Study on toughness and toughening mechanism of cement-based fiber materials

In order to improve the character of common cement mortar which are brittle and easily cracked, and to take advantage of the local materials, abandoned fishing nets was used toform get the cement-based fiber materials. In this study, the influence of the amount and length of abandoned fishing nets on the mechanical properties of cement mortar and its mechanism were discussed. Four different lengths of abandoned fishing nets were adopted under four kinds of dosages to mix with mortar and make a total of 16 test blocks. Then the flexural strengths of the prepared test blocks were tested, and the load-deflection curve of the test block was obtained. The results show that the residual strength after the test block after which reaches the peak load in the flexural test generally increases with the increase of the fiber content and the length of the fiber, but when the dosage and length increase to a certain range, the influences becomecomplicated. The crack propagation simulation byAbaqus further revealed the reinforcement mechanism, and it can be concluded that the bridging between the fiber and the matrix has a great influence on the character toughness.


Introduction
As the main construction engineering material, mortar has some character defects during construction, such as brittleness, low tensile strength, large dry shrinkage and poor impact resistance. These defects make the mortar products susceptible to cracks and reduce the durability of the building. Adding fiber is a good way to improve the toughness of the mortar product and it has been reported that polypropylene fiber can effect the mechanical properties of cement-based materials [1][2] . To obtain better building materials, many researchers have studied the cement-based fiber toughening materials [3][4] and some have been applied to practical projects in the United States and Japan [5,6] .
Fiber reinforced cement-based composite material is a composite material composed of cement paste, mortar or cement concrete as the substrate, and a combination of non-continuous short fibers or continuous long fibers as reinforcing materials [7] . The fiber can prevent the expansion of micro-cracks in the cement matrix and the tensile load across the crack, which can improve the composite tensile strength, flexural strength and fracture energy of the materials significantly compared to the common cement.
In this study, we combined the local available material, abandoned fishing net wires, to make the cement-based fiber toughened materials, and used the bending test and the method of numerical simulation to study the property of this kind of materials and the mechanism of reinforcement and toughening. Then the influence of the length, content of the fiber under different ages to the fiber and cement matrix was discussed and the efficiency of adding fiber was evaluated by the factor of destruction.

Experiment material
The curing agent used in the test was Conch brand No. 425 ordinary Portland cement, and the new standard (ISO) sand was used in the cement test. The mass ratio of themortar is made by the ratio of cement: standard sand: water = 1:3:0.5. One pot of mortar can make three test pieces. The required amount of each pot material is as follows: cement (450±2) g, standard sand (1350±5) g, water (225±1) mL. The reinforced material is made of discarded fishing net wire (polyethylene fiber) as shown in Figure 1. It was added in an amount of 0.1%, 0.15%, 0.2%, and 0.25% by mass in four different fiber lengths.

Preparation of mortar
Under different fiber lengths, the fiber content is 2.03g, 3.04g, 4.05g, and 5.06g, respectively. The water, cement and standard sand was weighted with a balance under the accuracy of ±1g. The accuracy of the balance used for weighing discarded fishing nets was±0.01g. Automatic dropper plus 225mL water with the accuracy of 1mL.
Each pot of mortar is mechanically agitated using a mixer with the following procedure: Pour the standard sand into the sand tank on the upper part of the mixer, then add the water to the mixing pot, then add the cement, and place the pot on the holder and raise it to a fixed position. Press the start button and the machine will automatically stir at a low speed for 30s, automatically add standard sand at the same time as the second 30s, then enter the third 30s at a high speed stirring. In the first 15s of the static stop for 90s, the mortar on the blade and the wall of the pot was scraped into the middle of the pot with a rubber scraper, then stirred at high speed for 60 s. For each mixing phase, the time error should be within±1s.

Test block preparation
The mortar was formed immediately after preparation.
The oil-coated test piece was fixed on the forming vibration The prepared sample was placed in a standard curing box, and the size of the sample was 40 mm*40 mm*160 mm, and the curing period was 3 days, 7 days, and 28 days, respectively.

Bending test
The equipment of the flexural test is shown in Figure 2.
At the beginning of the test, the sample was placed on a fully automatic cement load compression and compression tester, and two displacement meters were

The influence of fiber lengths
After the test block was prepared well, the flexural test was carried out by a fully automatic cement load compression and compression tester. During the test, the split hydraulic jack was manually loaded slowly, and the force of the pressure sensor and the displacement of the displacement gauge during the test were recorded. The specific sample size and the number of effective load deflection curve groups were shown in Table 1.

Table.1. The number of production test pieces and the number of load and deflection curves of each group of flexural test pieces
The load-deflection relationship of test pieces with different fiber lengths of 28 days is shown in Fig. 3. 3-3(28d-0.25%)

Fig. 3.The load and deflection curves of different fiber length
It can be seen in Figure 3 that the load and deflection curves differed with the fiber lengths under the same age and the same fiber content. and it has the following regulations: (1)When the fiber length is 10 mm and 15 mm, the curve tends to be about the same, but a test piece incorporating a fiber of 15 mm provides greater residual strength. So the longer fiber length has a larger residual strength and a better toughening effect on the cement-based material.
(2)When the blending amount is low and the fiber length is 25mm, the load area under the peak and the deflection curve are larger, and the flexural toughness after peak is better. However, in the case of a high amount of blending, when the length of the incorporated fiber is also large, the toughness property of the test piece has little influence by the fiber length.
As reviewed, the toughness properties of cement-based fiber toughened materials are related to the length and amount of fiber blended. In the case of small dosage, the toughness performance is mainly related to the length of the blended fiber. When the length of the blended fiber is increased, the toughness is also increased.
When the blending amount is large, the toughness property increases with the length of the blended fiber.
When increased to a certain extent, the length of the incorporated fiber has little to do with the toughness properties.

The influence of the fiber content
The load-deflection relationship of test pieces with different fiber content for 28 days is shown in Fig. 4. is not well improved. However, when the fiber content is increased, the toughness is enhanced.
(2)When the length of the incorporated fiber is long, the flexural strength after the peak is large, and sometimes the flexural strength can approach to the peak.
In the post-peak stage, the ductility of a certain load can still increase. The area under the load and deflection curve is larger, and the flexural toughness can be better improved after the peak. As summarized in the review, when the length of the blended fiber is short, the toughness property is closely related to the fiber content, and the toughness of the blending amount is also increased. When the length of the incorporated fiber is long, the toughness performance generally increases as the fiber content increases.
However, at lower dosages, the toughness properties have little to do with the amount. Test phenomena and results analysis: (1)When the test block with the age of 7 days and 28 days is cracked, the bearing capacity of the test block is greatly reduced instantaneously, while for the 3 days old, the peak bearing capacity of the test block appeared after the crack. The reason for this phenomenon may be that the strength of the cement-based material is much smaller than the strength of the abandoned fishing net line when the test piece is 3 days old.

The influence of the test ages
(2)The toughness of the test piece with an age of 28 days was significantly greater than that of the test piece with an age of 3 days. As the age increases, the bond between the cement-based material and the fiber is better.
The greater the force required to pull the fibers out of the matrix, the greater the toughness increases.
( 3 ) When the age is short, the strength of the cement-based material is small, the fiber is distributed in the 3D chaotic direction in the cement-based material, the distribution is not completely uniform, the data obtained by the experiment is more discrete, and the accuracy of the data is lower.
As summarized in the review, the greater the age of the test block, the better the adhesion of the cement-based material to the fiber, the higher the

Discussion on toughening mechanism
Through careful observation of the anti-folding test process, it is found that after the cracking of the large-scale test block, the bearing capacity of the test block drops instantaneously. However, as the load was continuously loaded, the bearing capacity of the test block arose. When the fiber in the test block was pulled out, a sound was made and the value of the pressure sensor has dropped slightly and then rose again, the process will repeated until most of the fibers were pulled out, and the test piece was almost cracked. During the entire flexural test, the bridging effect of the fiber is obvious. As shown in Fig. 5, it can be observed that many fibers in the cross section of the test piece was pulled out during the test. The test results showed that the load-deflection curve is not a smooth curve. After the peak value, the load size fluctuates continuously. The description of the rise is to overcome the bridging force of the fiber. Once the fiber is pulled out, the bearing capacity will decrease and the crack will continue to expand. When the crack propagation encounters the fiber resistance, the bearing capacity will rise again. Therefore, the relationship between load and deflection reflects the expansion of cracks on the test block to some extent.

Analyze test results from the aspect of energy
Since the fiber distribution of most cement-based fiber toughening materials tends to be distributed randomly in different directions [8][9][10][11] , there is still no perfect method to accurately determine the bending toughness of cement-based fiber toughened materials. Starting from the experimental phenomena and results, the relative area enclosed by the load and deflection curves before the peak load capacity is again greatly reduced, and the failure energy is introduced. The plotted curve is shown in Fig. 6.  Combined with the two relationship diagrams of Figure 7 and Figure 8, the following conclusions can be drawn: (1)When a large amount of fibers are incorporated in the test piece and the fiber length is 20 mm, the crack width at the time of initial cracking is the smallest, and the line length to be dialed is also the longest.
(2) In general, it can be concluded that the higher the fiber content, the longer the length of the pull-out line and the smaller the crack width at the initial crack.
In summary, the fiber is blended into the cement mortar to prevent cracking. When the fiber length is 20 mm, the crack resistance is the best.

Numerical simulation of the flexural test
In order to further explain the influence of the blended fiber content on the toughness of the test piece, a four-point bending beam crack propagation model was established, as shown in Fig. 9.  According to the volume percentage of the fibers in the matrix, the empirical formula of the surface [12]

Conclusion
The toughness of the cement-based fiber toughening materials is related to the length and amount of fiber blended, which plays a role of "micro-reinforcing". With the results of the experimental study and numerical simulation, the conclusion are as follows: (1)In the case of small dosage, the toughness performance is mainly related to the fiber length. When the length of the incorporated fiber increases, the toughness also increases.
(2)When the amount is large, the toughness performance increases when the length of the incorporated fiber (3) When the length of the incorporated fiber is short, the toughness performance is closely related to the fiber content. When the fiber content is increased, the toughness of the test piece is also increased.
(4) When the incorporated fiber is long, the toughness performance generally increases with the fiber content.
However, at lower dosages, the toughness properties are not related to the amount.