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sale@lscmagnetics.comNuméro de contact
+86 -13559234186
In reinforced concrete construction, the thickness of the concrete cover directly determines the structure’s durability, fire resistance, and load‑bearing safety. Achieving a precise cover thickness often depends on those inconspicuous small accessories—rebar positioning devices. Currently, the two most common products on the market are steel rebar chairs and plastic spacers. Each has its advocates and its limitations. This article provides a comprehensive comparison in terms of material properties, mechanical performance, ease of construction, and economy.
Steel rebar chairs are typically made from round or deformed steel bars, bent and welded into a “Π” shape or a continuous wave shape. They support the top layer of reinforcing mesh, maintaining the required distance between the mesh and the formwork.
Common forms: single‑leg chairs, double‑leg chairs, continuous chairs, and high chairs specifically for composite slabs
Core function: to bear the weight of the reinforcement and construction loads, preventing the top layer from sagging or deforming
Typical applications: floor slabs, foundation mats, box‑culvert top slabs, two‑way slab reinforcement mesh, etc.
Plastic spacers are injection‑molded from high‑density polyethylene (HDPE) or polypropylene (PP). They come in various shapes—washers, wheels, cones, and trestle types—and are used to maintain the gap between the reinforcement and the formwork.
Core function: to control the clear distance between rebar and formwork, ensuring the specified cover thickness
Typical applications: beams, columns, wall panels, precast elements, and components with high aesthetic requirements for exposed surfaces
| Dimension | Steel Rebar Chair | Plastic Spacer |
| Compressive strength | Very high – can support rebar self‑weight and construction foot traffic | Medium – standard spacers have limited capacity; heavy‑duty versions offer improved strength |
| Stiffness | High – resists deformation, maintains flatness of the rebar mesh | Low – prone to plastic deformation or crushing under heavy pressure |
| Applicable span | Large‑span slabs, heavy rebar mesh (e.g., double‑layer mesh with bars > Ø25) | Small‑span slabs, walls, columns, conventionally reinforced members |
Conclusion: For thick foundation mats, heavy industrial floors, and large‑span slabs, steel rebar chairs are almost irreplaceable. Plastic spacers are more suitable for conventional members with lighter loads.
This is the most debated aspect.
Hazards of steel chairs:
Steel can suffer pitting corrosion after concrete carbonation if the cover thickness is not properly controlled.
In areas with fluctuating water levels, marine environments, or chemical attack, steel chairs may become corrosion pathways, leading to rust‑induced cracking.
Standards generally require steel chairs to be made of bars with corrosion resistance at least equal to that of the main reinforcement, and they must not be exposed on the component surface.
Advantages of plastic spacers:
Good chemical stability – resistant to acids and alkalis, non‑conductive, and no new corrosion risk.
Good adhesion to concrete – no electrochemical corrosion issues.
Some premium products use fiber‑reinforced composites (e.g., glass‑fiber spacers) that balance strength and durability.
In highly corrosive environments (marine works, chemical plants, bridge piers) and for exposed architectural concrete, plastic or composite spacers are the more reliable choice.
| Aspect | Steel Rebar Chair | Plastic Spacer |
| Installation speed | Requires tying or spot welding – more labor time | Snap‑on design – ready to use, significantly faster |
| Positioning accuracy | Relies on worker skill – spacing consistency is moderate | Factory‑made with precise dimensions – easy to control spacing |
| Adaptability | Can be bent and adjusted on site – highly flexible | Fixed dimensions – poor adaptability for irregular parts |
| Formwork damage | Rigid steel feet may scratch wood or aluminum formwork | Soft material – formwork‑friendly |
For projects that prioritize construction speed and standardized operations, plastic spacers have the edge. However, when dealing with irregular components and frequent on‑site adjustments, the flexibility of steel chairs is more valuable.
Steel chairs: The contact area with formwork is small, but chairs may leave rust stains or slight bumps on the concrete surface – undesirable for exposed architectural concrete.
Plastic spacers: Well‑designed contact surfaces leave minimal marks after form removal. However, low‑quality plastic spacers may cause color differences or indentations (commonly called “spacer marks”), especially in high‑strength concrete or thin‑topping applications.
Recommendation: For architectural concrete, precast facade panels, and decorative elements, choose high‑quality concrete spacers or color‑matched plastic spacers that blend with the structural member. Always check the fit between spacer and formwork before pouring.
| Cost Item | Steel Rebar Chair | Plastic Spacer |
| Material unit price | Steel prices fluctuate – generally higher than plastic | Mass‑produced by injection molding – lower unit price |
| Labor cost | Tying takes time – labor share is high | Fast installation – low labor cost |
| Transport & storage | Heavy – higher transport and on‑site handling costs | Lightweight, compressible packaging – low logistics cost |
| Overall cost | Good value for heavy members | Significant cost advantage for conventional members |
Note: If plastic spacers cause rebar sagging or inadequate cover, the repair costs will far exceed the initial material savings. Cost control should never come at the expense of quality.
Steel chairs: Fully recyclable, but production is energy‑intensive (steelmaking CO₂ emissions). Some projects have started using recycled steel.
Plastic spacers: Made from petroleum‑based materials – not easily biodegradable. However, some manufacturers now offer recycled‑plastic spacers or reusable spacers. From a life‑cycle perspective, if plastic spacers help ensure structural durability (reducing maintenance and reconstruction), their indirect environmental benefit is significant.
| Application Scenario | Recommended Choice | Reason |
| Basement mat / pile cap (thick reinforcement, heavy loads) | Steel rebar chair | High load‑bearing capacity needed – plastic spacers may crush. |
| High‑rise floor slab (conventional reinforcement) | Plastic spacer or light‑duty steel chair | Efficiency and economy are priorities. |
| Precast composite slab / wall panel | Plastic spacer | Standardized production and high surface quality. |
| Bridge pier / marine structure | Plastic or composite spacer | Corrosion prevention is key – avoid introducing rust sources. |
| Architectural concrete / decorative member | Concrete spacer or color‑matched plastic spacer | Avoid color differences and rust stains. |
| High‑temperature workshop / fire‑rated structure | Steel chair or concrete spacer | Plastic softens and fails under high heat – steel chairs provide better fire resistance. |
If steel chairs must be used, ensure they are hot‑dip galvanized, powder‑coated, or painted with anti‑rust coating. Rusty chairs are strictly prohibited – rust not only stains the formwork but also creates a risk of corrosion‑induced cracking inside the concrete.
The market for plastic spacers is flooded with substandard products. Before purchase, request a compressive strength test report from the manufacturer and conduct on‑site sampling and pressure testing. For high‑strength concrete (C50 or above) or heavy rebar, ordinary plastic spacers are often inadequate – choose heavy‑duty reinforced plastic spacers or steel chairs.
Spacer spacing must comply with specifications. The general principle: the reinforcement must not deflect beyond allowable limits under construction loads. Typical maximum spacing: bottom spacers for slabs ≤ 1.0–1.2 m; top steel chairs for slabs ≤ 0.8–1.0 m. Always refer to the design codes and product technical manuals.
Never place rebar directly on formwork without any support – it may tip over during concrete placement.
Never use stones, broken bricks, wood blocks, etc., as substitutes for approved spacers. This is a common quality defect that seriously affects cover uniformity and concrete density.
Never use plastic spacers that have been stored in direct sunlight for a long time – UV exposure significantly reduces their mechanical properties.
Steel rebar chairs and plastic spacers are not a simple “one replaces the other” situation; rather, they complement each other.
Choose steel chairs for heavy loads, large spans, and severe construction loads – their rigidity and reliability are the safety baseline.
Choose plastic spacers when you prioritize construction speed, corrosion resistance, durability, and surface quality – their standardization and material properties are very attractive.
As a project manager, move beyond a “price‑first” mentality. Consider four factors comprehensively: structural type, environmental conditions, construction methods, and quality targets. Sometimes the most practical solution is to use both materials in the same component (e.g., steel chairs for the base slab and plastic spacers for the side walls).
Ultimately, regardless of your choice, strictly following specifications, procuring qualified products, and strengthening on‑site inspection are the fundamental ways to ensure that the concrete cover meets standards and that the structure achieves century‑long durability.
1. Q: What are the main uses of plastic spacers?
A: They control the thickness of the concrete cover, fix the position of reinforcing bars, and prevent corrosion due to insufficient cover.
2. Q: What are the advantages of plastic spacers?
A: Factory‑produced with high dimensional accuracy, lightweight, easy to install, corrosion‑resistant, and low cost.
3. Q: What are the main problems with plastic spacers?
A: Low strength – easily deformed or damaged during construction; weak bond with concrete; prone to aging; poor long‑term durability; and may create water seepage paths.
4. Q: What can be used as a substitute if plastic spacers are banned?
A: High‑strength precast concrete spacers (round, G‑type), prefabricated metal supports, F‑type rebar positioning clips, or ladder‑type supports.
5. (Number corrected) Q: What are the standard specifications for plastic spacers?
A: Internationally accepted standards include: BS 7973:2001 (British Standard, covering product performance and installation, with a requirement that spacers have open areas for aggregate passage); AS/NZS 2425:2015 (Australian/New Zealand Standard for plastic rebar chairs); JIS A 5390:2001 (Japanese Industrial Standard for plastic spacers in precast concrete); and the CRSI Manual (US standard).
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