Preparation and flexural fatigue resistance of self-compacting road concrete incorporating nano-silica particles
Introduction
Self-compacting concrete is a kind of sustainable and energy-saving material suitable for pavement construction, which can achieve a compacted state relying on gravity without vibration during field construction, thus reducing energy consumption and noise pollution[1], [2]. This type of concrete can not only improve the shape-holding ability by adjusting the content of attapulgite and other clay particles for slip form paving construction, but also alleviate the problems of bleeding and segregation for formwork construction[3], [4]. At the same time, self-compacting rigid pavement presents a superior self-healing ability under the effect of overloading, its strength as well as durability can be restored after curing when internal damage is relatively slight[5]. Currently, some studies highlighted that when recycled rubber powder[1], [2], [6], recycled concrete aggregates[7], [8], [9], [10], ladle furnace slag[11] and recycled polypropylene plastic particles[12] were added to replace part of the aggregates, the environmental pollution and potential health threats brought by solid waste could be reduced, and the lack of natural aggregate resources would be mitigated. In addition, the service life of the roads can also be extended by applying the high-performance SCC cover or overlay on the surface of old pavement[13], [14], [15].However, due to the omission of the vibrating process during the construction of SCC, it is difficult to avoid the existence of unstable air bubbles, which forms a weak link in the hardened concrete and weakens its fatigue resistance[1], [16]. Fatigue resistance is one of the most important performance requirements for the concrete pavement structure, which is subjected to the repeated action of vehicle wheel load and periodic temperature change for a long time[16], [17]. Insufficient fatigue resistance would make the pavement more prone to permanent through cracks in the early stage of service, which facilitates the penetration of rainwater into the base layer and subgrade, and induces a series of diseases such as mud, dislocation, and vacancy at the bottom of the board, resulting in the reduction in pavement service life greatly. Meanwhile, it brings bad driving experience to drivers and passengers, leaving hidden dangers for driving safety[16]. Therefore, how to improve the fatigue performance of SCC pavement deserves more attention from researchers.
At present, there are mainly the following methods to improve the fatigue performance of ordinary SCC: (a) Incorporating an appropriate amount of waste tire rubber particles; (b) Adding a proper amount of fiber material; (c) Constructing a multi-element cementitious system. Rubber particles could increase the deformation resistance and ductility of SCC, but this practice caused a decline in the mechanical properties of concrete[18]. Chen carried out four-point flexural fatigue tests at different stress levels, loading frequencies and stress ratios, the results showed that the replacement of fine aggregates by equal volume rubber particles could effectively improve the flexural fatigue performance of SCC, but the addition of rubber particles would reduce its mechanical properties[1]. The cracks could be dispersed and the crack propagation could be delayed by bridging effect between fiber and mortar, so as to enhance the fatigue resistance. However, the uniform dispersion of fiber in SCC is difficult to be guaranteed[19], [20]. The research results of Ganesan showed that the flexural fatigue performance of ordinary SCC can be significantly increased by 15%–25% with the existence of steel fibers[21]. A multi-element cementitious system has also been proved to be an effective and simple way to strengthen the fatigue resistance of the SCC through the physical and chemical reactions[22]. Common supplementary cementitious materials include fly ash, silica fume and a variety of nano materials[14], [23], [24].
Generally, cement, fly ash, silica fume, and nano-silica are utilized to form a multi-element cementitious system in SCC, which has promising value in the mechanical strength, fracture performance and durability[22], [25]. Fly ash, a kind of solid waste produced by coal-fired power plants, can save energy and reduce costs, and improve the internal microporous structure of SCC, thus having a beneficial effect on its mechanical properties and abrasion resistance[26]. However, the early strength of SCC increases slowly when only fly ash is added, which hinders its promotion and application in the field of road transportation. Silica fume plays a similar role to fly ash, and by virtue of its high pozzolanic activity, it can also promote the hydration of fly ash[25]. In addition, with the increasing application of nano-silica, its use in concrete modification has attracted more and more attention[24]. With high surface activity, nano-silica can promote the secondary hydration of fly ash and silica fume with calcium hydroxide, thus effectively improving the strength and durability of concrete, and the improvement effect is better than that of silica fume. By comparing the effects of Class F fly ash, silica fume and nano-silica particles on different properties of high-performance SCC, Jalal summarized that high content mineral admixtures cooperated with a low dosage of nanoparticles would become key materials of high-performance SCC[25]. However, no matter in the case of single or compound doping, different nanomaterials always exert different effects on the performance and microstructure of SCC, and the corresponding optimal dosages also differ[27], [28], [29]. Baloch found that nano-silica can optimize the microstructure of the self-compacting pastes, which is conducive to reduce the adverse effect of super absorbent polymer on the mechanical properties, and the best effect is obtained when the content is 2%[30].
Previous studies have mostly verified that the addition of an appropriate amount of nanomaterials can ameliorate the mechanical strength and durability of cement-based materials, and pointed out that nanoparticles are prone to agglomerate and reduce the fluidity of cement-based materials[22], [31], [32], while there are relatively scattered studies on the fatigue performance of nano-silica modified self-compacting road concrete. In view of this, nano-silica was selected as the modified material, cooperated with other mineral admixtures such as fly ash and silica fume, to prepare self-compacting road concrete, and after optimizing the parameters of mix proportion, four-point flexural fatigue tests were carried out to obtain the fatigue lives of concrete specimens with various mix proportions at different stress levels. The two-parameter Weibull distribution was utilized to process the data to predict the fatigue lives at survival rates of 50% and 95%, respectively, and the corresponding probabilistic fatigue equations were established. Finally, the mechanism of nano-silica improving flexural fatigue performance of SCC was discussed through microscopic analysis.
Section snippets
Cement (CE)
The ordinary Portland cement P·O42.5, produced by Hunan Pingtang South Cement Co., Ltd., was used in the experiment. The specific surface area is 330 m2/kg, the density is 3.10 g/cm3, the compressive strength measured on 28 days is 49.6 MPa, the flexural strength is 7.2 MPa. Its chemical composition is shown in Table 1.
Silica fume (SF)
The silica fume used in the experiment was produced by Shandong Boken Silicon Material Co., Ltd., the specific surface area is 2.7 × 104 m2/kg, the density is 2.2 g/cm3. Its
Orthogonal test design
According to the relevant material parameters and target strength C55, referring to JGJ/T283-2012 Technical Specification for Application of Self-compacting Concrete, the initial mix ratio was calculated as m(fly ash):m(cement):m(fine aggregate):m(coarse aggregate) = 165.5 kg:386.3 kg:795.0 kg:928.2 kg, the water-binder ratio is 0.26. The SCC was prepared according to the initial mix proportion, and the filling performance of the mixture was ensured to meet the requirements by adjusting the
Conclusions
The main conclusions of this research are summarized as follows:
- (1)
Taking the filling performance and 28-day compressive strength as evaluating indicators, combining grey relational analysis and comprehensive weighting method to analyze the orthogonal test results, the optimal mix proportion of nano-silica modified self-compacting road concrete was determined: water-binder ratio is 0.26, in the cementitious material, the content of fly ash is maintained at 20%, and the content of nano-silica and
Funding
The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China and Civil Aviation Administration of China (U1833127), National Natural Science Foundation of China (General Program; 51978080), Hunan Province Natural Science Foundation (2018JJ4016), the Key Scientific Research Projects of Hunan Education Department (18A129) and Hunan Graduate Research and Innovation Project (CX20190668).
CRediT authorship contribution statement
Yingli Gao: Conceptualization, Project administration, Funding acquisition. Wenjuan Zhou: Formal analysis, Writing - original draft, Writing - review & editing. Wei Zeng: Data curation, Validation. Ganpeng Pei: Visualization, Investigation. Kairui Duan: Formal analysis, Writing - original draft, Writing - review & editing.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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