1. Remove oil stains

During the concrete lining construction, small oil stains may adhere to the concrete surface due to trolley positioning, mechanical repairs, etc. After the lining and underlay concrete construction is completed, the tunnel will become relatively dry. The smoke and dust from the machinery used in subsequent construction, as well as the raised dust, will attach to the concrete surface, affecting the mortar.

Bonding strength with fire-retardant coatings and concrete: First, remove oil stains with coarse sandpaper, then wash the lining surface with a high-pressure water jet. After cleaning the surface crystalline matter with a wire brush or scraper to remove scaling, wipe away the remaining dust with a damp sponge, and ensure the wall's pH value is greater than 10; weak acid treatment is mandatory.

2. Seam Treatment

Some gaps on the wall are caused by trapped air, which can create bumps on the surface. These bubbles need to be removed first, then sanded and filled with mortar. Spackle can also be used to even out the area. Besides these gaps, there are also expansion joints and other construction gaps that require spackling as well.

3. Grinding construction joints

Due to the segmental construction of the concrete lining using lining trolleys within the tunnel, construction joints are inevitably left, resulting in minor misalignment. Additionally, during the operation of the concrete pump, a small amount of concrete may splash onto the finished lining surface. For larger misalignments at the construction joints, flatness is achieved by chiseling, followed by using an angle grinder.

The slightly misaligned areas can be smoothed with an angle grinder; for small clumps of concrete splattered on the lining surface, use a small shovel to remove them, then smooth with coarse sandpaper or an angle grinder.

4. Clear away debris

Clean cement slurry and other debris. When cleaning, use a brush dipped in oxalic acid solution. This solution is highly effective for alkaline debris. However, be mindful of the solution ratio; avoid excessive acidity. Maintain it around 3% to prevent wall corrosion and pit formation.

5. Coating with surface paint

After the lining surface is ground smooth, apply the cement mortar base coat to achieve overall flatness. The requirement for the flatness of the cement mortar surface is ≤3mm, and it is coated with a specially formulated interface paint. First, prepare the slurry by mixing the interface paint and water at a weight ratio of 1:0.3, then spray it on the surface to a thickness of 1-2mm. After curing and drying, proceed with the fire-resistant paint application.