conditions and procedure as it will work on site. Application process is made by applicator teams that will carry out the job at site in actual work.

3. Samples for Compressive Strength Tests

For pre-construction studies, test panels simulating actual job conditions provide a sufficiently reliable indication of the quality to be expected in the structure. A panel is fabricated by gunning onto a back form of heavy plywood or steel plate.

A separate panel should be fabricated for each mixture proportion being considered, and also for each shooting position to be encountered in the structure, down, horizontal, and overhead. The mixture proportions should be overdesigned to produce sample strengths higher than the design strength.

The panel should be large enough to indicate what quality and uniformity may be expected in the structure; generally it should be not less than 30 inches (750 mm) square. The thickness should be the same as in the structure except that it should normally be not less than 3 inches (75 mm).

Cubes or cores should be obtained from the panels for testing. Cores should have a minimum diameter of 3 inches (75mm) and an length/diameter (L/D) of at least 1. The specimens should be tested in compression at the required ages. Core strengths should be corrected for L/D as described in ASTM C 42. Cube strengths may be reported as determined, or converted to cylinder strengths by multiplying by the factor 0.85.

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