Variable requirements for early strength development have to be met, depending largely on the point of use of the sprayed concrete or mortar. A distinction is made between:
very early strength development in the range of a few minutes to about 1 hour
early strength development in the range of about 1 hour to max. 1 day
After that we require of normal strength development, comparable with that of structural concrete. The strength development is influenced by the same factors:
cement type and content
temperatures in the concrete and the environment (substrate)
For sprayed concrete there is the added strong influence of the accelerator, which is intended to greatly increase the strength from the first few minutes to the first few hours.
Sprayed concrete or shotcrete is mainly used for stabilisation, but also frequently to grout or fill cavities. Mainly for rock and soil support and overhead spraying requirements for very early and early strength development are crucial and are generally specified.
Very early strength development
In the first few minutes after application of the sprayed concrete, the adhesive strength is decisive. Accurate dosage of the amount of air has here a great influence. It determines the rate of application (thickness). The consequence of insufficient air is insufficient concrete compaction which in its turn negatively influences final strength of the sprayed material.
Too much air produces much dust and high rebound losses. Fine cement and accelerator particles lost in the dust are important components missing for optimal strength development. Dust emission must also be avoided as much as possible for reasons of work hygiene (health protection). In any case, it is never possible to apply more sprayed concrete than the substrate is capable of absorbing, even as initial tensile force on the surface. The very early strength development determines the speed of advance and therefore the performance of the contractor .
A measurable compressive strength is obtained after about 1 hour (in special cases or in immediate stabilisation after only a few minutes). This strength development determines when heading can continue to advance. The early strength development determines the progress with tunnelling.
Alongside the very early and early strength required specifically for sprayed concrete, there are mechanical requirements for the hardened sprayed concrete, just as there are for conventional concrete, generally after 28 days. The level of strength is based on the engineering by the design requirements. The compressive strength is measured on cores taken from the structure or from sprayed panels. Cube samples of the base concrete are sometimes used as controls, but they cannot give meaningful results for the sprayed concrete application because the characteristics may be changed considerably by the spraying process. The setting accelerators used and the skill of the nozzle man have a huge influence on the final strength obtained. Sprayed concrete is normally designed as a thin load-bearing skin and should therefore have ductile load-bearing properties. These can be obtained with reinforcing mesh, but the use of fibres for sprayed concrete and mortar reinforcement is ideal for flexible forming of the material. Steel-fibre-reinforced sprayed concrete is an extremely high-performance, load-bearing material.
The properties of the sprayed concrete are tested on samples taken directly from the structure or from panels sprayed parallel to the application under conditions of maximum similarity and then taken for sampling without destroying the structure. Sprayed panels with defined dimensions are also used for the plate test to determine the tensile strengths and the ductilty of the reinforced sprayed concrete.