In recent years, the concept of high-performance concrete has been deeply rooted in the hearts of the people, but the concrete strength index is not enough to reveal the durability performance of concrete. Therefore, to study high-performance concrete, we must pay attention to the impermeability properties of concrete. The impermeability properties of concrete (i.e. water permeability, air permeability and resistance to chloride ion permeability) are the first line of defense for the durability of concrete. Improving the resistance of concrete Permeability is key to improving the durability of concrete. The durability of concrete is closely related to its impermeability. If the impermeability is insufficient, some harmful liquids and gases will penetrate into the concrete and corrode, which will not only destroy the internal structure of the concrete, but also cause the steel bars to rust and expand, causing the concrete protective layer to crack. Or spalling, and at the same time due to the increase in free water inside the concrete, the ability of the concrete to resist freezing and thawing is significantly reduced.
(1) Cement dosage and water-binder ratio
In recent years, the fineness of cement products has become thinner and the activity has increased, which has caused the acceleration of hydration reaction, increased heat release, and increased drying shrinkage. , ultimately leading to an increase in cracks caused by temperature shrinkage and dry shrinkage of concrete. The increase in micro-cracks and macro-cracks is detrimental to the impermeability and durability of concrete.
The total volume of cement decreases during the hydration process when it encounters water. Full hydration means that the volume of cement decreases by about 7% to 9%. An increase in the amount of cement will inevitably lead to an increase in volume reduction. When the cement slurry is hydrated, about 6% of the water will evaporate to generate capillary pressure, which will cause concrete cracking and reduce the impermeability of the concrete. To improve the impermeability of concrete, we should mainly improve the impermeability of the cement stone itself and the contact surface between the cement stone and the aggregate. To this end, the volume change caused by the hardening of the cement mortar should be reduced. In addition to filling and bonding functions, cement mortar should also form a certain quantity and quality of wrapping around the stones. On the one hand, the wrapping layer prevents water from penetrating inside the cement stone; on the other hand, it is firmly bonded with the stones and sand to prevent water from penetrating along the cement stone aggregate contact surface. Therefore, in order to achieve satisfactory anti-permeability performance, the cement dosage cannot be too small. Too low cement dosage may easily lead to insufficient hydrated slurry to fully wrap the coarse and fine aggregates, resulting in a decrease in the bonding strength between the aggregate and the slurry, and interface transition. The performance of the area will also be worse, and the impermeability performance of the concrete cannot be guaranteed. However, if too much cement is used, the fluidity of the concrete mixture will be too great, and the stones will sink unevenly, resulting in poor homogeneity and large shrinkage of the hardened concrete, thereby reducing the impermeability of the concrete.
The water-cement ratio has an important impact on the impermeability of concrete. The water consumption required for cement hydration only accounts for about 23% of the cement consumption. In order to meet the needs of concrete workability, the water consumption is often higher than The amount of water required to hydrate the cement. The water above the hydration level of the cement evaporates to form capillary pores, reducing the impermeability of the concrete. Generally speaking, as the water-cement ratio increases, the anti-penetration ability of concrete decreases. Especially when the water-cement ratio exceeds 0.55, the concrete’s ability to resist penetration will decrease significantly. When the water-cement ratio is lower than 0.38, mature hardened cement The permeability coefficient of slurry is even higher than that of granite. When the water-cement ratio changes from 0.4 to 0.7, the permeability coefficient will increase at least a hundred times, while when the water-cement ratio changes from 0.65 to 0.55, the permeability coefficient can be reduced by more than half. This shows that in order to make the concrete structure itself have good impermeability properties, in addition to related In addition to improvement measures, it is very important to reduce the water-binder ratio in the mix design. .
(2) Coarse and fine aggregates
The sand and gravel aggregates are composed into a mixture of different particle sizes. It is generally believed that the smaller the void ratio of the sand and gravel, the greater the density of the prepared concrete. The better the impermeability of concrete is. Therefore, the pores of aggregates are regarded as the only important factor affecting the impermeability of concrete. To obtain concrete with good impermeability properties, it is necessary to increase the density of aggregates in concrete to reduce the voids in the aggregates, and to fill the voids in the concrete mixture with the maximum amount of solid particles to obtain concrete with good impermeability properties. Practice has proved that the impermeability performance of concrete prepared with aggregates with large volume, small voids and well-graded aggregates may not necessarily be good. On the contrary, concrete with large aggregate voids and small bulk density has better impermeability performance, that is, the quality of sand and gravel gradation has little impact on the impermeability performance of concrete. Whether it is continuous grading or discontinuous grading, the resulting concrete with high density can only reduce the possibility of water penetration along the gaps between aggregates after the sand and gravel are increased to a certain extent, thereby increasing the effective water-blocking cross-section. However, the main reason for concrete water permeability is that the capillary pores of the cement stone itself and the contact surface of the cement stone aggregate produce cracks. Therefore, regardless of whether the concrete aggregate gradation is good or bad, the aggregate voids and bulk density are large or small, and the aggregate’s own anti-permeability ability Strong or weak, as long as the pores of the cement stone itself and the cement stone aggregate contact surface are permeable, the impermeability performance of concrete will still be poor. On the contrary, if the contact surface between cement stone and aggregate in concrete has good waterproof performance, cement stone aggregates are firmly bonded to each other and have good water blocking performance, the impermeability performance of concrete will also be very good. Therefore, in order to make concrete have good impermeability properties, we should mainly control the quantity and quality of mortar in concrete, control the formation of concrete structure, and improve the internal structure of concrete structure.
Common fine aggregates include natural sand and manufactured sand. Compared with river sand, manufactured sand contains some stone powder. An appropriate amount of stone powder can…The use of mineral admixtures can improve cement gradation, fill cement voids, and secondary reactions can improve later performance. Generally speaking, the activity of mineral admixtures is lower than that of cement. The use of mineral admixtures other than silica fume will reduce the early strength and anti-penetration ability of concrete to varying degrees. Even if the strength is not significantly reduced, the anti-penetration performance In terms of strength, concrete will also be weakened. The reason is that the mineral admixture cannot effectively provide sufficient hydration products for the slurry that wraps the aggregate in the early stage, resulting in poor performance in the early interface transition zone and reduced strength and impermeability properties. Although the performance of the concrete decreases in the early stage after adding mineral admixtures, as the age increases, the filling effect and activity effect of the admixtures in the middle and later stages will gradually appear, which can improve the anti-penetration performance of the concrete in the later stage.
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