**Comparison of Material Properties of SCC Concrete with Steel Fibres Related to Ingress of Chlorides**

#### **Petr Lehner 1,\*, Petr Koneˇcný <sup>1</sup> and Tomasz Ponikiewski <sup>2</sup>**


Received: 23 February 2020; Accepted: 18 March 2020; Published: 20 March 2020

**Abstract:** The paper focuses on the evaluation of chloride ion diffusion coefficient of self-compacting concrete with steel fibre reinforcement. The reference concrete from Ordinary Portland Cement (OPC) and Self-Compacting Concrete (SCC) with several values of added steel fibres—0%, 1% and 2% of weight—were cast in order to investigate the effect of fibres. The three procedures of diffusion coefficient calculation are presented—rapid chloride penetration test, accelerated penetration tests with chloride as well as the surface measurement of electrical resistivity using Wenner probe. The resulting diffusion coefficients obtained by all methods are compared and evaluated regarding the basic mechanical properties of concrete mixtures.

**Keywords:** SCC-SFR; chlorides; diffusion; mechanical properties; concrete

#### **1. Introduction**

Self-compacting concrete (SCC) [1,2] allows the simplification of concrete processing technology and the production of complex cross-sections and structural shapes. The difference compared to standard concrete is mainly in mixture composition. The proportion of cement and small aggregate is higher, and the amount of plasticizer is significantly greater. The basic properties and advantages of self-compacting concrete are mentioned in [3–6].

Other options for improvement of the mechanical properties of concrete is the use of steel fibres [7,8]. The selection of quantity and type of fibres depends mainly on the purpose of application and is a matter of intensive research [9–11]. Due to this, there are also several special tests for steel fibre reinforced concrete (SFRC), which are codified in recommendations and national standards [12,13]. A strong emphasis on the correct description of the material properties is important [14].

The application of self-compacting concrete (SCC) [5] brings simplification of the concrete processing technology due to the lack of need for vibration. Improved properties are achieved by modification of the formula, i.e., by appropriate use of aggregate, cement content and plasticizers [1]. Another rapidly growing area of concrete technology is steel fibre reinforced self-compacting concrete (SCC-SFR), which is classified within the group of composites that combine conventional typical structural concrete and fibres [15–17].

The purpose of the use of fibres is that it eliminates one of the greatest concrete disadvantages–low tensile strength. Fibre concrete application examples can be found particularly in tunnel lining, industrial floors, foundations, or structural elements of carrier systems. The most demanding use is in the latter group, where it is also necessary to find a suitable design procedure while providing enough reliability and safety of the structure. These may be for instance cases of lightweight structural elements where fibres are to eliminate shear destruction/failure. Input parameters of the calculation, fibre concrete properties are verified mainly by laboratory testing included in suggestions, standards, and design code [12,13]. It is well known that electrical properties are influenced by conductive materials. Thus, it is interesting to record the electrical conductivity values of SCC-SFR mixtures.

Furthermore, the development of tools for predicting the durability of concrete structures that help to develop durable concrete and construction systems is useful. Increased focus on higher durability helps maintain the required level of safety and maintenance for a longer period of time, which saves the cost related to premature repair and reconstruction [18,19]. A correct description of the properties of the composite materials can be combined with modelling of structures within several available models [20–23].

The article aim is to determine and extend the knowledge about the properties of self-compacting concrete with steel fibre reinforcement mixture with a fibre content of 0%, 1%, and 2%. This is due to their use in the field of reinforced concrete structures exposed to chlorides, where it is necessary to properly model diffusion processes [24–27]. Moreover, three methods for evaluation of the diffusion coefficient related to a concrete's ability to resist chloride ingress are compared—electrical resistance measurements [28], the rapid chloride permeability test [29], and accelerated penetration tests with chloride [30].

#### **2. Mixtures Properties and Samples Settings**

The evaluated experiment, calculations and results are part of the extensive campaign dealing with the durability of concrete structures. The laboratory experiments were conducted at the laboratories of Silesian University of Gliwice and VSB - Technical University of Ostrava. The set of laboratory samples consisted of seven large cylinders (diameter 150 mm, height 300 mm), six smaller cylinders (diameter 100 mm, height 250, 200 and 100 mm), four cubes (dimension 150 mm), and three beams (150 × 150 mm, length 450 mm) for each mixture. (see Figure 1a).

**Figure 1.** (**a**) Example of preparation of laboratory samples for one set of mixtures; (**b**) Steel fibres KE20/1.7—shape and dimension [31].

The reference concrete was formed from Ordinary Portland Cement (OPC). The Self-Compacting Concrete (SCC) with several values of added steel fibres—0%, 1%, and 2%—was used in order to investigate the effect of fibres. The steel fibres were of type KE20/1.7 (see Figure 1b). The composition of the mixtures is shown in Table 1, and it is based on earlier SCC research at the SUT in Gliwice [1,19,31,32]. It needs to be noted that the cement applied in the SCC mixture had been in laboratory storage for more than two years, and partial hydration had already started. However, the concrete slump test was executed, and all mixtures have the same value of workability.


**Table 1.** The concrete mixtures components [31] based on from Ordinary Portland Cement (OPC) and Self-Compacting Concrete (SCC) with steel fibres.

#### **3. Experimental Tests Range and Results**

A comprehensive range of tests comprised from basic mechanical properties; fracture test, as well as electrical resistance measurements [28], the rapid chloride permeability test [29], and accelerated penetration tests with chloride [30] were conducted.

#### *3.1. Mechanical Properties*

Compressive strength measurements were performed on standard cubes and cylinders, and also the tensile strength and modulus of elasticity were determined (see Figure 2) [31]. Firstly, it is possible to compare the results between an OCP and SCC mixture. In all properties, the SCC values are higher than the OPC. Because the SCC mixtures are not prepared in the way typical for high-performance concrete, it was expected that there would not be much difference in mechanical properties.

**Figure 2.** Results of material characteristics for all mixtures: (**a**) cylinder compressive strength; (**b**) cube compressive strength; (**c**) tensile spitting strength; (**d**) modulus of elasticity [31].

The second possibility is comparison related to the number of steel fibres in the SCC mixture. It shows that the higher the relative weight of the steel fibres, the higher the tensile splitting strength. However, other mechanical properties show that, when the amount of fibres exceeds a certain value, the properties no longer improve. The modulus of elasticity of the mixture SCC 1% is about 15% higher compared with other mixtures. This is in line with the typical limitation of the use of steel fibres up to 1.5% of the weight of concrete [33].

#### *3.2. Di*ff*usion Properties*

The three procedures of detection of concrete diffusion coefficient were used. Two indirect electrochemical methods—rapid chloride permeability test (RCPT) [29] and surface measurement of electrical resistivity using Wenner probe (Resistivity) [28]—and a method based on the evaluation of chloride action—accelerated penetration tests with chloride (NTBuild 443) [30].

#### 3.2.1. Rapid Chloride Permeability Test

The RCPT test [29] was conducted at the SUT in Gliwice. Due to the influence of fibre scattering, one cylindrical core for each mixture was cut into three slices of testing and marked Upper, Middle and Lower. All three slices from one cylinder were analysed. Test specimens were prepared and test procedures were performed according to ASTM C1202 [29]. The resulting passing charges, which indirectly evaluated the ability of the concrete to withstand an aggressive environment, are shown in Figure 3. The results show that there is a negligible difference in the reference mixture in terms of individual slices. There are some deviations (approx. 15%) in SCC mixtures without wires, which can be influenced by shrinkage of the mixture. For a mixture of SCC with 1% of fibres, the differences are more significant, and the most significant differences are found for a mixture of 2% of fibres. The problem of the test is a rapid increase in temperature when using fibre reinforced concrete. From this point of view, the test is not an ideal solution for such an SCC-SFR mixture. Taking statistical variance into consideration, the average value was chosen for derivation of diffusion coefficients.

**Figure 3.** Results of the rapid chloride permeability test for three slices of testing and marked Upper, Middle and Lower, and mean values.

#### 3.2.2. Electrical Resistivity

The standard test method for surface resistivity of concrete [28] is non-destructive; therefore, repeated measurements are possible to determine the time dependency of the diffusion. Unfortunately, this measurement method may have a relatively large variability, partly due to the heterogeneity of concrete and also due to the use of relatively uncontrollable contact conditions. It should be noted that the values of the volumetric resistivity were calculated from surface resistivity based on the

relationship obtained from [34] and after that, to the resulting passing charges (equivalent to the results from RCPT) [35]. The test is non-destructive, and it is possible to measure the electrical properties during concrete ageing (see Figure 4). Results were partly published in [19,31].

**Figure 4.** Results of time-dependent surface electrical resistivity of concrete.

Looking at the change of the passed charge, we may derive the following findings. The OPC mixture has standard behaviour whereby charge decreases over the concrete maturation, thus reflecting an improved ability of the concrete to withstand the aggressive environment. The SCC mixtures have similar behaviour, reflected in the similar shape of the curves. Although the relationship of shapes between 14 and 28 days are visible in all three SCC mixtures, their absolute difference cannot be considered as constant. If the influence of passed charge is based on the amount of added steel fibres, the shape of curves is proportional. However, there are also other influences that affect the readings, such as the initiation of corrosion of steel fibres.

#### 3.2.3. NTBuild 443

The third test method was based on the modified NORDTEST NT Build 443 [13]. Concrete specimens were immersed in the saline solution (see Figure 5a), and then sampling of concrete powder in respective layers of chloride profile was conducted by drilling (see Figure 5b), thus the suitable period for chloride penetration was selected as 90 days. The concrete powder was subsequently evaluated for the presence of chloride ions in the laboratory, the obtained chloride profile was analysed, and the diffusion coefficient was calculated. The whole test procedure and the process of calculating the diffusion parameter are described in detail in [36].

*Crystals* **2020**, *10*, 220

**Figure 5.** The process of the modified NORDTEST NT Build 443: (**a**) samples submerged in the chloride solution; (**b**) samples after drilling.

#### **4. Comparison of Di**ff**usion Coe**ffi**cients**

The resulting values of diffusion coefficients calculated from direct chloride profiling, and indirect electrochemical methods (RCPT and resistivity), are given in Figure 6 and in Table 2. The calculation of the diffusion coefficient *Dc(t)* is based on the procedures given in [35,37] and is very well described in [36,38,39]. The number of analysed samples mean value and standard deviation is given in Table 2. Figure 6 shows mean value and T-plot of minimum and maximum values.

**Figure 6.** Diffusion coefficient *Dc(t)* derived from surface resistivity, rapid chloride permeability test (RCPT) and chloride profile analysis (NTBuild 443). The minimum and maximum values are shown in the form of a T-graph.


**Table 2.** Statistical data of diffusion coefficient *D*c(*t*) derived from surface resistivity, rapid chloride permeability test (RCPT) and chloride profile analysis (NTBuild 443).

It should be noted that the values of the diffusion coefficient from resistivity are based on measurement at 28 days, from RCPT are based on measurement at 56 days, and from chloride profile, approximately 118 days. All these values are precisely determined according to the standards. It is worth noticing that the different level of maturation at the time of testing each method does not affect the comparison between specific mixture design that matters.

It is necessary to explain the meaning of the diffusion coefficient value. A diffusion coefficient closer to zero shows better diffusion resistance and hence better resistance to chloride ion penetration. Looking at the results of diffusion coefficients (Figure 6), the reference mixture (OPC) reports almost the same values for all methods. On the other hand, SCC 0% mixture has larger differences, but is still within limits of the inaccuracies of each method. In this case, the results of SCC 0% are worse than OPC, which is probably influenced by chemical additives. This is also because it is not a mixture that has been prepared as high resistance to the chloride. This is observed even though the mechanical properties (see Figure 2) are better for SCC mixtures.

Subsequent evaluation of mixtures with 1% and 2% fibres is interesting in terms of two hypothetical effects. The first effect is related to the electrical conductivity of steel wires in concrete. This effect should be reflected in two methods—RCPT and surface resistivity. The second effect is related to the real acceleration of diffusion along the fibres at the transition level with other material with respect to a possible influence of the microscopic void structure and porosity [6,40]. This should affect all three methods. Considering the possible scatter of measurement, it is possible to evaluate the effect of the amount of fibres on the two mentioned effects accordingly. For the purpose of comparison, SCC 0% mixture results were considered as the base values (0%), and differences against the SCC 1% and 2% mixtures are shown in Figure 7. It is worth mentioning that the results of measurement of the RCPT is based on three cut out samples from one concrete cylinder. It can be seen, that the results are consistent within the OPC, SCC 0%, 1%. However, the scatter for the 2% fibre content shows much higher scatter, indicating that this amount of steel fibres is too high. The finding is consistent with recommendation that the highest applicable amount of fibres is 1.5%. It seems that the steel fibres were not spread out uniformly thought the cylinder.

**Figure 7.** Percentile differences of the diffusion coefficient of the SCC 1% and 2% mixtures compared to mixture SCC 0%.

Looking at the percentage results, the resistivity, RCPT and NTBuild 443 are noticeably affected by the amount of steel fibres as expected. The most significant influence is observed with a resistivity test, which is based on the direct current measurement, and the steel material is, therefore, a significant factor. The RCPT method is less affected but is also based on the charge and use of the penetration of a salt solution (in all samples of the same amount), so the effect of the amount of the steel fibres is lower. In the third method, the evaluation of the chloride profile, it is possible to consider only the effect of increased diffusion along the fibres or the possible influence of interface and micro-cracks along the fibres.

#### **5. Discussion and Conclusions**

The article evaluates durability related to concrete material parameters. There are relative values for cube and cylinder compression strength, tensile splitting strength, and modulus of elasticity. Three approaches are discussed for the computation of diffusion coefficient applicable to the numerical modelling of chloride ion ingress to concrete. Studied approaches are chloride profiling, electrical resistivity measurement, and rapid chloride permeability test. The comparison of the relatively fast method (resistivity and RCPT) for the evaluation of concrete ability to resist aggressive agents was conducted on the sample of self-compacting concrete. There was a correlation between the amount of steel fibres and conductivity, as expected. A larger amount of the steel fibres increases the calculated diffusion coefficient, both in electrochemical methods and in chloride profile evaluation. From this point of view, it is necessary to investigate, for example, porosity in further research. It needs to be proved if it causes some worse mechanical properties and, on the contrary, may tend to increase the results of all diffusion values (reducing the resistance against chloride penetration).

From the point of view of the amount of fibres in the concrete, it would be advisable to prepare more a graduated set of mixtures, e.g., 1.2%, 1.4%, that can lead to finding the threshold at which the increasing properties change to decreasing. The experiment also confirmed that diffusion coefficients obtained by the electrochemical approaches are influenced by the steel fibres from both standpoints—electrical conductivity, as well as pore structure.

**Author Contributions:** Conceptualization, P.K. and T.P.; methodology, T.P., P.L. and P.K.; software, P.L.; validation, P.K., P.L. and T.P.; data curation, P.L.; writing—original draft preparation, P.L.; writing—review and editing, P.K.; visualization, P.L.; project administration, P.K. All authors have read and agreed to the published version of the manuscript.

**Funding:** Financial support from VSB-Technical University of Ostrava by means of the Czech Ministry of Education, Youth and Sports through the Institutional support for conceptual development of science, research and innovations for the year 2020 is gratefully acknowledged.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

*Article*

## **Experimental Study on Unconfined Compression Strength of Polypropylene Fiber Reinforced Composite Cemented Clay**

#### **Qiangqiang Cheng 1,2, Jixiong Zhang 2,\*, Nan Zhou 2, Yu Guo <sup>1</sup> and Shining Pan <sup>1</sup>**


Received: 3 March 2020; Accepted: 23 March 2020; Published: 26 March 2020

**Abstract:** The effects of three main factors, including polypropylene fiber content, composite cement content and curing time on the unconfined compressive strength of fiber-reinforced cemented clay were studied through a series of unconfined compressive strength tests. The experimental results show that the incorporation of fibers can increase the compressive strength and residual strength of cement-reinforced clay as well as the corresponding axial strain when the stress peak is reached compared with cement-reinforced clay. The compressive strength of fiber-reinforced cement clay decreases first, then increases with small-composite cement at curing time 14 d and 28 d. However, fiber-reinforced cement clay's strength increases with the increase of fiber content for heavy-composite cement. The compressive strength of fiber-composite cement-reinforced marine clay increases with the increase of curing time and composite cement content. The growth rate increases with the increase of curing time. The failure mode of composite cement-reinforced clay is brittle failure, while the failure mode of fiber-reinforced cemented clay is plastic failure.

**Keywords:** fiber-reinforced cemented clay; unconfined compression strength; fiber content; composite cement content; curing time

#### **1. Introduction**

Marine clays have the characteristics of large pore ratios, high water content, low shear strength, soft sensitive and high compressibility [1,2]. Soft clays are widely present in offshore areas and cannot be used directly in geotechnical engineering activities, such as subgrade engineering, embankments, deep excavation and underground construction. Considering the economy and effectiveness, using cement to reinforce soft clay is of great popularity [3–8], compared with other chemical stabilization methods. Though cement-stabilized clay has the advantages of rapid formation, good plasticity and high compressive strength, it also has the disadvantages of low tensile strength and flexural strength. A number of studies that used fly ash, a by-product of coal or solid waste, to partially replace cement for improving the mechanical strength of cement-stabilized clay have been carried out [9–11]. Zentar et al. [12] conducted experimental investigations into the tensile strength and unconfined compressive strength of solidified marine sediments using siliceous-aluminous fly ash and cement. Through laboratory-unconfined compression tests, split tensile tests, bender element tests and isotropic compression tests conducted by Xiao et al. [13], a semiempirical relationship between compressive strength and curing time for fly ash-blended cement-stabilized marine clay has been obtained. A series of laboratory experiments, including isotropic compression, triaxial drained shearing, unconfined

compression and bender element testing were carried out by Cheng et al. [14], in which the primary yielding and yield locus of fly ash cement-stabilized marine clay were investigated.

Polypropylene fiber is characterized by light weight, high tensile strength and low energy consumption. The strength and deformation resistance of cement-stabilized soils can be improved by incorporating appropriate fibers [15–17]. Many studies have been carried out on the properties of polypropylene fiber-reinforced cement-stabilized soils. Correia et al. [18] conducted experiments and concluded the compressive and tensile strength characteristics of polypropylene fiber-reinforced blast furnace slag solidified Portuguese soft soils. Through laboratory-split tensile tests conducted by Xiao et al. [19]—as well as stochastic finite element theory—a prediction model for tensile strength of polypropylene / polyvinyl alcohol fiber-modified cement-stabilized clay was established considering the fiber length and content. Ding et al. [20] studied the effect of freeze-thaw cycles on the mechanical properties of polypropylene fiber-reinforced cement-stabilized clay; the relationships among sample size, residual stress ratio, tangent modulus, cement content, fiber content and number of freeze-thaw cycles were established. The tensile strength of cement-stabilized marine clay reinforced by short waste fibers was investigated by Li et al. [21]. Through experimental and numerical methods, a numerical simulation of a single fiber pullout from a matrix was established by using a cohesive contact model.

Compared with the prevalence of studies on unconfined compression strength, there have been fewer studies on the compressive characteristics of fiber-reinforced fly ash-cemented clay. This paper presents a study on the effect of polypropylene fiber content, composite cement content and curing time on the unconfined compression strength of polypropylene fiber-composite cement-reinforced marine clay by using experimental tests, in order to provide reference for the future application of polypropylene fiber in soft clay foundations.

#### **2. Materials and Experimental Methodology**

#### *2.1. Materials*

The studied materials mainly included marine clay, fly ash cement and polypropylene fibers. The clay used in the test was Singapore marine clay excavated from a subway station with characteristics of grayish brown, saturated, obvious rheological properties, high compressibility and low bearing capacity. The main physical characteristics of the marine clay are presented in Table 1. The marine clay was put into a PVC plastic shading bucket and transported to the laboratory for backup. The water content of marine clay was approximately 88%. The liquid limit and plastic limit of marine clay were, respectively, 73% and 32%.


**Table 1.** Physical characteristics of marine clay.

The test cement is CEM II / B-V commercial composite cement and its physical and mechanical indexes are shown in Table 2. The mass ratio of fly ash to ordinary cement in CEM II / B-V commercial composite cement is 0.345:0.655; its main chemical components are SiO2, Al2O3, Fe2O3, MgO and CaO.


**Table 2.** Indexes of mechanical properties of fly ash cement.

The polypropylene fibers used in the test were chopped 6-mm commercial fibers. The main physical and mechanical characteristics of polypropylene fibers are shown in Table 3.


**Table 3.** Physical and mechanical characteristics of polypropylene fibers.

#### *2.2. Experiment Scheme*

The definition of compound composite content, polypropylene fiber content and water content are shown in Equations (1)–(3), respectively. The fly ash cement content was adopted based on applications in practice, such as the improvement of an ending shaft to supporting a tunnel boring machine. The fiber content was determined as the common values used in foundation pit project and subgrade engineering.

$$C\_c = M\_c / M\_s \times 100\,\%\tag{1}$$

$$C\_f = M\_f / M\_s \times 100\% \tag{2}$$

$$C\_{w} = M\_{w}/(M\_{s} + M\_{c}) \times 100\% \tag{3}$$

where *Cc* is fly ash cement content, *Cf* is polypropylene fiber content, *Cw* is water content, *Mc* is the quality of fly ash cement, *Ms* is the quality of dry soil in the marine clay, *Mf* is the quality of polypropylene fiber, *Mw* is the quality of water.

An orthogonal design method was used to study the influence of factors such as fiber content, composite cement content and curing time on the unconfined compressive strength of polypropylene fiber-reinforced fly ash-cemented marine clay. The test scheme is shown in Table 4. Considering test error factors such as sample heterogeneity, 5 samples were conducted in each group.


**Table 4.** The test scheme of the unconfined compressive strength.

#### *2.3. Sample Preparation and Testing*

The obvious plant roots, shells and other debris were removed from the marine clay at the site. Moreover, the sand particles are sieved with a 1-mm sieve. Samples were prepared in accordance with a procedure described by Chin et al. [22]. The reshaping of marine clay and the preparation method of triaxial samples were consistent with references [6,14,23]. According to the content of polypropylene fiber, composite cement and water in the test scheme, the required quality of marine clay, composite cement, fiber and water were weighed with a high-precision electronic scale. First, appropriate amount of water was added to the marine clay and stirred well. Secondly, the weighed composite cement was added and stirred well. Finally, the weighted polypropylene fibers were added and stirred well. The stirred fiber-reinforced flay ash cemented clay was put into a sealed plastic bag for later use.

The PVC plastic mold with an inner diameter of 50 mm and a height of 100 mm was fastened and cleaned. One end of the mold with a plastic film was sealed and then a release agent evenly inside the mold was applied. The mixture of fiber-composite cement-reinforced clay was stirred evenly into the mold for 5 times. We ensured the mixture previously squeezed in was shaken evenly before the next squeeze. After the top of the sample that has been shaken uniformly was scraped with a spatula, the two ends of the mold were completely wrapped with labeled water-permeable filter paper and placed horizontally in the conservational water tank for indoor conservation to ensure that the specimen was not affected by any external force during the conservational process.

The cylindrical samples were taken out from the conservational water tank and wiped off the surface water when the curing time reached 7 d, 14 d and 28 d, respectively. The experiment of unconfined compressive strength was conducted using a triaxial test device with a load at a constant rate of 1 mm/min after the samples were scraped well. The relevant data during the test were collected. An unconfined compression test was performed by following the procedures prescribed in ISO/TS 17892 (2004) [24].

#### **3. Results and Discussion**

#### *3.1. Stress-Strain Behavior*

Figures 1–3 present the curves of axial strain ε – δ with 0%, 0.5% and 1% polypropylene fiber content of the fiber-reinforced fly ash-cemented marine clay, respectively, depending on different mix ratio and curing time. All stress-strain curves show the peak strength of fiber-reinforced fly ash-cemented marine clay was significantly higher than that without polypropylene fiber, except the condition of 20% compound fly ash cement content. Compared with fly ash cement-reinforced marine clay, the residual strength deformation stage of the stress-strain curve of fiber-reinforced composite-cemented clay was more significant and the retention time of residual strength was much longer after reached the peak strength. Taking the 50% fly ash cement content and 0.5% fiber content as an example, the residual strength of the fiber-reinforced fly ash-cemented marine clay increased by 133.3%, 145.6% and 224.5% when the curing age was 7 d, 14 d and 28 d, respectively, which shows that adding certain quality polypropylene fiber can effectively improve the residual strength of fly ash cement-reinforced marine clay. Moreover, the trend for the changes of curves of axial strain ε – δ—as well as strength and stiffness without fiber with fly ash cement content and curing time—is similar to the observation reported in a previous study [6,9,14]. The researchers analyzed the structure with respect to the unconfined compression strength of fly ash cement-reinforced clay.

**Figure 1.** Curves of axial strain ε – δ without fiber depending on fly ash cement content and curing time.

**Figure 2.** Curves of axial strain ε – δ with 0.5% fiber content depending on fly ash cement content and curing time.

**Figure 3.** Curves of axial strain ε – δ with 1% fiber content depending on fly ash cement content and curing time.

The axial strain corresponding to the failure time of fiber-reinforced composite-cemented marine clay is shown in Table 5. Taking the 50% fly ash cement content and 0.5% fiber content as an example, the axial strain at the failure time of the fiber-reinforced fly ash-cemented marine clay increased by 253.5%, 197.3% and 172.9% when the curing age is 7 d, 14 d and 28 d, respectively. The axial strain at the time of failure of fiber-reinforced composite-cemented marine clay was much larger than that without polypropylene fiber under the same conditions, which indicates that the incorporation of polypropylene fiber can effectively improve the brittleness and toughness of the composite cement-reinforced marine clay. The main reason is that the composite cement-reinforced marine clay particles attached to the fiber surface can increase its cohesion and friction. The tensile stress between the fiber and the reinforced clay may continue for a long time though the specimen is already damaged, which means that the fiber can effectively reduce the deformation and improve the toughness and residual strength of the fly ash cement-reinforced marine clay.


**Table 5.** The axial strain at failure time of the fiber fly ash cement-reinforced marine clay.

#### *3.2. E*ff*ect of Fiber Content on Unconfined Compressive Strength*

The relationship between the unconfined compressive strength and the fiber content of each group of samples depending on fly ash cement content and curing time is shown in Figure 4. The figure indicates that the unconfined compressive strength of polypropylene fiber-reinforced composite-cemented marine clay increases at first and then decreases with the increase of fiber content when the composite cement content is 20%, however, it keeps increase with the increase of fiber content when the composite cement content is 50% and 100%. Taking the 50% fly ash cement content and 0.5% fiber content as an example, the unconfined compressive strength of the fiber-reinforced fly ash-cemented marine clay at curing time 7 d, 14 d and 28 d is 1.11 times, 1.27 times and 1.06 times, respectively, that of without polypropylene fiber. When the fiber content is 1% with the 50% fly ash cement content, the unconfined compressive strength of the fiber-reinforced fly ash-cemented marine clay is 1.20 times, 1.63 times and 1.31 times that of without polypropylene fiber, which indicates that with low fly ash cement content, the structure of composite cement-reinforced marine clay is not fully formed; the modification effect of polypropylene fiber on composite cement-reinforced clay is not obvious. However, when the content of fly ash cement is higher, adding appropriate amount of fiber can effectively increase its cohesion of the fly ash cement-reinforced marine clay, which can make the reinforcing effect of polypropylene fiber obvious. The mechanical performance of fiber-composite fly ash cement-reinforced clay with 0.5% fiber content is basically consistent with previous studies, however, it shows different characteristics with 1% fiber content [16,17,22].

**Figure 4.** Relationship between unconfined compressive strength and fiber content depending on fly ash cement content and curing time.

#### *3.3. E*ff*ect of Fly Ash Cement Content on Unconfined Compressive Strength*

In order to study the effect of fly ash cement content on the unconfined compressive strength, the relationship depending on fiber content and curing time was studied, as shown in Figure 5. The figure indicates that the unconfined compressive strength of polypropylene fiber-reinforced fly ash-cemented marine clay increased with the increase of fly ash cement content. The increase trend of 14 d to 28 d was more significant than that from 7 d to 14 d. Taking the 0.5% fiber content as an example, the unconfined compressive strength of 50% fly ash cement content of the fiber-reinforced fly ash-cemented marine clay at curing time 7 d, 14 d and 28 d was 2.23 times, 2.47 times and 2.52 times that of 20% compound cement content respectively. As a comparison, the unconfined compressive strength of 100% fly ash cement content of the fiber-reinforced fly ash-cemented marine clay at curing time 7 d, 14 d and 28 d was 4.67 times, 5.13 times and 5.48 times, respectively, that of 20% compound cement content. It was shown that the fly ash cement content had a significant effect on the unconfined compressive strength of fiber-reinforced composite-cemented marine clay: the unconfined compressive strength increased significantly with the increase of curing time and the polypropylene fiber content.

**Figure 5.** Relationship between unconfined compressive strength and fly ash cement content depending on fiber content and curing time.

#### *3.4. E*ff*ect of Curing Time on Unconfined Compressive Strength*

The relationship between the unconfined compressive strength and the curing time depending on different fiber content and fly ash cement content is shown in Figure 6. The figure indicates that the unconfined compressive strength of polypropylene fiber-reinforced fly ash-cemented marine clay increased with the increase of curing time. Moreover, the growth rate increased more obviously when the curing time increased. Taking the 50% fly ash cement content and 0.5% fiber content as an example, the unconfined compressive strength of the fiber-reinforced fly ash-cemented marine clay at curing time 14 d and 28 d was 1.29 times and 1.57 times, respectively, than that of 7 d curing time. The relationship between the unconfined compressive strength and the curing time at 7 d, 14 d and 28 d can be expressed by Equations (4)–(6), respectively.

$$f\_{ts} = 176.4 \text{e}^{0.16t} \tag{4}$$

$$f\_{ts} = 373.9 \text{e}^{0.23t} \tag{5}$$

$$f\_{ls} = 373.9 \text{e}^{0.23t} \tag{6}$$

where *fts* is unconfined compressive strength, *t* is curing time.

**Figure 6.** Relationship between unconfined compressive strength and curing time, depending on fiber content and fly ash cement content.

#### *3.5. Failure Modes*

Taking the 7 d curing time as an example, the failure photos of fiber-composite cement-reinforced marine clay with 0.5% polypropylene fiber content are shown in Figure 7.

**Figure 7.** Failure photos of the specimens with 0.5% fiber content and 7 d curing time. (**a**) 20% fly ash cement content; (**b**) 50% fly ash cement content; (**c**) 100% fly ash cement content; (**d**) 20% fly ash cement content with fiber; (**e**) 50% fly ash cement content with fiber; (**f**) 100% fly ash cement content with fiber.

As can be seen from Figure 7, the unconfined compressive strength of the fly ash cement-reinforced marine clay increased with the increase of fly ash cement content; the failure of the specimen gradually showed the characteristics of brittle failure. When 20% fly ash cement was added, there were no obvious through-cracks in the fly ash cement-reinforced clay. However, samples with 50% and 100% fly ash cement content showed obvious through-cracks in a short period of time; specimens with 100% fly ash cement content showed surface shedding during compression.

Compared with fly ash-cemented marine clay, fiber-reinforced fly ash-cemented marine clay had a longer crack generation time and a slower crack propagation speed under the same conditions. The specimen of fiber-reinforced composite-cemented marine clay showed good plastic deformation after compaction and failure without phenomenon of surface shedding. The tensile stress between the fiber and the fly ash cement-reinforced clay could continue for a long time under the continuous compressive stress though the specimen was already broken. It was affected by the mechanical behavior of the interface between the fiber and the soil particle matrix microscopically. The results were similar to those in reference [16]. The specimen cracked, instead of broke, macroscopically.

#### **4. Conclusions**

The following conclusions can be drawn according to the tests results:

The peak strength of fiber-reinforced fly ash-cemented marine clay is significantly higher than that without polypropylene fiber except on the condition of 20% fly ash cement content. The axial strain at the time of failure of fiber-reinforced composite-cemented marine clay is much larger than that without polypropylene fiber under the same conditions, which indicates that the incorporation of polypropylene fiber can effectively improve the peak strength and toughness of the composite cement-reinforced marine clay.

The unconfined compressive strength of fiber-reinforced fly ash-cemented marine clay shows different trends with the increase of fly ash cement content: it increases at first, and then decreases with 20% fly ash cement content, while it appears an increasing trend with 50% and 100% fly ash cement content. The unconfined compressive strength of fiber-reinforced composite-cemented marine clay increases with the increase of fly ash cement content and curing time. The increase trend of the unconfined compressive strength from 14 d to 28 d is significantly larger than that from 7 d to 14 d. The growth rate of the unconfined compressive strength increases with the increase of the curing time and the unconfined compressive strength shows a growth trend of power function.

Composite cemented marine clay forms through-cracks in a short time and shows a brittle failure mode during compression, while fiber-reinforced fly ash-cemented marine clay shows a plastic failure mode which appears that it can continue for a long time after the formation of through cracks. On the microscopic level, it shows continuous tensile stress between polypropylene fiber and composite cement-reinforced marine clay; on the macro level, it became cracked and continuous.

**Author Contributions:** Q.C. and J.Z. conceived and designed the study; Q.C. and N.Z. performed the experiments and wrote the paper; Y.G. and S.P. reviewed and edited the manuscript. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work is funded by the Research Fund of The State Key Laboratory of Coal Resources and Safe Mining, CUMT (SKLCRSM19KF012); Qing Lan Project (2018); Ministry of Housing and Urban-Rural Development Technology Plan (2018-K7-004); Science and Technology Project of Construction System in Jiangsu Province (2018ZD021, 2019ZD083); and the Doctoral Special Fund of Jiangsu Vocational Institute of Architectural Technology (JYBZX18-02).

**Acknowledgments:** The authors gratefully acknowledge the support provided by National University of Singapore and the Jiangsu Overseas Research & Training Program for University Prominent Young & Middle-aged Teacher and Presidents.

**Conflicts of Interest:** All the authors declare that there are no conflicts of interest regarding the publication of this paper.

#### **References**


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