In order to improve the comprehensive performance of pervious concrete, nano-silicon and polypropylene fiber were added to pervious concrete to study the change of performance of pervious concrete. Firstly, the effect of single doped nano-silicon on the properties of cement slurry and pervious concrete was studied, and the optimal water-binder ratio and nano-silicon content were determined. Based on this, mixed polypropylene fiber with different proportions of length of 18 mm to determine the reasonable amount of polypropylene fiber. The results showed that the compressive strength of pervious concrete was the highest when the nano-silicon content was 0.5% and the water-binder ratio was 0.32. Based on this ratio, the maximum compressive strength can be obtained by adding 1.0 kg/m3 polypropylene fiber, and the compressive strength of 7d and 28d increased by 29.9% and 42.2%, respectively. Adding 1.5 kg/m3 polypropylene fiber was the most beneficial to improve the freezing resistance of pervious concrete. For example, after 300 freeze–thaw cycles, the compressive strength residual rate was 62%. That's much higher than the 40 percent that was found when nano silicon was mixed alone.
1 Introduction
Pervious concrete is a kind of multi-air mixed material, which is widely used in sponge city construction because of its advantages such as permeability, air permeability and noise absorption. Due to its many connected voids, its strength is low, which limits its application in the field of practical engineering. Therefore, many scholars began to add admixture materials such as fibers and nanomaterials [1, 2] into pervious concrete to achieve the purpose of improving the performance of pervious concrete.
Nano-silicon is an inorganic material with small particle size and has strong pozzolanic activity, nucleation and micro-aggregate filling effect, which is conducive to improving macro mechanical properties and durability of pervious concrete [3, 4]. For example, Tarangini [5] added nano-silicon with 3% cement mass to pervious concrete to study the durability of pervious concrete. The results showed that compared with ordinary pervious concrete, the freeze–thaw resistance of pervious concrete modified by nano-silicon had been effectively improved, and the durability coefficient was up to more than 70%. Fiber can improve the performance of pervious concrete to a certain extent [6‐9]. Under the action of external forces, fibers will be in different stress states such as stretching, pulling out and breaking, thus consuming part of the energy transferred under the action of external forces, restraining the expansion of cracks, and making concrete structures have certain flexibility. For example, Banthia [10] studied the relationship between shrinkage cracks and polypropylene fiber size, and the study showed that the incorporation of fine fibers was more effective in suppressing cracks than that of coarse fibers, while the incorporation of long fibers was more effective in suppressing cracks than that of short fibers. In order to comprehensively utilize admixture materials to improve the performance of pervious concrete, many scholars began to study the influence of admixture materials on the performance of pervious concrete. Ali [11] added volcanic ash and fiber into the permeable concrete structure, and the results showed that the mechanical properties of the concrete structure were effectively improved and basically met the requirements of practical application without changing the permeability.
Anzeige
To sum up, when different admixtures are added to the pervious concrete structure, the comprehensive performance of pervious concrete will be significantly improved if the advantages of each additive material can be brought into play. Therefore, nano silicon and polypropylene fiber were added to pervious concrete in this paper to explore the effect of composite additive on the performance of pervious concrete.
2 Test Raw Materials and Forming Methods
(1)
Test raw materials
The cement used for the permeable concrete was ordinary Portland cement P42.5R, the water was local tap water, the water reducing agent was high efficiency polycarboxylate concrete water reducing agent, the aggregate was basalt aggregate, the gradation is 4.75~9.5 mm (80%): 9.5~13.2 mm, (20%). The nano silicon is Degussa A380 vapor phase nano silica, and the fiber is 18 mm polypropylene fiber.
(2)
Test block forming method
The test block of pervious concrete was compacted by Marshall automatic compactor. The weight of the instrument drop hammer was 10.21 kg. According to practical experience, the striking times were set to 100 times.
3 Effect of Single Doped Nano Silicon on Cement Slurry and Pervious Concrete
3.1 Effect of Nano-Silicon on Water Consumption of Standard Consistency of Cement Paste
Take the cement weight of 500 g for the test, added nano silicon at the same time, reduced the same weight of cement. The concrete cement paste test mix was divided into eight groups, as shown in Table 1.
Table 1
Experimental results
Serial number
Cement/g
Nano silicon/%
Water consumption/g
Water-binder ratio
1
500
0
129.6
0.26
2
497.5
0.5
141.6
0.28
3
495
1
154.6
0.31
4
492.5
1.5
167.5
0.34
5
485
3
191.5
0.38
It can be seen that with the increase of nano-silicon content, the water consumption of the standard consistency of the cement paste increases significantly. When the nano-silicon content was 3%, the water consumption of the cement paste was 192 mL, 47.7% higher than the water consumption of the cement paste without the nano-silicon content. Therefore, from the perspective of cement slurry to reach the standard consistency, there should be different water-binder ratio under different nano-silicon content.
Anzeige
3.2 Effect of Nano-Silicon on the Performance of Pervious Concrete
From the perspective of cement slurry to reach the standard consistency, there should be different water-binder ratio under different nano-silicon content. The strength and porosity of pervious concrete with different nano-silicon contents (0%, 0.5%, 1%, 1.5%, 3%) were tested under the condition that the cement slurry reached the standard consistency. Considering that the aggregate has certain absorbability, after the preliminary test, the water-binder ratio under different nano-silicon content was slightly larger than the value in Table 1 and the specific water-binder ratio was (0.28, 0.30, 0.32, 0.36, 0.4). The influence of water-binder ratio on compressive strength and voidage is shown in Figs. 1 and 2.
Fig. 1
Influence of nano-silicon content on compressive strength
Fig. 2
Effect of nano silicon dules on void ratio
×
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That the compressive strength of both 7d and 28d increased firstly and then decreased with the increasing of nano-silicon content. The compressive strength of 0.5% nano-silicon concrete at 7d and 28d was the highest, which increased by 12.66% and 7.14% compared with that of ordinary concrete, respectively. As can be seen from Fig. 3, the variation law of nano-silicon pervious concrete was higher than that with ordinary concrete. For example, the maximum total voidage occurred when the nano-silicon content was 1.5%, which increased by 24%. The maximum effective void fraction occurred when the nano-silicon content was 0.5%, which increased by 25.8%. Analysis the reasons: when the content of nano-silicon is appropriate, the surface effect plays a dominant role, which promotes the hydration reaction and improves the distribution state of cement slurry in the vertical direction of pervious concrete. With the increase of the content, the flocculation effect plays a dominant role, and the hydration reaction begins to be hindered. In conclusion, from the perspective of strength and voidage, nano-silicon content of 0.5% is optimal.
Fig. 3
Effect of glue ratio on compressive strength
×
Based on the nano-silicon content of 0.5%, the influences of different water-binder ratios (0.28, 0.30, 0.32, 0.34) on the compressive strength and porosity of concrete were studied. The results were shown in Figs. 3 and 4.
Fig. 4
Effect of glossy rate
×
It can be seen that the 7d compressive strength first increases and then decreases with the increase of water-binder ratio from the test results. When the water-binder ratio is 0.28, the strength was 16.6 MPa the lowest, which was the lowest. This is because the water-binder ratio is too small, resulting in insufficient water required for hydration reaction and low adhesion between aggregate particles. With the increasing of water-binder ratio, the overall compressive strength of pervious concrete gradually increased. When the water-binder ratio was 0.32, it reached the extreme value of 18.7 MPa, which increased by 12.7% compared with the compressive strength of concrete with the water-binder ratio of 0.28. When the water-cement ratio exceeded 0.0.32, the overall compressive strength of concrete began to decrease gradually due to excessive moisture, which was not conducive to the uniform coating of aggregate by slurry. At this time, the total void fraction and effective void fraction of pervious concrete had an obvious downward trend with the increasing of water-cement ratio.
In conclusion, from the perspective of improving the comprehensive performance of nano-silicon pervious cement concrete, the best water-binder ratio is 0.3~0.32.
4 Pervious Concrete Modified by Nano-Silicon + Polypropylene Fiber Composite
In this section, polypropylene fiber with length of 18 mm was mixed on the basis of 0.5% nano-silicon content concrete, to explore the effect of nano-silicon + fiber composite modification on the performance of concrete. The fiber content ranges from 0 to 2 kg/m3. Considering that the amount of water used by adding fiber increases further, the water-cement ratio of 0.32 was adopted for the experimental study.
4.1 Physical and Mechanical Properties
The variation law of compressive strength and voidage of nano-silicon pervious concrete with polypropylene fiber is shown in Fig. 5 and 6. The voidage first decreases and then increases with the increase of fiber content, while the compressive strength is on the contrary. For example, when the dosage was 1.0 kg/m3, the decrease of total void fraction and effective void fraction was 2% and 1.8%, which was the lowest respectively, and the compressive strength was the highest, which was increased by 29.9% and 42.2% respectively. When the content exceeds 1.0 kg/m3, the void fraction began to increase and the compressive strength began to decrease. This is because the appropriate fiber content and cement slurry formed a fiber network, and the surface effect of nano silicon can enhance the strength of the network structure and reduce the porosity. However, with the addition of too many polypropylene fibers, the fiber is too dense and easy to cluster, thus weakening the compressive strength, and the porosity began to rise.
Fig. 5
The effect of fiber doping on the gap rate
Fig. 6
Effect of fiber dosage on compressive strength
×
×
4.2 Frost Resistance
It can be seen from the results of Figs. 7 and 8 that the incorporation of polypropylene fiber greatly improves the frost resistance of pervious concrete. After 75 freeze–thaw cycles, the compressive strength of pervious concrete in each group decreased less. For example, the compressive strength of nano-silicon pervious concrete mixed with 1.0 kg/m3 polypropylene fiber decreased by 0.6 MPa, only 2%. After 150 freeze–thaw cycles, the decline in compressive strength began to accelerate. After 225 freeze–thaw cycles, the compressive strength of nano-silicon pervious concrete mixed with 1.5 kg/m3 polypropylene fiber was larger than that of the nano-silicon pervious concrete mixed with 1.0 kg/m3 polypropylene fiber pervious concrete, and the compressive residual rate was 84% and 75%, respectively. After 300 freeze–thaw cycles, the residual rate of compressive strength of pervious concrete doped with nano-silicon stabilized at about 40%. The nano-silica permeable concrete mixed with 1.5 kg/m3 polypropylene fiber had the highest compressive strength and the compressive residual rate was 62%.
Fig. 7
The impact of compressive intensity of cyclic frozen fusion
Fig. 8
The impact of the residual rate of the reconciliation intensity of the cycle frozen fusion
×
×
5 Conclusion
(1)
Nano-silicon content directly affects the water consumption of cement slurry at standard consistency, and different nano-silicon content corresponds to different water-binder ratio.
(2)
By comparing the compressive strength and porosity of pervious concrete, the optimal content of nano-silicon is determined to be 0.5%. On this basis, the influence of water-binder ratio was further analyzed, and the optimal water-binder ratio range was determined to be 0.30–32.
(3)
When the nano-silicon content is 0.5% and a certain amount of 18 mm polypropylene fiber is mixed, the porosity of pervious concrete changes little, and the compressive strength is significantly improved, especially the late compressive strength, compared with the nano-silicon concrete only mixed 0.5%, the strength is increased by 42.2%.
(4)
After mixing 18 mm polypropylene fiber on the basis of 0.5% nano-silicon content, the freezing resistance of concrete can be effectively improved. The compressive residual rate of the composite fiber concrete is stable at about 50~60%, which is higher than the single nano-silicon concrete compressive residual rate of 45%.
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