Minerals Engineering 21 (2008) 761–765

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Minerals Engineering

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Application of DEM model to breakage and liberation behaviour of recycledaggregates from impact-breakage of concrete waste

Hoon Lee *, Ji Hoe Kwon, Kwan Ho Kim, Hee Chan ChoEnergy System Engineering, Seoul National University, 135-502 Shinlim 9th Kwanak, Seoul, Republic of Korea

a r t i c l e i n f o a b s t r a c t

Article history:Received 24 December 2007Accepted 12 June 2008Available online 31 July 2008

Keywords:Discrete element modellingRecycled aggregateLiberation

0892-6875/$ - see front matter � 2008 Elsevier Ltd. Adoi:10.1016/j.mineng.2008.06.007

* Corresponding author.E-mail address: robbed7@snu.ac.kr (H. Lee).

For the recovery of aggregates from construction waste, the principle process is a separation of aggregatesby breakage from the cement paste. In this study, a model based on DEM (discrete element method) wasused to simulate the breakage behaviour of a block of concrete waste in repeated free fall tests. With 3DDEM model, 73 mm � 53 mm concrete block was modelled with macro-particles formed by a group of970 spherical sub-particles and the sample block free fell 30 times from a 2 m height. The results of sim-ulation were used to analyze the breakage characteristics of bonds between particles according to thenumber of free fall impacts. And, liberation ratio showed that high-quality recycled aggregates from con-crete waste can be produced.

� 2008 Elsevier Ltd. All rights reserved.

1. Introduction

In Korea, the amount of construction waste has steadily in-creased to about 53 million tons in 2003, 60% of which was concretewaste. This trend is expected to continue with the growth of theKorean economy and the subsequent increase in construction.Moreover, the supply of the natural aggregates has been depleted,causing an imbalance between the supply and demand for aggre-gates. In addition, it is becoming increasingly difficult to excavatenatural aggregates due to environmental restrictions. Conse-quently, the industry is turning to technologies related to recycledaggregates, and about 90% of construction waste is being recycled.

At present, there are over 200 recycled aggregate productioncompanies, and their production procedure mainly consists of acrushing process, using 2–3 crushers, and a classifying process.However, the recycled aggregates produced from the crushing pro-cess have large amounts of cement paste attached to the surface;and when they are used to manufacture concrete the quality is poordue to low strength and a high water absorption rate. Therefore, inorder to produce high-quality recycled aggregates from waste con-crete, it is very important to understand the breakage and libera-tion characteristics when waste concrete undergoes an impact.

There have been many studies dealing with the analysis of thecomminution process required for the production of recycledaggregates. Kiss and Schönert (1980) conducted single-particlecrushing tests to determine whether impact or compression crush-ing is more suitable for liberation crushing. They used 10 mmspheres consisting of cement stone and embedded quartz particles.

ll rights reserved.

Tomas et al. (1999) examined how a crack is initially formed andpropagated when a spherical B35 concrete sample (35 N/mm2

compressive strength, 0.5 water–cement ratio, over 180 days stor-age time) is hit against a wall. They performed experiments tostudy how cracks necessary for liberation are most efficiently cre-ated when impact, double impact or compression stress is applied.

Other studies involved simulating the concrete comminutionprocessing using the DEM. To study the variations in breakagemechanisms and patterns according to the variables that deter-mine the level of comminution, Potapov and Campbell (1994) per-formed a computer simulation of breakage caused by collision.Schubert et al. (2005) constructed a concrete block that consistedof various sized particles with non-uniform properties and con-ducted both experiments and discrete element simulations tostudy the cracking phenomena of the aggregates.

In order to enhance the quality of recycled aggregates, the goalof this study is to examine the liberation behaviours of the cementmortar from aggregates according to the number of impacts, basedon free fall tests of concrete waste. The computer simulation tech-nique known as the DEM was used for the purpose of performinganalysis of (1) the breakage of bonds between particles, (2) the lib-eration ratio and (3) size distribution between the simulation andthe actual experiment.

2. Theory

2.1. The linear contact model

When a collision takes place between two particles, they mutu-ally affect each other’s movement. The interactive forces betweenthe particles can be represented using the Kelvin–Voigt model that

Particle i Particle j

Fb,n

Fb,s

Mb

Area

Fig. 2. Sketch of bond model in normal tangential and moment component.

762 H. Lee et al. / Minerals Engineering 21 (2008) 761–765

consists of a parallel link of a spring, indicating elasticity, and adashpot, indicating damping (Fig. 1, Mishra, 2004). Here, Kn, Ks

and gn, gs denote the contact stiffness and the damping coefficientsin the normal and tangential directions, respectively. ~vn, ~vs and un,us are the relative velocity between the two particles and the over-lap magnitude, and u shows the angular displacement of the par-ticle. Furthermore, a slider with a friction coefficient l is addedto the tangential component in order to represent the additionalfriction at the contact point of the solid particle cluster.

The magnitude of the interactive force between the particles isdefined as the sum of the normal force component ~fn and the tan-gential force component ~fs, as indicated in Eq. (1). Here,~n,~s are theunit vectors in the normal and tangential directions between theparticles, respectively

f!¼ f!

n þ f!

s

f!

n ¼ Knun n!þgn v

!n

f!

s ¼ Ksus s!þgs v

!s

ð1Þ

The contact forces in the normal and tangential directions canbe calculated using the model in Eq. (1). Once the magnitude andthe direction of the force generated by the collision are deter-mined, the acceleration,~a can be calculated by using Newton’s Sec-ond Law (Eq. (2.1)). Then, the acceleration can be used to calculatethe velocity ð~vÞ and displacement (~U) of the particles to determinethe location of the particles at the subsequent time step using Eqs.(2.2) and (2.3)

½~a�t ¼½~f �tm

ð2:1Þ

½~v�t ¼ ½~v�t�Dt þ ½~a�tDt ð2:2Þ½~U�t ¼ ½~U�t�Dt þ 0:5½~v�tDt ð2:3Þ

2.2. Particle bonding model

The parallel-bonding model was applied to represent the bondbetween the particles, which can be indicated with springs of nor-mal, tangential and moment components denoted as ~fn;b, ~fs;b and~Mb, calculated according to Eq. (3) (PFC manual, 1999; Antonyuk

et al., 2005). And, the Sketch of bonding model between two parti-cles is shown in Fig. 2

fn;b

!¼ Ks;bAun~n ðNormal componentÞ

fs;b

!¼ Ks;bAus~s ðTangential componentÞ

Mb

!¼ Kn;bID h

!ðMoment componentÞ

ð3Þ

Here, Kn,b, Ks,b are bonding stiffness (N/m3) in the normal andtangential directions, respectively. A is the area of the bond surface,I is the moment of inertia of the bond surface, and D h

!is the angu-

n nK

nu

s

ssu

nη

nu

sη

ϕsK

slider

μ

su

(a) normal force (b) tangential force

Fig. 1. Kelvin–Voigt contact model.

lar displacement. When the normal or shear stress applied to thebond between the particles reaches the max bond strength thresh-old, the particle bond breaks.

2.3. Determining the stiffness and damping coefficients

One of the important aspects of the DEM simulation is estab-lishing practical stiffness and damping coefficients. As explainedabove, the DEM model used in this study is a linear spring-dashpotmodel, and the stiffness and damping coefficients vary according tothe material characteristics. Misra and Cheung (1999) used Pois-son’s ratio and Young’s modulus to calculate the normal stiffnessusing the following equations:

Kn ¼ 0:094E�R�

1E�¼ 1� mA

2GAþ 1� mB

2GB

1R�¼ 1

2RAþ 1

2RB

ð4Þ

where mA, mB and GA, GB are Poisson’s ratio and Young’s modulus (N/m2) of the two materials, respectively, and RA, RB are the radii of thetwo particles. The normal damping coefficient gn was determinedusing the restitution coefficient, e and the mass of the two materialsMA, MB as follows:

gn ¼� ln e

ffiffiffiffiffiffiffiffiffiffiKnMp

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffip2 þ ðln eÞ2

q ð5Þ

1M� ¼

1MAþ 1

MB

From the Hertz contact theory, it is known that the tangentialstiffness, Ks may have from 0.67 to 1 of the normal stiffness, Kn

(Rajamani et al., 2000). In this study, 0.67 times of Kn is chosenas the value for Ks. Accordingly, the normal stiffness can be calcu-lated using Young’s modulus and Poisson’s ratio of each material,and the stiffness, damping coefficient and particle mass can beused to obtain the final damping coefficient.

3. Experimental and simulation conditions

Products of a primary jaw crusher from a concrete waste recy-cling plant were used as the test concrete samples for this study. Asample that corresponded with the particle size fraction of73 mm � 53 mm was selected to observe the breakage and the lib-eration properties displayed by the free fall tests. The samples

Table 1Major parameters for free fall test simulation

Particle number 970 –

Young’s modulus(Cements paste) 16 GPa(Aggregate) 100Normal stiffness(Cements paste to cement paste) 4.4 M N/m(Cement paste to aggregate) 7.5(Aggregate to aggregate) 27Poisson’s ratio 0.3 –Coefficient of restitution 0.18 –Coefficient of friction 0.3 –Normal and shear stiffness of bond model 44 M N/m3

Maximum normal and shear strength of bond(Cements paste to cement paste) 2.0(Cement paste to aggregate) 2.3(Aggregate to aggregate) 8.0Time step 1.0 lS

SIZE, mm

0.1 1 10 100WEI

GH

T PE

RC

ENT

LESS

TH

AN S

IZE

0.1

1

10

10010 FREE FALL IMPACTS20 FREE FALL IMPACTS30 FREE FALL IMPACTS

Fig. 3. Size distribution of free fall test: experimental.

Fig. 4. Photograph of aggregate products from free fall test.

H. Lee et al. / Minerals Engineering 21 (2008) 761–765 763

were KS (Korea Standard) recycled aggregates class 1 with thecompressive strength of 30–35 N/mm2 and the water adsorptionof 3%. They included a total of 15 aggregates: 13 aggregates of13.5–19.1 mm and 2 aggregates of 19.1–26.5 mm. The subject con-crete waste samples were heat treated to augment the liberationprocess. When treated with heat, the concrete waste undergoes areaction opposite to the hydration which occurred when the con-crete was formed, increasing the probability of separation between

Fig. 5. The photo of sample material for free fall test: (a), a snapshot of th

the aggregate and the cement paste. From previous studies, it hasbeen determined that a heat treatment temperature between400 �C and 500 �C is appropriate to instigate an opposite reactionto concrete hydration without causing degradation of the aggre-gates (Ahn et al., 2001). The samples for this study were heat trea-ted at 400 �C for 1 h. The free fall test was conducted by droppingthe samples from a height of 2 m, and the free falls were repeateduntil clean aggregates were produced. The particle size distribu-tion, according to the number of free fall impacts, was examined.Upon completion of the experiment, the product samples weresieved into particle size fractions of the

ffiffiffi2p

Series for analysis.The concrete block used for the DEM simulation consisted of

970 particles of a 2.5 mm size. In order to correspond with the ac-tual experimental conditions, 1, 2 and 7 aggregates, constituted of70, 30 and 20 particles, and 700 uniform cement paste particleswere prepared to simulate a 40 � 40 � 60 mm concrete block. Be-cause a waste concrete block is not round, we simulated it bycomparing it with the picture in Fig. 5a. The bonds betweenthe particles used for the simulation can be classified intocement–cement, cement–aggregate and aggregate–aggregatebonds. The simulation constants of the three bond types can be cal-culated using the equations in Section 2.3 as well as Young’s mod-ulus E and Poisson’s ratio. Table 1 summarizes the major DEMsimulation parameter values. Furthermore, it is very important toidentify the accurate material bond strength to compare the DEMsimulation results and the results of the actual experiment. Khanalet al. (2004) conducted the simulation using 4.1 MPa as the bondstrength value. This study referred to Khanal’s study and

e simulation (b) and its bonding network diagram for simulation (c).

THE NUMBER OF FREE FALL IMPACTS

0 5 10 15 20 25 30 35

TH

E N

UM

BE

R O

F B

ON

DS

0

500

1000

1500

2000

2500

3000INTRA-CEMENT PASTECEMENT PASTE-AGGREGATETOTAL

Fig. 7. The number of bonds between particles with the number of free fall impacts.

764 H. Lee et al. / Minerals Engineering 21 (2008) 761–765

conducted around 30 DEM simulations in the range of 100–101 MPa, and selected the value that most agreed with the resultsof the actual experiment (Table 1). The free falling tests were sim-ulated based on Eqs. 1–5 and used a self-developed 3D DEM pro-gram running on a Compaq Fortran 6.5. The movement ofparticles was visualized from the simulation data in the C++OPENGL graphic library.

4. Results and discussion

4.1. Free fall test results

Fig. 3 displays the particle size distribution curve of the productof concrete waste samples according to the number of free fall im-pacts. The samples go through comminution as the number of freefall impacts increases to yield smaller particles, and the accumu-lated distribution curve moves into the smaller particle size area.After 20 free falls, the accumulated ratio of a particle size of10 mm or less was about 20%. The result after 30 falls was not sig-nificantly different from that of 20 falls, indicating that a substan-tial amount of cement was peeled off from the aggregate surfacebetween the 10th and 20th falls. After 30 free fall impacts, cleanand liberated recycled aggregates were obtained, as shown inFig. 4.

4.2. DEM simulation results

Fig. 5a is an actual photo of the concrete block. Fig. 5b is thenearest simulation of the rectangular block consisting of 970 parti-cles of a 2.5 mm size, where the aggregates are indicated in 3D asblack spherical particles and the cement as light grey particles. TheBond-Network-Diagram exhibiting the internal structure of thebonds among particles is shown in Fig. 5c. In the figure, the aggre-gates are indicated as bright red spheres, cement appears as darkblue spheres and the aggregate–cement bonds are shown withwhite lines.

Fig. 6 presents a captured image of the simulation video at eachprogressing free fall impact to assess the comminution and libera-tion behaviour of the block according to the number of free fall im-pacts. The video analysis showed a few small grey cement particlesfalling off the block up to the 8th free fall impact. At the 9th free fallimpact, the first large particle lump, including a big aggregate, wasseparated from the main block. At the 17th free fall impact, thelargest aggregate lump, consisting of 70 particles, was removedfrom the main block, and as the number of free fall impacts in-creased, the cement particles were peeled off from the aggregate.

Fig. 6. The snapshots of the simulation results cor

After the 30th, and final, free fall impact it was confirmed that 6of the 10 aggregates were separated.

The number of bonds within the block, according to the numberof free fall impacts (Fig. 7), showed a similar tendency. The impactenergy generated by the free fall impacts was transferred through-out the block, breaking the bonds. The number of bonds decreasedfrom 2245 to about 1300 by the 30th impact. While the number ofcement–cement bonds linearly decreased during all 30 of the freefall impacts, the number of cement–aggregate bonds that deter-mine the actual quality of recycled aggregates rapidly decreaseduntil the 10th free fall impact, but, after the 15th impact, the rateof decrease was reduced. The results indicate that most aggre-gate–cement bonds are broken by the 15th free fall impact, evenfor the aggregates separated from the main block between the15th and 30th impact. Therefore, it can be inferred that the separa-tions simultaneously take place when the cement paste that sur-rounds the aggregates is broken.

Also, the purity of recycled aggregates was analyzed in terms of‘liberation ratio’. In the numerical simulation, the liberation ratiocan be defined as the ratio of the particle number of aggregatesto the particle number of the sample group. The liberation ratiois shown in Table 2 with the number of free fall impacts. Untilthe 10th free fall impact, because the necessary energy to generatethe breakage and liberation behaviour was not applied to the con-crete block, a large group which contained 3–4 aggregates and ce-ments paste was observed. Therefore, there were only six groups

responding to the number of free fall impacts.

Table 2Liberation ratio, %

The number of free fall impacts 10 20 30

Group which contains the aggregates 1 82.8 100 1002 62.5 76.8 1003 41.6 62.5 90.94 35.7 60 88.25 33.3 55.5 81.086 18.9 26.8 71.47 58.88 509 28.6

SIZE, mm1 10 100W

EIG

HT

PE

RC

EN

T L

ES

S T

HA

N S

IZE

0.1

1

10

100

10 FREE FALL IMPACTS, DEM20 FREE FALL IMPACTS, DEM30 FREE FALL IMPACTS, DEM10 FREE FALL IMPACTS, EXPERIMENTAL30 FREE FALL IMPACTS, EXPERIMENTAL

Fig. 8. Size distribution of product: experimental and simulation.

H. Lee et al. / Minerals Engineering 21 (2008) 761–765 765

and the liberation ratio was low. On the other hand, the results of30 free fall impacts show that about 40% of recycled aggregateshave a very clean quality (liberation ratio; 91–100%).

Fig. 8 displays a graph comparing particle size distribution be-tween the simulation and the actual experiment. In the experimen-tal data, up until 10 free fall impacts, aggregates are yet to beseparated from the block and only some cement paste is detached.However, the simulation results indicate that a single concrete par-ticle is separated, resulting in a particle size discrepancy of around53 mm. Nevertheless, it was confirmed that a particle size of lessthan 53 mm showed similar results. After the 30th free fall impact,there were particle size differences in the 19–37 mm region. Thiscan be explained by the fact that most aggregates were separatedin the actual experiment whereas the simulation results indicatedthat 4 out of 10 aggregates were still attached to the main block.Such discrepancies can be attributed to the differences in the con-

figurations of the concrete block samples used in the actual exper-iment and the simulation. However, the overall results displaysimilar tendencies between the simulation and the experiment re-sults, confirming that the DEM model will be able to simulate ac-tual concrete breakage characteristics with a proper adjustmentof model parameters.

5. Conclusion

The DEM was used to create a 3D model of a concrete block con-sisting of 10 aggregates with 970 particles. The model free fell 30times from a 2 m height to examine the breakage and liberationcharacteristics according to the number of free fall impacts, andthe results were compared to those of an actual experiment. Thecomparison of size distribution showed similar tendencies be-tween the simulation and the experimental results. Also, it wasconfirmed that the DEM with proper model parameters will beable to simulate actual concrete breakage characteristics. Fromthe simulation results, the number of bonds between particles de-creased by 42% from 2245 to 1300 by the 30th impact. In particu-lar, the aggregate–cement paste bonds decreased by over 75% from443 to 112 by the 15th drop, which confirmed that a very effectiveseparation between aggregate and cement paste took place in earlystage of free falling tests. Moreover, the results of liberation ratioindicated that 40% or more aggregates showed a 90% or higher lib-eration ratio, demonstrating that high-quality aggregates can beproduced.

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