In tunnel construction, the early strength of the shotcrete is of prime importance:

1. At low temperatures, it must be possible to apply an overhead layer of concrete up to 15 cm thick (possibly with added bonding agent).
2. On wet ground, it must be strong enough to hold back the water.
3. After 2 to 3 hours, the concrete must not be damaged by drilling roof holes or anchor holes or by the vibration of the steel wire reinforcement during drilling.
4. After about 5 to 6 hours, it must stand up to blasting.
5. It must rapidly develop resistance to interior work.

It must be possible to measure the early strength easily and accurately at any time when faulty production is suspected, without disturbing the work flow in the driving of headings.

The Manual Shotcrete Penetrometer has been developed for measuring early strength in the first 15 to 120 minutes.

This method measures the force required to push a needle of certain defined dimensions a given distance into the shotcrete. The device, which indicates resistance by compressing a spring, is equipped with a sliding indicator. The point is specially made to snap or screw onto the instrument.

A user guide is available to download from the Impact web site www.impact-test.co.uk