Causes and Prevention of Sanding in Cement-Sand Mortar Floor Surfaces
Release Time:
2020-08-17
Source:
Causes and Prevention of Sanding in Cement-Sand Mortar Floor Surfaces
Cement-sand mortar surface layers are commonly used in construction projects, particularly in resettlement projects for relocated residents, new rural development initiatives, dilapidated housing renovation programs, and affordable housing and low-rent housing projects—all of which are vital to improving people’s livelihoods. This type of flooring system is simple in structure, durable and wear-resistant, and cost-effective; however, it is often prone to… These issues give rise to three common quality defects—cracking, hollowing, and sanding—which adversely affect both the aesthetic appearance and the normal functionality of the building. Residents are deeply dissatisfied and have voiced strong complaints; therefore, they must be given close attention and thoroughly addressed.
Analysis of the causes of sanding:
1. The water-cement ratio of the cement mortar mix is too high, resulting in excessive consistency. The water-cement ratio is inversely proportional to the strength of cement mortar; as the water-cement ratio increases, the mortar strength decreases. If too much water is used during construction, the mortar strength will be significantly reduced; moreover, bleeding may occur during placement, further degrading the surface of the floor. Surface strength: once walked on and worn after completion, it will generate dust.
2. Improper sequence of operations. A lack of understanding of the cement hardening process, coupled with inappropriate scheduling of the troweling operation and issues such as an excessively dry or overly wet underlying layer, can result in troweling being carried out either too early or too late. If troweling is performed too soon, cement hydration has just begun, the gel has not yet fully formed, and there is still a significant amount of free water; even after troweling, the surface will still be… Applying a floating layer of water is detrimental to the strength and wear resistance of the surface mortar; if burnishing is delayed too long, the cement will have already reached final set and hardened, making the work not only difficult and preventing the elimination of capillary pores and trowel marks on the surface, but also disturbing the already hardened surface, thereby significantly reducing the strength and wear resistance of the surface mortar.
3. Improper curing of the surface layer. After completion of the cement floor, if no curing is performed or the curing period is insufficient, the moisture in the surface layer will evaporate rapidly in a dry environment, which will impair the hydration of the cement, slow down the hardening process, and in severe cases even halt hardening altogether. This can lead to dehydration of the cement mortar, thereby compromising its strength and wear resistance. In addition, If the floor is watered for curing less than 24 hours after troweling, the relatively “fresh” surface can lead to widespread peeling, exposure of sand particles, and dusting after use.
4. Inadequate protection of the finished floor. If a cement floor is put into use or subjected to subsequent construction operations before it has attained sufficient strength, the surface may be subjected to friction and other forms of wear, which can easily lead to sanding. This problem is particularly pronounced in low-temperature conditions.
5. Raw materials do not meet the requirements. First, the cement has a low strength grade, or expired, damp, and caked cement is used; such cement has poor activity, which adversely affects the strength and wear resistance of the floor finish. Second, the sand particle size is too fine, requiring a large amount of water during mixing, which increases the water-cement ratio and reduces strength; excessive silt content in the sand also impairs the cement’s performance. Its bonding strength with sand easily leads to sanding on the floor surface.
Preventive Measures for Sanding
1. Strictly control the water-cement ratio. The consistency of cement mortar used for floor finishes shall not exceed 35 mm, and the slump of concrete and fine-aggregate concrete used for flooring shall not exceed 30 mm. The leveling layer must be thoroughly wetted, the cement slurry applied evenly, and the screed leveled immediately after spreading. The concrete finish shall be vibrated to achieve full compaction; for fine-aggregate concrete, rolling with a roller is required. Press down, or tamp with a wooden trowel, to bring up a slurry on the surface, thereby ensuring the strength and compactness of the finishing layer.
2. Master the timing of surface troweling. For cement floors, troweling should generally be performed no fewer than three times. The first pass should be carried out immediately after the surface layer has been laid. Begin by using a wooden trowel to evenly rub and pat the surface, ensuring that the surface material is uniformly distributed and tightly compacted, with smooth and even finishing—ideally without any visible water film on the surface. The second troweling pass should be completed after the cement has reached initial set but before final set ( It is generally advisable to achieve a slight foot impression that is not obvious or results in only minimal sinking; the surface should then be compacted and leveled. The third pass of troweling is primarily intended to eliminate trowel marks and seal fine pores, further compacting and smoothing the surface (the timing should be such that no footprints are visible upon walking on the surface, or only very faint ones remain). It is crucial to avoid troweling after the cement has reached final set.
3. After the cement floor has been troweled smooth, water curing should be carried out after one day and one night, depending on the ambient temperature; alternatively, the surface may be covered with straw mats or sawdust and then watered for curing. Where conditions permit, a threshold can be constructed at the doorway using yellow clay or lime paste, followed by water-retention curing. For floors made with ordinary Portland cement, the curing period shall be no less than seven days and nights; for those made with slag… The curing period for cement-concrete floors made with Portland cement shall be no less than 10 days and nights.
4. Arrange the construction sequence reasonably to avoid premature foot traffic. Cement flooring should, whenever possible, be laid only after wall and ceiling plastering and other decorative works have been completed, so as to prevent contamination and damage to the finished surface. If it must be installed prior to other decorative works, effective protective measures shall be implemented, such as laying bamboo mats or straw curtains, Asphalt felt and other materials shall be used, and a curing period of 7 to 10 days and nights shall be ensured.
5. Cement should preferably be high-early-strength silicate cement or ordinary silicate cement, with a strength grade of no less than 32.5. Cement that is past its shelf life or has become caked due to moisture absorption must not be used. Sand should preferably be coarse or medium sand, with a silt content not exceeding 3%. The nominal size of fine aggregate and crushed stone used for the surface layer shall not exceed 15 mm, nor shall it It shall be greater than two-thirds of the surface layer thickness, and the silt content shall not exceed 2%.
Treatment of Sanding on Concrete Floors
1. Sanding on small areas. The sanded-off portions can be wet-ground with a grinding stone until the hard underlying surface is exposed. Alternatively, repair can be carried out by applying a skim coat of neat cement slurry, following this sequence: clean the substrate → thoroughly rinse and moisten → apply a 1–2 mm layer of neat cement slurry (or sprinkle dry cement over the surface) → trowel and polish 2–3 times → cure. If the surface It’s not smooth, but it can be wet-ground once.
2. Severe sanding. Renovation treatment is required: remove the entire surface layer and thoroughly rinse with clean water. Before laying the new surface layer, the roughened surface must be kept moist, and a coat of plain cement slurry with a water-to-cement ratio of 0.4–0.5 should be applied (with an appropriate amount of 108 adhesive added to enhance adhesion). Subsequently, a new layer of 1:2 cement mortar should be laid. Apply the surface layer immediately after brushing on the mortar. After the surface layer is laid, proper troweling and curing shall be carried out.
3. Extensive sanding of the surface. This can be repaired using a 108 adhesive cement slurry; the specific method and precautions are as follows:
(1) Use a wire brush to remove loose, powdery sand from the deteriorated areas, then rinse thoroughly with clean water. If the floor has cracks or obvious depressions, first fill and smooth these areas with a putty made by mixing cement with a small amount of 108 adhesive.
(2) Mix 108 adhesive with water (approximately twice the volume of adhesive), stir thoroughly, and then apply it to the floor surface to enhance the bond between the 108-adhesive cement slurry and the finishing layer.
(3) The 108 adhesive cement slurry shall be applied in layers, with each layer approximately 0.5 mm thick; typically 3–4 coats are required, for a total thickness of about 2 mm. The mix ratio for the base coat is cement : 108 adhesive : water = 1 : 0.25 : 0.35. After thorough mixing, apply the slurry to the prepared substrate. During application, use a trowel or scraper to smooth and level the surface. The base layer is typically applied in 1–2 coats. The mix ratio for the finishing adhesive mortar is cement : 108 adhesive : water = 1 : 0.2 : 0.45, and it is generally applied in 2–3 coats. After application, cure the surface according to standard cement floor curing procedures; after 2–3 days, gently polish away any trowel marks using a fine grinding wheel or oilstone, then apply a coat of wax before the floor is ready for use.
