Effect of fiber content on mechanical parameters and crack development of two kinds of reinforced cement soil

PVA fiber and basalt fiber are two kinds of common fibers used to reinforce cementitious materials and are widely used in engineering, therefore it is of great interest to study the effect of the content of the two kinds of fibers on the strength change of the cementitious materials. In this study, the unconfined compressive strength (UCS) test and digital image correlation (DIC) test of cement soil with different contents (0,0.25%, 0.5%, 0.75% and 1%) were carried out. The following conclusions were drawn: in the process of uniaxial compression, the curve of specimens can be roughly divided into five stages: compaction, elasticity, plastic yield, failure and residual stage; the UCS of the soil specimens increased with the increase with the content of the two kinds of fibers, the UCS of 1% PVA fiber can be increased to 179.32% of the control group, but when the content is greater than 0.75%, the development of strength was limited by fiber aggregation; The modulus of deformation and the compressive toughness index of the soil specimen are linearly related to the compressive strength; DIC technology can simply and efficiently monitor the horizontal strain field changes and crack development of specimens in several stages, which can be extended to the actual project.


Introduction
Adding fibers to the cement soil material is a simple and efficient method to improve the strength, and a large number of scholars have done research on the various strengths of fibrous cement soils (Thanushan and Sathiparan 2022, Xu et al 2022, Li et al 2022a, Yang et al 2023a), PVA and basalt fiber are widely used in the reinforcement process of cement soil materials as common fiber. PVA fiber is a kind of super-extension material. The fracture elongation of qualified PVA fiber can reach 6.9%, thus avoiding the destruction of cementitious materials in the process of damage only through a crack, but first formed numerous micro-cracks, and finally the overall damage occurred (Li and Leung 1992), Li et al (2022b) found that PVA fiber can be combined with hydration products to improve the crack resistance of cement soil materials and enhance the durability of materials, Mercuri et al (2023) added PVA fibers with different contents and lengths to mortar, and obtained the change rule of compressive strength of mortar material with fiber, and fitted the formula by linear regression, Yang et al (2023b) added PVA fibers and expanders to cement grout and found that it improved the mechanical properties and prevented brittle damage, but reduced the thermal aging bond strength of the grout, Yao et al (2021) found that the addition of PVA fibers can improve the compressive strength, flexural strength and tensile strength of the cement soil, and can effectively reduce the width of cracks, however, when the content of fiber is more than 1%, fiber aggregation will occur inside the specimen, which will affect the improvement of strength. Mei et al (2022) found through experiments that adding 0.1% 6 mm PVA fibers to cement soil can improve the static strength best, and adding 0.3% 12 mm PVA fibers can improve the dynamic strength best. Basalt fibers, as an environmentally friendly fiber with good mechanical properties, are also often used to Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.
improve the properties of cementitious materials (Zheng et al 2021, Aishwarya andPriya Rachel 2023, Navaratnam et al 2023), Owino and Hossain (2023) found that the longer the length of the basalt fibers the more significant the increase in the bias stress in the cement soil material, Zhou et al (2020) found that when the basalt fiber content is 0.3% and 0.4%, the improvement of concrete toughness is the best. Zhuang et al (2022) obtained through experiments that when the basalt fiber content is 1.2%, the UCS of engineering cement-based composites reaches a maximum of 46.37 MPa.
The DIC technique is a non-contact method of measuring the strain field on the surface of a specimen, and has advantages in macroscopic and microscopic dimensions that are unmatched by other methods (Alhakim et al 2023, Allain et al 2023, Holmes et al 2023, Wang et al 2023a, 2023b, 2023c, 2023d, 2023e), Miao et al (2023 monitored the crack development of marble specimens during compression using the DIC technique and found that tensile cracks developed from the clear narrow strains in the strain field diagram and shear cracks developed from the fuzzy wide strains in the strain field. Kinda et al (2023) investigated the drying creep and shrinkage of cement paste from a microscopic point of view using the DIC technique under SEM and established a model between creep modulus, drying rate and relative humidity. Wang et al (2023aWang et al ( , 2023bWang et al ( , 2023cWang et al ( , 2023d added corn straw fibers to gangue powder concrete and monitored the surface of the specimens using the DIC technique, and the results of the tests showed that the addition of corn straw fibers improved the stability of the cement soil. Guo et al (2023)  In this study, DIC technology was innovatively introduced into the strength test of cement soil specimens. In order to comprehensively analyze the mechanical parameters of cement soil, the stress-strain curve, unconfined compressive strength, deformation modulus, compressive toughness index and other aspects were analyzed. The variation law of these mechanical parameters with fiber content and the mutual law between them can provide a good reference for practical engineering. As a simple and efficient method, DIC technology can be used to analyze the change of the surface strain field of the specimen in this experiment. In the future, it can be extended to other experiments that need to analyze the change of the surface strain field and the change of the surface displacement, which is theoretically feasible.

Materials
The soil sample used in the test is clay deep in the foundation pit of a construction site. The physical and mechanical properties of the soil are tested according to Standard for geotechnical testing method (GB/T 50123-2019). The test results are shown in table 1. The plastic limit of the soil is 24.6%, and the liquid limit is 44.8%. According to the USCS, it can be classified as inorganic clay (CL) with low to medium plasticity (Shu et al 2023). The cement uses P·O42.5 ordinary Portland cement, and the fiber uses basalt fiber and PVA fiber, as shown in figure 1. The fiber properties were tested according to the Synthetic fibers for cement concrete and mortar (GB/T 21120-2018), and the basic parameters are shown in table 2. According to the mix ratio shown in table 3, a cube cement soil specimen with an edge length of 70.7 mm is made, the prepared specimens were placed in a sealed bag and placed in a standard curing room ( temperature 20 ± 2°C, relative humidity >95% ) for 28 days.

Test method
After the cement soil specimens are cured, the unconfined compressive strength test is carried out on them, and the DIC test is carried out to analyze the strain field and displacement field on the surface.

Unconfined compressive strength test
The test system is shown in figure 2. The loading equipment is a microcomputer-controlled electronic universal testing machine produced by Jinan Zhongluchang Testing Machine Manufacturing Co., Ltd. The displacement closed-loop control is adopted, and the loading rate is 1 mm min −1 .

DIC characterization
Before conducting the DIC characterization test, the specimens to be tested needs to be sprayed with paint, select a surface to be sprayed with paint, first spray white primer uniformly sprayed on the surface of the specimen, after the white primer dries naturally, spray the black paint to create a scattered spot, the scattered spot production process is shown in figure 3. The basic principle of DIC is shown in figure 4. This method is to measure the change of gray value mode in adjacent small areas called pixel subsets by high-speed camera tracking during the deformation process. f (x, y)   Figure 5 shows the stress-strain curves of the specimens with different content of fiber after 28 d of curing, and it can be seen that for the specimens with fiber added, there is a similar curve change law. We can divide the whole   process of UCS of the cement soil specimen into 5 stages. In order to make the description more concise, we choose the stress-strain curve of the specimen with 1% PVA fiber as a typical curve for analysis.
(1) Compacting stage (OA section): The strain range occurring in this stage is about 0 ∼ 1.5%, the slope of the stress-strain curve gradually increases and the curve is concave. In this stage, the pores in the specimen are gradually compacted under the action of load, the soil particles and hydration products are rearranged in the specimen, the specimen becomes more dense and the resistance to load deformation is gradually increased.
(2) Elastic stage (AB section):The strain range occurring in this stage is about 1.5 ∼ 3.7%, the slope of the stressstrain curve at this stage remains basically unchanged, showing an inclined straight line, the internal structure of the specimen develops steadily, and cracks begin to appear on the surface of the specimen and develop steadily, where the stress corresponding to point B is called the proportional limit, and for the cement soil mixed with two kinds of fibers, the proportional limit of two kinds of cement soil both increase with the increase of the content of fiber.
(3) Plastic yielding stage (BC section): The strain range occurring in this stage is about 3.7 ∼ 4.0%, the cracks on the surface of the specimen accelerate to expand in this stage, the deformation changes obviously with the stress, after the stress reaches the proportional limit, the slope starts to decrease until it reaches the peak stress (C point), as for the two kinds of fiber cement soil, the peak stress increases with the increase of the content of fiber.
(4) Damage stage (CD section): The strain range of this stage is about 4.0 ∼ 8.2%. When the stress reaches the peak stress, it enters to the damage stage, at this time, with the increase of strain, the stress decreases rapidly, and this stage is the most obvious stage of damage occurring in the unconfined compressive strength test, and the cracks on the surface of the specimen expand rapidly until they penetrate the whole interface.
(5) Residual stage (DE section): The strain range of this stage occurs after 8.2%, the stress-strain curve at this time is roughly a horizontal line, which means that the cement soil specimen is not completely lost the bearing capacity after the damage, for the cement soil with PVA fiber added, the residual stress can reach 36.53%∼42.25% of the peak stress, the residual stress of the cement soil with basalt fiber added can reach 14.95%∼25.95% of the peak stress, it can be seen that the addition of PVA fiber has better help to improve the residual stress of the specimen, which can show that the addition of PVA fiber in the cement soil can improve the reliability of the cement soil material.

The relationship between the content of fiber and the unconfined compressive strength of cement soil.
The results of the unconfined compressive strength (UCS) experiments were processed to obtain the relationship between the content of basalt and PVA fiber on the unconfined compressive strength of the cement soil, respectively, and the results are shown in figure 6. As can be seen from figure 6, the UCS of the specimens increased continuously with the fiber mass fraction at 0 ∼ 1%, regardless of whether the fibers added were basalt fibers or PVA fibers. When PVA fibers with mass fractions of 0.25%, 0.5%, 0.75%, and 1% were added, the UCS of the cement soil specimens improved by 14.93%, 24.05%, 61.14%, and 79.32%, respectively, compared to the UCS of control group (PC). When basalt fibers with mass fractions of 0.25%, 0.5%, 0.75%, and 1% were added, the UCS of the specimens improved by 16.25%, 25.07%, 50.01% and 68.56%, respectively, compared to the UCS of PC group. It can be seen that the addition of fibers can improve the UCS of cement soil specimens, but after adding fibers, the value of UCS increases first and then decreases for each increase of the same content of fibers (0.25%), when the added mass fraction is greater than 0.75%, the reinforcing effect of the fiber will decrease, this is because after adding too much fiber, it may cause part of the fiber united together because of uneven mixing, so that the soil and hydration products in the specimen can not well wrap the fiber to play the role of micro reinforcement. On the contrary, due to the occurrence of fiber agglomeration, weak surfaces will appear in the place where the fibers are united under the action of load. Therefore, when the content of fiber is higher than 0.75%, the speed of the content of fiber to increase UCS of cement soil will be limited.

The change of deformation modulus E 50 and compression toughness index with the content of fiber
Fiber cement soil is an inelastic material., and the deformation modulus E 50 is an important parameter of cement soil material in practical engineering. E50 is calculated by equation (1), which is the ratio of the 50% peak stress point σ 0.5 to its corresponding strain ε 0.5 on the stress-strain curve of cement soil. The stress-strain curves were analyzed, and the stress-strain curves derived from the cement soil specimens at each doping level were selected for calculation, and finally the average value of each group of data E 50 was plotted as figure 7, in which the black dots are the original data and the red curve is the fitted curve, from which it can be seen that: The addition of fibers affects the deformation capacity of the cement soil. For the specimens mixed with PVA fibers and basalt fibers, the E 50 shows a similar pattern with the increase of the content of fibers, the E 50 of the specimens increases with the increase of the content of fiber, however, the growth rate of E 50 increases and then decreases with the increase of the content of fiber, and finally the growth rate flattens out.
UCS and E 50 are two important parameters of cement soil materials in engineering applications. The relationship between UCS and E 50 is obtained by analyzing UCS in figure 6 and E 50 in figure 7. As shown in figure 8, it can be seen that E 50 = ( 27.64 ∼ 32.80 ) UCS of cement soil specimens with PVA fiber and E 50 = ( 44.03 ∼ 60.24 )UCS of cement-soil specimens with basalt fiber. E 50 increases with the increase of UCS, and the two are roughly linearly related, which is similar to the results of Asgari et al (2015).
Compression toughness index (CTI) refers to the ability of a material to absorb energy before damage, and is usually determined by the ratio of the area of the stress-strain curve of the specimen with added fibers to that of the specimen without added fibers. There are few studies describing the CTI of cement materials, with reference to concrete and other materials on CTI (Marara et al 2011), So we refer to the concept of axial compressive strength design value fc in the 'Code for the Design of Concrete Structures', and similarly define a f cs = 0.67f pk for cement soil ( f pk is the UCS of the cement soil specimen). Based on this, we propose a new method for calculating the CTI with the calculation area schematic shown in figure 8 and the calculation formula shown in equation (2), where I = 0%, 0.25%, 0.5%, 0.75%, 1%. It can be seen from figure 8 that the CTI of the fiber specimens increases with the increase of the content of fiber, and mainly due to two reasons, on the one hand, the addition of fibers raises the peak stress of the specimens, and the area S in figure 9(a) is larger for the same strain span, On the other hand, the higher the content of fiber, the smaller the slope of the stress-strain curve of the specimen in the damage phase, which will result in a greater strain for the same strain of decay, leading to an increase in S. The combined effect of two kinds of reasons makes the CTI of the specimen increase with the increase of the content of fiber.
In addition, it can be found from figure 9(b) that the CTI of the specimen is improved by adding the same content of PVA fiber is greater than that of basalt fiber, and the maximum CTI of the cement soil with PVA fiber is 3.85, which is 285% higher than that of the plain cement soil, while the maximum CTI of the cement soil with basalt fiber is 1.68, which is 68% higher than that of the plain cement soil. It can be shown that the improvement of PVA fiber on the CTI of the cement soil is relatively obvious. Comparing the CTI with the UCS of the cement soil mixed with the two fibers, it can be seen that the CTI and unconfined UCS approximately satisfy the positive correlation relationship, which indicates that the present method to define the CTI is reasonable.

Damage process study based on DIC
The DIC technique is a method that can easily analyze the full-field strain on the surface of an object and is very versatile when analyzing strain fields. Due to the large number of test specimens in this test and the similar pattern of fiber cement soil specimens in the process of unconfined compressive strength test, we selected a representative specimen (adding 1% mass fraction of PVA fiber) to analyze the horizontal strain e xx of the whole process of unconfined compressive strength test, and the principle of calculating e xx is shown in equation (3) (Blaber et al 2015). The points taken throughout the process of unconfined compressive strength test of specimens are analyzed, and the critical points of each stage of unconfined compressive strength are selected, where point A is the end of the compacting stage and the beginning of the elastic stage, point B is the end of the elastic stage and the beginning of the plastic yielding stage, point C is the peak stress point and also the end of the plastic yielding stage and the beginning of the damage stage, point D is the end of the damage stage and the beginning of the yielding stage, and point E is the end of the yielding stage.  Figure 10(b) shows the horizontal displacement field at point A in the stress-strain curve, it can be seen that after the compression-density stage, the strain concentration part of the horizontal displacement field on the surface of the specimen is mainly distributed at the top and bottom of the specimen, and it is roughly symmetrically distributed, the strain field on the surface is smaller, and the maximum horizontal strain is about 7.8 × 10 -4 , The whole strain field is in a uniformly distributed state, and with the increase of load, the specimen goes through the elastic phase, and the specimen is uniformly deformed throughout the process. Figure 10(d) shows the horizontal displacement field of point B in the stress-strain curve, and it can be seen that after the elastic stage, four obvious strain concentrations appear on the surface of the specimen, and the maximum horizontal strain at this time is about 0.012, As the load gets continued to increase, the peak stress point C is reached at the end of the yielding stage, at which time the area of strain concentration is further expanded, and it is difficult to see cracks on the outer surface of the specimen, but the cloud diagram of horizontal strain shows that the surface of the specimen has undergone a large deformation, and it can be seen that DIC has very good advantages in the detection of cracks, After the peak stress, the specimen entered the damage stage, the development of cracks in this stage all appeared at the strain concentration in figure 10(f), as the load continued to increase cracks appeared through the surface of the cement, and the specimen underwent a large deformation, as can be seen from the horizontal strain cloud at point D, the maximum value of strain in this stage was about 0.47, In the residual stage after point D, the stresses applied to the cement specimen remain basically unchanged due to the fiber reinforcement, but the strains develop rapidly in this stage, as can be seen from the horizontal strain cloud in figure 10(j), where the horizontal strain at point E is about 2.7, which is 5.74 times higher than the horizontal strain at point D. Figures 10(c), (e), (g), (i) and (k) show the real pictures of the compression process of the unconfined compressive strength test. The strain changes and microcracks on the surface of the specimen before the damage stage are difficult to observe with the naked eye due to the reinforcing effect of the fibers, but the changes in strain on the surface of the specimen can be easily seen in the DIC strain cloud diagram, which shows that the DIC technique has the prospect of being widely used in the field of structural inspection, and is very advantageous compared with traditional contact measurements, especially in dealing with the surface deformation of materials, so that the development of cracks and the dynamic response can be clearly determined and relevant measures can be taken in time.

Conclusion
In this study, the unconfined compressive strength, stress-strain curve, deformation modulus and compression toughness index of the specimens were analyzed by adding two types of fibers, PVA and basalt fiber, to the cement soil material, and finally, the whole process of the specimens under unconfined compressive loading was analyzed by combining the DIC technique, and the following conclusions were drawn: (1) When the content of fiber is 0 ∼ 1%, the UCS of specimens increases with the increase of the content of two kinds of fiber, the UCS of the cement specimens with 1% PVA fibers can reach 179.32% of the control group, but when the fiber content is greater than 0.75%, the strength will be limited due to the effect of fiber aggregation. The reinforcement effect of PVA fiber on specimens is higher than that of basalt fiber.
(2) According to the stress-strain curve, the uniaxial compression process of fiber specimens can be roughly divided into five stages: compaction, elasticity, plastic yield, failure and residual.
(3) The deformation modulus (E 50 ) and compressive toughness index (CTI) of specimens increase with the increase of the content of the two kinds of fibers, and they are roughly linear with the compressive strength.
(4) The DIC technique can monitor the change of strain field on the surface of the specimen simply and efficiently, and the location of crack development can be identified at the strain concentration in the horizontal strain field cloud diagram, itcan be well used and promoted for testing the mechanical properties of cementitious materials.