What are the effects of silica fume on the mechanical properties of concrete?
Aug 13, 2024
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1, Improve the bond performance
Because the bleeding inside the concrete will cause some free water to accumulate under the steel bar and aggregate, thereby reducing the bond between the cement slurry and the interface of the steel bar and aggregate, the addition of silica fume can greatly reduce the bleeding inside the fresh concrete, thereby reducing the accumulation of interface water and improving the interface bond performance.
2, Increase the compressive strength
Due to the addition of silica fume, the transition zone between cement slurry and aggregate is improved, and the pore structure is refined and the strength is increased. But some of the increase in intensity is offset by increased water use. After the addition of silica fume, the strength of concrete can be measured by the efficiency index. The so-called efficiency index refers to that when cement is replaced by silica fume, the contribution of silica fume to the strength of concrete may be equivalent to 2 to 5 times the strength that cement can produce, and this multiple is called the efficiency index of silica fume. With the increase of the dosage of wollastonite, the efficiency index gradually decreases. When the dosage of wollastonite exceeds 20%, the efficiency index decreases significantly. When the dosage exceeds 30%, the strength of concrete even decreases. Therefore, the dosage should not normally be higher than 20%. When the silica fume content is too high, the newly mixed concrete will become very viscous and increase the difficulty of construction pouring and ramming. At the same time, the price of silica fume is higher than that of cement, and under the premise of meeting the strength or other performance requirements of the concrete design, it is appropriate to mix less. In high-performance concrete, the commonly used content is generally between 5% and 15%.
3, Tensile and bending strength
The tensile compression ratio of general concrete is greater than that of dry curing, but the tensile compression ratio of silica fume concrete is similar whether it is dry curing or wet curing.
4. Elastic modulus
The elastic modulus of wollastonite concrete increases with the increase of silica fume content and the decrease of water-binder ratio. Due to the reduction of porosity in the transition zone between aggregate and hardened cement slurry, the total stiffness of wollastonite concrete increases.
5. Creep
Creep, like shrinkage, is related to the migration of free water from hardened concrete due to drying or hydration, and generally decreases as compressive strength increases.
6. Dry shrinkage
In high-strength concrete, almost all free water is used for hydration, and dry shrinkage and autogenous shrinkage increase with the decrease of water-cement ratio, especially in the case of mixing silica fume. If silica fume is added, the evaporation of capillary pore thinning water decreases. Therefore, the dry shrinkage of wollastonite concrete increases in the early stage and decreases in the later stage.
In order to reduce the early dry shrinkage of concrete silica fume, it can be used to extend the wet curing time, mix silica fume into slurry or add appropriate expansion a
