Causes and Prevention of Hollowing and Cracking in Cement-Sand Mortar Floor Surfaces
Release Time:
2020-08-21
Source:
Causes and Prevention of Hollowing and Cracking in Cement-Sand Mortar Floor Surfaces
Cement-sand mortar floor finishes are a common and time-honored construction technique that is both economical and practical. They are extensively used in livelihood projects such as resettlement projects for migrants under the South-to-North Water Diversion Scheme, new rural development initiatives, dilapidated housing renovation programs, affordable housing projects, and low-rent housing schemes. However, due to poor construction practices, they are prone to cracking, hollowing, and sanding, which can lead to serious Such defects adversely affect the aesthetic quality and normal use of the property, and complaints from residents are frequent; therefore, these common quality problems must be eliminated to ensure the functional performance of the building. The cement-sand mortar finish can be divided into three layers: the mortar layer, the cement slurry layer, and the bleeding layer. Notably, the cement slurry layer is troweled up with a trowel, which is essential for achieving a smooth and durable cement-sand mortar finish.
The key layer, which is also the most vulnerable part during maintenance, often suffers structural damage due to erosion by bleeding water.
Causes of Hollowing and Cracking
1. The surface of the leveling (or base) layer has not been thoroughly cleaned, with loose dust, slurry film, or other contaminants remaining, which severely compromises the bond between the leveling layer and the finish layer, leading to hollowing and cracking.
2. During the construction of the surface layer, if the surface of the cushion layer (or base layer) is not adequately wetted or is insufficiently moistened and remains excessively dry, the mortar laid on it will lose water too rapidly due to absorption by the cushion layer, resulting in low strength and poor bond between the surface layer and the cushion layer. In addition, if the dry cushion layer (or base layer) is not rinsed beforehand, dust on its surface will be difficult to Scouring provides a certain isolating effect on the surface mortar layer.
3. If standing water accumulates on the surface of the cushion layer (or base layer), the water-cement ratio in the pooled areas will suddenly increase after the finish layer is laid, compromising the bond between the finish layer and the cushion layer and thereby increasing the likelihood of hollowing and cracking in the finish layer.
4. Improper application of the cement slurry bonding layer. For example, if the slurry is applied too early, by the time the surface layer is laid, the applied cement slurry has already dried and hardened, losing its bonding strength and instead acting as a separating layer. Alternatively, if the method of first spreading dry cement powder and then watering (or first watering and then spreading dry cement powder) is used for brushing the slurry, because the dry cement surface does not It is easy to spread unevenly, and watering can be either excessive or insufficient, which can readily lead to the formation of a dry powder layer and puddles, thereby creating potential risks of delamination and cracking in the subsequent surface layer.
5. The backfill soil on the first-floor slab is of poor quality; large areas of the floor lack expansion joints; the cushion layer lacks sufficient strength; and structural deformation has occurred.
Preventive Measures for Hollowing and Cracking
1. Strictly treat the base layer (underlayment or subbase). First, thoroughly remove loose dust, laitance, and other contaminants from the surface, and rinse it clean. If the base surface is excessively smooth, it should be roughened by chiseling. Second, control the flatness of the subbase; its unevenness shall not exceed 10 mm to ensure uniform and consistent finish-layer thickness and prevent excessive variation in thickness. This can lead to uneven shrinkage during setting and hardening, resulting in cracking and hollow sounds. Third, 1–2 days before applying the finish layer, the base layer should be thoroughly wetted to ensure it has a clean, moist, and rough surface.
2. Pay close attention to the quality of the bonding layer construction. The plain cement slurry bonding layer should be evenly brushed after the slurry is prepared; the method of first spreading dry cement on the surface and then watering it is not recommended. The water-cement ratio of the plain cement slurry should be between 0.4 and 0.5. The application of the plain cement slurry must be closely coordinated with the laying of the surface layer, ensuring that it is applied and the surface layer is laid immediately afterward. When laying the surface layer, if the plain cement slurry If it has dried and hardened, it should be scraped off and repainted.
3. Ensure the construction quality of the concrete leveling layer. The concrete leveling layer shall be compacted using a plate vibrator, and any uneven areas shall be leveled with cement mortar or fine aggregate concrete.
4. Prior to construction on highly compressible soft soil foundations, ground improvement measures shall be implemented first. For areas with locally high equipment loads, pile-cap foundations may be adopted to support the structure and thereby eliminate potential post-construction settlement issues.
Treatment of delamination and cracking in flooring
1. For localized hollows and cracks, the hollow and cracked sections shall be chiseled out, with the surrounding area chiseled into a square or circular shape and extended 30–50 mm into sound, well-bonded substrate; the edges shall be chiseled into a sloping profile. The underlying surface shall be appropriately roughened by chiseling. After chiseling, the area within a 100-mm radius around the repair site shall be thoroughly cleaned. One to two days prior to repair, the surface shall be rinsed with clean water to ensure Ensure thorough wetting. During repair, first brush a coat of plain cement slurry with a water-to-cement ratio of 0.4–0.5 over the base surface and surrounding areas, then fill the defect with a mortar mix made from the same material as the existing surface layer. If the original surface layer is relatively thick, the repair should be carried out in multiple passes, with each pass not exceeding 20 mm in thickness. After final setting, immediately cover the repaired area with wet sand or wet straw bags for curing.
Strictly prevent the formation of early-age shrinkage cracks.
2. For extensive hollowing and cracking, the entire surface layer shall be chiseled away, the underlying surface roughened, and a new surface layer laid. The requirements for cleaning, rinsing, applying primer, laying the new layer, and curing remain as specified above. In summary, to prevent and control cracking, hollowing, and sanding of cement-sand mortar floor finishes, thorough cleaning of the base layer is essential, and the use of qualified materials is a prerequisite. Meticulous construction is the key, and strengthened management is the guarantee.
