In the coming year, the focus will be on industry-wide awareness of the proper use of reinforcing steel.
By Phillip Cutler, P.E.
According to the Chinese calendar, 2012 will be the year of the dragon. For NPCA, 2012 is shaping up to be the year of concrete reinforcement.
Raising the bar, pun intended, means NPCA will be on a campaign to educate members about the proper quality control documentation and placement of reinforcement in precast concrete structures. In 2012, there will be a significant change in the NPCA Plant Certification Program in regard to reinforcement. This initiative is in response to demands from the specifier community as well as the precast industry to tighten up requirements on quality inspection of reinforcement.
Since the beginning of the NPCA Plant Certification Program in 1989, the Quality Assurance (QA) Committee has focused on quality. Improving the value of the program has been the focus of each edition of the “Quality Control Manual for Precast and Prestressed Concrete Plants,” and the 2012 program will continue this trend.
What can NPCA certified plants and their prospective customers expect in 2012?
In June, during NPCA’s Committee Week sessions in Indianapolis, the QA Committee voted to make the manual’s section 4.3.3, “Positioning of Reinforcement,” a “critical requirement section” for plant certification beginning in January 2012 (see the sidebar “Critical Requirements”). Representatives from certified plants heard details about this change and the rationale behind it at a luncheon during NPCA’s 46th Annual Convention in Coeur D’Alene, Idaho.
Why should reinforcing steel be a critical section requirement?
For plant engineering, quality, estimating and management personnel, the reasons and needs for proper selection, positioning and documentation of reinforcing steel characteristics (type, size, shape, steel area, condition and grade) in structures may be quite obvious. However, to fabricators and other plant personnel assembling and placing the reinforcing steel, these fabrication details may be viewed as just another component part on a print and the rationale behind it might not be as obvious. In order to meet design loading and performance requirements, the reinforcing steel for a given structure must be validated, which is of vital importance.
Precast concrete structures under load experience compressive forces that are easily tolerated due to concrete’s inherent nature to withstand compression. However, in many load cases a structure will also experience tensile forces on the opposite side from the compression force. A simple example is a flat-top slab, where the top surface of the slab under load is in compression and, as the element has a tendency to bow under load, the bottom surface experiences “stretching” or tensile forces. Concrete structures do not tolerate tensile forces without proper reinforcing steel to support these loads. In this case, the reinforcing steel needs to be as close to the bottom surface as possible to provide the maximum benefit. Good design practice places reinforcing steel close to the bottom surface of the slab, but always maintains the proper minimum concrete protection over the reinforcement. This distance is commonly known as adequate cover, and is usually specified in project documents or referenced in the standards that plants must conform to in manufacturing. Adequate cover is usually in the range of ¾ in. to 1 in., although more cover may be required for structures exposed to harsh weather or corrosive environments.
Also under the critical requirements of this section, plants must maintain rebar and/or reinforcing steel details for all precast structures in the plant files and ensure it is available to QC and plant personnel on the production floor. Plants will be required to maintain documentation of reinforcing cage inspections with information on the required cage design versus the actual cage used, including the following specific elements:
• Bar size and/or welded-wire reinforcement (WWR) bar diameter;
• Bar spacing and/or WWR style;
• Steel area (As);
• Effective depth (d), (d = the distance from the compressive face to the centroid of the tensile reinforcement member);
• Concrete cover, never less than ½ in. clear;
• Development length;
• Quantity and spacing of bars;
• Cage dimensions: length, width, height, and/or diameter, as applicable;
• Reinforcement condition:
– Clean or light red rust, not flaking or pitted;
– Free from oil, dirt and other contaminant;
– If welded, meets the requirements in Section 4.2.2 (proper grade and/or carbon equivalence);
– If welded, does not contain any damage such as gouges
• Reinforcement hooks and bends (90° and 180°). If design requires a bend in reinforcement around a corner, it is not acceptable to tie straight reinforcing corners together.
Included at the conclusion of Section 4.3.3, a detailed inspection is required on each piece produced unless the product is machine-cast or dry-cast (see Section 4.1.5 for reinforcement in machine-cast or dry-cast products). Documentation of the inspection can be on a piece- or production-shift basis and must be recorded daily.
What is a critical requirement section?
All critical requirement sections of the NPCA QC manual require a minimum passing grade of 75% in order to achieve normal certification status. Plants failing to achieve the minimum passing score for critical requirement sections are eligible for probationary certification, and must take corrective actions and pay for a reinspection of the plant within 90 calendar days from the previous inspection. A plant failing reinspection will be required to reapply to the program.
An educated work force is an effective work force. The worker on the floor places rebar on the form every day, ties it in place and uses the right spacers, but does the worker truly know why rebar is placed at a specific location? Do workers understand that placing the rebar ½ in. from the specified location could reduce the tensile capacity of the final precast product by as much as 10%? Do workers know how to read the symbols on a stick of rebar?
Sometimes manufacturing plants may depend a little too much on the QC staff. Sure, the QC staff understands the rationale behind steel detailing, but can they be reasonably expected to catch each misplaced rebar on a pre-pour inspection? Perfection is not likely. A production worker armed with the basic knowledge of steel reinforcing behavior in concrete and the rationale behind these requirements can provide additional insurance against misplaced, or worst yet, missing reinforcement.
NPCA will be launching educational opportunities for members to use to teach their own workers about the importance of reinforcement, as well as how to place it, tie it, and inspect it. Mel Marshall with Mel C. Marshall Industrial Consultants Inc. will be presenting a course on reinforcement at The Precast Show in Orlando, Fla., March 1, 2012, called the “Whys, Wheres and Hows of Reinforcement.”
Reinforcement is critical to precast concrete durability. This cannot be overstated. Improper steel selection and placement can seriously impair a precast concrete structure’s ability to withstand handling and service loads and can even lead to failure in the field. NPCA is committed to doing everything possible to maintain solid quality control in the proper selection and placement of reinforcing steel in all precast concrete products. Quality is something everyone in the precast industry should strive for, and 2012 promises to be a year that will raise the bar on industry-wide awareness of the critical importance of the proper use of reinforcing steel.