Ductile Evaluation and Mechanical Performance of Recycled Aggregate Concrete Using PVA and Steel Fibers

In recent years, a high toughness cement composite material (HTCCM) has been developed, which has far more performance than existing fiber reinforced concrete. HTCCM is a composite material made by reinforcing cement-based materials with fibers. It exhibits multiple crack characteristics under bending stress and greatly improves toughness during flexural, tensile, and compressive fracture. In this study, it is examined the mechanical properties of high fluidity and high toughness concrete (HFHTC) using fly ash as an admixture and recycled fine and coarse aggregate as an aggregate. From the standpoint of durability, it is necessary to fully examine the long-term properties of HFHTC using recycled fine and coarse aggregate, therefore, it is examined the strength and shrinkage of HFHTC using recycled fine and coarse aggregates.


Introduction
Many paper and research on recycled aggregate have been actively carried out in the concrete industry. To promote the recycling of concrete more extensively, it is necessary to develop new technology for effectively using recycled aggregate. As an example, research on ductile-fiber-reinforced cementitious composites (DFRCC) using recycled fine aggregate has been reported. DFRCC are composites of cementitious material reinforced with fibers, which have multiple cracking characteristics and much improved toughness during bending, tension, and compression fracture. [1][2][3][4] However, due to workability-related defects and so on, there are only a limited number of examples of construction using DFRCC. If DFRCC with excellent workability characteristics can be developed, those problems would be solved. As this material overcomes the brittle properties of general concrete, the performance and durability of concretebased structural elements are expected to be greatly improved, and various new applications such as high-performance repair materials and shock-absorbing materials are expected to replace conventional cement-based materials. However, although actual examples of construction using DFRCC have been reported, the number is still small. [5][6][7][8] The reasons for this are construction performance problems, high cost compared to other materials, and large effects due to hydration heat and dry shrinkage compared to general concrete because mortar and cement paste are mainly used as matrices.
To promote the use of DFRCC in the future, it is considered necessary to develop new materials, including improvements to existing materials. By the way, in implementing production activities, efforts to deal with global environmental problems are an important issue. In the concrete field, research on recycled aggregate concrete, which is used to manufacture concrete again using recycled aggregate taken out of disassembled concrete blocks, is being actively carried out, and research results and construction examples have been reported. To further promote recycling of concrete in the future, it is necessary to develop new effective utilization technologies for recycled aggregate. [9][10][11][12][13][14] Therefore, this study focused on high-fluidity concrete, and examined the material properties of high-fluidity ductilefiber-reinforced concrete (HFDFRC) using recycled aggregate. To apply HFDFRC using recycled aggregate (R-HFDFRC) to RC structures, it is necessary to clarify the long-term material properties (such as strength development and shrinkage behavior) of R-HFDFRC. To evaluate the long-term material properties of R-HFDFRC, we first conducted compressive test, 3-point bending test and shrinkage tests on the R-HFDFRC for materials that had aged 7, 28 and 91 days. Then we tried to apply the approximation formulae which are based on conventional strength development formulae. It is concluded that the overall trend in strength development in R-HFDFRC can be broadly approximated with our equation proposed. In this study, uniaxial compression test, 3-point bending  test, and shrinkage test of HFDFRC shown in Table 1 were performed, however, RC50P10S0 is a shrinkage test only.

Experimental Outlines
It is also tested the shrinkage of the mortar based DFRCC (DFRM). There are four types of HFDFRC and DFRM: HFDFRC (R-HFDFRC) using recycled fine aggregate, high flow DFRM (R-HFDFRM) using recycled fine aggregate, and high flow DFRM (NHFDFRM) using recycled fine aggregate.

Used Materials
A list of the physical properties of the aggregate used in this study is shown in Table 2.
Cement is used ordinary portland cement (density: 3. Used admixtures are high performance AE water reducing agents and separation reducing agents and so on.

Combinations of Specimen
In this study, the water binder ratio (W/B) of R-HFDFRC and R-HFDFRM was 40, 50 and 60%. R-C50P10S0, R-M50P10S0 and N-M50P10S0 have only 50% W/B. The fine aggregate ratio was 85% for R-HFDFRC and 100% for DFRM.
In addition, the preparation of HFDFRC and DFRM used in this study was determined after many trials. The mixing of coarse aggregate is aimed at reducing shrinkage caused by dry shrinkage. However, when the aggregate ratio is close to the general concrete level, the R-HFDFRC results in material separation and not satisfying the target slump flow.
Therefore, even in the preparation of R-HFDFRC used in this study, the fine aggregate ratio was 85%. For the fibers, P and S were used, the fiber volume fraction was 3.0%, and the fiber volume fraction ratio (P:S) between P and S was 7:3 and 10:0.

Uniaxial Compressive Strength
The outline of the uniaxial compressive test is shown in Figure 1. The loading was carried out using a 2,000 kN pressure-resistance tester. The test specimen was a cylindrical test specimen of Φ 100 ××200 mm. Five factors were manufactured. The measurement items were the load, the longitudinal/lateral strain in the center of the test specimen by the compressometer, and the displacement between the loading plates.
Each data was taken in using a data logger. The specimen was demolded two days after implantation, and standardized until the test (age 7, 28 and 91).
In addition, compressive fracture energy (GFc) is limited to 3.0 mm according to plastic deformation.

3-point Bending Test
The outline of the 3-point bending test is shown in Figure  2. The specimen was made into a prismatic specimen measuring 100×100×400 mm, and five of each factor was manufactured. The 3-point bending test was conducted in accordance with JCI specification, and the measurement items were load, deflection, and curvature of the center of the span. Each data was taken in using a data logger.
Also, after the test, the number of cracks generated in the net bending section was visually measured, and in this study, this number was defined as the number of cracks.
In addition, the specimen was demolded two days after implantation, and standardized until the test (age 7, 28 and 91 days). Flexural toughness was evaluated by the following method and the flexural strength was determined by the following formula. [15] (1) Here, : Flexural strength (MPa), : Load (kN), : Span (mm), : Width in case of fracture, : Height of fracture section Fracture toughness is then expressed in terms of flexural toughness coefficient and obtained by the following formula. (2) Here, : Flexural toughness (MPa), : Area below curve from origin to , : Deflection at center of span (mm) In this study, is used as the value when becomes 7.5 mm.

Shrinkage Test
In this study, it is conducted shrinkage tests for HFDFRC and DFRM with a drying initiation age of 7 days. The specimen was made into a prismatic specimen measuring 100×100×400 mm, and two of each factor was manufactured. Shrinkage strain was measured by installing an embedded strain gauge with a length of 100 mm in the center of the specimen.
Each data was captured using a data logger, and the test specimen was demolded two days after implantation.
After that, it was standard curing and air curing was performed in a constant temperature and humidity chamber (20°C, 60% RH) at the age of 7 days.  Figure 3 shows the relationship of compressive strength (Fc), Young's modulus (E), and GFc of R-HFDF up to 91 days obtained by the uniaxial compression test. According to Figure 3, the Fc, E and GFc of R-HFDFRC up to 91 days are increasing with age regardless of the difference in W/B.    Figure 4 shows the mechanical properties of R-HFDFRC up to 91 days old and the number of cracks by results of 3point bending test. First, according to Figure 4 On the other hand, W/B=40% is the highest value in 28 days, and it is declining at 91 days. This is because the matrix strength of R-HFDFRC increased with the difference in W/B and the progress of age, but it became difficult to bridge effect with fibers. However, detailed examination is required in the future.

Flexural Strength
In addition, the fracture toughness of R-HFDFRC up to 91 days varies with the progress of age due to differences in W/B, but after 28 days, it is about 4 MPa or more, indicating that it has sufficient flexural toughness. Next, according to Figure 4(d), the ultimate tensile stress of R-HFDFRC up to 91 days is the minimum at 28 days regardless of the difference in W/B, but it is more than 1% regardless of the difference in W/B and age. Finally, according to Figure 4(e), the number of cracks in R-HFDFRC up to 91 days is more than 6 cracks although the tendency with the progress of age varies due to differences in W/B. In other words, R-HFDFRC was found to have sufficient flexural toughness and crack dispersibility even at 91 days. From now on, it is necessary to examine in detail the effects of differences in W/B on the process of age according to factors mentioned above.

Compressive Strength
For Fc and E of R-HFDFRC up to 91 days, concrete standards specification of Japan society of civil engineers (JSCE) and CEB-FIP Model Code 1990(MC90) try approximation with an approximate expression. [16][17] From JSCE's formula 28 ∕ +     Figure 7 shows the relationship between flexural strength and tensile strength-compressive strength obtained by the uniaxial compressive test and the 3-point bending test of R-HFDFRC.

Flexural Strength
In addition, the curves in the figure are approximate results according to the following formula, (7) and (8).
According to Figure 7(a), the flexural strengthcompressive strength relationship is generally approximated by a single curve. In addition, the tensile strengthcompressive strength relationship in Figure 7(b) is also considered to be approximate by a single curve. Flexural strength and tensile strength of concrete are generally represented as the power function of compressive strength. Therefore, in this study, we will try to approximate flexural strength and tensile strength of R-HFDFRC by the following approximate formula. [18] 5 6 (7) 7 8 (8) Here, 5, 9, 7 and : are material constants.  The approximate error of flexural strength by formula (7) to the result of the experiment up to 91 days is +10.7 to -9.28%, and the approximate error of tensile strength by formula (8) is +19.0 to -11.5%. Flexural strength and tensile strength of each R-HFDFRC up to 91 days are approximate by formula (7) and (8) regardless of the difference of W/B.
In other words, it was found that there is a high possibility that flexural strength and tensile strength can be estimated from the compressive strength of various R-HFDFRCs according to the different ages.
However, since flexural strength and tensile strength of R-HFDFRC tend to decrease when compressive strength exceeds 50MPa the range of application of this approximate expression should be more than 50MPa on compressive  Figure 8 shows the change over time in shrinkage strain of HFDFRC obtained by a shrinkage test with a drying starting 7 days.
If we focus on the preparation in this study (see Table 1 above), the unit weight of water is the smallest when W/B=40%. Therefore, the shrinkage strain was minimized when W/B=40%.
In the case of W/B=50 and 60%, the unit weight of water is about the same. However, the unit weight of coarse aggregate is W/B=60% lager than W/B=50%. Therefore, the shrinkage strain is thought to be the largest when W/B=50%.  If we focus on the preparation in this study (see Table 1 above), the unit weight of water is the smallest when W/B=40%. Therefore, the shrinkage strain was minimized when W/B=40%. Also, when W/B=50% and 60%, the unit weight of water is about the same.
However, the unit weight of fine aggregate (fine grain) is W/B=60% larger than W/B=50, and it is thought that the shrinkage strain becomes the largest when W/B=60% due to the influence of fine particles in the fine aggregate.
According to Figure 9(b), the shrinkage strain of R-M50P10S0 and N-M50P10S0 on 420 days of drying age is R-M50P10S0 (4048 ;) larger than N-M50P10S0 (3740 ;). If we focus on the preparation in this study (see Table 1 above), the unit weight of water and fine aggregate of R-M50P10S0 and N-M50P10S0 are about the same.
However, if we pay attention to the physical properties of aggregates in this study (see Table 2), the surface dry density of fine aggregates is N-M50P10S0 larger than R-M50P10S0, the water absorption ratio is N-M50P10S0 larger than R-M50P10S0. Therefore, it is thought that the shrinkage strain is R-M50P10S0 larger than N-M50P10S0.
Focusing on the combinations in this study (see Table 1), used fine aggregate is medium and fine grain in R-M50P10S0, and medium grain in R-M50P10S0. Due to the influence of fine grain in R-M50P10S0, the shrinkage strain is assumed to be R-M50P10S0 larger than R-M50P10S0.   mixing steel fiber as a reinforcing fiber, shrinkage strain can be reduced by 182 ;. According to Figure 10(b), the shrinkage strain of R-C50P10S0 and R-M50P10S0 on 49 days of drying age is R-M50P10S0 (2443 ;) larger than R-C50P10S0 (1997 ;). With the addition of coarse aggregate, it was found that the shrinkage strain was reduced by 446 ; by reducing mortar paste.
In other words, by mixing steel fiber and coarse aggregate into HFDFRM, it has been found that shrinkage strain can be greatly reduced.

Conclusions
The conclusions obtained within the scope of this research are shown below. 1) High fluidity and high toughness concrete made from recycled fine and coarse aggregate has sufficient flexural toughness and crack dispersibility even at 91 days.
2) From the results of expression of compressive strength and long-term strength of Young's modulus of high fluidity and high toughness concrete using recycled fine and coarse aggregate, it can be approximated from the experimental formula shown in this study.
3) There is a high possibility that flexural strength and tensile strength can be estimated from the compressive strength of high flow and high toughness concrete using recycled fine and coarse aggregate of different ages within the range of 50 MPa or less. 4) By mixing steel fibers and recycled coarse aggregate into high flow and high toughness cement composite materials using recycled fine aggregate, shrinkage strain can be greatly reduced.