Management of concealed works

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Management of concealed works I. Concept of concealed works Concealed works refer to those sub-projects or components that are covered by subsequent construction activities; in other words, they are those elements that become difficult to see after the construction is completed. Such works are known as concealed works. They exist throughout the entire project lifecycle, but those related to the construction of the main structure are particularly important as they affect the structural safety and functional capabilities of the building. Moreover, it is difficult to inspect these elements, and direct testing methods are costly and not precise enough. For this reason, these types of concealed works are especially well-known, including pile foundations, foundation works, rebar in reinforced concrete structures, connections in prefabricated structures and composite components, as well as embedded piping systems. Since concealed works are difficult to inspect upon project completion, to ensure the quality of the project, quality inspections should be carried out promptly during their construction. Quality inspection documents and records must be kept for future reference, and acceptance records for these concealed works should be prepared before they are covered up after completion. II. Hidden project items and inspection contents 1. Foundation pit inspection: Buildings require foundation pit inspections, which include checking the location of the foundation pit, its planar dimensions, the bearing stratum, the absolute and relative elevation of the base, as well as the soil quality of the foundation pit and the groundwater level. For projects with pile support or pile foundations, pile inspections are also necessary. The records of foundation inspection should be approved and signed by the construction party, surveying party, design party, supervision party, and construction contractor together. When the foundation requires treatment, the survey and design agencies shall submit suggestions for such treatment. 2. Earthwork: Before backfilling the foundation trench and the area around the building, check the cleanliness of the foundation base, its elevation, and the condition of the foundation treatment. 3. Support works: Number the anchor rods, and check the type, specifications, quantity, location, insertion length, drill diameter, depth, and angle of the anchor rods and soil nails. Inspect the trenching width, depth, and verticality of the diaphragm wall, as well as the specifications and position of the rebar cage, the cleanliness of the trench bottom, the thickness of the sediment layer, and the slope gradient. Other supports are also subject to hidden inspection in the same manner. 4. Reinforced concrete cast-in-place pile project: Check the specifications and dimensions of the rebar cage, the thickness of the sediment layer, the condition of hole cleaning, as well as the rock property report for rock-embedded piles. 5. Underground waterproofing works: Check the location, design, and construction of concrete deformation joints, construction joints, post-cast strips, wall-penetrating sleeves, embedded components, etc. Waterproofing method for civil air defense exits. Base layer of the waterproofing layer, specifications of waterproofing materials, thickness, laying method, treatment of internal and external corners, overlap sealing treatment, etc. 6. Reinforcement work: Check the type, specification, quantity, position of the reinforced bars, as well as the locations of anchorage and joints, the lap length, the thickness of the protective layer, and the degree of rust and dirt removal ； Replacement and modification of rebar ； Reinforcement treatment of lap joints, lintels at openings, additional reinforcements, etc. The drawing number and acceptance comments should be indicated, and illustrations should be attached if necessary. Inspect the type of reinforcement connection, the types of connections used, the location and quantity of joints, as well as the electrodes, flux, weld length, thickness, surface cleaning, and the quality of the connections. Installation of seismic reinforcement in seismic-resistant structures. Check the specifications, quantity, location, shape of the reserved channels, as well as the embedded end plates ； Length of prestressed tendons to be cut, cutting method, vertical position deviation, fixation, integrity of the sheath ； Anchors, fixture connection points assembly, etc. 8. Prefabricated structure: Quality of the rough concrete surface, dimensions, quantity, and location of the key pins ； Grade, specification, quantity, position, spacing of rebar, as well as the bending angle and straight section length of stirrup hooks ； Reinforcement connection method, joint location, number of joints, percentage of joint area, lap length, anchoring method, and anchoring length ； Specifications, quantities, and locations of embedded parts and reserved pipelines; ； Construction methods for waterproofing, fire protection, etc. at the joints of precast concrete elements ； Thermal insulation and construction of its joints. 9. Construction details for thermal insulation on the exterior (and interior) walls, as well as sound insulation measures. 10. Floor and ground surface works: Inspect various sublayers (underlayment, leveling layer, isolation layer, waterproof layer, filling layer), as well as the type and specifications of floor joists, the thickness and method of installation, the slope, elevation, surface condition, sealing treatment, and bonding quality. 11. Plastering work: The condition of the interface agent should be checked. Reinforcement measures when the total thickness of the plastering is 35 mm or more, as well as reinforcement measures at the joints between different material substrates. 12. Door and window works: Check the specifications, quantities, positions, spacing, installation methods of embedded parts and anchors, bolts, etc., as well as their connection to the frames, the anti-corrosion treatment applied, the filling of gaps, and the bonding of sealing materials. 13. Ceiling installation: Check the material, specifications, spacing, connection methods, fixing techniques, as well as the fire and corrosion protection treatments of the ceiling joists and hanging components. Appearance, condition of joints and seams, types and specifications of filling and sound-absorbing materials, as well as their installation and fixation. 14. Light partition wall project: Check the specifications, positions, quantities, and connection methods of embedded parts, connectors, and tie rods; as well as their connections to the surrounding walls and ceiling, the connections between the studs, the spacing between them, fireproofing and anti-corrosion treatments, and the use of filling materials. 15. Cladding panel (tile) works: Inspect the specifications, quantity, location, connection methods, and anti-corrosion treatment of embedded parts, post-installed anchors, and connectors. For areas with waterproofing features, the construction of the leveling layer and the waterproofing layer should be inspected, in the same manner as is done for floor installations. 16. Roofing work: Check the type, specifications, thickness, laying method, overlap width, joint treatment, and adhesion of the materials used for the subbase, leveling layer, insulation layer, waterproof layer, and separation layer ； Additional layers, gutters, eaves drains, flashing and expansion joints, details of roof protrusions, installation of isolation layers, areas that require sealing, separation joints in rigid roofs and the condition of the seals, etc. 17. Curtain wall project: Check the specifications, quantity, location, connection methods, and anti-corrosion treatment of embedded parts, post-embedded parts, and connectors ； Check the installation and anti-corrosion treatment of the connection joints between components as well as between the components and the main structure ； Treatment of gaps around the curtain wall and between the curtain wall and the main structure, installation of seals ； Installation of expansion joints, settlement joints, seismic joints in curtain walls, as well as wall corner connections ； Installation of lightning protection and grounding connections for curtain walls, etc ； Installation and treatment of the fireproof layer structure in curtain walls. 18. Steel structure projects: Check the specifications, quantities, connection methods, and anti-corrosion treatment of embedded parts, post-embedded parts, and connectors. Check the specifications, location, embedding method, and tightening of the foundation bolts ； Welding and insulation measures for steel structures. III. Construction and acceptance of concealed works: The construction of concealed works should be carried out in accordance with the relevant construction standards and proper techniques. During the construction process, technical personnel from the contractor must be present to supervise and guide the work, and they must keep proper records and related documentation to ensure the quality of the concealed works as well as the progress of construction. This also ensures that there is evidence available once the concealed works are completed and covered up. After the concealed works are completed, they should be inspected promptly before being covered. This is because if quality issues arise after the works are concealed, it will be necessary to dig them out again and cover them over, resulting in significant losses such as the need for rework. To avoid waste of resources and losses for both parties, as well as to ensure the quality of the project and its successful completion, the contractor should notify the client or the client’s representative, namely the supervision engineer, to conduct an inspection. Only after the supervision engineer approves the work and signs off on the relevant inspection documents can the concealed works be covered. In practice, when the project conditions permit covering or concealing certain elements, the contractor should first conduct a self-inspection. Once this inspection is successful, the contractor must promptly notify the client or the supervision engineer before those elements are concealed, so that they can inspect the conditions and attend the process of concealing them. The notice includes the contractor’s self-inspection records, the items that are concealed, as well as the time and location of the inspection. Upon receiving the notification, the constructor or supervising engineer shall arrive at the site where the work is to be concealed within the specified time frame in order to inspect the conditions of that work. If the inspection shows that the conditions are satisfactory, the constructor or supervising engineer signs the inspection report; only then can the contractor proceed with covering up the work. If the supervising engineer finds that the conditions of the work are not satisfactory, he/she has the right to require the contractor to rectify the issues within a specified period. The construction of concealed works complies with the relevant specifications. Once the constructor or the supervision engineer gives approval after inspection, if they refuse to sign the inspection report, it can be considered in practice that the client has given approval, and the constructor may proceed with covering the concealed works. If the supervision engineer fails to inspect the conditions of the concealed works within the prescribed time after receiving the notification, the contractor shall urge the supervision engineer to conduct the inspection within a reasonable period. Since the supervision engineer fails to conduct inspections, the contractor cannot carry out concealed work; therefore, if the contractor notifies the employer to conduct an inspection and the supervision engineer fails to do so in a timely manner, the contractor has the right to suspend construction. The contractor may extend the construction timeline and demand compensation from the employer for losses resulting from delays in work, idle time, and accumulation of materials and components. If the contractor carries out concealed work without notifying the supervision engineer for inspection, the employer and the supervision engineer shall have the right to request an inspection of the concealed work afterward. The contractor must expose it as required, and then re-conceal it or repair it before concealing it again. If the concealed works fail to meet the requirements upon inspection, the contractor shall redo the work and conceal it again. Under such circumstances, the costs incurred in inspecting the concealed works – such as inspection fees, rework costs, and material costs – shall be borne by the contractor, who shall also bear the liability for any delay in the project timeline. IV. Points to Note in Hidden Works During the construction of hidden works, it is necessary not only to carry out on-site supervision, self-inspection, and guidance, but also to strictly control the construction procedures as well as the materials used and their proportions. It is important to keep an eye on the progress of work on site at all times, and to maintain proper records and documentation related to the construction. When keeping these records, it is essential to have both paper-based records as well as photographic evidence, so that everything can be referred to more easily in the future. Once the concealed work is completed, our on-site technical staff must first conduct a self-inspection. Once the inspection is successful, they should promptly notify the supervision engineer to come to the site for inspection, so as to ensure the continuous progress of subsequent work. Additionally, the relevant construction documents must be signed for future reference.