From "Gunite & Shotcrete" by the Gunite Contractors Association.

When properly mixed and applied, Gunite is extremely strong, dense and highly resistant to weathering and many forms of chemical attack. It is heat resistant to a high degree and can be made more so by substituting refractory aggregates for part or all of the sand. Resistance to abrasion is extremely high. The bond to other Gunite, well cleaned masonry or other materials is equal to or greater than the shearing strength of the material to which it is applied.

OPERATING REQUIREMENTS.
Not less than 365 cubic feet of actual free air per minute, at a minimum pressure of 45 pounds per square inch in the gun chamber, is necessary for proper Gunite placement and adequate "blow-out" jet requirements. Water under a pressure of at least 15* pounds in excess of air pressure is also required. Pressure requirements increase with the height of operation above the gun and length of hose required.
The cement gun should be operated at a minimum air pressure of 45 pounds per square inch on the gun tank when 100 feet or less of material hose is used and the pressure should be increased 5 pounds for each additional 50 feet of hose required.

MIXTURES.
Structural Gunite proportioned on the basis of one part of cement by volume to various parts of sartd by volume will easily attain the following compressive strengths in 28 days:
1:4.....................4,000 lbs. per square inch
1:41/2.................3,000 lbs. per square inch
1:5.....................2,500 lbs. per square inch
1:6......................2,000 lbs. per square inch

Under proper curing conditions Gunite will attain at least 60% of the above compressive strengths in 7 days.
 

CONTROL OF CONSISTENCY.
Consistency of the Gunite is controlled at the nozzle by the nozzleman.

PREPARATION OF SURFACES.
If bond is required, surfaces to be Gunited must be thoroughly cleaned of all dirt, oil or foreign matter and all loose, scaly, or unsound material removed.
Gunite cannot be applied to a surface containing frost or ice.
Gunite must be protected against freezing.

In some instances, such as in relining of existing concrete reservoirs, bond to existing material may be undesirable. In such cases, the usual procedures should be followed to prevent bond.

CURING.
Proper curing is essential. Alight spray of water should be applied as soon as is possible without damage to the surface. After the surface has hardened, it must be kept moist for a period of from five to seven days, depending upon atmospheric conditions.
Satisfactory   curing   can  be   obtained  by  proper application of an approved sealing compound.

DESIGN.

Load tests on slabs show Gunite conforms to the same design assumptions as ordinary concrete of equivalent compressive strength.

The fundamental principles used in the design of reinforced Gunite are the same as the fundamental principles for the design of reinforced concrete in accordance with the rules of the American Concrete Institute.

Wire mesh, when used as reinforcement, should have a minimum area in each direction of .0025 times the cross sectional area of the Gunite section.

NOTE: Inasmuch as Gunite is readily adaptable to rounded corners without the use of forms, it is suggested that this added feature be utilized for the inside corners of reservoirs or similar locations.

NOTE: Inasmuch as Gunite is readily adaptable to rounded corners without the use of forms, it is suggested that this added feature be utilized for the inside corners of reservoirs or similar locations.

General Information

HYDRATION.
Only that amount of water necessary for the proper hydration of the mix is used. This water is added to the dry mix at a special premixing nozzle immediately prior to placing on the desired surface.
COMPRESSIVE STRENGTH.
A mix of I'AVz usually produces a 7-day strength of 2400 psi and a 28-day strength of 4000 psi. As indicated under "Gunite Characteristics" the more conservative design strength of 3000 psi at 28 days is advocated for this mix. In many cases "rich" mixes such as 1:3 are specified when the required design strength could be adequately met by a lAVz mix. Besides being uneconomical, such specifications may produce Gunite which tends to be brittle and more susceptible to fine cracking. The specifications of the Association are based on a lAVz mix.

WEIGHT.
Gunite will weigh 140 to 152 pounds per cubic foot
depending upon the type of sand used. Lightweight
aggregates can be used to further reduce weight as in
concrete.
DENSITY.
Because it is applied under pressure with a rejection of excess material, Gunite is an extremely dense product.
ELASTIC PROPERTIES.
Tests indicate that the modulus of elasticity of Gunite approaches 4,670,000 pounds per square inch (see "Engineering News Record," August 31, 1933).
EXPANSION.
Numerous tests have indicated that the coefficient of expansion of Gunite is almost identical to that of low carbon steel.


REBOUND - RELATION TO STRENGTH.
Due to the method of placement at fairly high velocities a certain proportion of material is rejected. This rejected material is called "rebound" and consists mainly of sand particles. It can be seen that if a1:4/1/2 mix is passed through the cement gun, the resultant mix in place is much "richer", hence of higher strength, than the original mix.
BULKING - RELATION TO STRENGTH.
In many cases the effect of sand bulking is not taken into consideration in the proportioning of Gunite. A bulking factor of twenty percent is not uncommon. If this fact is not considered where materials are proportioned by volume, the actual mix in place is much "richer" than that specified. This condition is often undesirable as explained under "Compressive Strength" above.
BOND STRENGTH.
Gunite bonds perfectly to properly prepared surfaces of other materials such as concrete, brick, rock, tile, stone or steel. Numerous tests have been made which indicate that the bond strength of Gunite exceeds the shearing strength of good quality brick or concrete against which it is applied. In tests, loads in excess of 600 pounds per square inch in shear were sustained by the bond between brick and Gunite, final failure being in the brick.

DIMENSION.
Gunite can be successfully applied to any desired
thickness with uniform quality throughout.
FIRE RESISTANT QUALITIES.
Gunite is a highly economical medium of fireproofing for steel structures, both because of its natural fire resistant qualities and because of the fact that it is adaptable to placing on irregular shapes and can be finished to true and accurate lines. Gunite is well adapted to firewall construction in buildings, bridges and piers.
RESISTANCE TO CORROSION.
Gunite adheres better than poured concrete to structural steel and is, therefore, a better form of protection from corrosion.
STRENGTHENING OF STEEL BEAMS.
The strength of steel beams and girders can be increased within reasonable limits by the use of reinforced Gunite.
ECONOMY.
Gunite application is based upon the transportation of material through a hose and is consequently more economical in many- instances than other types of concrete construction.
The fact cannot be too thoroughly stressed that Gunite is a specialty product and that its design and application must be executed by men thoroughly-experienced in its characteristics and uses.

Historical Background of GUNITE
Because of the rapid growth of the Gunite industry immediately following World War II, it is often thought of as a post-war development. However about 1895, Dr. Carlton Akeley, curator of the Field Museum of Natural Science in Chicago, developed the original cement gun. Dr. Akeley was searching for a method to apply mortar over skeletal matrices to form the shapes of prehistoric animals, since conventional troweling could not form the convoluted shapes of the musculature systems of these giant reptiles and mammals. His search for a solution to the problem led him to try to spray a cement, lime and sand mortar. He failed with this method but his next attempt resulted in the original cement gun. In a single-chambered pressure vessel, he placed a mixture of sand and cement. Compressed air was then pumped into the chamber containing the mixture, forcing the material through an opening and into a material-conveying hose. As the material was ejected from the end of the hose, it passed through a spray of water which hydrated the mixture.
This crude apparatus was adequate to prove the theory of the process. Later refinements, such as another compression chamber and the addition of an agitating geared feed wheel made it possible to achieve a continuous flow of material. The process was patented in 1911, and except for the improvements in ancillary equipment, such as portable, large size air compressors, mixing and conveying devices which have increased production capacities, there has been literally no change in the Gunite method of construction to date.

of the ASSOCIATION
Immediately following World War II the use of Gunite in every facet of construction increased by leaps and bounds. It also became apparent that architects, engineers, designers and specification writers needed a source of expert information to enable them to design for Gunite construction. In order to meet this demand, individuals started meeting informally to pool their knowledge and establish basic design criteria. Most of these individuals had been directly involved with the industry for a minimum of twenty years.
This tremendous pool of first-hand knowledge was incorporated into a set of recommended practices and general specifications. In 1951, the Gunite Contractors Association was formed by active, experienced firms. In February 1952, the Association was incorporated as a non-profit corporation for the express purpose of disseminating standards of the industry to anyone in need of this information. The Association has remained in continuous operation to date, and is internationally recognized as the fount of information and expertise. All requests for information are handled with the greatest dispatch.
The Association has, over the years, spent a great amount of money on research and development and has accumulated a vast reservoir of technical data to aid engineers in design. The Association also has expert speakers available to schools, colleges or other interested groups.
All of the above listed services are available at no cost to members of the architectural, engineering and general construction industries.

In order to reproduce lifelike prehistoric animals and giant reptiles such as the one pictured above, the original cement gun was conceived and developed. This is why it all started.


GUNITE: General Specifications

A-1 DEFINITION:
Gunite as herein specified is a trade name used to designate a mixture of Portland cement and sand thoroughly mixed dry, passed through a cement gun and conveyed by air through a flexible tube, hydrated at a nozzle at the end of such flexible tube and deposited by air pressure in its place of final repose.
A-2 PROPORTIONS:
Unless otherwise specified, all Gunite shall be mixed in the proportions of 1 part of cement to 41/2 parts of sand based on dry loose volume.
A-3 MATERIALS
CEMENT:
Only Portland cements of American manufacture complying with the current issue of "Standard Specifi¬cations for Portland Cement," A.S.T.M. C-150-67 shall be used. Type I or II Portland cement shall be used unless otherwise specified in the detailed specifications.
SAND:
Fine aggregate shall consist of washed sand and shall be hard, dense, durable, clean, sharp and graded evenly from fine to coarse in accordance with the "Standard Specifications for Concrete Aggregates," A.S.T.M. Designation: C 33-67. It shall be free from organic matter and shall not contain more than 5% by weight of deleterious substances.
LIMITS OF GRADING OF FINE AGGREGATES

 

                
 
For proper placement of Gunite, sand should contain between 3% and 6% moisture by weight. Sand and cement proportion may be corrected to provide for bulkage due to sand moisture- content. Percentage of bulking can be easily determined in the field, using a process based on the theory that 100% surface saturation by water will develop a material density equal to that of loose dry sand. To run this test simply fill any vertical sided watertight container level full of sand and fill container with water.

Physically measure the settlement of the sand and calculate the percent of shrinkage to vertical depth of the container.
Lightweight aggregates and refractory aggregates may be used in accordance with recommendations of the manufacturer.
WATER:
Water used for hydration at the nozzle shall be fit for drinking and shall be maintained at a uniform pressure which shall be at least 15 pounds per square inch above air pressure at the nozzle.
A-4 REINFORCEMENT:
All reinforcement shall be clean and free from loose mill scale, loose rust, oil or other coatings interfering with bond.
Reinforcement shall be rigidly secured in the position indicated on the drawings. Reinforcing bars shall be at least two and one-half (21/2) inches apart in the clear. Minimum cle ar distance between forms and reinforcement and for cover shall be as shown on the drawing. Space shall be provided for splicing bars in an approved manner. Only round bars or wire mesh shall be used.
A-5 OPERATING REQUIREMENTS:
For lengths of hose up to 100 feet, air pressure at the gun shall be 45 pounds per square inch or more. Where length exceeds 100 feet, pressure shall be increased 5 pounds per square inch for each additional 50 feet of hose required. Constant pressure must be maintained. Nozzles used for structural Guniting shall have a maximum size of 1-5/8".
A-6 REBOUND:
Rebound, recovered clean and free of foreign matter, may be reused as sand in a quantity not to exceed 20% of total sand requirements.
A-7 CONSTRUCTION JOINTS:
Particular care shall be given to formation of construction joints. They shall be sloped to a thin edge and the entire joint shall be thoroughly wetted before adjacent Gunite is placed. No square joints will, be allowed, unless specifically required.
A-8 CURING:
Gunite shall be damp cured for at least 7 days after placing or by proper application of an approved sealing compound. It shall be mandatory for the Gunite Contractor to perform the curing operation. No Gunite shall be placed during freezing weather except when protective measures are taken as with ordinary concrete work. Gunite shall not be placed against frosted surfaces.

GUNITE: General Specifications

A-9 GUNITE QUALITY:
The minimum ultimate 28 day compressive strength shall be 3000 pounds per square inch (Based on a nominal l:41/2mix).
The representative of the Engineer may require two test cylinders of Gunite each day as a material control. Test cylinders shall represent the quality of Gunite being placed in the structure, and if there is more than one crew or nozzleman on the work, test cylinders shall be made by each nozzleman in rotation so that the tests shall represent the quality of Gunite being placed by each nozzleman, all as determined by the representative of the Engineer. Each cylinder shall be dated, given a number, the name of the nozzleman making the cylinder and the point in the structure represented by the cylinder.
The Gunite Contractor shall furnish at his own expense, especially constructed cylinders 6 inches in diameter and 12 inches high, made of % inch square mesh hardware cloth.
The test cylinders of Gunite shall be made with the same air pressure, nozzle tip and hydration as the Gunite in the structure at the point where the cylinder was made.
At the end of 24 hours after cylinders are made, the hardware cloth form shall be removed and the cylinders stored in the testing laboratories in accordance with the current issue of "Standard Method of Making Compression Tests of Concrete," A.S.T.M. Designation C 39.
Separate tests of Gunite cylinders taken at the same place and time shall be made at the ages of 14 days and 28 days and shall be used for correlative purposes only. Compressive strength of structural Gunite in place shall be determined by cores as specified below.
A-10CORETESTS:
For structural Gunite a minimum of three cores shall be taken for each 250 cubic yards or fraction thereof, of structural Gunite deposited. Cores shall be obtained and tested in accordance with A.S.T.M. C 42-55. One core shall be removed and tested at a Gunite age of 14 days, the other two cores at a Gunite age of 28 days.
Fourteen day cores shall develop a minimum strength of 2200 psi. Twenty-eight day cores shall develop a minimum strength of 3000 psi.
If Gunite cores show deficient strength, additional cores shall be taken at the Contractor's expense from adjacent areas. Two cores shall be required for each deficient core. Should either additional core prove deficient, the defective Gunite shall be removed and replaced. Should such deficiency be evident in 14 day cores, on approval of the Engineer, the Contractor may proceed* with the work on his own responsibility until the 28-day cores are tested.

Where conditions preclude the possibility of obtaining cores from the Gunite in place, the Engineer may approve cores taken from a representative test panel made at the same time and under the same conditions as the structural Gunite.
A-11 WORKMEN:
Only foremen, nozzlemen, gunmen and rodmen with at least three years of structural experience shall be employed and satisfactory written evidence of such experience shall be furnished the Engineer or his representative upon demand.
A-12 ELIGIBLE GUNITE CONTRACTORS:
The Contractor, to be eligible as a bidder, must have had at least five (5) years' experience in Gunite construction and must list at least twenty significant structural Gunite installations which he has constructed and which, on investigation, have been found to be completed in a satisfactory manner. Bidders with limited experience are advised that very close scrutiny will be given all phases of this work. Unsatisfactory work will be immediately rejected. The Contractor is cautioned against attempting to substitute for specific equipment, items which have not been previously approved and items which may not meet all requirements of design and quality. Inferior equipment will not be accepted.
NEW GUNITE STRUCTURES Specification B B-1 SCOPE:
This specification with the General Specifications shall govern in all cases where Gunite is placed in a new structure.
B-2 FORMS:
Forms shall be adequately braced to insure against excessive vibration. Forms shall be built so as to permit the escape of air and rebound and to facilitate the placing of Gunite. Wall intersections shall be formed in such a manner as to afford a minimum loss of time in Guniting the intersection. This may be accomplished by the installation of short removable bulkheads at these points. Free standing columns may be formed on three sides or two adjacent sides whichever is practicable. Pilasters may be formed on two opposite sides. Forms for beams may be constructed of a soffit and one side or a segmented soffit and one side to permit Gunite placement in supported layers. Bond beams shall be formed with a soffit only. Shores shall be provided below the soffit in such a manner that no deflection will occur under the load to be superimposed. Bucks shall be installed around all openings.
Sufficient time shall be allowed other crafts for installation of equipment or materials which must be fastened to the forms. Form surfaces shall be cleaned prior to application of Gunite.

B-3 REINFORCEMENT:
See General Specifications. (Special requirements for any one job may be listed here.)

B-4 GROUND WIRES:
Adequate ground wires, to be used as screeds, shall be installed to establish the thickness and surface planes of the Gunite work. Ground wires shall be placed so that they are tight and true to line and in such a manner that they may be easily tightened.

B-5 PLACING OF GUNITE:
Whenever possible, except when enclosing reinforcing steel, the nozzle shall be held at right angles to the Gunite surface at a distance of 2Yz to ZVz feet. When enclosing steel, the nozzle shall be held so as to direct the material around the bars. A nozzleman's helper equipped with an air jet shall attend the nozzleman and blow out all rebound, sand, etc., which may have lodged on the forms, steel or Gunite. Gunite material shall emerge from the nozzle in a steady, uninterrupted flow. When flow becomes intermittent for any cause, the nozzle shall be diverted from the work until the flow again becomes constant. Hydration shall be thorough and uniform without the use of excessive water.
In shooting walls, columns and beams, application shall begin at the bottom and shall completely embed the reinforcement. The limit of the thickness and height has been exceeded when the material begins to sag.
In shooting beams, the nozzle shall be held at right angles to the surface of application.
In shooting formed slabs, the nozzle shall be held at a slight angle to the work so that rebound is blown onto the completed portion from where it shall be removed. The air jet shall be constantly employed to keep the area of placement free of rebound and all loose material. Wherever possible, slabs shall be completed in one operation.
Reinforcement shall be cleaned of any previously deposited Gunite which might prevent proper bond to reinforcement. Sufficient time shall be allowed between layers for the material to set. Before set has taken place, and before placing any succeeding layer, laitance shall be removed by brooming. Any laitance which has set shall be removed by sandblasting. Surfaces shall be damp at all times.
Rebound pockets, sags or other defects shall be carefully cut out and replaced with new Gunite-or hand patched in a manner satisfactory to the Engineer.

B-6 FINISHING:
Upon reaching the thickness and planes outlined by forms and ground wires, the. surface shall be rodded to true lines. Upon completion of rodding, ground wires may be removed. If possible, the finish coat shall be applied so that Gunite is not shot over the finished work. All exposed surfaces shall be finished to straight and true lines, as indicated on the drawings. Finish shall be as indicated below:
a. Steel Trowel
b. Wood Float - Granular texture
c. Rubber Float - Coarse texture and finish
d. Sack - Coarse sand texture with wavy outline
e. Broom - Natural finish broomed
f. Rodded - Natural finish removed by use of a rod
g. Gun - Natural finish as left by nozzle
It should be pointed out that all Gunite finishes tend toward a coarser texture than plastered surface due to coarser aggregate.

REHABILITATION OF CONCRETE
AND MASONRY STRUCTURES

Specification C

C-1 SCOPE:
This specification with the General Specifications shall govern in all cases where Gunite is used to rehabilitate concrete or masonry structures.

C-2 CUTTING AND STRIPPING:
Where plans indicate that concrete or masonry is to be cut or stripped, this work shall be done by experienced workmen equipped with suitable power tools. Where, in the opinion of the Engineer, the use of power tools may damage the structure, the cutting shall be done by hand. Extreme care shall be taken that portions to remain are trimmed in such a manner so as to facilitate placing of Gunite and to present a neat finished appearance, if exposed.

C-3 PREPARATION OF SURFACES:
All concrete or masonry surfaces to receive Gunite shall be thoroughly cleaned by sandblasting. Sandblasting shall be done by experienced workmen using approved equipment and suitable sandblasting materials. Prior to receiving Gunite, all surfaces shall be cleaned of dust and* debris, using compressed air and water. Concrete and masonry shall be thoroughly wetted before application of Gunite, but shall not be so wet as to overcome suction. Free water shall not remain on the surface to be Gunited, nor shall surfaces be so dry that there is excessive absorption of moisture from the Gunite.

C-4 PROTECTION OF SURFACES:
Surfaces which do not receive Gunite, such as wood framing, etc. shall be protected with waterproof paper or other adequate means.
C-5 BONDING TO STRUCTURAL STEEL:
Steel to be embedded or fireproofed with Gunite shall be cleaned of all substances which may prevent bond. Steel members shall be sandblasted where necessary to remove paint or scale.
C-6 FORMS: Same as B-2 C-7 REINFORCEMENT: Same as B-3 C-8 GROUND WIRES: Same as B-4 C-9 PLACING GUNITE: SameasB-5 C-10 FINISHING: SameasB-6
EARTH LININGS Specification D
D-1 SCOPE:
This specification with the General Specifications shall govern in all cases where Gunite is placed against earth for channel, ditch, reservoirs, lake and pool linings.
D-2 PREPARATION OF SUBGRADE:
The surfaces against which Gunite is to be applied shall be presented in a thoroughly compacted condition and shall be accurately trimmed to line and grade as shown on drawings. All dry surfaces shall be wetted before application of Gunite, but Gunite shall not be placed on any surface which is saturated, spongy or where free water exists.
D-3 REINFORCEMENT:
Reinforcement shall be as indicated on the plans, however, there shall be a minimum of .0025 times the cross sectional area of the lining in each direction. Wire mesh shall lap a minimum of one mesh spacing and laps shall be securely tied.
D-4 GROUND WIRES:
Ground wires, if required, shall be installed in such a manner that they accurately outline the section of the linings as indicated on the plans. They shall be located at intervals sufficient to insure proper thickness throughout and shall be maintained tight.
D-5 HEADERS:
Headers shall be installed where required or indicated on the plans and shall be securely set to line and grade.
D-6 PLACING OF GUNITE:
Gunite shall be placed in accordance with the Specifications. There shall be a nozzleman's helper continuously in attendance to accomplish at the

direction of the nozzleman the proper positioning of the reinforcement and cleaning of joints. Rebound shall not be incorporated in the work. Whenever possible, Gunite shall be installed to the full thickness of - the lining in one application. Where required, Gunite shall be carried over the side in a berm not less than the lining thickness and/or a cut-off wall constructed to prevent water seeping under the lining.
D-7 FINISHING:
Gunite shall be placed to the thickness indicated on the plans. The surface shall then be finished as specified.
D-8 WEEP HOLES:
Where weep holes are required they shall be installed in accordance with the drawings.
D-9 CURING:
Gunite shall be promptly cured in accordance with Paragraph A-8 of the General Specifications.
D-10OVERBREAK:
Overbreak is defined as excess material that is used to establish line or grade on any application where the original thickness has been increased due to over-excavation, erosion or lack of header boards. When present, overbreak shall be determined by a joint survey of the owner and/or general contractor with Gunite contractor representatives. Costs for overbreak shall be determined and set forth in writing acceptable to both parties prior to placement of any materials.
FIREPROOFING Specification E E-1 SCOPE:
These specifications shall govern work in which Gunite is applied to steel columns, girders and beams for purposes offireproofing.
E-2 PREPARATION OF SURFACE:
All steel surfaces to be encased with Gunite shall be thoroughly clean and free of rust, paint scale, oil, grease, dirt or other materials which would prevent bond.
E-3 REINFORCEMENT:
Reinforcement shall consist of a system of electrically welded wire mesh and/or bars. The mesh shall be furred out from the member to be encased so that the mesh occupies a position in the center of the encasement. Mesh laps shall be a minimum of 4 inches and shall be securely tied with wire at intervals of 12 inches. In general, the mesh shall follow the outline of the member.
E-4 GROUNDS:
Grounds or forms shall be placed at corners and along plane surfaces in such a manner that the full thickness is insured at all points.

LAST UPDATED 12-29-08