1. Overview of Plain Concrete Formwork
Concrete, professionally known as concrete, can be categorized into ordinary plain concrete, decorative plain concrete, and decorative plain concrete with unique finishes. Decorative plain concrete, or decorative concrete, offers a distinctive aesthetic effect. It is poured in one continuous process, preserving the natural color and texture of cast-in-place concrete to serve as a building’s decorative surface. The result is fresh, elegant, bright, and modern in appearance. In recent years, this technique has gained significant praise and has become widely used in construction projects.
Colored clear water concrete presents even greater challenges due to its complex process and the difficulty of achieving a beautiful finish. Unlike ordinary concrete, plain concrete is more akin to a craft that demands precision and excellence. Clear faced concrete utilizes the natural texture of the formed concrete surface as its decorative feature. To meet the structural and aesthetic requirements, formwork for plain concrete can include large steel formwork, steel-wood formwork, composite ribbed plastic formwork, aluminum alloy formwork, and polyurethane-lined formwork.
Architects first transform the mold into a “work of art,” pour the concrete into this mold, and then remove the formwork without applying any additional decoration. Every step in the creation of fair faced concrete must be executed with care and precision. Since concrete flows as a liquid before solidifying, it is essential to maintain the desired shape by selecting appropriate formwork and securing it properly before the concrete hardens. During the design and selection of templates, the expected finish of the plain concrete must be thoroughly considered. Templates should be configured carefully according to their reuse and the building design requirements.
Innovations in construction have led to the development of a comprehensive “leak-proof” template system, effectively addressing quality issues like water loss and exposed reinforcement. Currently, most fair faced concrete buildings domestically are gray, with only a few using colored concrete in white, red, yellow, or up to two colors, such as the Wooden Heart Art Museum in Wuzhen, the Museum of the Yuan Dynasty Shangdu Capital Site in Zhenglan Banner, Inner Mongolia, and Zhejiang Yuexiu International Studies University.
2. Construction Techniques for Plain Concrete Formwork
2.1 Template Selection
In building projects, construction methods must align with the project’s characteristics, addressing key challenges effectively. Choosing a reasonable and scientific formwork approach ensures the shapes are practical and meet specific building requirements. To guarantee formwork quality, materials must meet quality standards and employ advanced construction techniques to ensure sufficient stiffness. Besides strength and structure, cost-effectiveness of formwork materials is crucial. For prefabricated residential buildings, steel formwork is preferred due to its stability.
2.2 Template Design
Template design is a critical step that depends on actual site conditions. The main strategy involves dividing the overall module into smaller units for assembly. This approach ensures that the formwork installation adheres to construction standards. The joints between formwork panels must align perfectly in straight lines or curves to achieve a flawless clear water finish. While guaranteeing load-bearing capacity, the stiffness of the formwork can be enhanced to facilitate smooth surface transitions.
2.3 Template Fabrication
Template panels are fabricated from hot-rolled steel plates. Dimensions and surface forming adjustments are determined through experiments. CNC cutting, mechanical bending, and rolling processes shape the multi-dimensional panels. The layout is planned based on curvature changes. Components like supports, fasteners, ribs, and panels are initially spot welded, calibrated, then bolted together. After further calibration, carbon dioxide shielded welding ensures uniform and secure welding of the complete template units. Prefabricating the entire template assembly allows precise control over overall accuracy.
2.4 Template Joint Processing
Template panels are assembled by splicing multiple steel plates using semi-hard splicing techniques. The steel plate edges are mechanically ground to ensure tight joints. Only the back side of the joint is welded, preventing color and flatness inconsistencies that double-sided welding and grinding might cause.
2.5 Template Surface Treatment
Considering the nature of steel formwork, different brands of formwork paint and release agents were tested. High-quality formwork paint was selected for surface treatment to minimize and disperse concrete surface pores and improve texture uniformity. Painting is done before installation, emphasizing thorough rust removal, polishing, timely paint application, and ensuring strong adhesion of the paint film to the steel base.
2.6 Technical Specifications for Plain Concrete Formwork
- Surface flatness of finished plain concrete formwork: 2mm
- Surface flatness for ordinary plain concrete formwork: 3mm
- Height difference between adjacent panel joints: ≤ 0.5mm
- Gap between adjacent panel joints: ≤ 0.8mm
- Installation section size tolerance for decorative plain concrete formwork: ±3mm
- Verticality tolerance for decorative plain concrete formwork installation (up to 5m height): 3mm
3. Case Studies Demonstrating Innovative Formwork Technology for Plain Concrete
Located in Shaoxing, Zhejiang Province, the Jinghu Campus Library of Zhejiang Yuexiu Foreign Studies University stands as the world’s only green building constructed with four colors of plain concrete. This landmark campus building features a frame shear wall structure with one basement and four above-ground floors, totaling 18,916 m² of construction area on a 4,768 m² site.
During construction, the team overcame numerous challenges by integrating multiple innovative techniques, perfectly realizing the design vision with oversized natural and colored plain concrete. This project marks China’s first large-scale use of colored plain concrete, a process that demands precise and stable color tone, purity, brightness, and also ensures the load-bearing strength and chemical durability of the structure.
Plain concrete is poured in one continuous process without additional decoration, relying on the concrete’s texture, patterns, and carefully designed construction joints to create a natural decorative surface. Controlling the concrete’s state during pouring is difficult, often leading to uneven color. However, the clear water concrete name wall at Zhejiang Yuexiu Foreign Language College’s east gate was a remarkable success. The design team combined traditional methods with modern technology to craft the most beautiful clear water school name wall.
3.1 Innovative Template Plugs
To prevent surface contamination caused by water loss at template holes, the project improved an earlier independently developed template plug. By embedding rubber gaskets on the plug, adhesion issues were resolved. The plug nests elastic rubber gaskets of the same size, effectively preventing “panda eyes” caused by water loss at bolt holes. Additionally, the elastic gasket cushions the force when bolts are removed, avoiding “goldfish eye” deformation of the template.
3.2 Cross-Buckle Steel Reinforcement Protective Layer Cushion Block
A new cross-buckle steel reinforcement protective cushion block replaced the traditional version during construction. This cushion contacts the formwork at four points, greatly increasing load capacity and ensuring even force distribution. Clamped at steel bar intersections, it enhances stability and solves common issues such as missing reinforcement and steel bar impressions on the concrete surface. Made from nylon, the cushion block protects positioning steel bars while matching the concrete color, seamlessly integrating into the structure.
3.3 Combining Traditional Lost Wax Method with Clear Water Concrete Technology
Before construction, the team researched school logos nationwide and found that engraved characters require a 90-degree angle with the monument surface for clear calligraphy strokes. Achieving this with plain concrete is challenging because it must be poured in one go, making repairs difficult, and demolding can damage the strokes. Facing this challenge, the designer proposed combining the ancient lost wax casting method with plain concrete construction.
Project members practiced by melting ordinary white candles and applying the wax to the template edges of each character. After demolding, the nine large characters shone brightly with every detail preserved. This innovative approach successfully addressed issues of expansion during pouring and mold removal impact, achieving outstanding results.
4. Conclusion
In conclusion, the widespread use of plain concrete formwork in construction projects has significantly enhanced both the aesthetics and cost-efficiency of buildings. Successful plain concrete formwork construction requires strict adherence to plans, appropriate technique selection, and continuous innovation tailored to project needs. These efforts contribute to improving construction quality and advancing the development of China’s construction industry.
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