According to the provided reference, you can increase the permeability of concrete, particularly in the top layer, by having more cement paste fill the voids between aggregates and by having a weak bond between coarse aggregates and the concrete matrix.
Understanding Concrete Permeability
Concrete permeability refers to how easily fluids (like water or chemicals) and gases can pass through its pore structure. High permeability is generally undesirable in typical structural concrete as it can lead to durability issues such as corrosion of reinforcement, freeze-thaw damage, and sulphate attack. However, the question asks how to increase it, based on the provided context, which seems to focus on factors specific to certain concrete characteristics or layers.
Factors Increasing Concrete Permeability Based on Reference
Based on the given reference, two primary factors contribute to increased concrete permeability:
1. Increased Cement Paste in Aggregate Voids
The reference states: "More cement paste filled the voids between the aggregates in the top layer means more permeable of the concrete."
- Explanation: While cement paste itself has pores, having an excess of paste that doesn't fully hydrate or compact properly, especially in the surface layer, can create a more interconnected pore network or lead to microcracking pathways, thus increasing the ease with which substances can flow through. It suggests that simply having more paste filling spaces doesn't necessarily lead to a denser, less permeable material in all contexts, especially if the quality of the paste or its interaction with aggregates is compromised.
2. Weak Bond Between Aggregates and Matrix
The reference notes: "The weak bond between the coarse aggregates and the matrix also offers an easier flow path around the aggregates and the increase the permeability of the top layer."
- Explanation: The Interfacial Transition Zone (ITZ) is the area surrounding aggregate particles within the concrete matrix. This zone is typically less dense and more porous than the bulk paste. A weak bond in this area means the ITZ is more pronounced or contains more defects, creating preferential pathways for fluid movement around the aggregate particles. This breakdown in the bond effectively bypasses the denser bulk concrete structure, leading to higher overall permeability.
Practical Implications (Based on Reference)
While typical concrete design aims to reduce permeability, the factors highlighted in the reference point to conditions that could increase it, particularly near the surface. These might include:
- Poor Compaction: Inadequate vibration can leave voids (bug holes) and lead to a less dense, more permeable structure, potentially contributing to weaker ITZs and more interconnected pores.
- High Water-Cement Ratio: While not explicitly mentioned, a high water-cement ratio generally results in a more porous cement paste and a weaker ITZ, aligning with the conditions described as increasing permeability.
- Aggregate Properties: Smooth or dusty aggregate surfaces can lead to poor bonding with the cement paste, weakening the ITZ.
In summary, according to the reference, focusing on aspects like having more cement paste filling voids in the top layer and creating a weak bond between aggregates and the surrounding matrix are ways permeability is increased.
