How to Prevent Hollowing and Cracking in Floor Mortar
Release Time:
2020-08-28
Source:
How to Prevent Hollowing and Cracking in Floor Mortar
I. Strict Control of Raw Materials
1. Cement: Silicate cement or ordinary silicate cement that has passed re-inspection for soundness shall be selected. These two types of cement exhibit low shrinkage, resulting in minimal drying shrinkage of cement mortar during hardening. However, these two types of cement must never be mixed. Furthermore, the use of expired, damp, or caked cement is strictly prohibited. When cement fails to meet soundness requirements, the free calcium oxide and magnesium oxide it contains hydrate very slowly during setting. After the cement mortar or concrete has hardened, these free substances continue to hydrate, causing the solid-phase volume to increase by 2 to 2.5 times, which can damage the hardened structure and lead to cracking. For flooring works, slag silicate cement, fly-ash silicate cement, and pozzolanic silicate cement—characterized by low early strength and poor wear resistance—should not be used.
2. Sand: Use clean medium-to-coarse sand with a silt content of less than 3%. Mortar mixed with fine sand or muddy sand exhibits greater drying shrinkage and overall shrinkage during hardening, making the floor prone to shrinkage cracks.
II. Ensure Proper Cleaning and Moisturizing at the Grassroots Level
1. Subbase: refers to the structural layers beneath the surface layer, including the filling layer, isolation layer, leveling layer, cushion layer, and subgrade, among others.
2. Substrate Preparation: Prior to commencing floor construction, the substrate must be thoroughly prepared. The quality of the substrate shall be inspected to ensure compliance with design requirements, including material specifications, strength, thickness, surface flatness, and slope. Any defects must be rectified to meet the required standards and pass inspection and acceptance before construction can proceed, as the quality of the substrate directly affects the quality of the top finish layer.
3. Substrate Preparation: Remove all debris from the substrate surface; scrape off any adhered slurry, mortar, or free silica gel film on cement-based substrates; and thoroughly sweep and rinse the surface to eliminate dust, oil stains, and other contaminants. Prior to construction, no standing water shall be present on the substrate surface. This is critical for ensuring the quality of the flooring work.
4. Moisture of the substrate: This is critical for bond strength between the upper cement-sand mortar or concrete layer and the underlying substrate. When work is carried out on a dry substrate, the dry surface will absorb moisture from the cement-sand mortar or cement concrete layer, thereby inhibiting further hydration and hardening of the cement and reducing the bond between the substrate and the upper layer. Moreover, as the finishing layer shrinks and undergoes drying shrinkage, it may delaminate from the substrate, leading to spalling and cracking. Therefore, 1–2 days before applying the finishing layer, the clean substrate should be sprayed or drenched with water to maintain moisture and then allowed to air-dry, ensuring no standing water remains so that the substrate reaches a state of dry saturation. This step is essential for achieving strong adhesion between the finishing layer and the underlying layer, as well as for preventing hollows and cracks.
5. Pipeline Installation: Concealed pipelines in the floor should, whenever possible, be installed within the base layer. If any pipelines protrude from the base layer, the surface mortar at that location will be excessively thin, lose moisture rapidly, and become prone to cracking. When water and electrical embedded pipelines slightly project above the base layer, a layer of steel wire mesh with a width greater than 500 mm or a reinforcing steel mesh with a diameter of 4 mm should be laid in the middle of the cement-sand mortar layer above the pipeline to prevent cracking.
III. Properly Control the Construction Ambient Temperature
Ground-level construction involves a large surface area, and the optimal ambient temperature for such work is between 5°C and 35°C. If the temperature drops below 5°C, doors and windows should be promptly closed to prevent early freezing of the floor finish, which could lead to sanding or chalking.
IV. Strictly Control the Bonding Layer
1. Bonding layer: refers to the intermediate layer that connects the surface layer to the underlying structural layer.
2. Construction requirements: Prior to applying the surface layer, a coat of cement slurry shall be brushed onto the base layer as a bonding layer to enhance the bond strength between the surface and base layers. However, the bonding layer often ends up acting as a barrier due to improper construction practices.
(1) If cement slurry that has been applied too early has already hardened, reapplying a cement mortar topping will only serve as a barrier. Therefore, the existing layer must be chipped away and thoroughly swept clean, after which a new coat of qualified cement slurry must be applied before the cement mortar topping can be laid.
(2) It is prohibited to first apply water at the base layer and then sprinkle dry cement, or to first sprinkle dry cement and then apply water (the “sweeping slurry” method), because standing water and unevenly swept cement slurry can actually act as a barrier, leading to delamination and cracking of the surface layer.
(3) The cement slurry shall be prepared with a water-to-cement ratio of 0.4–0.5, based on the moisture content of the substrate and local climatic conditions. Thoroughly mix the specified amount of water with cement to obtain a uniform, pure cement slurry, then apply it evenly to the substrate surface to be constructed (ensuring no areas are missed). After approximately 30 minutes, the cement-sand mortar topping can be laid.
V. Strict Control of Cement Mortar Mix Proportions
1. The water-cement ratio for ordinary cement-sand mortar on the ground should be controlled within 0.4, and the consistency should not exceed 55 mm. When the mix proportion of each batch of cement-sand mortar is correct and the water content is accurately measured, the mortar will exhibit consistent consistency, uniform strength, and even shrinkage.
2. If water is added indiscriminately, resulting in an excessive water content in the mortar, the hardened cement mortar will experience excessive moisture evaporation and a high volumetric shrinkage rate, which in turn reduces the strength of the floor finish, diminishes its wear resistance, and increases the likelihood of shrinkage and drying shrinkage cracks.
3. Test data indicate that, with a constant cement-sand mortar mix ratio, its strength is related to consistency (i.e., the amount of water used): for example, when the consistency increases from 30 mm to 70 mm, the compressive strength decreases by approximately 30%. Moreover, excessive bleeding during construction can reduce the strength, hardness, and wear resistance of the floor finish.
VI. Mastering the Compaction and Troweling of the Floor Surface Layer
The troweling and compaction of cement mortar and cement concrete surface layers are generally carried out in three stages.
1. First, fully lay the cement-sand mortar screed, then use a screed board to level and compact it, using the screed lines as the reference. In general rooms, construction joints should be avoided; it is essential to ensure surface flatness. After the cement-sand mortar has begun to set, smooth the surface with a power trowel or wooden trowel, ensuring uniform density and thoroughly filling in any sand holes, depressions, footprints, and other imperfections to achieve a smooth, even finish.
2. The second troweling should be carried out at the initial set, when a person standing on the surface leaves a foot impression about 5 mm deep; use a steel trowel to smooth and compact the surface, or use a power trowel for finishing. The primary purpose is to eliminate surface air bubbles. The finished surface after troweling must be free of defects such as trowel marks, footprints, and sand holes.
3. The third troweling should be carried out before the cement reaches final set—specifically, when a light press with the finger leaves no fingerprint. Use a steel trowel to perform the third pass, applying slightly more pressure to smooth and polish the surface until it is free of sand holes and trowel marks, thereby further sealing the capillary pores. Working from the interior walls toward the doorway is essential for ensuring a level and smooth floor.
VII. Ensure Proper Curing of the Cement Mortar Finish Layer
Upon completion of the surface layer construction, promptly close all doors and windows and maintain moist curing by spraying or drenching with water for no less than 7 days. During the curing period, foot traffic is strictly prohibited, and no other trades or heavy materials may be placed on the floor. Adequate thermal protection against low temperatures must be provided. Inadequate curing, early use, or exposure to freezing can all lead to cracking and sanding of the floor surface.
In summary, only by rigorously controlling construction raw materials, the construction process, and subsequent curing can we ensure that cement-sand mortar floors are free from quality defects such as sanding and dusting, cracking, and delamination.
