431Pneumatically Placed Concrete

2.1 Exceptions to Item 421, “Hydraulic Cement Concrete.” Provide a fine aggregate that meets the

requirements of Item 421, Table 6, Grade 1, and a coarse aggregate that meets the requirements of Item 421, Table 4, Grade 7, unless otherwise shown on the plans.

2.2 Exceptions to Item 440, “R einforcement for Concrete .” Provide mushroom -headed steel anchors or

expansion anchor hook bolts with a minimum diameter of 1/8 in. and a minimum length of 2 in. to attach reinforcement for the repair of concrete structures as shown on the plans or as directed. Reinforcing steel may be either welded wire fabric or reinforcing bars unless otherwise shown on the plans.

2.3 Exception to DMS -6310

, “Joint Sealants and Fillers.” Provide a preformed bituminous fiber material unless otherwise noted on the plans.

2.4 Proportioning and Mixing. Submit for approval a proposed mix design conforming to the basic mix design requirements shown in Table 1 unless otherwise shown on the plans.

Table 1 Classes of Concrete Class Ratio of Cement to Total Aggregate1 Minimum 7 -Day Compressive Strength (psi)2 I 1:4 3,000 II 1:5 2,500 1. More cement may be used when approved. 2. Higher minimum strengths may be specified. 601 Measure the cement and aggregates by volume and mix with enough water to achieve the desired consistency. Use as little water as possible to achieve sufficient adhesion. Mix concrete sufficiently dry so it will not sag or fall from vertical or inclined surfaces or separate in horizontal work. Prepare test panels using the same air pressure, nozzle tip, and position to be used for the production work to verify the mix design before approval. Apply a 3-in. layer of concrete to a plywood sheet with minimum dimensions of 18 in. × 18 in. for each test panel. Cure the test panels in the same manner as the proposed work. Take three cores, each 2 in. in diameter, out of each test panel and test in compression at 7 days in accordance with Tex -424-A . The mix design will be approved when the average strength of the three cores conforms to the strengths shown in Table 1 or strength shown on the plans. Provide additional test panels as directed if there are any changes in materials, equipment, or nozzle operator during the work.

3.1 Qualification . Provide experienced personnel able to produce concrete satisfying plan requirements and of

uniform quality as required. Provide documentation of nozzle operator’s qualification for the process proposed and orientation of the application meeting the minimum requirements when shown on the plans. Demonstrate nozzle operator’s abilities by constructing test panels before commencement of work. Orient test panels to match application direction of placement. Include reinforcing steel in the test panel with similar spacing as in member. Qualification test panels may be used for mix verification in accordance with Section 431.2.4., “Proportioning and Mixing.”

3.2 Surface Preparation . Grade the area of proposed work accurately to the elevation and dimensions shown

on the plans when concrete is to be placed against soil. Compact with sufficient moisture to provide a firm foundation and to prevent absorption of water from the concrete but without free surface moisture. Remove paint, rust, loose mill scale, grease or oil, and all other foreign materials that may reduce the bond of the concrete to the steel when concrete is used to encase structural steel members. Remove all deteriorated or loose material by chipping with pneumatic, electric, or hand tools when concrete is placed against concrete or rock. Cut square or slightly undercut shoulders approximately 1 in. deep along the perimeter of repair areas. Sandblast the surface to clean all rust from exposed reinforcing steel and to produce a clean rough- textured surface on the concrete or rock. Place the concrete while the substrate is in a saturated surf ace-dry (SSD) condition. Achieve SSD conditions by high- pressure water blasting 15– 30 min. before placing the repair material. An SSD condition is achieved when the surface remains damp until the concrete is applied. The surface must be free of standing water. Remove all free (ponded) water just before placing concrete. Provide joints, side forms, headers, and shooting strips for backing or paneling. Use ground or gauging wires where necessary to establish thickness, surface planes, and finish lines.

3.3 Reinf orcement . Place and secure reinforcement to ensure there is no displacement from impact of applying

pneumatically placed concrete. Place reinforcing bars at a spacing not less than 2- 1/2 in. Support reinforcing wire fabric or bars using mushroom -headed anchors, expansion hook bolts, or grouted rebar capable of resisting a pullout force of 2,500 lb. Space anchors no more than 12 in. center -to-center on overhead surfaces, 18 in. center -to-center on vertical surfaces, and 36 in. center -to-center on top horizontal surfaces. Use at least three anchors in each patch area. Do not use explosive force to shoot anchors into concrete. Check the resistance to pullout of the reinforcing anchors when directed. Notify the Engineer before installation of the anchors. Locate anchors so there is no damage to prestressing tendons or conduits embedded in the concrete. Use reinforcement when performing repair work in all areas where the thickness of the concrete will exceed 1-1/2 in. Use a single layer of either 2 × 2— W1.2 × W1.2 or 3 × 3 — W1.5 × W1.5 of welded wire fabric, or 602 approved equivalent, unless otherwise shown on the plans. Use a single layer of wire fabric to reinforce each 4-in. thickness of patch or fractional part in areas where the concrete thickness exceeds 4 in. Encase completely each layer of wire fabric in concrete that has taken its initial set before installing the succeeding layer of wire fabric. Place the reinforcing fabric parallel to the finished surface and support it so it will be at least 3/4 in. out fro m the surface to be covered. Provide at least 1 in. of clearance between the finished concrete surface and all steel items , including anchors, reinforcing bars, and wire fabric. Lap adjacent fabric sheets at least 6 in. and tie together securely at a spaci ng of no more than 18 in. Pre -bend fabric before installing to fit around corners and into re- entrant angles. Pre-bend the welded wire fabric for encasement of steel members using a template to conform as nearly as possible to the outlines of the members t o be encased. Drill holes between 1/2 and 1 in. in diameter in the webs of the members as close as possible to the flanges to allow for attachment of the reinforcing fabric. Space these holes at approximately 3 ft. on center. Use 3/8- in. diameter rods plac ed through these holes to secure the reinforcing fabric. Hold the reinforcing fabric at least 3/4 in. out from the surface of the steel member. Lap adjacent fabric sheets at least 6 in. and tie together at a spacing of no more than 12 in.

3.4 Pneumatic Placement of Concrete . Pneumatically applied concrete can be either dry -mix or wet -mix. The

dry-mix process consists of dry- mixed fine aggregate and hydraulic cement to which water is added immediately before its pneumatic expulsion from a nozzle. The wet -mix pro cess consists of mechanically premixed concrete pneumatically applied through a nozzle.

3.4.1 General . Place the concrete when the ambient temperature is above 35°F and rising and material

temperature is between 50°F and 90°F for wet -mix and below 100°F for dry -mix. Do not place concrete against a surface containing frost, ice, or standing water. Protect concrete from freezing or quick drying after placement. Apply the concrete using pneumatic equipment that sprays the mix onto the prepared surface at a veloc ity less than 100 ft. per sec ond for construction of portions of structures, repairing concrete structures, or encasement of structural steel members. Minimize rebound and produce a compacted dense homogenous mass. Do not apply concrete if high winds will prevent proper application or if rain could wash out the concrete. Hold the nozzle approximately 2– 4 ft. from the surface and position it so the concrete impinges nearly at right angles to the surface being covered. Use shooting strips to ensure straight lines, square corners, and a plane surface of concrete. Place to keep the trapping of rebound to a minimum. Slope the concrete off to a thin edge at the end of each day’s work or at similar stopping periods requiring construction joint. Thoroughly clean and wet previously placed concrete before placing an adjacent or additional section. Apply a sufficient number of coats to obtain the required thickness. Place coats on vertical and overhead surfaces in layers of such thickness to prevent sloughing, sagging, tearing, or debonding. Provide a sufficient interval between successive layers in sloping, vertical, or overhead work to allow initial but not final set. Clean the surface to remove the thin film of laitance to provide for a bond with succeeding applications. Remove rebound and accumulat ed loose sand from the surface to be covered before placing the original or succeeding layers of concrete. Correct any sags or other defects to the proper section as directed. Place concrete to completely encase reinforcing steel. Encase reinforcing steel by shooting with sufficient velocity and plasticity that material flows around and behind reinforcement. Apply the concrete using either the wet -mix or dry -mix process unless otherwise shown on the plans. Mix the materials thoroughly and uniformly using a paddle or drum -type mixer designed for pneumatic application. Wet-mix process applications can use transit -mix concrete. Do not use the wet -mix process for repair of damaged concrete. Clean mixing and placing equipment at regular intervals. Inspect the noz zle liner and water and air injection system daily; replace worn parts as necessary. Do not reuse rebound or overspray concrete.

3.4.2 Dry-Mix Process. Use a compressor or blower capable of delivering a sufficient volume of oil -free air at the

pressure shown in Table 2. Maintain steady pressure throughout the placing process. 603 Use a water pump with the size and capacity to deliver water to the nozzle with a pressure at least 15 psi more than the required air pressure. The values shown in Table 2 are based on a hose length of 150 ft. with the nozzle less than 25 ft. above the delivery equipment. Increase operating pressure approximately 5 psi for each additional 50 ft. of hose and approximately 5 psi for each 25 ft. the nozzle is raised. Table 2 Compressor Capaciti es Compressor Capacity, cu. ft/min. Hose Diameter, in. Maximum Size of Nozzle Tip, in. Operating Air Pressure Available, psi 250 1 3/4 40 315 1-1/4 1 45 365 1-1/2 1-1/4 55 500 1-5/8 1-1/2 65 600 1-3/4 1-5/8 75 750 2 1-3/4 85

3.4.3 Wet-Mix Process. Operate the pump at a line pressure between 100 psi and 300 psi. Use delivery hoses

between 1- 1/2 in. and 3 in. in diameter. Use mixing equipment capable of thoroughly mixing the materials in sufficient quantity to maintain continuous placement.

3.5 Construction Joints . Use a square butt joint where the joint is subject to compressive stress or is over

existing construction joints , unless otherwise shown on the plans. Use tapered or square butt joints at other locations. Square the outside 1 in. of tap ered joints perpendicular to the surface.

3.6 Finish . Use a sharp trowel to cut off all high spots after the concrete has been placed to the desired

thickness or screed to a true plane as determined by shooting strips or by the original concrete surface. Light ly apply cutting screeds, where used, to all surfaces so as not to disturb the concrete for an appreciable depth. Work in an upward direction when concrete is applied on vertical surfaces. Give the finished concrete a final flash coat of approximately 1/8 in. unless directed otherwise. Obtain a uniform appearance on all exposed surfaces unless otherwise shown on the plans.

3.7 Curing. Cure encasements with water for 4 days. Cure repairs and structural construction using either a

piece of wet burlap taped over the repaired area with a covering of 4- mil minimum plastic sheet also taped in place or membrane curing as approved. Overlap the burlap with the plastic sheet and continuously tape the edges with a tape at least 3 in. wide (air duct tape or better) to completely enclose the mat and hold in moisture. Cure in this manner for 4 days. Curing is not required for soil -nail walls unless walls are the final exposed surfaces ; in this case, cure at least 4 days in accordance with Item 420. Apply membrane curing in accordance with Section 420.2.7., “Curing Materials ,” for tunnel and ditch linings and vertical or overhead patches as approved.

3.8 Repair of Defects. Repair or replace debonded areas as directed.