Introduction – Inspections are an essential part of the precast concrete production process. Items such as sound walls, traffic barriers, and retaining walls are a few types of precast concrete products. Precast concrete differs from standard poured concrete in that the concrete is cast into forms in a shop, allowed to cure, and transported to the job site for erection. Poured concrete is formed at the job site in its permanent location. Also referred to as “cast in place” (CIP), it is not moved after pouring into the forms and finishing. A variation of precast concrete is prestressed concrete, which can also be fabricated in a shop depending on the product and application. Prestressing is used for items like bridge girders, wall panels, and foundation piles where rebar alone cannot provide the tensile strength that is needed. Prestressed concrete products are either pretensioned or post-tensioned. The pretensioning process involves the strands being stretched before the concrete is cast into forms in a shop. Post-tensioning is the process of stressing the strands after the concrete hardens. Because post-tensioning typically takes place at the jobsite, it is not addressed in this article.
In order to ensure the precast/prestressed products perform as designed, the fabrication process should be closely monitored. The quality inspection process involves multiple checks during fabrication, including pre-pour and post-pour inspections, concrete mix, batching, testing, placement, finishing, and curing. The scope of inspections will vary depending on the products being cast and the requirements of the Contract Documents. The scope of inspection for prestressed concrete products requires additional checks including pretensioning, strand placement and detensioning. The main components of precast/prestressed concrete fabrication inspection are detailed in this article to provide a basic understanding of the quality inspection process and the vital role it plays in achieving products that meet specified requirements.
The Inspection Process
The Inspection Process
The specific inspection requirements for precast/prestressed can vary by owner, but are often based on technical information provided by the Precast/Prestressed Concrete Institute (PCI), such as:
- Manual for Quality Control for Plants and Production of Structural Precast Concrete Products (MNL-116)
- Tolerance Manual for Precast and Prestressed Concrete Construction (MNL-135)
- Quality Control Technician/Inspector Training Manual (TM-101) for Certification of Inspection Personnel
Typical inspection requirements include the following:
1. Shop Drawings and Certifications
The beginning of the precast/prestressed inspection process starts long before the concrete is cast. Maintaining proper documentation and records are of utmost importance during fabrication. The first step in the process is to review shop drawings and specified tolerances. Materials being utilized in the production process need to be checked for compliance by reviewing mill certificates, such as reinforcing steel, cement, and aggregates. All material testing required must be recorded for compliance of quality of the products.
2. Pre-pour Inspection
Most of the precast/prestressed concrete products are produced using the wet casting method. Wet cast concrete is a flowable form of concrete cast into forms, then stripped and finished prior to shipment to the job site.
During the pre-pour inspection stage, forms and strand/reinforcements are inspected for proper dimensions, positioning, cleanliness, and use of release agents as listed in Table 1. For prestressed pretensioned concrete, strand tensioning is also inspected at this stage.
3. Concrete Mix, Placement and Finishing
Once the forms are accepted for use, the next step is to verify that the mix meets the approved design, applicable ASTM tests are performed, and that the pour is continuous with appropriate vibration and consolidation as outlined in Table 2.
4. Concrete Curing, Stripping and Detensioning
Once the concrete cures, the forms are stripped, strands are detensioned for prestressed pretensioned products, and the tests and documentation listed in Table 3 are completed.
5. Post-Pour Inspection
The last step is to confirm dimensions; placement of inserts, anchors, embeds, and holes; conduct final tests, identify deficiencies, and confirm proper storage and loading for transportation as listed in Table 4. The stamp indicating approval of the products is also affixed at this stage.
Conclusion – For precast/prestressed products to perform as required, it is important that inspections of the fabrication process be made at key points as listed above. Depending on the concrete product being fabricated and owner-specific requirements, additional inspections may be necessary. The American Concrete Institute (ACI) and PCI have many resources available on the topic of quality inspection for various products. In the end, quality inspection throughout the entire precast/prestressed concrete fabrication process will not only help to confirm that the final product meets all specified requirements but can also save a considerable amount of time and money on rework and negative impact on the schedule.
Topics in this precast concrete series:
Topic 1 – Inspection of the Precast/Prestressed Concrete Fabrication Process
Topic 2 – Preparation of Precast Concrete for Painting
Topic 3 – Coatings, Stains, and Water Repellents for Precast Concrete
Topic 4 – Inspection of Surface Preparation and Coating Application