Shotcrete Mix Design for 9 cubic foot mixers

David B. South • Published • Shotcrete Articles

This is for a seven sack mix used for Shotcrete for maximum strength utilizing Kel-crete as and admixture. This Mix Design is for 3/10’s of a cubic yard – using two sacks of cement. It is formulated especially for the Monolithic Portable Concrete Mixer, but will work well in most 9 cubic foot mixers.

Cement — 188 lbs Two sacks of standard Type I or II or I-II Portland cement. This is what the lumber yard will carry. It is not mortar mix.

Water — about 80 lbs (10 gallons) Water must be clean (potable). This will vary from job to job and must be adjusted. The water in the aggregate will cause a difference in the amount of water needed. Adjust to a proper slump. 2" to 6" as needed.

Concrete Sand — 690 lbs (Ten, 5 gallon buckets is a good approximate measure).

Pea rock — 140 lbs (Two, 5 gallon buckets.) The amount of pea rock is adjusted per gradation of the sand. If the sand is high in larger aggregate the pea rock may be left out. If it is low, then more may be needed. This is a place to start for maximum strength. The total of the sand and pea rock should equal the 830 pounds (twelve, 5 gallon buckets).

Kel-Crete — use 2 oz minimum per batch. Try up to four ounces per batch. Adjust between for best results. We normally use 2.5 ounces. Add the Kel-Crete additive in with the water for easier dispersion. If the Kel-Crete is not added, another type may be substituted. The Kel-Crete adds lots of features including “air entrainment”. Air entrainment is very important where the finished concrete is subject to severe freeze and thaw cycles.

Kel-Crete is a highly concentrated liquid magic:

  • It adds air entrainment. Most concrete and especially shotcrete should have an air entraining compound added for flowability and most especially freeze/thaw protection.
  • It is a water reducing agent. Concrete is generally stronger if made with less water. Kel-Crete reduces the amount of water needed in the mix.
  • Costs less than other additives both to purchase and to handle. It is highly concentrated. The one gallon (128 oz) containers can be shipped easily. It is non corrosive. The dosage rate is from 1 oz to a maximum of 7 oz per yard.
  • Long shelf life. It can be stored for years and still work.

When to use What

Obviously, if the job uses thousands of yards of concrete, a ready mix plant is the only way to go. But for the small jobs, we suggest you consider mixing on site. Mixing on site with small mixers must be well organized.

The mixers can be a skid-loader mixing unit, or a plaster mixer, or small line concrete mixer. The biggest advantage of on site mixing is the ability to control speed and delivery. There is nothing worse than waiting hours for the ready mix truck for a yard or two of concrete.

Click here for downloadable PDF containing more detail for shotcrete mix.