Mechanical properties of recycled aggregate concrete under multiaxial compression

Abstract

The mechanical properties of recycled aggregate concrete under multiaxial compression were tested by servo-controlled setup (TAWZ-5000/3000). Properties of strength and stress–strain relation were obtained, and the influence factors of stress ratio and recycled coarse aggregate replacement ratio were analyzed. The results show that the strength of recycled aggregate concrete under multiaxial compression is higher than that of under uniaxial state, the stress ratio and recycled coarse aggregate replacement ratio have obvious effect on strength, and the shape of stress–strain curve is also varied with different levels of the two factors. Failure criterion can reflect the strength relation for recycled concrete under multiaxial stress state. Kupfer’s failure criterion is selected to describe strength properties under biaxial stress state, and the failure envelope reflects the energy absorption of different mix series. Based on octahedral stress theory, the tensile and compressive meridians have been proposed to analyze the strength characteristics under triaxial compression, and the theoretical values are well coherent with the test data.

Keywords

failure criterion multiaxial compression recycled aggregate concrete (RAC)recycled coarse aggregate (RCA)stress–strain relation

Introduction

Recycling of construction and demolition (C&D) wastes attracts enough attention in recent decades. Recycled concrete has been widely used in subbase, sidewalk, parkways, and many other nonstructural constructions in many developed countries (Katz, 2004; Nagataki et al., 2004; Sagoe-Crentsil et al., 2001; Tavakoli and Soroushian, 1996). The government in these countries takes some administrative enforcement measures such as tax and fine to prevent the dumping of C&D wastes while using economic regulatory measures to promote the recycling of wastes. In order to efficiently reveal the structural properties of buildings constructed with recycled concrete, the material properties of recycled concrete were systematically studied in scientific research institutions. Lower density, higher porosity, and weaker interfacial transition zones (ITZs) of recycled concrete have been identified by many researchers, and most of them found that the strength of concrete made with recycled aggregate is lower than the concrete made with natural aggregates (Etxeberria et al., 2006; Huda and Alam, 2014; Kong et al., 2010; Lee and Choi, 2013; Rao et al., 2011). Poon (2004a, 2004b) found that the loose and porous aggregate obtained from the crushed construction waste has an adverse effect on concrete strength. Xiao et al. (2005, 2006) found that the replacement ratio of recycled coarse aggregate (RCA) influences the strength of recycled concrete and pointed out that the strength value decreases with the increase in RCA replacement ratio. Hansen (1990) found that the strength of recycled concrete is greatly affected by the quality and the replacement ratio of the RCA. The complex ITZs were considered to be the main factor affecting the strength of recycled concrete, and some researchers have taken some methods to improve the strength (Dong et al., 2017; Otsuki et al., 2003; Rajhans et al., 2018). Tam et al. (2005; Tam and Tam, 2008) proposed a two-stage mixing approach (TSMA) to improve the ITZs of recycled concrete. The feasibility of the TSMA method is verified by Duan and Poon (2014). TSMA method can improve the properties of recycled concrete; however, for concrete under multiaxial compression, the constraint from lateral force has a greater effect on strength. In order to conveniently compare the mechanical properties between ordinary concrete and recycled concrete under multiaxial compression, conventional mixing method is used in this study. Mechanical properties under multiaxial stress state is the basic theory for learning and using recycled concrete, as in real structural engineering, most of the frame members are in multiaxial state and the uniaxial strength theory is not suitable for explaining the mechanical properties of structures. With the application of recycled concrete in structural engineering, study on multiaxial properties of recycled concrete is very necessary.

Tests on mechanical properties of ordinary concrete have proved that lateral force has great influence on concrete properties, strength, and stress–strain relation of concrete under multiaxial stress, which are quite different from those under uniaxial stress state (Folino and Xargay, 2014; He and Song, 2010; He and Zhang, 2014; Shang and Song, 2006; Tschegg et al., 1995). Mechanical properties of recycled concrete under uniaxial stress state have been studied by many researches, but there are few studies on multiaxial properties. The effect of RCA property and replacement ratio on strength and stress–strain relation can provide valuable basis for analyzing the failure criterion and constitutive model of recycled concrete under multiaxial stress state. The existing research on failure criterion and constitutive law provides a reference for this study (Candappa et al., 2001; Chi et al., 2014a, 2014b, 2017; Lim and Nawy, 2005). This article will focus on the mechanical properties of recycled aggregate concrete (RAC) under multiaxial compression and will reveal the effect of RCA replacement ratio and stress ratio on these properties, finally giving a good suggestion for structural design with recycled concrete.

Experimental procedures

Materials and mix proportions

Two kinds of coarse aggregate are used in this study, one is natural coarse aggregate (NCA) which is obtained from local quarry, and the other is RCA, which is sourced from demolished urban pavement. The physical properties of the two coarse aggregates are shown in Table 1.

Table 1.

Physical properties of coarse aggregates.

Classification	Gradation(mm)	Bulk density(kg/m³)	Apparent density(kg/m³)	Crush index(%)	Water absorption(%)	Residual mortar content (%)
NCA	5–20	1618	2708	4.6	0.3	–
RCA	5–20	1413	2485	13.4	3.0	30.55

NCA: natural coarse aggregate; RCA: recycled coarse aggregate.

The cementitious material is ordinary Portland cement P.O 42.5. The replacement ratio of RCA is determined according to mass substitution principle. The fine aggregate is ordinary river sand with a fineness modulus of 2.8. The water dosage consisted of two parts, one part is calculated by mix proportion of common concrete, and the value is shown in Table 2; the other part named additional water is determined by water absorption of RCA. The mix design of this study is shown in Table 2.

Table 2.

Mix proportions.

Unit weight (kg/m³)
Specimens	RCA%	Cement	Sand	Water	NCA	RCA
RAC0	0	415	631	195	1121	0
RAC30	30	424	643	195	767	329
RAC50	50	433	656	195	536	536
RAC70	70	443	669	195	314	733
RAC100	100	453	681	195	0	1021

RCA: recycled coarse aggregate; NCA: natural coarse aggregate; RAC: recycled aggregate concrete.

Specimen and test procedure

In this study, 100 ×100 ×100 mm³ are prepared for testing the mechanical properties of recycled aggregate concrete under multiaxial compression. Concrete specimens are cast and cured in curing chamber with a relative humidity of 95% and temperature of 20°C ± 3°C. The test is carried out on a servo-controlled setup (TAWZ-5000/3000), which can offer 5000 kN loading force in principle direction and 3000 kN in other two directions. Linear vaiable differential transformers (LVDTs) are symmetrically arranged, and strength and deformation can be accurately measured. Test specimens are wrapped by steel plate to reduce friction resistance, and a thin layer of grease is applied between the specimen and the plate. The test setup and measure equipment are shown in Figure 1.

Figure 1.

Test setup and measure equipment: (a) TAWZ-5000/3000 and (b) measure equipment.

Force loading method is used in this test: $0.3 MPa / s$ for biaxial test and $0.5 MPa / s$ for triaxial. Four levels of stress ratio are used under multiaxial compression separately: 0.25:1, 0.5:1, 0.75:1, and 1:1 for biaxial compression, 0.1:0.25:1, 0.1:0.5:1, 0.1:0.75:1, and 0.1:1:1 for triaxial compression. The strength and deformation are obtained during the test procedure.

Test results and discussion

Strength

Figure 2 displays the strength of test specimens under biaxial and triaxial compression. There is a great difference between these groups.

Figure 2.

Strength of RAC under multiaxial compression: (a) biaxial strength and (b) triaxial strength.

From Figure 2, it can be seen that the uniaxial strength is about 40 MPa, the biaxial strength is between 50 and 90 MPa, and the triaxial strength is almost higher than 170 MPa. Under biaxial compression, loads are applied simultaneously in two directions. A mutual constraint is formed and gives a positive effect on strength. A free surface still exists under biaxial state; tensile stress is equivalent to exert on free surface when loads are applied on the other two directions. When exceeding the ultimate tensile stress, the test specimen will be damaged, and biaxial strength is only a bit higher than uniaxial strength due to the effect of the free surface. Under triaxial compression, no free surface exists, specimen is compacted by lateral forces, and the strength of the concrete has been improved remarkably. The test data illustrate that constraint plays an important role in strength whenever it is natural aggregate concrete or recycled aggregate concrete, and the greater the constraint, the higher the strength. There is an interesting phenomenon that the strength of RAC30 is a little higher while that of other specimens are lower than strength of RAC0 whether under biaxial or triaxial compression. When RCA replacement ratio exceeds 30%, the defect of RCA will adversely influence the strength of concrete whether under uniaxial or multiaxial compression. When RCA replacement ratio is less than 30%, the lateral restraint may compress the micro-cracks of recycled aggregate concrete when subjected to multiaxial compression. In order to get an obvious comparison, relative strength $σ_{3} / f_{c}$ is used to represent the ratio of multiaxial strength to uniaxial strength, where $σ_{3}$ represents the biaxial or triaxial strength and $f_{c}$ represents the uniaxial strength. The relative strength $σ_{3} / f_{c}$ under multiaxial compression is shown in Figure 3.

Figure 3.

Relative strength of RAC under multiaxial compression: (a) $σ_{3} / f_{c}$ under biaxial compression and (b) $σ_{3} / f_{c}$ under triaxial compression.

From analysis of the strength value under biaxial and triaxial compression, it can be seen that $σ_{3} / f_{c}$ under biaxial compression is between 1.2 and 2.0, and more than 70% of the values are concentrated in the interval of 1.4–1.8. The $σ_{3} / f_{c}$ under triaxial compression is between 3.5 and 6, and almost 90% of the values are concentrated in the interval of 4–6. There is a great increase in strength with stress state changing from biaxial to triaxial. Figure 3(a) shows that the maximum biaxial strength is obtained at stress ratio $σ_{2} / σ_{3} = 0.5$ , and the minimum strength is obtained at $σ_{2} / σ_{3} = 0.25$ . The variation of biaxial strength is not obvious with RCA replacement ratio changing from 0% to 50%. When replacement ratio exceeded 50%, the strength value significantly reduced, and the minimum strength is obtained at 100%. Figure 3(b) shows the influence of stress ratio and RCA replacement ratio on triaxial strength. The maximum strength obtained at stress ratio 0.1:0.5:1 represented by intermediate stress ratio is $σ_{2} / σ_{3} = 0.5$ . The strength value decreases gradually as the RCA replacement ratio increases from 30% to 100%.

Stress–strain relationship

Force-control loading method is used in this test, and stress–strain curves of RAC under biaxial and triaxial compression are shown in Figure 4.

Figure 4.

Stress–strain curves of RAC under biaxial compression: (a) RAC0 under biaxial compression, (b) RAC0 under triaxial compression, (c) RAC50 under biaxial compression, (d) RAC50 under triaxial compression, (e) RAC100 under biaxial compression, and (f) RAC100 under triaxial compression.

For concrete under multiaxial compression, with the combined action of $σ_{2}$ and $σ_{3}$ , tensile strain along the direction of $σ_{1}$ is produced. From Figure 4, it can be seen that $ε_{1}$ is negative for all groups when under triaxial compression and $ε_{2}$ is negative at stress ratio $σ_{2} / σ_{3} = 0.25$ when under biaxial compression. The stress–strain curve of concrete under biaxial compression is parabolic in shape, similar to the ascending section of stress–strain curve under uniaxial compression. The strength $f_{3}$ and peak strain $ε_{3}$ in the maximum principal compressive direction are greater than the corresponding values under uniaxial compression. Under biaxial compression, the maximum strain $ε_{3}$ occurs at stress ratio $σ_{2} / σ_{3} = 0.25$ , and with the increase in stress ratio, the strain $ε_{2}$ turns from tensile to compressive. For concrete specimens under triaxial compression, it can be seen that the strength $f_{3}$ sharply increases with the increase in lateral force, the maximum $f_{3}$ is 5.99 times of uniaxial strength, and the strain $ε_{3}$ is up to 40–50 × 10⁻³, that is, about 20–30 times of uniaxial strain. The second principal stress $σ_{2}$ has a significant effect on the triaxial compressive strength, with $σ_{1} / σ_{3} = constant$ , and the maximum strength occurs at 0.1:0.5:1, followed by 0.1:0.75:1. RCA also has influence on the shape of stress–strain curve; the initial slope of the curve decreases with the increase in RCA replacement ratio, and porous and loose properties of RCA make a greater deformation at the initial loading stage. According to the variation of strength under multiaxial compression, it can be seen that the effect of RCA replacement ratio on biaxial strength is greater than that on triaxial strength.

Biaxial failure criterion

The classical failure criterion, proposed by Kupfer et al. (1969), can also be used to explain the strength relation for recycled aggregate concrete under biaxial stress state. The failure criterion equations are listed as follows

{(\frac{σ_{3}}{f_{c}} + \frac{σ_{2}}{f_{c}})}^{2} + a \frac{σ_{3}}{f_{c}} + b \frac{σ_{2}}{f_{c}} = 0

(1)

\frac{σ_{3}}{f_{c}} = \frac{a + b \times α}{{(1 + α)}^{2}}

(2)

where $σ_{3} / f_{c}$ and $σ_{2} / f_{c}$ represent the strength ratio, $α$ is the stress ratio, and $a$ and $b$ are the unknown parameters which can be determined by regression analysis.

Table 3 shows the regression results, where R² represents the correlation coefficient between test results and the calculated value, R² > 0.9, shows that the calculated value is very close to the test value. Figure 5 shows the strength relation changes with the change in stress ratio and RCA replacement ratio. It can be seen that the test data distribute along the envelope, which indicates that the failure criterion is suitable for describing the strength failure characteristic of recycled aggregate concrete under biaxial compression. The area encircled by the envelope varies with the change in RCA replacement ratio. The encircling area reflects the absorption energy when concrete reaches the critical point of strength. The larger the area, the more energy it will absorb. With stress ratio $σ_{2} / σ_{3} = 0 - 0.25$ , biaxial strength increases rapidly; with $σ_{2} / σ_{3} = 0.25 - 0.5$ , there is a slow increase in strength; and when $σ_{2} / σ_{3} = 0.5 - 1$ , there is a gradual decrease in strength. Figure 5(a) shows the comparison between test data of RAC0 and data from other researchers (Kupfer, 1973; Kupfer et al., 1980; Mills and Zimmerman, 1970). It can be seen that the data are very similar for concrete with natural aggregate. Figure 5(b) to (e) shows the change in the failure envelope caused using different replacement ratios of RCA. With RCA replacement ratio less than 30%, the biaxial strength is slightly higher than that of natural concrete, and the failure envelope of RAC30 is little bigger than that of RAC0. When replacement ratio exceeds 30%, the strength value decreases with the increase in RCA replacement ratio, and the failure envelope reduces gradually. The phenomenon of the reduction in encircling area illustrates that concrete with RCA is more fragile than that with natural aggregates when RCA replacement ratio is more than 30%.

Table 3.

Regression coefficient of the failure criterion for RAC.

	RAC0	RAC30	RAC50	RAC70	RAC100
a	1.003	1.039	1.027	1.002	0.957
b	5.753	6.120	5.545	4.898	4.725
R ²	0.985	0.966	0.981	0.998	0.928

RAC: recycled aggregate concrete.

Figure 5.

Comparison between test data and calculated value: (a) comparison between test data and reference data, (b) comparison between data of RAC0 and RAC30, (c) comparison between data of RAC0 and RAC50, (d) comparison between data of RAC0 and RAC70, and (e) comparison between data of RAC0 and RAC100.

Triaxial failure criterion

By describing the triaxial strength in octahedral stress space, the octahedral normal stress and shear stress can be expressed as

σ_{oct} = \frac{1}{3} (σ_{1} + σ_{2} + σ_{3})

(3)

τ_{oct} = \frac{1}{3} \sqrt{{(σ_{1} - σ_{2})}^{2} + {(σ_{2} - σ_{3})}^{2} + {(σ_{3} - σ_{1})}^{2}}

(4)

Table 4 shows the change in octahedral normal stress $σ_{oct}$ and shear stress $τ_{oct}$ with stress ratio and RCA replacement ratio. It can be seen that $σ_{oct}$ increases with stress ratio from 0.1:0.25:1 to 0.1:0.75:1. $σ_{oct}$ and $τ_{oct}$ of RAC30 are slightly highter than that of RAC0, the value decreases gradually as the RCA replacement ratio increases from 30% to 100%. It illustrates that with less micro-cracks in aggregate, the strength of recycled aggregate concrete can be improved by lateral force, and when it exceeds some degree, the aggregate defects will give an obvious adverse effect on strength.

Table 4.

Octahedral normal stress $σ_{oct}$ and shear stress $τ_{oct}$ .

Specimen	Stress ratio	$σ_{oct}$ (MPa)	$τ_{oct}$ (MPa)	Stress ratio	$σ_{oct}$ (MPa)	$τ_{oct}$ (MPa)
RAC0	0.1:0.25:1	109.87	96.12	0.1:0.75:1	156.34	96.17
RAC0	0.1:0.5:1	146.03	100.81	0.1:1:1	155.64	94.33
RAC30	0.1:0.25:1	112.13	98.10	0.1:0.75:1	163.17	100.37
RAC30	0.1:0.5:1	149.35	103.10	0.1:1:1	158.72	96.20
RAC50	0.1:0.25:1	97.99	85.69	0.1:0.75:1	141.15	86.80
RAC50	0.1:0.5:1	126.27	86.68	0.1:1:1	139.09	82.91
RAC70	0.1:0.25:1	92.55	80.97	0.1:0.75:1	129.14	79.44
RAC70	0.1:0.5:1	113.33	78.23	0.1:1:1	127.18	77.08
RAC100	0.1:0.25:1	84.35	73.79	0.1:0.75:1	118.46	72.87
RAC100	0.1:0.5:1	107.11	73.94	0.1:1:1	119.00	72.12

RAC: recycled aggregate concrete.

The failure criterion reflects the conformity between tensile or compressive meridian and test data. According to octahedral stress theory, the tensile and compressive meridian equations can be expressed as

σ_{0}^{t} = a_{1} {(τ_{0}^{t})}^{2} + k_{1} τ_{0}^{t} + c_{1} (θ = 0^{0})

(5)

σ_{0}^{c} = a_{2} {(τ_{0}^{c})}^{2} + k_{2} τ_{0}^{c} + c_{2} (θ = 60^{0})

(6)

where $σ_{0}^{t}$ and $τ_{0}^{t}$ represent the normal and shear stress on tensile meridian, respectively; $σ_{0}^{c}$ and $τ_{0}^{c}$ represent the normal and shear stress on compressive meridian, respectively; and $a_{1}$ , $a_{2}$ , $k_{1}$ , $k_{2}$ , $c_{1}$ , and $c_{2}$ are the unknown parameters.

According to failure envelope characteristic, the octahedral strength can be expressed as

σ_{0} = \frac{σ_{oct}}{f_{c}}, τ_{0} = \frac{τ_{oct}}{f_{c}}

where $σ_{oct}$ and $τ_{oct}$ represent the octahedral normal and shear stress, respectively, and $f_{c}$ is the uniaxial strength.

The unknown parameters can be determined by geometric and physical characteristics of the failure envelope:

When $σ_{1} = σ_{2} = σ_{3} = f_{ttt}$ , the parameters $c_{1}$ and $c_{2}$ are equal which can be expressed as $c_{1} = c_{2}$ , where $f_{ttt}$ represents the tensile stress under triaxial stress state. The tensile or compressive meridian and the hydrostatic pressure axis just intersect in one point, that is, $τ_{0}^{t} = τ_{0}^{c} = 0$ and $σ_{0}^{t} = σ_{0}^{c}$ .

The closed curve of deviatoric plane is 70% off-symmetric. With the decrease in hydrostatic pressure or octahedral normal stress, the shape of the curve transits from an approximate triangle $(r_{t} / r_{c} \approx 0.5)$ to a circle $(r_{t} / r_{c} \approx 1)$ . At this point

lim_{ξ \to \infty} \frac{r_{t}}{r_{c}} = 1 \Rightarrow a_{1} = a_{2}

Equations (5) and (6) can be converted to

σ_{0}^{t} = a {(τ_{0}^{t})}^{2} + k_{1} τ_{0}^{t} + c (θ = 0^{0})

(7)

σ_{0}^{c} = a {(τ_{0}^{c})}^{2} + k_{2} τ_{0}^{c} + c (θ = 60^{0})

(8)

The two equations can be merged into one expression

σ_{0} = a {(τ_{0})}^{2} + k τ_{0} + c (0^{0} ⩽ θ ⩽ 60^{0})

(9)

Based on the calculation of triaxial strength under different stress ratios, the deviatoric plane equation can be expressed as

k = k_{1} (\cos 1.5 θ)^{1.75} + k_{2} (\sin 1.5 θ)^{2} (0^{0} ⩽ θ ⩽ 60^{0})

(10)

The parameter values of $a$ , $k_{1}$ , $k_{2}$ , and $c$ can be determined by four characteristic values as follows

Uniaxial tension ( $f_{t}$ , $θ = 0^{0}$ ), assume $f_{t} = 0.1 f_{c}$ ;

Uniaxial compression ( $f_{c}$ , $θ = 60^{0}$ );

Biaxial isobaric ( $σ_{2} = σ_{3}$ , $θ = 0^{0}$ );

Triaxial compression ( $σ_{2} > σ_{2} = σ_{3}$ , $θ = 0^{0}$ ).

These four parameters can be calculated based on regression analysis, and the calculated results are shown in Table 5.

Table 5.

Parameters of failure criterion for recycled aggregate concrete.

Parameters	RAC0	RAC30	RAC50	RAC70	RAC100
$a$	−0.1033	−0.1062	−0.1164	−0.1073	−0.1153
$k_{1}$	−1.4291	−1.4123	−1.3988	−1.4264	−1.4192
$k_{2}$	−0.9016	−0.8912	−0.8834	−0.8994	−0.8838
$c$	0.1147	0.1104	0.1090	0.1145	0.1089

RAC: recycled aggregate concrete.

Regression analysis of the parameters can provide the relationship between the parameters and RCA replacement ratio, and the calculated result is shown in Figure 6. The general trend is that, parameters a and c decrease with the increase in the RCA replacement ratio, and parameters k₁ and k₂ increase with the increase in the RCA replacement ratio.

Figure 6.

Regression analysis of parameters.

Figure 7 shows the tensile and compressive meridian of test specimens. The test data suit well with the tensile and compressive meridian, and most of the data are located on tensile line. The results show that this failure criterion can be well used to express the strength characteristic of recycled aggregate concrete under triaxial compression.

Figure 7.

Comparison between test data and failure criterion curves. Test data and simulated curves of (a) RAC0, (b) RAC30, (c) RAC50, (d) RAC70, and (e) RAC100.

Conclusion

Based on the experimental results and the analysis, the following conclusions are drawn:

The stress ratio and RCA replacement ratio have obvious effect on strength of concrete under multiaxial compression; lateral force plays an effective role in promoting concrete strength; and the strength value under triaxial compression is much higher than that under uniaxial and biaxial compression. Using RCA takes a negative effort on multiaxial strength when the replacement ratio is higher than 30%, and the general trend of strength value is RAC30 > RAC50 > RAC70 > RAC100.

Using RCA has a great effect on stiffness of concrete, and the initial slope of stress–strain curve is declined with the increase in the RCA replacement ratio. The shape of stress–strain curve is also influenced by stress ratio, where greater stress ratio leads to higher initial slope. The acquisition of stress–strain relationship of RAC under multiaxial compression provides a basis for finite element analysis and structural design.

Classical Kupfer’s failure criterion can be used to explain the strength characteristic of RAC under biaxial compression; material defect of RAC leads to less energy absorbed to get failure; and the failure envelope decreases with the increase in the RCA replacement ratio. Based on octahedral stress theory, the triaxial failure criterion is proposed, the tensile and compressive meridian equations are determined according to geometric and physical conditions, and the theoretical results are well coherent with test data.

Footnotes

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This study was supported by the National Natural Science Foundation of China (Grant No.s 51478126 and 51661145023).

ORCID iD

Jianzhuang Xiao

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