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Experimental study on the behavior of Recycled Coarse and Fine Aggregates concrete columns under eccentric loads

	Abstract 1. Introduction 2. Experimental program 3. Test results 4. Eurocode calculations 5. Conclusion 6. References
Experimental study on the behavior of Recycled Coarse and Fine Aggregates concrete columns under eccentric loads



Rémi Boissière, Firas Al Mahmoud, Mais Ghassoun, Abdelouahab Khelil, Achraf Hamaidia



Abstract This study investigates with the mechanical behavior of reinforced concrete columns made from Recycled Coarse and Fine Aggregates (RCFA). The columns are loaded in eccentric static compression load with different RCFA replacement ratios in their composition using the same compression strength class. An experimental program followed by an analytical analysis is developed. Four concrete compositions are tested: one control column with Natural Aggregate (NA) and three RCFA columns composed of different replacements ratio (gravel and sand) as described in the National Project RECYBETON in France. Results seem to underline that a low RCFA replacement ratio exhibits a low change in the ultimate strength of RCA columns compared to the NA ones. On the other hand, total use of RCFA induces light differences in mechanical behavior.

Keywords: Recycled concrete; columns; eccentric load; experimental



1. Introduction
The construction industry produces about 900 million tons of waste each year; essentially consisted of inert materials which include concrete. Respecting the standards provides opportunities for road undercoats to replace Natural Aggregates (NA). Similarly, the gravel can be reused in structures to be incorporated into the sand and cement and produce new concrete. The question is to define the acceptable fraction of Recycled Coarse and Fine Aggregates (RCFA) to implement to maintain the quality of the final concrete. This is precisely the aim of the project RECYBETON in France. The study of the properties of RA has been ongoing over the last few decades [1–5], leading several countries to establish standards or recommendations supporting their use. XIAO Jian zhuang [6] showed that the stress-strain curves of recycled aggregate concrete are similar to those of conventional concrete. [Liu et al, 2010] [7] conducted tests on six NA and RA concretes, the replacement ratios were 0 and 100%, with eccentricities values of 60 and 150 mm. The results obtained show that the behavior and the failure mechanism of the RA concrete column are similar to those of NA concrete columns. The bearing capacity of NA concrete columns is higher than that of RA concrete columns. Zhou et al. [8] studied the behavior of RCA concrete columns under large eccentric compressive loads with different replacement ratios of RA. They found that the smaller the load that is applied to a recycled concrete column, the more recycled aggregate can be used up to 80% RA. However, the ductility of RA concrete columns is shown to be slightly higher than that of NA concrete columns. They suggest that concrete structures that contain 50% RA could have practical applications in engineering. The following study deals with the mechanical behavior of reinforced concrete columns made from RCFA. These columns are loaded in eccentric static compression load with different RCFA replacement ratios in their composition with the same compressive strength class. Four concrete compositions are tested.

2. Experimental program
2.1 Test geometry
The experimental program consisted of four-square columns tested under eccentric loads. All columns have cross-sectional dimensions of 150 x 150 mm and a height of 1200 mm. Longitudinal steel reinforcements were 4Ø10 mm. Stirrups were Ø10 mm with a constant spacing of 150 mm. The concrete external cover was 15 mm. All tested columns were loaded as pinned-end columns with an eccentric load. The columns were designed to study the effect of different replacement ratios of RCFA. The dimensions, and reinforcement details of the columns are shown in Figure 1-A. In order to control strains and displacements of the column, strain gauges and Linear Voltage Differential Transducers (LVDT) were used (Figure 1-C). The columns were subjected to a quasi-static compressive loading with a measuring step of horizontal displacement of 30 kN up to 300 kN. Then, the measuring step was reduced to 15 kN up to 350 kN. When the applied load was close to the ultimate load loading rate was reduced again. The strain measurements and vertical displacement were recorded automatically every 5 seconds. Figure 1-B shows details and setup for tested columns.
2.2 Material properties
Four concrete of C25/30 (according to Eurocode standard) series were made. The formulations include a reference concrete with NA and three concretes with RCFA (table 1). The designation of each formulation is as the following, i.e. Column xR-yR: x is the replacement ratio of recycled fine aggregate and y is the replacement ratio of recycled coarse aggregate. Mechanical properties of the concrete for all columns are given in table 2.

3. Test results
3.1 Observations and load measurements
The failure occurred because of buckling for all specimens due to concrete crushing in the most compressed zone. Buckling of the steel bars was located in the upper third of the columns 0R-0R, 30R-30R and 0R-100R. For column 100R-100R, buckling occurred close to the specimen center as shown in Figure 2. With a dissymmetry between the faces parallel to the rotation axis, all columns continue to be mostly in a compressive state. Early on in the test, the fully recycled specimen (100R-100R) showed tension in the one of sides. This is a result of a formulation's lower Young's modulus value, which can enable second-order displacement and increased ductility.
These findings lead to the conclusion that concrete columns with a low substitution rate displayed little variations in their mechanical characteristics. Furthermore, column's behavior was unaffected significantly by the amount of recycled gravel when mixed with natural sand. On the other hand, a complete replacement of sand and gravel can result in a change in mechanical characteristics. Table 3 summarizes the ultimate load for each column. At around 65% of the ultimate load, the first cracks in every column started to show up in an area adjacent to the left side of the specimens' upper half. The loading caused cracks to enlarge and exacerbated lateral displacement. The recycled column had the most significant lateral displacement (100R-100R). Transverse cracks were observed at the ultimate load in the upper third of the other columns as well as the central zone of the less compressed side of the column 100R-100R. These cracks spread to the other side. The concrete cover was abruptly lost due to the rapid expansion of plastic stresses on the crushed side.
3.2 Strains and displacements
During the test, strains along the steel bars and concrete columns were measured. Assessments have also been made of the column's lateral horizontal displacement and vertical displacement. The horizontal displacement in the center of the column during loading is depicted in Figure 3. Two stages could be distinguished: first, when the load was applied, the displacement is mostly linear; second, the displacement increased rapidly due to plastic strain in the compressed reinforced concrete,
Figure 3 shows that, globally, columns made from recycled granulates 30R-30R, 0R-100R, and 100R-100R exhibit more lateral displacement than the control column (0R-0R). Displacements at the load of 400 kN were 1.1, 2.25, 1.2, and 3.3 mm for each column 0R-0R, 30R-30R, 0R-100R, and 100R-100R, respectively.
The ultimate displacements for the 0R-0R, 30R-30R, 0R-100R, and 100R-100R columns were 2, 4.48, 2.96, and 4.85 mm, respectively. These values exhibit an increase of the horizontal displacement close to +124, +48, and +143%, respectively, in comparison with NA columns.
Figure 4 shows the evolution of the strains in steel bars located on the less compressed side according to applied load. As expected, it was observed that steel bars exhibited linear compression behavior up to failure, except for the 100R-100R column, in accordance with concrete strains as developed downwards. Figure 5 describes the values of the strains measured on the most compressed side according to the applied load (for 75% of ultimate load, 400 kN and ultimate load (Ned) for columns 0R-0R, 30R-30R, 0R-100R and 100R-100R. Figure 5 shows that the strain measurements exhibit the same tendency that the one observed for the transversal displacement. Values from 0R-0R and 0R-100R are close. The 100R-100R column has an important difference compared to the others for the same load. Strains measured at 400 kN are 1150, 1500, 1250, and 2150 micro strains for columns 0R-0R, 30R-30R, 0R-100R and 100R-100R respectively. These values correspond to a difference of +30, +7 et +87% respectively compared to 0R-0R column.
4. Eurocode calculations
In this section, experimental results were compared with design calculations of Eurocode2 [9] to verify if the use of recycled concrete still fits the resistance requirements. FAESSEL method was used for this calculation. This method allows calculating reinforced concrete structures with eccentric loading.
First, the eccentricity must be determined. There are three levels of eccentricity: initial eccentricity, geometric eccentricity and second order eccentricity.
The first one (e0) is known following the set up and is equal to 15 mm.
e0 = 15 mm
The second one is the geometric imperfection ei and is equal to the maximum between 20 mm e_i= (θ_i×L_0)/2 and L0/400.
The third eccentricity e2 is the second order eccentric that is active for slender structures. Then the eccentricity taken into account for the design Eurocode calculation is
e = e0 + ei + e2
L0 : effective length of a bi-articulated column L0 = L = 1.2 m
And θ_i=θ_0×α_h×α_m
with : α_h= 2/√L ; 2/3 ≤α_h≤1
α_m=√(0.5×(1+1/m) ) ; m = 1 number of elements
Hence α_m=1
θ_i=5×〖10〗^(-3)
and e_i=(5×〖10〗^(-3)×1.2)/2=3×〖10〗^(-3)m
e_i=max⁡〖(0.003,0.002)=0.003 m〗
It is found that the eccentricity of the first order
e1 = 0.003+0.015
Therefore e_1=0.018 m
Criterion for taking into account the second order:
λ=L_0/i with i= h/(2×√3)=0.0433 m
then λ=27.713
η= ((20×1×1.28×0.7)/27.713)^2=0.418
corresponding to N_Ed=η×A_c×f_cd=0.418×〖0.15〗^2×18.92×〖10〗^3
N_Ed=177.94 kN
In this study, the load has reached 500kN so it can be established that second-order phenomena cannot be neglected. As a consequence, the use of a calculation device is compulsory to solve this request. Using Faessel table or committed software, the critical load requested by Eurocode can be determined. The calculation gives a critical design load Nrd = 235 kN. Figure 6 shows the different ultimate loads measured in experimental tests compared with the design predictions of Eurocode2. It can be concluded that despite minor changes in the mechanical behavior, recycled concrete fits the requirements of Eurocode2 with still a large safety factor.

5. Conclusion
This study deals with the mechanical behavior of recycled aggregate concrete of four formulations with the same normalized mechanical compression resistance. According to the obtained results, the following conclusions could be drawn:
Low replacement rate (30R-30R) leads to few modifications in the ultimate loading compared to a control natural column.
The ultimate strength of the column doesn't vary significantly when gravel is replaced with natural sand at a high replacement rate (0R-100R; loss of 5% relative to the control column).
In both previous cases, measured strains are different and vary according to the recycled rate.
Mechanical proprieties alter when the natural coarse aggregate is completely replaced. The young modulus has decreased, and there have been larger strains and displacements.
The ultimate design strength calculation mandated by Eurocode has been carried out by employing the standard Faessel method. The calculated force continues to be significantly less than the obtained and tested columns' force. In this case, Eurocode 2 can provide a sound design normative safe prediction for recycled concrete column structures.

6. References
[1] Yagishita F, Sano M, Yamada M. Behavior of reinforced concrete beams containing recycled coarse aggregate. Demolition and reuse of concrete &masonry RILEM proceeding, vol. 23. E&FN Spon; 1994. p. 331–42.
[2] Ajdukiewicz A, Kliszczewicz A. Influence of recycled aggregates on mechanical properties of HS/HPC. Cement Concr Compos 2002;24(2):269–79.
[3] Fathifazl G, Razaqpur AG, Isgor OB, Abbas A, Fournier B, Foo S. Creep and drying shrinkage characteristics of concrete produced with coarse recycled concrete aggregate. Cement Concr Compos 2011;33:1026–37.
[4] Kou S, Poon C, Wan H. Properties of concrete prepared with low-grade recycled aggregates. Constr Build Mater 2012;36:881–9.
[5] Kou SC, Poon CS. Enhancing the durability properties of concrete prepared with coarse recycled aggregate. Constr Build Mater 2012;35:69–76.
[6] XIAO Jianzhuang « Experimental investigation on complete stress-strain curve of recycled concrete under uniaxial loading ». J. Tongji Univ. 35(11), 1445–1449, 2007.
[7] Liu Chao LIU, Guoliang BAI, Letian WANG, and ZonggangQUAN,« Experimental study on the compression behavior of recycled concrete columns » 2nd International Conference on Waste Engineering and Management – ICWEM, 2010.
[8] Zhou JH, He HJ, Meng XH, Huan S. Experimental study of recycled concrete columns under large eccentric compression. Proceedings of the 12th international conference on engineering, science, construction, and operations in challenging, environments; 2010, March 14–17. doi:10.1061/ 411096(336)54.
[9] Eurocode 2. Calcul des structures en béton. Paris: AFNOR; 2005.

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Experimental study on the behavior of Recycled Coarse and Fine Aggregates concrete columns under eccentric loads