10-20 Section 1002
1002 -1 DESCRIPTION This section addresses shotcrete to be used for temporary support of excavations and other applications in accordance with the contract. Produce shotcrete by ei ther the dry -mix or wet-mix process composed of Portland cement, fine and/or coarse aggregates, water and at the Contractor ’s option, SCMs . Include chemical admixtures as required or needed for shotcrete produced by the wet -mix process. SCMs may be subs tituted for a portion of the Portland cement. Type IL, IS, IP or IT blended cement may be used instead of Portland cement. Mixes for all shotcrete shall be designed by a Certified Concrete Mix Design Technician or an engineer licensed by the State of Nort h Carolina. Dry-mix shotcrete shall be on the NCDOT APL. Shotcrete shall be applied by a nozzelman certified as an ACI Shotcrete Nozzelman in accordance with ACI Certification Publication CP-60. Nozzlemen sh all be certified in either dry-mix or wet -mix shotcrete based on the process to be used for the work. 1002 -2 MATERIALS Refer to Division 10. Item Section Chemical Admixtures 1024 -3 Coarse Aggregate 1014 -2 Fine Aggregate 1014 -1 Fly Ash 1024 -5 Ground Granulated Blast Furnace Slag 1024 -6 Portland Cement 1024 -1 Silica Fume 1024 -7 Type IL Blended Cement 1024 -1 Type IP Blended Cement 1024 -1 Type IS Blended Cement 1024 -1 Type IT Blended Cement 1024 -1 Water 1024 -4 1002 -3 SHOTCRETE FOR TEMPORARY SUPPORT OF EXCAVATIONS
A.Composition and Design Submit proposed shotcrete mix designs for each shotcrete mix to be used in the work. Mix proportions shall be determined by a testing laboratory approved by the Department. Submit shotcrete mix designs in terms of saturated surface dry weights on Materia ls and Tests Form 312U at least 35 days before proposed use. Adjust batch proportions to compensate for surface moisture contained in the aggregates at the time of batching. Changes in the saturated surface dry mix proportions will not be permitted unles s revised shotcrete mix designs have been submitted to the Engineer and approved. The Engineer will review the shotcrete mix design for compliance with the contract and notify the Contractor as to its acceptability contingent upon compressive strength test results for cores from preconstruction test panels. Do not use a shotcrete mix until written notice has been received. Acceptance of the shotcrete mix design does not relieve the Contractor of his responsibility to furnish a product that meets this cont ract.
B.Chemical Admixtures Use a quantity of chemical admixture within the range shown on the current list of approved admixtures issued by the Materials and Tests Unit.
10-21 (C) Strength of Shotcrete
Provide shotcrete with a compressive strength at 3 and 28 days of at least 2,000 psi and 4,000 psi, respectively. The compressive strength of the shotcrete will be considered the average compressive strength test results of 3 cores from the same test panel at each age.
D.Preconstruction Test Panels Before beginning construction, provide one preconstruction test panel for each shotcrete mix design and nozzlemen using the same equipment that will be used for the work. Use 3 feet x 3 feet forms at least 3.5 inches thick for preconstruction test panels. Batch, deliver, mix and apply shotcrete in accordance with Subarticles 1002- 3(E) and 1002- 3(F) and the contract. Make preconstruction test panels in the presence of the Engineer with forms in a vertical position and from the same shooting position anticipated for construction. Do not disturb test panels for the first 24 hours after shotcreting.
E.Mixing and Delivery Produce shotcrete of required strength, consistency, quality and uniformity with minimum rebound. Do not use rebound or previously expanded material in the shotcrete mix. Thoroughly mix materials in sufficient quantity to place shotcrete continuously. Regulate the delivery so the maximum interval between the shooting of batches at the work site does not exceed 20 minutes. Comply with Articles 1000 -10 through 1000- 13 to the extent applicable for shotcrete instead of concrete.
F.Shooting Requirements Use equipment capable of handling and shooting shotcrete at a steady uninterrupted flow. Use air supply systems that supply clean, dry air free of contamination and capable of maintaining sufficient nozzle velocity at all times. Apply shotcrete with the same equipment and methods as used for the preconstruction test panels. The shotcrete temperature at the time of shooting shall be not less than 50° F or no more than 90° F. Do not apply shotcrete during heavy rains or runoff or high winds so the nozzle stream separates during shooting. Do not apply shotcrete if surface to receive shotcrete is frozen or the air temperature measured at the location of the shotcreting operation in the shade away from artificial heat is below 40°F. Apply shotcrete before the time between adding the mixing water to the shotcrete mix and shooting the shotcrete exceeds 60 minutes.
G.Production Test Panels Provide one production test panel for every 33 cy of shotcrete with at least one test panel for each day shotcreting occurs. Use 18 inch x 18 inch forms at least 3.5 inches thick for production test panels. Make production test panels with forms in a vertical position from the same shooting position and at the same time as shotcreting is done. Do not disturb test panels for the first 24 hours after shotcreting.
H.Handling and Storing Test Panels Notify the Area Mat erials Engineer when preconstruction or production test panels are made within 24 hours of shooting the panels. Field cure and protect test panels from damage in accordance with ASTM C1140. The Contractor shall core the panels in the presence of the Engi neer. T he Department will transport the cores to a Materials and Tests Regional Laboratory for testing .
Source: North Carolina Standard Specifications for Roads and Structures, 2024 Edition. Pages 436–437 of 857.