Questions from the Field is a selection of questions NPCA Technical Services engineers received from calls, emails and comments on blogs or magazine articles on precast.org.
If you have a technical question, contact us by calling (800) 366-7731 or visit precast.org/technical-services.
Thomas writes:
Under what conditions can concrete reinforcing steel be used safely as lifting devices?
NPCA Technical Services engineers answered:
Use of rebar as lifting devices is not recommended. Lifting inserts used in precast concrete products must be verified for capacity and must have an adequate factor of safety for lifting and handling products. Safety factors include the various forces acting on the device, including form release suction, impact and the various positions of the product during handling. The capacity of commercial lifting devices must be marked on the devices or posted in production areas.
Lifting inserts, which are embedded or otherwise attached to precast concrete members, must be capable of supporting at least four times the maximum intended load applied or transmitted through them, as required in OSHA 29 CFR 1926.704 (c).
Lifting hardware such as slings, lift bars, chains, hooks, etc., must be verified for capacity and must have an adequate factor of safety for lifting and handling products. Lifting hardware must be capable of supporting at least five times the maximum intended load applied or transmitted to it, as required in OSHA 29 CFR 1926.704 (d).
Non-commercial lifting inserts and hardware must be proof-tested by a certified testing lab for the rated working load limit. OSHA requirements for lifting devices and apparatuses are documented in Title 29 of the Code of Federal Regulations.
Eric writes:
Are there any concerns regarding the use of clay veneers or cladding cast within precast concrete panels?
NPCA Technical Services engineers answered:
The short answer is no. Clay brick materials have been successfully bonded with cement-based mortars for centuries. The basic constituent differences between the mortars and concrete would be the addition of a coarse aggregate. Consequently, the primary chemical makeup and corresponding potential negative reaction is very low or non-existent. However, good testing practices for uniformity of raw materials used along with proper batching and curing are critical to ensure the finished product is consistent and meets the project team’s expectations.
NPCA file photo
Bob writes:
Can reinforcing steel be tack welded as an alternative to using wire ties or plastic-type clips?
NPCA Technical Services engineers answered:
Yes, this is a typical practice when building reinforcement cages with reinforcing bars or welded wire fabric.
However, it’s important to differentiate tack welds from structural welds. Structural welds occur when the weld is expected to transfer design loads from one bar element to another. Another example requiring a structural weld could be attaching an embedded steel item, like a lifting hook or weld plate, to the internal reinforcement as part of the structural function of the product. Those scenarios require welding practices that follow the AWS Structural Welding Code D1.4, which also addresses the weldability of different types of steel rebar (ASTM A615 vs. ASTM A706, for example).
The condition you described, to simply hold the bar in place in lieu of wire ties or plastic clips, doesn’t require structural welds. However, tack welding reinforcement assemblies does require trained welders and best welding practices.
NPCA file photo
Leave a Reply