13 Common Defects and Solutions in Tunnel Kiln Construction
(1) Obvious foundation settlement in a section after the main structure is completed
Cause: The backfilling depth of the foundation in this section is relatively large. The construction team failed to ensure a sufficient width of the replacement filling under the ultra-deep foundation, and the backfill layers were too thick with inadequate compaction.
Solution: The construction team should pay attention to ultra-deep backfilling areas, ensuring safe width and process quality. The owner should strengthen management and simultaneously implement on-site drainage measures.
(2) Misalignment between the kiln centerline (kiln walls on both sides) and the kiln car track axis; the straightness deviation of the kiln's long axis exceeds the specified requirement, which affects the operation of kiln carriages to varying degrees.
Cause: Only a theodolite was used for setting out, and cable-stretching was adopted for inspection during construction, with insufficient re-inspection.
Solution: Use a theodolite for positioning and a level instrument with a fixed ±0 origin. Set out, inspect, and re-inspect jointly with the owner (coordinated inspection by moving empty kiln cars into the kiln) for processes such as foundation, track beam, curved sealing wall, cantilever brick, and kiln wall. This ensures consistent gaps between the kiln car and the curved seal/kiln wall, as well as stable operation.
(3) Inadequate control of internal kiln elevation: In some areas, kiln cars were used for measurement. The distance from the upper edge of the carriage surface to the lower edge of cantilever bricks ranges from 1.5 cm to 4 cm, and the maximum height error from the lower to upper edge of the kiln wall is 3 cm.
Cause: Insufficient measurement and positioning checks by the construction team; the layer-counting benchmarks failed to function properly; some refractory bricks have non-standard thicknesses.
Solution: The construction team should use a level instrument for measurement and re-inspection. Set up standard layer-counting rods for each construction section to control elevation and layer count. Refractory bricks with large errors should be sorted out and used in designated sections at the kiln head or kiln tail.
(4) Severe damage to some cantilever bricks and wall damage near the contraction joints of air outlets was found at the maintenance site.
Cause: Inspection revealed that the cantilever bricks are 350 mm long, and areas with severe damage have poor mortar fullness (some ongoing projects also have insufficient mortar in cantilever brick joints). Large or incomplete mortar joints accelerate damage during the production cycle. The contraction joints of walls adjacent to air outlets are too close, increasing the extrusion force on the wall when the air outlet bricks get stuck.
Solution: The length of cantilever bricks should be >390/400 mm; control mortar joints within the specified range and ensure fullness; process cantilever bricks with large errors. Adjust the distance between air outlets and adjacent contraction joints appropriately.
(5) Settlement and deformation of the vault (3.6 m curved arch) under maintenance, with inconsistent heights in the same firing section, affecting the normal service life of the kiln.
Cause: The arch support beam has low strength, a small cross-section, insufficient reinforcement, and large spacing between supporting walls; the mortar for vault masonry is incomplete; the wedges and grouting were not properly installed after vault bricklaying.
Solution: Strengthen inspection during construction to ensure full mortar in masonry and proper wedging of wedge positions. The concrete mix ratio, compactness, and reinforcement of the arch support beam cross-section should meet requirements. Increase the density of supporting walls in the middle section. If there is no design or supervision unit, the owner and construction team should strengthen inspection of this critical process and rework any substandard parts immediately.
(6) Problems in refractory brick inspection during construction: Incomplete testing data for refractory bricks; internal blackening in broken bricks; insufficient weight of pipe-shaped bricks; non-conforming dimensions.
Cause: Inadequate inspection; unclear contractual requirements or low unit prices.
Solution: Refractory brick masonry is a key part of the kiln structure. Select reputable suppliers with good cost-effectiveness; clarify contractual terms and ensure thorough inspection (random sampling for testing). The Al₂O₃ content, refractoriness, compressive strength, and bulk density of refractory bricks should meet requirements. Sort refractory bricks before masonry.
(7) Problems in fire hole construction: Fire holes exceed the planned position (beyond the marked line); misaligned fire hole arrangement; insufficient inspection holes (inadequate arrangement at both ends of the kiln); fire hole positions and construction sections (expansion joint positions) exceeding the line.
Cause: Some construction teams believe minor deviations in fire hole positions are acceptable for internal combustion kilns; inadequate management during kiln roof construction.
Solution: Attach importance to kiln roof construction (especially for projects using mechanical brick setting). Control the position of fire holes in each construction section, ensure accurate arrangement of kiln roof bricks, layer count, and mortar joint size, and conduct thorough inspection.
(8) Problems with kiln cars and carriage surfaces: There are several large-scale kiln car manufacturers in the industry that can design standard kiln cars matching the kiln dimensions specified in the design or plan, along with supporting operation equipment-allowing the owner to ensure usability without extra effort. However, many owners opt for self-made kiln cars with contracted labor costs to save funds or utilize self-owned materials and personnel, leading to defects in kiln cars and supporting carriage surfaces. Issues include unreasonable front/back sealing of kiln cars, improper size of steel plates at traction stress points (either too small or too thin), insufficient strength of diagonal supports, and 10 cm-thick insulation layers on carriage surfaces (insufficient for heat preservation)-all affecting subsequent normal use.
Cause: Pursuit of convenience and cost reduction; following contractors' suggestions without thorough investigation.
Solution: Rectify and reinforce self-made or already produced kiln cars: For projects without track beams and sand seal grooves, lower the elevation appropriately to increase insulation layer thickness. For non-standard designed kilns and kiln cars, the frame bricks (refractory bricks or custom-processed red bricks) on the kiln car surface should be cut 50–80 mm in height and 20–25 mm in width at the lower outer side, then embedded into the car. Cut a 20 mm-deep flat surface 100–120 mm from the edge at the upper part of the frame bricks to fix the lower cushion bricks of the green body (preventing loosening and extrusion beyond the car edge, which may scratch the kiln wall). This minimizes green body collapse and extends the service life of the kiln car surface. It is reported that poor handling of kiln car surfaces in many factories increases maintenance costs and affects kiln car turnover.
(9) Problems with sand filling pipes: Some projects have insufficient sand filling pipes (5–6 pairs should be installed); some pipes have small diameters (select 120–160 mm based on kiln cross-section size). The slope of sand filling pipes and the distance from the pipe outlet to the bottom of the sand seal groove should be appropriate to ensure effective sand sealing during production.
(10) Reinforcement binding defects: In some reinforced structures, joints are not staggered; some joints are tens of centimeters longer than required, while others are more than 10 cm shorter.
Solution: Bind reinforcement strictly in accordance with construction specifications.
(11) Mortar and concrete quality issues: Mortar plasticizers (to improve mortar workability and prevent sedimentation) are added to cement mortar for masonry, but most projects use excessive amounts (even several times the standard). This significantly reduces the strength of mortar masonry. Some self-mixed concrete for foundations, structural beams, slabs, and track beams has insufficient strength.
Solution: The owner and construction team should strengthen inspection and management. The construction team must strictly follow the mix ratio.
(12) Waterproofing and insulation defects in drying chambers: No waterproofing treatment at the openings of drying chambers, leading to condensed water seeping into the kiln; no or excessively thin insulation layers on the drying chamber roof, affecting heat preservation.
Solution: The construction team should implement waterproofing and roof insulation strictly in accordance with design or service requirements. No process omissions or quality standard reductions are allowed.
(13) Other recommendations
① Some factories only focus on construction costs and use excessively short kilns. To ensure firing and drying cycles, comprehensively consider designing the firing kiln to accommodate 30–33 kiln cars and the drying kiln for 20–23 kiln cars. Implement proper site drainage and slope safety measures.
② The construction team should strictly control process quality, learn construction specifications and new construction techniques, and proceed to the next process only after passing inspection of the previous one. Protect finished construction materials, and cooperate with the owner and on-site representatives in inspection and supervision.