【Haichuan Construction Technology】Why cannot plastic spacers be used for the rebar cover?
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On February 28 of this year, the Fujian Provincial Department of Housing and Urban-Rural Development issued the \"Notice on Strengthening Precise Control over the Quality of Reinforcement Installation Work\" (Min Jian Ban Jian Han [2025] No. 6), which stipulates that the use of plastic shims and stirrups is prohibited throughout the province. Subsequently, the Xiamen Housing and Urban-Rural Development Bureau also issued a corresponding document (Xia Jian Zhu Gong [2025] No. 40) to further refine and implement this policy. Why are plastic spacers, which are commonly used as auxiliary materials for the reinforcement cover, being banned? In fact, this policy in Fujian Province was not introduced suddenly; rather, it represents a further development and implementation of item 1.1.1 from the \"List of Construction Techniques, Equipment, and Materials That Pose a Risk to Production Safety in Housing Construction and Municipal Infrastructure Projects (First Batch)\」issued earlier by the Ministry of Housing and Urban-Rural Development, which already prohibits the use of the technique of manually manufacturing simple steel bar protection pads on-site. In engineering practice, three types of ready-made spacers and shims are used for protecting rebar: concrete-based, rebar-based, and plastic-based ones. The ban issued in Fujian Province this time applies to plastic shims and stirrups. Why can’t plastic shims be used? It mainly boils down to the following three points.
Why can’t plastic shims be used? It mainly boils down to the following three points. (1) The plastic shims lack sufficient strength. Plastic spacers are prone to deformation and damage during construction, leading to misalignment of rebar or uneven thickness of the protective layer. The thickness of the rebar cover refers to the distance from the outer edge of the stressed rebar to the outer surface of the concrete, and it directly affects the load-bearing capacity and safety of the structure. During construction, actions such as workers stepping on the rebar while pouring, as well as the impact from concrete pouring, exert pressure on the spacers. Plastic spacers, due to their lower strength, are prone to deformation or damage, which prevents the accurate positioning of the rebar. (2) Poor durability. Plastics tend to age and become brittle over time, which shortens their service life. When plastic spacers age, they can create voids or cracks within the concrete; these defects may serve as pathways for water and other corrosive substances to enter, leading to rusting of the rebar inside and affecting the structural safety and durability. (3) The adhesive strength of plastic spacers is weak, creating pathways for water leakage. Plastic and concrete are two different types of materials with poor bonding properties, which inevitably leads to a loose connection between them; the interface between the two can thus easily become a pathway for water leakage. During long-term use, if there are continuous pores or channels within concrete structures, water and corrosive substances can reach the surface of the rebar, causing rusting and reducing the structural lifespan. This problem is particularly prominent in humid environments or coastal areas. In contrast, concrete spacers have the same material properties as the main concrete structure, so they do not create the aforementioned weak points. Lastly, plastic shims are not environmentally friendly. Plastics are difficult to degrade, and they can cause environmental pollution when discarded, which does not meet the requirements of sustainable development.
Since plastic shims cannot be used, what material should be used as a substitute? In accordance with the requirements of the Fujian Provincial Department of Housing and Urban-Rural Development, the shims should be prefabricated circular or G-shaped shims made of high-strength concrete ； Metal brackets, F-shaped rebar positioning clips, or ladder bars are suitable for use as shims. (1) High-strength concrete spacers are the most common alternative at present, with the main types being round and G-shaped. The greatest advantage of concrete spacers is that they are compatible with the materials of the main structure, thus avoiding the creation of weak points, and their durability is comparable to that of the concrete structure itself. (2) Metal shims and positioning elements. The finished metal stirrups are made of steel, offering high strength and good stability, which can effectively ensure the accurate positioning of the rebar in the upper layer. (3) F-type rebar positioning clip, specifically used to fix the position of rebar and ensure precise control of the cover thickness. (4) Ladder ribs can fix the position of the steel bar grid as a whole, thereby enhancing overall stability.
After selecting the type of shims, attention must also be paid to the requirements regarding their placement. The placement of shims should follow these principles: (1) Shims should be placed between the rebar and the formwork to ensure that the rebar does not come into contact with the formwork. For different components, the key points for placing shims vary. There should be no less than 2 shims on each side of the beam bottom, with a spacing between the shims not exceeding 800 mm ； The vertical spacing between column elements is 600–1000 mm, with no fewer than 2 elements on each side ； The spacing between plate members is generally 600-1000 mm. (2) The number of shims should be sufficient to ensure that the rebar does not shift during concrete pouring. Not less than 4 per square meter, with increased density in critical areas. (3) The shims should be firmly fixed to the rebar and must not become loose or fall off. For horizontal members, shims should be placed at the intersections of the rebar ； For vertical members, shims should be placed symmetrically on both sides of the rebar. During concrete pouring, a dedicated person should also be assigned to check the positions of the positioning elements and shims. If it is found that these positioning elements have shifted due to interference with the rebar, they should be adjusted promptly to ensure the proper protection layer around the rebar as well as the quality of the concrete pour.
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