During concrete pouring, expansion joints are usually reserved to accommodate the thermal expansion and contraction of concrete. The arrangement of expansion joints is closely related to the pouring process. For concrete poured in separate sections, expansion joints are generally set between each section. For integrally poured concrete, expansion joints are cut in designated areas after pouring is completed, commonly at a spacing of 6m × 6m. For special structural concrete, such as elastic crack-resistant concrete and reinforced concrete structures, expansion joints may be omitted or only arranged in larger areas.
When rigid epoxy floor coatings are applied to the concrete base, if the substrate concrete cracks due to thermal expansion and contraction, the limited elasticity of the floor coating will cause the coating to crack accordingly, and peeling and delamination are likely to occur at the cracks. For concrete with pre-set expansion joints, although cracks may still appear in other positions, most cracks occur along the original expansion joints. Therefore, during floor coating construction, the flooring contractor shall negotiate with the owner to properly treat the concrete expansion joints, so as to achieve an expected and acceptable result for both parties and avoid engineering quality disputes.
There are several common methods for treating concrete expansion joints during floor coating construction, each with its own advantages and disadvantages:
During floor construction, fill the expansion joints with rigid solvent-free epoxy floor mortar, then proceed with the overall floor coating process. Before any cracking occurs, the expansion joints are invisible on the finished floor surface. However, this method cannot prevent cracking or peeling at or near the joints. Since such rigid filling materials have no elasticity, they cannot resist the stress caused by concrete shrinkage, making cracking inevitable. If the owner accepts uniform and neat reserved joints on the floor, a cutter can be used to re-cut 2–5 mm wide joints along the original positions after coating completion. This reserves expansion space for the concrete, avoids irregular random cracks in the future, and reduces coating peeling along cracks. This approach is usually only accepted for places with low decorative requirements.
During floor coating, fill and level the expansion joints with elastic materials such as polyurethane elastic sealant, then apply the intermediate coat and topcoat. The specific operation steps are as follows:
First, inspect all expansion joints. For joints wider than 5 mm, clean out all internal debris, then prepare and fill the joints with elastic sealant and scrape flat. For joints narrower than 5 mm, which are too thin for direct filling, use a cutter to trim them into a neat trapezoidal shape with a width and depth above 1 mm before pouring the elastic sealant.
Follow the manufacturer’s instructions or use a caulking gun for filling. Avoid smearing sealant outside the joint; wipe off any overflow immediately. Fill the joint to a sufficient depth and scrape off excess material with a trowel. Thoroughly remove dust and debris inside the joint before filling, otherwise poor adhesion and detachment of the elastic sealant from the concrete may occur. Refer to the manufacturer’s guidelines for sealant mixing and application skills.
After the sealant is fully cured, proceed with subsequent coating procedures.
Compared with the first method, the elastic sealant disperses the stress generated by concrete shrinkage, greatly reducing the possibility of coating cracking over the joints and maintaining a uniform overall floor color. Nevertheless, the coating applied over the elastic sealant is still rigid. In cases of severe concrete shrinkage, the intermediate and topcoats on the sealant may still develop slight cracks, though the cracks will remain minor.
During coating construction, reserve expansion joints as much as possible. If temporarily covered, record their exact positions. After the overall floor coating is finished, trim the reserved joints neatly to a width of over 5 mm, preferably into an upward-widened trapezoidal shape. Clean all dust and debris inside the joints, paste protective tape on both sides, then fill the joints level with polyurethane elastic sealant, and remove the tape when the sealant is semi-dry.
This method is widely adopted in electronics factories, pharmaceutical factories and precision instrument manufacturing plants with high requirements for cleanliness and aesthetics. It can basically prevent coating cracking at expansion joints and delivers a neat appearance. Its disadvantage is the limited color range of existing polyurethane elastic sealants (mostly grass green and iron red), making it difficult to match the overall floor color. In addition, the sealant will bulge and sink constantly along with concrete thermal expansion and contraction.
For partial floor expansion joints, laying glass fiber cloth is also a common method to enhance crack resistance. First fill and level the joint, then paste one or two layers of about 10 cm wide glass fiber cloth over the joint during priming, followed by intermediate and topcoat application. This method ensures consistent floor color and resists minor cracks caused by slight concrete shrinkage. However, it is difficult to construct when the designed coating thickness is too high, and the cloth-laid area may end up slightly higher than the surrounding floor.
The above are conventional methods for treating ordinary small expansion joints and may not be effective for relatively large cracks. In any case, proactive communication and proper negotiation with the client is always the optimal solution.
