Early Strength Development

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 is a normal strength development required, comparable with that of structural concrete. The strength development is influenced by the same factors:

  • aggregate type
  • cement type and content
  • water content
  • temperatures in the concrete and the environment (substrate and ambient)
  • layer thickness
  • 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 is mainly used for stabilization, 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.

Early Strength Development

A measurable compressive strength is obtained after about 1 hour (in special cases or in immediate stabilization after only a few minutes). This strength development determines when heading can continue to advance. The early strength development determines the progress with tunneling.