Compared to ordinary concrete, fine aggregate concrete has the characteristics of smaller aggregate particle size (usually ≤16mm, commonly 5-10mm), better fluidity, and denser formation. Its construction thickness is mostly concentrated between 20-50mm, and it is often used for thin-layer structures such as ground leveling layers, waterproof protective layers, equipment foundation bedding layers, and roof slope-forming layers. Moreover, fine aggregate concrete is more expensive than ordinary concrete of the same grade.
However, in practical applications, fine aggregate concrete is very prone to cracking, ranging from surface cracking to through-cracks, affecting structural durability and functionality. Why is fine aggregate concrete more prone to cracking than ordinary concrete?
1. Mix proportion issue.
The water-cement ratio of fine aggregate concrete is too high, coupled with an excessive amount of cementitious materials. Fine aggregate concrete has a small aggregate particle size and a much larger specific surface area than ordinary concrete (the specific surface area of 5-10mm fine aggregate is 25%-30% larger than that of 10-20mm crushed stone). In order to ensure the fluidity of construction, more cement paste is needed to coat the aggregate, resulting in a generally higher amount of cementitious materials used (the cement usage of ordinary concrete is usually 300-350kg/m³, while that of fine aggregate concrete is mostly 350-400kg/m³, and some projects even exceed 400kg/m³).
Meanwhile, during on-site construction, the phenomenon of "arbitrarily adding water" is common, leading to an excessively high water-cement ratio (often reaching 0.55-0.60, far exceeding the standard recommended of ≤0.50). Excessive cement causes a concentrated release of hydration heat, and since thin-layer structures dissipate heat quickly, the temperature difference between the inside and outside easily generates thermal stress; it also exacerbates cement shrinkage. The drying shrinkage rate of fine-aggregate concrete can reach 1.2-1.5 times that of ordinary concrete. When the shrinkage stress exceeds the tensile strength of the concrete, cracking will inevitably occur. Failure to scientifically use admixtures in mix design will exacerbate this problem. For example, failing to use high-efficiency water-reducing agents to lower the water-cement ratio, or failing to incorporate shrinkage compensation/crack-resistant components (such as expansion agents and shrinkage-reducing agents) to offset some of the shrinkage stress, results in insufficient crack resistance of the concrete system itself.
2. Insufficient aggregate skeleton support.
Defects in fine stone gradation lead to weak shrinkage resistance. The crack resistance of concrete relies on the synergistic effect of "aggregate skeleton + cement paste binding". However, fine aggregate concrete has small aggregate size and uniform gradation, resulting in weak rigid support. Unlike the coarse aggregate in ordinary concrete, it cannot form "rigid support". It lacks effective restraint during cement paste shrinkage, resembling a "thin-walled structure without steel reinforcement", making it susceptible to cracking due to shrinkage stress. Some projects use single-size aggregates such as flakes, which have poor gradation continuity and insufficient skeleton density, further reducing crack resistance. In such cases, incorporating high-quality mineral admixtures (such as fly ash and mineral powder) and suitable thickeners/viscosity modifiers can improve the cohesiveness and encapsulation of the paste, optimizing the interface transition zone between aggregate and paste. While this cannot completely replace the rigid skeleton, it can enhance the overall uniformity and deformation resistance of the system to some extent.
3. Inadequate curing.
Rapid water loss in the thin-layer structure leads to plastic shrinkage.Fine aggregate concrete is mostly used for thin-layer construction, with a significantly increased surface area to volume ratio. For example, a 30mm thick leveling layer has a surface area to volume ratio that is more than three times that of a 100mm thick ordinary concrete. It also has a fast moisture evaporation rate, requiring extremely high timeliness and integrity in curing.
If the substrate is dry and not moistened, the internal moisture of the concrete will be reverse-absorbed by the substrate, leading to excessive surface water loss and the formation of plastic shrinkage cracks before initial setting. These cracks are mostly irregular network cracks, 0.1-0.3 mm wide. Insufficient curing time or curing period means the surface strength has not yet developed before being subjected to shrinkage stress, making it prone to through cracks. During construction in high temperatures and windy weather, without covering and moisturizing measures, the surface moisture evaporation rate can be up to twice that of normal environments, exponentially increasing the risk of cracking. To address the characteristic of thin layers easily losing water, internal curing agents or water-retaining components (such as cellulose ethers) can be added to the concrete. These admixtures can slow down the migration and evaporation rate of internal moisture, providing an internal water source for continuous cement hydration. Especially under limited curing conditions or in the early stages, they can effectively reduce the risk of plastic shrinkage and early drying shrinkage.
4. The concrete thickness is too thin.
Fine aggregate concrete is typically constructed with a thickness of 20-50mm, making it a typical "thin-walled component." Insufficient cross-sectional thickness means it cannot compensate for shrinkage deformation through its own stiffness, and there is insufficient concrete margin to disperse stress. Even minor deformations will manifest as cracks. Addressing this inherent structural weakness necessitates material-level optimization. Besides optimizing aggregate gradation and reducing binder usage, the use of composite admixtures is crucial. This includes compounding high-efficiency water-reducing agents (reducing water consumption), retarders (adjusting setting time for construction), expanding agents (generating appropriate compensatory expansion), and fibers (toughening and crack-resistant properties), thereby enhancing the crack resistance and toughness of thin-layer materials through multiple pathways.
In summary, cracking in fine aggregate concrete is a result of a combination of factors, including inadequate materials, mix proportions, and construction and curing techniques. Among these, the failure to scientifically design and apply a targeted concrete admixture system based on the material properties and construction characteristics is a key and often overlooked technical factor contributing to its insufficient crack resistance.
