Formation of Strength for Sprayed Concrete
Initial Strength Development: Variable plays an important role in forming initial strength and it should be met on the basis of the point of application of the sprayed concrete or mortar. The following discrepancies are made:
Initial strength formation at early stage in the range of a few minutes to about 1 hour
Initial strength formation in the range of about 1 hour to max. 1 day
After that formation of strength is needed in normal way, equivalent to that of structural concrete. The strength development is affected with the same factors:
cement type and content
temperatures in the concrete and the environment (substrate)
For sprayed concrete there is the extra strong influence of the accelerator, that allows to significantly raise the strength from the first few minutes to the first few hours.
Sprayed concrete or shotcrete is primarily applied for stabilisation, but it is also extensively used to grout or fill cavities. Primarily for rock and soil support and overhead spraying requirements for very initial and early strength development are important and are normally stated.
Very early strength development: In the first few minutes as soon as the sprayed concrete is employed, the adhesive strength becomes crucial. It is very important to maintain exact dosage of the amount of air. It defines the rate of application (thickness). The inadequate air may lead to deficient concrete compaction which consequently provides negative influence in final strength of the sprayed material.
If there is lots of air, abundant dust is created and high rebound losses. Fine cement and accelerator particles may lost in the dust and therefore vital constituents are missing for perfect strength development. Dust emission should be circumvented to highest possible way in order to attain work hygiene (health protection). In any case, it becomes impossible to employ more sprayed concrete except for the substrate has the ability for absorbing, even as initial tensile force on the surface. The very early strength development defines the speed of advance and consequently the performance of the contractor.
A quantifiable compressive strength is achieved after about 1 hour (in special cases or in immediate stabilisation after only a few minutes). This strength development defines when heading can continue to progress. The early strength development defines the progress with tunnelling.