Typical properties and parameters of sprayed concrete in mining | South Africa
mixtures are of different types, including plain (unreinforced), silica fume, blast furnace slag, fiber-reinforced, high-strength and high-performance, with different hardening properties.
The mixture composition should be properly designed to guarantee that the in-situ hardened sprayed concrete
will develop acceptable mechanical and physical properties. It will affect the properties of hardened sprayed concrete in the same manner as normal concrete. However, effects relating to the application process, i.e. compaction, rebound, and orientation of fibers, may influence the properties of the hardened sprayed concrete.
Two of the key parameters for both wet-mix and dry-mix sprayed concrete
are the water/cement ratio (w/c)
and the combined aggregate grading curve
Most properties of sprayed concrete improve with reduction of w/c, including its strength, permeability and durability. The addition of accelerators, silica fume, or other pozzolans changes the physical properties of sprayed concrete, particularly permeability and durability. Sprayed concrete’s resistance to freezing and thawing enhances with the use of air-entraining admixtures, while the use of fibers improves its toughness. Proper curing is important and improves the mechanical and physical performance of sprayed concrete.
It should be emphasized that the performance and properties of sprayed concrete depend mainly on the conditions of its application. The selected equipment and its particular characteristics as well as the competence and experience of the application crew also influence the performance and properties of the sprayed concrete
Water / cement ration
Fresh concrete properties
- Slump and slump retention
- Temperature of the sprayed concrete mix
Hardened concrete properties
- Early-age compressive strength
- Final compressive strength
- Flexural strength
- Modulus of elasticity
- Bond strength to substrate
- Resistance to water penetration
- Resistance to freezing and thawing
- Absorption and volume of permeable voids