19 June 2001
CPM-4: SHOTCRETE (GUNITE)
Description: Sprayed concrete applied directly by an air jet gun. Equipment
includes a mechanical feeder, mixer, and compressor. If using a dry-mix, water
is added at the nozzle.
is relatively economical where deterioration
is shallow and area
is large and irregular in shape. Shotcrete requires no form work and thin patches
can be made that have a high strength, is an efficient method for vertical and
overhead surfaces, and is also used for encasing timber and steel piling.
a. Must have enough repair work to justify the cost of equipment.
b. Normally restricted to a depth of 10 cm (4 inches) per lift.
c. Can have a high porosity if improperly applied.
d. Drying shrinkage rate and thermal expansion can be different than
concrete being repaired.
e. Can have wide variation in application/composition. Quality
depends almost entirely on the operator's skill. Follow ACI SP-14 guidelines.
a. Thoroughly remove all defective and loose concrete.
b. Clean all rust off exposed reinforcing steel.
c. Roughen all smooth surfaces.
d. Wire brush or abrasive blast exposed concrete surfaces to be
: A bonding agent
is not required.
a. Cement-to-aggregate maximum ratio 1 to 3.5. This will result in an
in-place mix of about 1 to 2.5 after rebound.
b. Minimize the water-to-cement ratio.
c. May use a non-chloride type admixture for rapid setting.
d. Fiber reinforcing may help minimize shrinkage cracks.