Shotcrete mix design and proportion

Aggregate

Aggregates for shotcrete my contain river sand, crushed sand and crushed stone with particle sizes up to 16mm, normally up to 9.5mm.

Here after is a typical and recommended gradation of combined aggregates for shotcrete that will resulted in perfect mix design.:

Water Cement Ratio

The water-cement ratio for wet shotcrete normally falls within a range of by weight and 0.40 to 0.55 for wet-mix shotcrete.

Dry mix design shall only include water content enough for the hydration process of cement needed for strength development.

In dry-mix shotcrete, the moisture content of the fine and coarse aggregates should be such that the aggregate-cement mixture will flow at a uniform rate, without slugging or hose plugging. The optimum moisture content is generally within the range of 3 to 6 percent. The sand should be dried or wetted as required to bring the moisture content within that range. Large fluctuations in moisture content should be avoided.

A crude but effective test for determining proper predampening is the “ball-in-hand” test. A small amount of mix is placed in the hand and squeezed tightly. When the hand is opened, the mixture may crumble into discrete particles which indicates too little predampening moisture and is usually light gray. If the material holds together, or cracks but remains essentially whole, there is enough moisture. If moisture comes off on the hand, there is too much moisture in the mix.

Unit Weight of Concrete

The unit weight of good shotcrete is usually between 2230 to 2390 kg/m3, about the same as conventional concrete.

Cement Content

OPC cement with surface Blaine above 4500 cm2/gr is most suitable for shotcrete. Typical and recommended cement contents is:


* It is in the best interest of the end user to perform trial mixes and shotcrete trial to determine the suitable cement content, to meet other parameters, above table is used only as a guide in preliminary design and shall be decided in the results of trial mixes and trial application.

Proportioning

Shotcrete design for wet process can be designed like conventional concrete but these mixes contains higher volume of fine aggregate and cement. Shotcrete process can be designed by absolute volume method or by weight.

Shotcrete design for dry process is designed using the bulk densities of the materials, usually the water content is not included in the calculation.