Section 2424. Shotcrete.

2424.01 DESCRIPTION.
This work shall consist of removing unsound concrete, preparing the concrete surfaces, and applying and curing shotcrete where indicated in the contract documents and where directed by the Engineer.

Shotcrete is mortar or concrete conveyed through a hose and pneumatically projected at high velocity onto a surface. Shotcrete shall be applied using the Dry Mix Process. This is a process in which the dry cement aggregate mixture is carried by compressed air to the nozzle where water is interjected and the resulting mixture is jetted from the nozzle onto the surface to be shotcreted.

A. Shotcrete Mortar.
Shotcrete mortar is a top or surface layer in which the aggregate is limited to sand with a maximum, nominal particle size of 1/4 inch (6.3 mm).

B. Shotcrete Concrete.
Shotcrete concrete contains fine aggregate and coarse aggregate with particle sizes in excess of 1/4 inch (6.3 mm). Procedures and quality of work shall, in general, comply with provisions of the current edition of ACI Standard 506, "Recommended Practice for Shotcreting," subject to approval of the Engineer.

2424.02 MATERIALS.
Materials for shotcreting shall meet the following requirements:

A. Portland Cement.
Portland cement shall meet requirements of Section 4101, Type I.

B. Water.
Section 4102 shall apply.

C. Fine Aggregate.
Fine aggregate for mortar or concrete shall be natural sand meeting requirements of Section 4110 or Section 4111.

D. Coarse Aggregate for Concrete.
Coarse aggregate for concrete shall meet requirements of Section 4115 and the following gradation requirements:

GRADATION
Sieve No. Percent Passing
3/4" (19 mm)
1/2" (12.5 mm)
3/8" (9.5 mm)
No. 4 (4.75 mm)
No. 200 (75 µm)		 100
97-100
40-90
0-30
0-1.5
The maximum percent passing the No. 200 (75 µm) sieve may be increased to 2.5%, provided the documented production limit agreed to and maintained is 1.0% or less and any increase up to 2.5% is due to degradation of the parent material and not to contamination by other material.

E. Wire Mesh.
Article 4151.04 shall apply. Mesh shall be 3 inches by 3 inches or 4 inches by 4 inches (75 mm by 75 mm or 100 mm by 100 mm) with nominal area of wire between 0.014 and 0.030 square inches (9 mm2 and 19 mm2), inclusive. The mesh shall be galvanized with a coating which is recognized in the industry as a "regular" coating. The coating shall be a minimum of 0.15 ounces per square foot (45 g/m2).

F. Concrete Anchors.
Concrete anchors, designed for use with 1/4 inch (6.4 mm) anchor bolts, shall be furnished. The 1/4 inch (6.4 mm) diameter, galvanized, hooked anchor bolts shall be furnished in lengths suitable for anchoring wire mesh to existing concrete.

2424.03 EQUIPMENT.

A. Power Driven Hand Tools
Power driven hand tools will be permitted with the following restrictions:

1. Jack Hammers.
Jack hammers heavier than a nominal 30 pound (with a mass greater than 14 kg) class shall not be used.

Jack hammers or mechanical chipping tools shall not be operated at an angle in excess of 45 degrees, measured from the surface of the concrete.

2. Chipping Hammers.
Chipping hammers heavier than a nominal 15 pound (with a mass greater than 7 kg) class shall not be used.

B. Hand Tools.
Hand tools, such as hammers and chisels, shall be provided for removal of final particles of unsound concrete or to achieve the required depth.

C. Sand Blasting Equipment.
Sand blasting equipment shall be capable of removing rust, oil, and concrete laitance from the existing surface.

D. Proportioning and Mixing Equipment.
Proportioning and mixing equipment for shotcrete shall meet requirements of Article 2001.20 and Article 2001.21, B. The mixing equipment shall be capable of thoroughly mixing the materials in sufficient quantity to maintain placing continuity.

E. Air Supply.
The compressor shall be of adequate capacity to maintain a sufficient, constant nozzle velocity for all parts of the work while simultaneously operating a blow pipe for cleaning away rebound. The air hose shall be equipped with a filter to prevent any oil or grease in the air stream.

F. Delivery Equipment.
The delivery equipment shall be capable of delivering a continuous, smooth supply of uniformly mixed material. The nozzle shall be equipped with a water ring and valve to permit adjustment of the water. The water added to the dry mix material at the nozzle shall be maintained at a pressure at least 15 psi (100 kPa) greater than the air pressure. The nozzle shall be capable of delivering a conical discharge stream.

G. Burlap.
Burlap for curing concrete shall meet requirements of Section 4104.

2424.04 QUALIFICATION OF OPERATORS.

A. Certification of Training and Experience.
The Contractor shall furnish to the Engineer a certified statement of training and experience in shotcreting for each of the individuals proposed to be utilized in the position of supervisor, shotcrete nozzle operator, and shotcrete gun operator.

B. Qualification Test.
Each nozzle operator, working with a certified gun operator, shall pass a qualification test prior to undertaking the shotcrete application in the project work. This test shall serve to qualify the shotcrete operator and shall be performed prior to beginning work. The test measures the percent by weight (mass) of rebound. If the rebound is over the allowable percentage, the operator shall be disqualified and shall be replaced by an operator who can pass this test.

The Engineer may call for additional qualification tests during the progress of the work if it is determined that the quality of the shotcrete operation is declining.

The test shall be as follows: The test panels shall be 30 inch by 30 inch (750 mm by 750 mm) plywood boards erected horizontally, vertically, overhead, or any combination of positions depending on the anticipated corresponding shooting positions. The shotcrete operator shall fill the middle 18 inch by 18 inch (450 mm by 450 mm) area of the test panel with shotcrete mortar to a minimum depth of 4 inches (100 mm). Drop cloths shall be arranged around and over the test panel to collect the rebound. The quantity of rebound and the applied shotcrete shall be determined and the percent rebound shall be computed by dividing the weight (mass) of rebound by the combined weight (mass) of rebound plus the weight (mass) of the applied shotcrete. The allowable percentage of rebound depends on the position of the surface and shall be as specified in the following table:

MAXIMUM REBOUND
Position of Surface Maximum Allowable Percentage of Rebound by Weight (Mass)
Horizontal
Vertical
Overhead		 15
30
50

The Contractor shall furnish the drop cloths, plywood, and all other material necessary for these tests.

2424.05 PREPARATION OF SURFACE FOR SHOTCRETE REPAIR.

A. Removals.
Concrete shall be removed from each area designated by the Engineer. Areas as shown in the contract documents are based on the best information available. Actual areas will be determined by the Engineer.

Concrete may be removed with power or hand tools, except that final cleanup shall be by the use of hand tools.

Removal shall extend at least to the level of reinforcing bars and shall extend deeper, as necessary, to remove all unsound concrete. In addition, further removal shall be accomplished, if necessary to assure a minimum replacement depth of 2 inches (50 mm). Boundaries of removal areas shall be cut to approximately a 45 degree bevel. They shall have neither feather edges nor square corners.

B. Cleaning Existing Reinforcement.
Existing reinforcement shall be cleaned of pack rust by hand methods and shall be sandblasted to sound metal. Where more than one-half of an existing reinforcing bar circumference is exposed or when a reinforcing bar is loose and unbonded, the old concrete shall be excavated 3/4 inch (20 mm) clear around the bar so that new concrete will bond around the periphery. Care shall be taken not to damage the reinforcement with tools.

C. Anchorage.
The principal anchorage of shotcrete to the existing structure shall be the bond of the shotcrete to the old concrete and to exposed existing reinforcement. Supplemental anchorage shall be provided as follows:

1. Where reinforcement has been exposed and clearance around the periphery of the bar is provided, no supplemental reinforcement need be provided, except where existing reinforcement density and pattern are such that individual open spaces between bars are of 1.5 square foot (0.14 m2) area or larger. For this condition, 1/4 inch (6.4 mm) diameter hooked anchor bolts shall be installed at the rate of one anchor bolt per each 1.5 square feet (0.14 m2) of area within each open space.

2. Where existing reinforcement is not exposed to provide clearance around the bar periphery, 1/4 inch (6.4 mm) diameter hooked anchor bolts shall be installed on an approximate 1 foot by 1 foot (300 mm by 300 mm) grid spacing. Mesh shall be fastened to each anchor and/or to any existing exposed reinforcement. Individual areas of 1 square foot (0.1 m2) or less shall have at least one anchor, although mesh will not be required. Wire mesh shall have a minimum of 3/4 inch (20 mm) clearance from the prepared surface.

D. Reinforcement.
Wire mesh reinforcement shall be secured to existing reinforcement and/or to hooked concrete anchors for areas described in Paragraph C above. Wire mesh shall be clean and free from coatings which will prevent adequate bond. Adjacent sheets of mesh shall lap at least 1.5 times the spacing of the mesh, and sheets shall be tied together at 1 foot (300 mm) intervals. Laps shall be aligned so that parallel wires are staggered, not placed adjacent to each other. Mesh extending around outside corners or re-entrant corners shall be bent to template before securing to the anchorage. All wire mesh, hook bolts, and existing reinforcement shall be positioned so that they will be covered by the application of at least 3/4 inch (20 mm) of shotcrete repair concrete.

E. Final Preparation.
Repair areas shall be given a final cleaning by sandblasting followed by air cleaning.

2424.06 PROPORTIONING AND MIXING.

A. Storage and Handling.
Storage and handling of cement and aggregates and measurement of materials shall be in accordance with Article 2403.06.

B. Mix Proportions.
The exact proportions of ingredients shall be determined on the basis of design mix proportions and a trial application of the design mix. Once approved by the Engineer, the field determined mix shall be used in the actual application of shotcrete and shall not be varied without written approval of the Engineer.

C. Field Test of Design Mixes.
The design mixes shall be field tested by shooting one or more test panels. The first panel shall be shot in the vertical position. If the first panel is satisfactory, a second panel shall be shot in the overhead position, if such position is applicable to the work.

A test for percentage of rebound shall be conducted with each test panel. Production of the test panel will be observed by the Engineer. The Engineer will approve the mix based on observation of the placement characteristics and appearance of the completed panels, or the Engineer will direct the testing of a new trial mix.

The test panels shall be 30 inch by 30 inch (750 mm by 750 mm) plywood boards. The panels for testing mortar shall be plain. The panels for testing concrete shall have one No. 5 (No. 15) reinforcing bar installed 1 inch (25 mm) from the surface of the backing board. This bar shall be parallel to and 6 inches (150 mm) from one side of the panel.

The middle 18 inch by 18 inch (450 mm by 450 mm) area shall be covered with shotcrete to a minimum depth of 4 inches (100 mm), and finished.

The test panels shall be cured according to Article 2424.08.

The operator qualification tests specified in Article 2424.04 may be conducted coincidentally with the field determination of mix for shotcrete.

D. Proportions for Shotcrete Concrete.
The design mix proportions for shotcrete concrete shall be 1 part cement to 3.5 parts aggregate. The aggregate fraction shall be proportioned 40% coarse and 60% fine by volume.

E. Proportions for Shotcrete Mortar.
The design mix proportions for shotcrete mortar shall be 1 part cement to 3.5 parts fine aggregate.

F. Proportioning.
Dry materials shall be proportioned by weight (mass). The moisture content of fine aggregates shall be in the range of 3% to 6% by weight (mass) and shall not vary more than ± 0.5% during a day's production.

G. Mixing.
Rebound materials shall not be reused. Each batch shall be mixed for at least 1.5 minutes. The mixer shall be cleaned as needed to remove all adherent material from the mixing vanes and from the drum. Batches in which the ingredients have been in contact with each other for 45 minutes shall not be used.

H. Admixtures.
Admixtures will not be specified. The Contractor may propose a specific admixture for either reducing rebound or improving the workability during finishing. The Engineer may approve such admixtures providing they are incorporated in the field determination of mix. Admixtures containing calcium chloride or polyvinyl acetate will not be allowed.

2424.07 PLACING AND FINISHING.

A. Prewetting.
Not less than 1 hour before the placement of shotcrete, the placement area shall be saturated and kept damp. However, shotcrete shall not be placed in standing water.

B. Concrete Placement.
Shotcrete concrete shall be applied in one or more layers to a total thickness as required to restore the repair areas to original lines and section or to construct a modified section as shown in the contract documents. Each layer should be 1 1/2 to 2 inches (40 mm to 50 mm) in thickness. Ground lines or other suitable devices shall be used as necessary to establish true lines and section. The final surface shall receive a wood float or broom finish, subject to approval of the Engineer.

The Contractor may use the following procedure in lieu of surface finishing shotcrete concrete as described above:

1. Follow the procedure described in A and B above, except stop the concrete filling and strike off the new concrete at about 1/2 inch (15 mm) below the final surface.

2. Change to shotcrete mortar and complete the repair, finishing the surface as described above.

C. Application Sequence.
The sequence of application for any layer, when shotcreting a vertical face, shall be from bottom to top with adequate attention to preventing incorporation of rebound.

2424.08CURING.
Repair areas restored with shotcrete shall be covered with wet burlap or other approved curing blankets. This cover is to be applied immediately following finishing and shall remain for at least 7 calendar days. Curing covers shall be maintained in a moist condition throughout this period.

The Contractor may submit, for approval by the Engineer, an alternate scheme to provide a wet cure through the minimum period stipulated above.

The Contractor shall submit this procedure for obtaining the wet cure, together with the schedule for application, to the Engineer for approval before beginning the work.

2424.09 LIMITATIONS OF OPERATIONS.

A. Temperature.

1. Shotcrete may be applied only when the ambient temperature is at least 40°F (4°C).

2. Shotcrete shall not be applied to any frozen or frosted surface.

3. Shotcrete, in place, shall be protected from freezing throughout the curing period.

B. Rain.

1. Shotcrete shall not be applied during periods of rain without shielding the work, the materials, and the batching and mixing facilities from the weather.

2. Shotcrete, in place, shall be shielded from rain until the curing blankets are in place.

C. Wind.

1. Sufficient screening or protection from wind shall be provided to prevent the cement or fines from being blown out of the jet.

2. Fresh shotcrete, in place, shall be protected from rapid cooling or drying out by shielding from the wind and/or application of fog mist until curing blankets are in place.

D. Containment.
The work area shall be screened sufficiently to:

1. Contain dust and rebound materials.

2. Protect nearby structures and vegetation.

E. Protection of Traffic.
Screening shall be mandatory between the work area and nearby traffic, as directed by the Engineer.

2424.10 METHOD OF MEASUREMENT.
Measurement of areas will be based on final surface dimensions to the nearest 0.1 foot (0.1 m), and the quantity will be calculated by the Engineer to the nearest square foot (0.1 m2) for each area.

2424.11 BASIS OF PAYMENT.
For the number of square feet (square meters) of shotcrete placed in a satisfactory manner and measured as provided above, the Contractor will be paid the contract unit price per square foot (square meters). This payment shall be full compensation for preparing the surface, installing anchors and reinforcement, testing, placing the concrete and mortar, curing and protection, and furnishing all materials, equipment, tools, labor, and incidentals necessary to complete the repair of the areas.