Shotcrete Components

shotcrete mix design

Shotcrete has been widely used for tunneling works and slope protection, it is also used for architectural purposes. There are two types of shotcrete application, the wet and dry process. Shotcrete is a concrete transported by means of air under pressure with high velocity. It is applied and compacted in the same time against a surface.

Shotcrete components

As mentioned earlier, shotcrete differ from normal concrete by the way to apply it. Here below some points that should be observed and respected when producing shotcrete mixes.

Cement

It is obvious that the cement quality properties play a dominant role in the high early strength behaviour. The specific surface (Blaine value) should be not lower than 3500 cm2/g. Compressive strength of the cement lime should be more than 10 MPa after 2 days and more than 35 MPa after 28 days. First stiffening of the cement lime should not be before 1,5 hours and not after 4 hours from the start of the mixing with water. The choice of cement however is at all the time governed by the required properties of the hardened concrete and not for their suitability for spraying.

The early strength requirement would determine the cement content. Usually for dry process we dose 280-350 kg for 1 m3 of dry bulk mix. For wet process, cement content can vary from 425 kg/m3 to 500 kg/m3. Suitable cement content regarding the strength required can be verified only by shotcrte trial. A slight retardation of the setting time is observed by the tri-calcium aluminates in case of sulfate resistant cement. Blast furnace cements cause the same problem.

Water

All water use for mixing cementitious components must be clean as it occurs naturally. Spring, waste- water must be analyzed in order to determine their compatibility with the other components of the mix. The water cement ratio should not exceed 0,40 if high strength or durable shotcrete is required.

Sand

Sand must be clean. It is always combined with aggregate. The S/A ratio varies from 55% – 65%. Despite to specify the sand grading itself according to a certain grading refereeing to a certain standards, it is the combination of the sand and aggregate together that must comply with the standard used.

Aggregate

Aggregate would be mostly crushed aggregate considering that they could be produced from the excavated material. However we should avoid flaky elongated aggregate.

Sand / Aggregate ratio

This ratio must satisfy a few conditions. The S/A ratio must allow easy shotcrete pumping or rotor filling. As a rule it is clear that mixes containing fines in excess would cause dust, increase the water demand. A contrary, mixes containing aggregates in excess would increase rebound. It has been observed that good pumping properties are achieved when the particles smaller than 1.18 mm represent around 40% of the sand / aggregate combination.

Superplasticiser/ stabilizer

They are taken in consideration for the production of wet mix only.
They are usually Naphthalene or polycarboxylate base superplasticiser.
Only some special modified Lignosulfate base superplasticiser could be used.
As normal concrete, their use allows a reduction of the water by still keeping suitable workability. However, it must be noted that lower the W/C is faster early strengths are reached. Dosage would depend on the workability required.
Usually they are dosed from 1,0-1,7 %.

Accelerator

They are definitely required for wet mix application. They are used in certain cases for dry process application too. They are added for various reasons. They allow the spraying of thick layer when out-put is high and consequently the shotcrete reaching the substrate would not fall down. They allow developing early strength that would minimize surrounding rock deformations. They limit the rebound formation. Certain type of accelerators allows sealing work when substrate shows water leakage. They exist in powder or liquid form. They are either Alkali or Alkali Free. Typical dosages of modern accelerator are from 3-8% by weight of cement