By Kayla Hanson, P.E.
Concrete is an inherently strong, durable and resilient material; however, its most impressive strength is displayed in compression. Its strength in tension tends to be only 10% of what it’s capable of enduring in pure compression. Conversely, steel exhibits incredible strength in tension and limited strength in compression. When these materials are used together strategically, each of their greatest strengths are activated, and the result is a uniquely capable and exceedingly strong construction material: reinforced concrete.
As with any design and manufacturing process, care must be taken throughout material procurement, storage and fabrication to optimize the benefits of concrete and steel together.
Whether your plant uses traditional black bar, epoxy-coated reinforcement, welded wire fabric or mesh, or other types of reinforcement, start by ensuring the reinforcement mill certificate for each shipment shows the material complies with the applicable ASTM standard. The most commonly referenced reinforcement standards in precast are:
- ASTM A615, “Specification for Deformed and Plain Carbon-Steel Bars for Concrete Reinforcement”
- ASTM A706, “Specification for Deformed and Plain Low-Alloy Steel Bars for Concrete Reinforcement”
- ASTM A775, “Specification for Epoxy-Coated Steel Reinforcing Bars”
- ASTM A1064, “Specification for Carbon-Steel Wire and Welded Wire Reinforcement, Plain and Deformed, for Concrete”
These standards ensure a consistent quality reinforcement material is used in your reinforced concrete products. For those projects complying to the Buy American Act, it is also very important the steel reinforcing documentation received clearly shows the country of origin.
Protect against contaminants
The bond between reinforcement and the surrounding concrete is critical to the reinforced concrete product’s performance and the steel’s ability to “activate” when necessary. Compromised bond between the two materials will prevent the structure from behaving as designed.
Regardless of the type of reinforcement, time of year or storage location, all reinforcement must be stored elevated from the ground, preferably in racks or on dunnage that sufficiently supports the reinforcement and prevents it from bowing considerably. When reinforcement contacts the ground, contaminants such as dust, dirt and oil can cling to the reinforcement’s surface and reduce its ability to bond with concrete. Refer to the Precast Inc. article “Bond, Reinforcement Bond” for more information.i
Protect against the elements
If the plant layout allows, consider storing reinforcement indoors or under cover to help protect it from the elements. Limiting the amount of exposure to sun, rain, snow, ice and temperature fluctuations will reduce the amount of weathering and oxidation the steel will experience prior to use. Epoxy-coated reinforcement is particularly susceptible to degradation and must be stored indoors or under a tarp or other means to provide protection from the sun’s ultraviolet rays, no matter the time of year.
Know acceptable levels of oxidation
Some reinforcement may be shipped from the supplier with minor spots of oxidation. Rough or irregular steel surfaces can enhance bond with concrete, so minor oxidation can actually benefit bond strength. However, if reinforcement shows signs of rusting to the point that the bar’s cross-sectional area is impacted even slightly, the reinforcement must not be used.
Ensure easy identification
All reinforcement supplies – whether bundles, mats or coils – must always be easily identifiable for as long as you have the material. The identification tag affixed to the shipment which shows the supplier name, steel grade, dimensions and/or bar size, heat number and other information must remain attached to the supply until the last piece is used.
Know the governing tolerances
All reinforcement must be fabricated in accordance with a detailed steel plan document, which must also show the steel dimensional tolerances, including length and spacing allowances. Certain ASTM standards as well as ACI 318, “Building Code Requirements for Reinforced Concrete,” outline product-specific tolerances. However, some projects or jurisdictions will specify different tolerances, so it is imperative the project documents clearly indicate the governing tolerance scheme.
Approve any variation
Any reinforcement substitution or variation from the detailed steel plan documents – in bar size, number of bars, steel grade, bar spacing, splice type, etc. – must be reviewed and approved by the appropriate plant personnel. Using a larger size bar or decreasing the spacing between bars does not always correspond to a stronger structure, so even the slightest variation or minor substitution must be approved before proceeding with the change.
Measure twice, cut and bend once
Use care when measuring, cutting and bending reinforcement to ensure accuracy. Bending or straightening reinforcement in extremely cold weather typically requires preheating of the bars to prevent cracking or brittle breakage. Most bars can be cut to size with an oxy-acetylene torch or even a bolt cutter. Epoxy-coated bars should be cut using a saw with a diamond-tip blade instead of being flame-cut.
Using reinforcement templates or jigs for frequently used or standard designs can help expedite the reinforcement cutting, layout and assembly process while also reducing possibilities for human error.
All reinforcement shall be bent in accordance with standard CRSI and RSIC/IAAC fabrication practices. Give special attention to the minimum bend diameters and hook dimensions associated with different bar sizes, lengths and steel types which are set forth by CRSI and RSIC/IAAC and reference ACI 318.ii Care should be given to the bending process to ensure the necessary bend is achieved on the first attempt rather than trying to re-bend the bar to the correct radius, since bending the bar back and forth multiple times can reduce its integrity.
If the design, project specifications or detailed reinforcing steel plans require a bend in reinforcing steel around a corner, substitution of straight sections tied or welded together is not an acceptable practice.
Ensure a rigid assembly
Reinforcement cages, bar mats and other configurations must be fabricated into rigid assemblies, meaning they will retain their shape, dimensions, spacing and integrity during handling, transport, positioning in the form and during concrete placement. Rigid assemblies are achieved by tying with wires, clipping with plastic clips or welding when allowed. Liberal use of tie wires, clips and welds is recommended. When using wire ties to assemble epoxy-coated reinforcement, ensure only epoxy-coated tie wires are used.
Generally, ASTM A615-compliant reinforcement is considered non-weldable unless carbon equivalence calculations are performed to determine the appropriateness of welding. So long as the carbon equivalence content falls within the target range set forth by American Welding Society D1.4, “Structural Welding Code – Steel Reinforcing Bars,” the bars may be welded. If the carbon equivalent falls outside of the target range, the bars may still be welded if the reinforcement is preheated adequately. Review Section 4.2.2 of the NPCA Quality Control Manual for guidelines, applicable equations and target ranges. ASTM A706-compliant reinforcement is considered weldable without performing carbon equivalent calculations. It is less brittle as a result of its lower carbon content, and therefore considered acceptable for welding.
Additionally, reinforcement that will be welded in temperatures less than 32 F should be allowed to reach 70 F prior to and during welding. When welding any type of reinforcement, be careful not to burn through the reinforcement or cause undercutting, which compromises the steel’s integrity and reduces the steel’s cross-sectional area. Refer to the Precast Inc. article “Practice What You Preach: Tips for Welding Rebar” for more information.iii
ASTM A775-compliant epoxy-coated reinforcement may also be welded, however this requires particular attention to detail to ensure a solid weld and adequate repair to the epoxy coating. Refer to the Precast Inc. article “Working with Epoxy-Coated Rebar” for further information.iv
Follow lap splice guidelines
Lap splices allow two lengths of reinforcement or two ends of welded wire reinforcement to be joined with the intent of acting as one continuous section of reinforcement. Minimum lap splice lengths are dictated by ACI 318 and depend on the concrete strength, steel grade, reinforcement bar size and spacing. Particular care is needed to ensure spliced reinforcement behaves as designed. Refer to CRSI’s Introduction to Steel Reinforcing Bar Splices for more information.v
Repair damage and watch for warning signs
Any damaged locations on epoxy-coated reinforcement must be repaired with an appropriate patching material in a manner conforming to the patching material manufacturer’s recommendations. When epoxy-coated reinforcement is cut or welded, repairs must be made to both the cut ends and to the compromised epoxy coating near the weld.
When bending bars, be mindful of the steel temperature as well as the ambient temperature. Watch for small cracks in the steel at the bend radius and discard any compromised reinforcement.
Combining the best of both materials
Reinforced concrete is nothing new. Mortar and concrete reinforced with materials like straw and hair has been used for millennia. However, only in recent history were two of earth’s most significant manmade materials combined to create modern steel-reinforced concrete. Taking special care to ensure the steel is handled properly, stored safely and fabricated with attention to detail will help optimize the performance of these materials and the structures they create.
Now is the time to review reinforcement best practices in your plant.
Kayla Hanson, P.E. is NPCA’s director of technical services.