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Shotcrete Quality Assurance and Quality Control

"Shotcrete is Concrete"

Definition of Shotcrete : "Concrete projected at high velocity onto a properly prepared surface. "

  • Concrete is vibrated to achieve consolidation.

  • Shotcrete is consolidated by material impact velocity.

When ACI 506.2-95, "Specifications for Materials, Proportioning, and Application of Shotcrete," was adopted in 1995, one of the most controversial sections required a visual core-grading system for evaluating reinforcement encasement. The core-grading requirement is still a contentious issue because some industry experts question its reliability.

Five grades were established, with grade 1 indicating the highest quality and grade 5 the lowest. (ACI 506.2-95)

Cores taken from a thick foundation wall

ACI 506.2-13:

In the revised ACI 506.2-13, ACI removed grading system and defers acceptance criteria to the Engineer.

Shotcrete Acceptance/rejection criteria:

Remove and replace shotcrete that exhibits laminations, voids, or sand pockets exceeding the specified quality. Remove and replace shotcrete that does not comply with the Contract Documents."

ACI 506.6T-17 Visual Shotcrete Core Quality Evaluation Technote:

ACI 506.6T-17 Visual Shotcrete Core Quality Evaluation Technote was published in 2017 and it is a reference document for a reasonable evaluation of shotcrete quality. Visual evaluation of core quality is based on two criteria:

  1. The degree of encapsulation of the reinforcing steel;

  2. The size and location of voids.

Qualification of the shotcrete contractor and nozzlemen, pre-construction mockup panels

Pre-construction mockup panels are used to qualify the shotcrete nozzleman, crew, equipment, and material. It takes a complete, experienced team for a successful concrete project using shotcrete placement.

The purpose of preconstruction testing is to ensure the shotcrete work will achieve the following objectives:

  1. The shotcrete placement can properly encase the reinforcement given the reinforcement design (size, spacing, and amount)

  2. The material meets the project specifications

  3. The finish will meet the owner's expectations

The pre-construction mockup panels should represent the most difficult-to-shoot part of the proposed project. This will often be the sections with the most congested reinforcing, large or irregular shaped block-outs or embeds, or a complicated geometry.

A “shadow” area occurs behind the steel rebar during shotcrete application. This shadow area is filled when using good shotcrete shooting technique to provide full encapsulation of the bar.
The nozzlemen qualification program is essential to a successful project.

Testing during construction: quality assurance and quality control

It is essential to establish quality assurance/quality control (QA/QC) procedures to ensure that the final product functions as designed ad has a satisfactory life expectancy.

Among the factors that determine the quality of the shotcrete are design, materials, application equipment, craftsmanship, and installation techniques.

Shotcrete application - shooting a beehive of plain concrete mixture for Crosslinx Project

Construction Testing

Shoot one construction test panel for each 40 cubic meters of shotcrete production, or for each day of shotcrete production, whichever is more frequent. Shoot the panel in the same orientation as the work being done.

Produce, store, handle, and cure construction test panels in the same manner prescribed for preconstruction test panels. Similarly, prepare, handle, cure, and test shotcrete in the same manner prescribed for the preconstruction test panels.

Shotcrete test panels:

Cores taken from Shotcrete production panels to evaluate compressive strength of the concrete material.

Reference Documents/Standards and Codes:

American Society for Testing and Materials (ASTM)

ASTM C1604 - Standard Test Method for Obtaining and Testing Drilled Cores of Shotcrete

ASTM C1602 - Standard Specification for Mixing Water Used in Production of Hydraulic Cement Concrete

ASTM C1550 - Standard Specification for Flexural Toughness of Fibre Reinforced Concrete (Using Centrally Loaded Round Panel)

ASTM C1436 - Standard Specification for Materials for Shotcrete

ASTM C1141 - Specification for Admixtures for Shotcrete

ASTM C1140 - Standard Practice for Preparing and Testing Specimens from Shotcrete Test Panels

ASTM C1116 - Specification for Fibre Reinforced Concrete

ASTM C642-82 - Standard Test Method for Specific Gravity, Absorption, and Voids in Hardened Concrete

ASTM C114 - Standard Test Methods for Chemical Analysis of Hydraulic Cements

American Concrete Institute (ACI)

ACI 506R-16 - Guide to Shotcrete

ACI 506.2-13 - Specifications for Materials Proportioning and Application of Shotcrete

ACI 506.6T-17 - Visual Shotcrete Core Quality Evaluation Technote

Canadian Standards Association (CSA)

CSA A3001-03 Cementitious Materials for Use in Concrete

CSA A23.2-04 Methods of Test and Standard Practices for Concrete

CSA A23.1-04 Concrete Materials and Methods of Concrete Construction


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