The status quo and development trend of composite manhole covers

l Preface

    The manhole cover is the closure on the top of the entrance and exit to the underground facility. It is necessary to install the manhole cover wherever public facilities such as tap water, telecommunications, electricity, gas, heating, fire protection, sanitation, etc. are installed. With the rapid economic development, the construction and investment of infrastructure has ushered in a new cycle, and the market demand for manhole covers in urban planning and road construction is huge. Most of the materials used for manhole covers were reinforced concrete materials. However, reinforced concrete has defects such as high brittleness, easy aging, and easy fracture, and has been replaced by materials such as cast iron and ductile iron. At present, the amount of cast iron (including ductile iron) manhole covers is more than 90%. However, cast iron manhole covers have recycling value and are often stolen, resulting in car crashes and deaths from time to time. In order to solve the theft problem, currently two methods are mainly adopted: one is to install an anti-theft device on the cast iron manhole cover, but the use of the anti-theft device increases the cost, but can not solve the problem fundamentally, and brings a lot of inconvenience to use; The effective way is to use composite materials to solve the theft problem fundamentally.

2 Current status of composite manhole covers

    There are three types of composite manhole covers, namely, steel fiber concrete manhole covers, recycled resin composite manhole covers, and fiber-reinforced resin-based composite manhole covers. These three kinds of manhole covers do not contain or contain a small amount of metal, and have no recycling value.

2.1 Steel fiber concrete manhole cover

    The steel fiber concrete (SFRC) manhole cover is an improvement and development on the basis of the original reinforced concrete manhole cover. It is to add steel fibers with a length of 10-20mm into the concrete. Because a large number of small steel fibers are evenly dispersed in the concrete, the melting surface of the steel fibers and the concrete is large, and they are distributed in an irregular radial pattern, almost in all directions. Both make the concrete reinforced, that is, it has an isotropic reinforcement, which greatly improves the performance of the concrete.

 Table 1 Ratio of mechanical properties between SFRC and plain concrete (ordinary concrete)

Compared with plain concrete (ordinary concrete), steel fiber concrete has the advantages of significantly improved toughness and good crack resistance. Its tensile strength, flexural strength, fatigue strength, etc. have been greatly improved, and the crack width can be reduced. On the other hand, the substantial increase in toughness means that it has good resistance to impact when applied to structures that are under bending or tension. Table 2 shows the performance comparison results of steel fiber concrete manhole covers, cast iron manhole covers, and ordinary concrete manhole covers.

 Table 2 Performance comparison of manhole covers

It can be seen from the performance comparison that the strength of the steel fiber concrete manhole cover is 30% higher than that of the ordinary concrete manhole cover under the same ratio. The strength and surface crack resistance are close to that of cast iron. Under the condition that the weight of the component is basically the same, its bearing capacity Approaching or exceeding cast iron parts. The most obvious defects of steel fiber concrete manhole covers are heavy weight, difficult to open, and serious collapse during use. The generally adopted improvement measure is to add a metal frame edge to the outer road of the manhole cover, and the collapse phenomenon has been improved. Because steel fiber concrete manhole covers have the advantages of low manufacturing cost, low recycling value and anti-theft, they can replace cast iron manhole covers in residential areas and green belts.

2.2 Recycled resin composite material manhole cover

    Recycled resin composite manhole covers are formed by mixing and pressing waste plastics and fly ash in a molten state. The materials used in this kind of manhole cover are basically waste utilization, the raw materials are cheap, and there is no recycling value, and they are quite popular with users after they are on the market.

    Some research institutes have started the research on the blending of fly ash and waste plastics earlier, and have summarized a set of effective formulas and processes for new materials of waste plastics and fly ash, and their research has reached the international advanced level. The resin-based composite manhole cover synthesized from fly ash and waste plastics is rigid and flexible, and has a beautiful appearance. It can also save a lot of energy and steel resources. It can digest a large amount of fly ash and waste plastics. It conforms to the national energy conservation and environmental protection policy. Require. More than 90% of this composite material manhole cover uses waste materials and does not have any steel or other materials as the framework. The cost is 10-20% lower than similar cast iron products, and the economic benefits are significant. The use of this "green manhole cover" not only solves the anti-theft problem of the manhole cover, but also has the advantages of no corrosion and no rust, so it has a strong market competitiveness.

      The manhole cover produced with fly ash and waste plastics has good comprehensive properties such as compression resistance, bending resistance and impact resistance. After actual use in various places, it has been well evaluated, and its quality has reached or exceeded the level of similar cast iron products. . However, in the use process of the recycled resin composite material manhole cover, especially in the environment with high temperature, the manhole cover will creep under the continuous rolling of the vehicle, causing the middle part of the manhole cover to sink continuously and finally to fall off. This will cause a great safety hazard. Generally, it can only be used as a substitute for cast iron manhole covers in places where there is no vehicle passing and the manhole covers have low requirements for force.

2. 3 Fiber reinforced resin matrix composite manhole cover

      The fiber-reinforced resin-based composite manhole cover is a glass fiber reinforced plastic (FRP) manhole cover, which is a manhole cover made of unsaturated polyester resin as a matrix material, glass fiber as a reinforcing material, and supplemented by a certain filler. Due to different processes, different structures, and different enhancement forms, the performance of the manufactured manhole covers are also very different. The process methods of domestically developed FRP manhole covers generally include ① Resin Transfer Molding (RTM) process. Due to the high resin content and average production efficiency, it is not suitable for large-scale production; ②Hand lay-up production process. The manufactured manhole cover is only suitable for use in the green belt due to insufficient carrying capacity, and has no competitive advantage; ③The molding process. The manhole cover is a load-bearing structure product, and FRP manhole covers are best to adopt a molding process, and the product structure is relatively dense. At present, there are many domestic companies that use the molding process to make manhole covers, and the molding process generally uses SMC molding compound, which is pressed under a certain temperature and pressure.

      The current manhole cover product structure is mainly flat-bottomed, ribbed, and fish maw (Figure 1). From the analysis of the force structure, the fish-maw-shaped structure design is economical and reasonable. While saving raw materials and simplifying the mold, it can also greatly improve the carrying capacity of the manhole cover.

      The reinforcing material for SMC compression molding is short fibers of 10-50mm, and the strength of short fibers is only 20-30% of continuous fibers. Therefore, it is difficult to make the load-carrying capacity of the manhole cover meet the requirements of using only short fibers. In this case, some companies add steel bars to the manhole cover. From the point of view of load-bearing capacity, it has been significantly improved, but because the materials of steel bars and composite materials are quite different, their thermal expansion coefficients are quite different, and the surface of steel bars is often rusted, resulting in poor bonding between the surface and resin. In terms of service life, this method is not ideal. Compared with the manhole cover reinforced with short fibers, the manhole cover reinforced with continuous fiber will greatly increase its bearing capacity, without additional reinforcement, and the service life is more secure.

      Layered composite is the use of different reinforcing materials in different parts of the manhole cover. Continuous fiber reinforcement is used in the lower part of the manhole cover to ensure its bearing capacity; a large number of fillers are used in the middle part to improve the product's resistance to deformation; the upper part uses materials with higher hardness, good wear resistance and weather resistance, and different designs and different designs colour. The manhole cover prepared in this way can meet the requirements of wear resistance and weather resistance, and has the characteristics of beautiful appearance. FRP manhole covers have the advantages of light weight and high strength, good fatigue resistance, good damage safety, superior forming technology, corrosion resistance, beautiful appearance, strong pattern designability, etc., and can well solve the problem of cast iron manhole covers being stolen. It has become an ideal substitute for cast iron manhole covers. Table 3 shows the performance comparison results of FRP, cast iron and cement manhole covers.

Table 3 Performance comparison of FRP, cast iron and cement manhole covers

2. 4 Comparison of three kinds of composite material manhole covers

     Composite manhole covers can be used in different conditions instead of cast iron manhole covers. High-performance fiber-reinforced resin-based composite manhole covers can replace cast iron manhole covers in various conditions. Table 4 shows the comparison results of the three composite manhole covers. It can be seen that FRP manhole covers have incomparable advantages over other composite manhole covers. With the deepening of research and development, this type of manhole cover will be more widely used.

      At present, although the comprehensive performance of steel fiber concrete manhole covers and recycled resin composite manhole covers is not as good as FRP manhole covers, their research and development are relatively early, and corresponding industry standards have been established, which can guide the production of corresponding composite material manhole covers in China and ensure the performance of the manhole covers. Quality and order of market competition. There are more and more companies engaged in the production of FRP manhole covers, but there is no industry standard for their manhole covers, and it is impossible to uniformly measure and evaluate the quality of manhole covers of different companies. In order to ensure the healthy development of FRP manhole covers, industry standards should be formulated as soon as possible.

Table 4 Comparison of three composite material manhole covers

3.Development trends and prospects

     At present, the rapid development of my country's national economy is vigorously promoting the construction of infrastructure, and the market prospects are very promising due to the extensive use of manhole covers in urban planning and road construction. Since the performance and quality of manhole covers have an important impact on public safety, the formulation and improvement of industry standards for manhole covers has practical urgency and broad social significance. Carrying capacity is one of the most important properties of manhole covers. Vehicles traveling on the road generate alternating loads on the manhole cover, and the fatigue damage of the manhole cover should be the most serious. Therefore, in addition to the static load test, the dynamic load test fatigue test should be carried out to evaluate the bearing capacity of the manhole cover. The fatigue test is helpful to evaluate the service life of the manhole cover, and the determination of the service life provides a basis for ensuring the safe use of the manhole cover.

      Manhole covers are mostly used in cities and towns, factories and mines, and large-scale material distribution centers. The use conditions are different, and the corresponding use level should be determined. The corresponding manhole cover products are prepared according to different use conditions, which can not only meet the use requirements of various roads and sites, ensure the safety and reliability of the manhole cover during use, but also make the most reasonable use of materials to avoid waste of resources and reduce The production and use cost of the manhole cover. The existing industry recommended standards of the Ministry of Construction divide the load-bearing grade of cast iron manhole covers into heavy-duty and light-duty. Drawing on the industry standards of cast iron inspection manhole covers, and fully considering many factors such as the traffic flow, durability requirements, saving raw materials, and facilitating manufacturing and installation of the road where the manhole covers are located, composite manhole covers should be divided into three different use levels to adapt to roads under different conditions. And venue. The production of manhole covers according to the load level is conducive to the safe use of manhole covers and the rational use of materials, and will produce good economic and social benefits.

      At present, there are a large number of patented technologies related to composite material manhole covers, and more and more companies are engaged in the development and production of composite material manhole covers. With the sustained and rapid economic development, manhole covers have been more and more widely used in urban planning and road construction. This heralds the rise of a new material manhole cover industry. Technically speaking, different composite material manhole covers can be used in different conditions instead of cast iron manhole covers, and high-performance FRP manhole covers can be used in various conditions instead . There is reason to believe that with the further improvement of the performance of composite manhole covers and the increase in promotion, its spring will definitely come.