89 (2007) 1–8www.elsevier.com/locate/enggeo

Engineering Geology

A laboratory and full-scale study on thefragmentation behavior of rocks

B. Bohloli a,⁎, E. Hoven b

a School of Geology, University college of Science, Tehran University, Tehran, Iranb Department of Geology, Chalmers University of Technology, Gothenburg, Sweden

Received 25 June 2005; received in revised form 13 May 2006; accepted 23 May 2006Available online 15 November 2006

Abstract

Rock fines produced in rock blasting and crushing processes is a major problem for aggregate producers. Laboratory and full-scale studies were carried out to explore the possible relationship between fines production and water content of rock material. TheBrazilian tensile test was selected for the laboratory study while a cone crusher machine was employed for the full-scale trials.Results of the laboratory work showed that the percentage of fines fraction produced was a function of the type of rock tested andthe tensile strength of individual specimens. Different rocks produced various amounts of fines, but for the rocks investigated, acorrelation between its strength and its fines generation could be attained. The influence of water content on production of fineswas also examined and showed that increased water content reduces both tensile strength and fines generation in the laboratory.The impact of water content on aggregate production was also studied in full-scale but the results were not as clear as those of thelaboratory tests.© 2006 Elsevier B.V. All rights reserved.

Keywords: Brazilian test; Failure mechanisms; Fines generation; Rock crushing; Rock fines; Tensile strength; Water content

1. Introduction

Crushed rock is one of the main raw materials used inthe construction of infrastructures such as roads, railroadsand buildings. The quality of these structures is partlydependent upon the quality of rock or aggregates used.Aggregates are produced in two main processes: blastingand crushing. Rock is separated from the rock mass inform of large blocks by blasting operation. These blocksare transported to crushing plants to be fragmented intothe desired fractions through multiple-stage crushing.

⁎ Corresponding author.E-mail address: bohloli@khayam.ut.ac.ir (B. Bohloli).

0013-7952/$ - see front matter © 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.enggeo.2006.05.010

During the crushing operation, a by-product called fines(fraction 0–4 mm) is generated. Today there is no directuse for fines in the industry and production of this fractionis an economic loss for aggregate producers. Reduction offines, even by a few percent, would result in significantcost reduction for the industry.

Several researchers have studied the problem of finesproduction (Berube et al., 1979; Kanda et al., 1988;Bohloli andRonge, 1999) and possible consumption areasfor this byproduct (Khachadoorian, 1997). Hoven (2001)carried out a laboratory study to find out a relationshipbetween the fines fraction and the tensile strength of rocks.Her study showed that for a given rock type, there is apositive correlation between the percentage of fines

Table 2Testing conditions for different groups of the specimens

Group Specimen condition Testing environment

A Dry AirB Saturated (5 h) WaterC Saturated (25 h) WaterD Dry Water

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generated and the tensile strength of the rock. Svahn(2003) also carried out an investigation on the generationof fines in bench blasting and concluded that the greaterthe specific energy is, the finer the fragmentation.

Most of the above mentioned references focused onfines generation in laboratory scale but no full-scalestudy have been carried out. We tested some rock typesin the laboratory and established a relationship betweenthe percentage of fines produced and the tensile strengthof the specimens. Subsequently this relationship wasemployed to reduce production of fines fraction in thelaboratory. In addition, we carried out a full-scale studyto explore the possible relationship between the strengthof rocks and the percentage of fines produced.

Generation of rock fines depends on at least two para-meters: the tensile strength and the mineral composition.This may be written as

Fines fraction ¼ f ðrt;minÞ ð1Þ

where σt stands for tensile strength and min stands formineral composition.

The influence of mineralogy on fines generation hasbeen studied by several authors (Stenlid, 1996; Svahn,2003). The dependence of fines fraction on the tensilestrength is discussed in this paper.

2. Laboratory work

Four rock types; amphibolite, diabase, diorite andgneiss were selected for laboratory testing (Table 1). Testspecimens were of 42 mm diameter and 21 mm length.Specimens of amphibolite and gneiss were randomlydivided into four groups; A, B, C and D while specimensof diabase and dioritewere divided into two groups; A andC. Specimens of groups A and D were dried in a 105 °Coven for 24 h and then put in a desiccator for several hoursto reach the room temperature. Specimens of groups Band C were immersed in water for 5 and 25 h respectivelyto be saturated.

Table 1Physical properties of the rocks tested

Rock type Density(g/cm3)

Porosity(%)

Water cont.5 h (%)a

Water cont.25 h (%)

Gneiss 2.62 0.40 0.118 0.125Amphibolite 3.20 0.23 0.046 0.053Diorite 2.83 0.12 – 0.03Diabase 2.96 0.55 – 0.13a Water content 5 h, means samples immersed in water for 5 h there-

after their water content was measured.

Specimens of groups A and D were taken from thedesiccators, quickly weighed and wrapped with tapeaccording to the ISRM recommendations (ISRM, 1978)and tested. When treating group A the testing apparatuswas placed in room conditions while for group D theapparatus was put under water and loading was per-formed in that condition. This was done to explore theeffect of water at the surface of the specimens on thefragmentation behavior. Saturated specimens of groupsB and C were taken from the water container, the surfacewas quickly dried, it was weighed, wrapped with tapeand tested. The apparatus was put under water andloading was carried out and failure occurred under water(Table 2).

The laboratory test used in this study was the Bra-zilian tensile test. The set-up was arranged according tothe recommendations of the International Society forRock Mechanics but in two different environments: inair and under water. Specimens were loaded diametri-cally and for the gneiss specimens the direction ofloading was selected perpendicular to the foliationplane. The tensile strength of the material was calculatedas:

rt ¼ 2P=ðkDLÞ ð2Þwhere P is the force at failure (maximum force reached),D is the diameter and L is the length of the specimen.Since Eq. (2) is valid only for isotropic material anddoes not account for anisotropy (Chen, 1996) the nor-malized failure load (Pn) was used instead of the tensilestrength.

Pn ¼ P=ðDLÞ ð3Þ

where P, D and L have been defined earlier.All tests were carried out according to the ISRM

suggestedmethod (ISRM, 1978) although the loading ratewas slightly faster. We used a force-controlled machinefor loading the specimens until failure occurred. Themaximum load reached was registered as failure load, P.The weight of the specimen before testing was referred toas W1 while W2 was the weight of all the rock particleslarger than 4 mm after failure. The following ratio, called

Fig. 1. Schematic set-up of the crushing plant used for the full-scaletests.

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fines fraction, is an indication of how much fine materialis produced due to failure of a rock specimen:

Fines;Fð%Þ ¼ ðW1−W2=W1Þ�100 ð4Þ

3. Full-scale study

Few tests were carried out in full-scale at Skanska rockcrushing plant located in Sperlingsholm, southern Swe-den. The objective was to explore whether water contenthas any (direct or indirect) effect on the production of acrusher machine. At the rock quarry adjacent to the plantamphibolite and gneissweremined.We tested only gneissmaterial since there was no access to amphibolite at thetime of trial. Three parameters versus the water content ofthe rock were studied. These parameters include the finesfraction, the production capacity, and the energy con-sumption of the crushing system. The two latter para-meters measure economic efficiency of the crushermachine. Higher output capacity and lower energy con-sumption will provide more economic production.

Fig. 2. A PVC circular hose for sprayin

3.1. Set-up of the trial crushing plant

A mobile crushing plant was employed for the full-scale trials (Fig. 1). The plant included a jaw crusher atthe beginning (A in Fig. 1), a cone crusher (E), two setsof screens (C and D), pre-weighed containers (F), aweigh and several conveyor belts.

The rock blocks quarried from the rock mass feed thejaw crusher. The crushed rock is transported to screen Cby a conveyor belt, where the fraction 0–8 mm is sepa-rated. In this way we eliminate the fines either producedduring blasting operation or by the jaw crusher. Aggregatelarger than 8 mm is further transported to screen D wherethree fractions are separated. Again material larger than25 mm is transported to the cone crusher (E). Herematerial enters a closed circle of sieving–crushing–siev-ing. Therefore the amount of fraction 0–8 mm obtainedfrom screen D has originated only from the cone crusher.

The feedmaterial included gneisswith size distribution30–90 mm. The material was sprayed with water for fewminutes before feeding the cone crusher. A circular PVChose was constructed and assembled at the inlet of the jawcrusher for spraying water into the machine during ope-ration (Fig. 2). A tank lorry delivered the required water.The material was weighed by means of the conveyor-weigh connected to the belt. The fractions separated byscreen D were transported to the pre-weighed containers.In order to be able to measure fines which possibly couldbe transported by water running out of the cone crusher asmall pool was built and its bottomwas covered by plasticto recover the fines.

3.2. Crushing procedure

Four tests were carried out on gneiss under differentconditions. The resultant parameter was the size distribu-tion of rock particles. The size distribution was obtainedthrough weighing different fractions. The four tests were

g water into the crusher machine.

Table 3The amount of each sample for different fractions

Fraction (mm) Amount (kg)

30–90 30–4016–25 208–16 150–8 10

Fig. 3. Failure load versus fines fraction of two rock types. The datapoints in both diagrams present the result of combined dry and saturatedspecimens tested in the laboratory.

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carried out under different conditions and between 37 and41 tonnes of rock was crushed in each test:

Test D Dry test, material contained natural moisture,amount used: 40 tonnes.

Test V1 Before feeding the crusher, material was sprayedwith water for few minutes and 0.5 l/s water wasadded into the crusher through the water hose,amount used: 41 tonnes.

Test V2 Before feeding the crusher, material was sprayedwith water for few minutes and 0.9 l/s water wasadded into the crusher through the water hose,amount used: 37 tonnes.

Test V3 Before feeding the crusher, material was sprayedwith water for few minutes and 2.5 l/s water wasadded into the crusher, amount used: 37 tonnes.

3.3. Sampling

A number of representative samples were taken toanalyze material before and after testing. The samples ofthe feed material were taken out of the rock stock andthose of the crushed rock from the containers. Theamount of samples for laboratory analyzing depends onthe largest nominal particle size (Table 3).

Samples of the crushed rock were directly taken aftercrushing and tied in plastic bags to preserve the moisturecontent they had during crushing. Table 4 shows thenumber of samples taken out of the various fractions ineach test. It was particularly difficult to take represen-tative samples for fraction 30–90 mm.

Each sample of fractions 0–8, 8–16 and 16–25 mm iscomposed of several sub-samples taken at different pointsinside the containers. In order to have a representative

Table 4The number of samples taken in different tests

Fraction (mm) Test D Test V1 Test V2 Test V3

30–90 4 4 3 316–25 3 3 3 38–16 3 3 3 30–8 6 4 6 4

sample, the rock stocks were dug out and the sampleswere taken from the middle of the stock.

The feed material was weighed by means of the weighinstalled under the conveyor belt. Accuracy of this weighis 1 ton. The produced fractions were led to pre-weighedcontainers. At the end of each test the containers wereweighed with an accuracy of 20 kg.

3.4. Determination of energy consumption

The amount of electricity consumed by the crushermachine is a measure of the energy required to crush thematerial and reflects material stiffness. Thus, we mea-sured the required data and calculated the amount ofelectricity consumed to crush a certain mass of rock indifferent trials to study the influence of water content onthe energy consumption. We used an electrical motor oftype ABB MAV 315L,Y and power consumption wasmeasured using an instrument called DIP 6000, con-nected to the motor. This instrument measures the elec-tricity current and voltage versus time. Knowing thecharacteristics of the motor, you can calculate the power(effect) using the following equation.

P ¼ffiffiffi

3p

:Uh:Ie:cosW ð5Þ

whereP is power inKW,Uh is voltage between phases in V,Ie is current load in A and cosΨ is the phase displacement.

Fig. 4. Failure load versus fines fraction for dry and saturated specimens of four rock types. Saturated specimens (25 h) have lower strength and produceless fines than the dry ones do.

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Energy consumption, W [KWh] is calculated bymultiplying the power by the crushing time.

W ¼ P:T ð6Þwhere T is crushing time [h] and P is power as definedearlier.

3.5. Production capacity of crusher machine

Capacity of the machine was determined throughdividing the total weight of the crushed material to thetime of crushing. Crushed material was led to pre-weighed containers. The containers were weighed at theend of the tests and the weight of the crushed rock wasdetermined with a relatively high accuracy.

4. Results

4.1. Results of the laboratory study

Around 70 gneiss, 40 amphibolite, 50 diabase and40 diorite specimens were tested. The results aresummarized in Figs. 3 and 4. Correlation coefficient

Table 5Mean value of failure load versus fines for different groups of gneisstested

Parameter Group A Group B Group C Group D

Water content (%) 0 0.118 0.125 0Failure load Pn (MPa) 31 24.5 22.5 30Fines, F (%) 12.5 9 6 10Correlation coefficient, R2 0.89 0.44 0.63 0.63

between the failure load and fines fraction wasdetermined according to the Pearson correlation testand shown to be relatively high (Fig. 3). Withincreasing water content (from group A to C, gneiss,see Table 5) the failure load and also the percentage offines fraction decreased. When dry specimens ofgroup D were loaded (while immersed in water) thefailure load and fines fraction showed a slight reductioncompared with those of group A. The correlation factorwas relatively high except for group B.

Results showed that with increasing water contentnot only failure load decreased but also less fines wereproduced. The fines generation could be reduced and themaximum reduction achieved was 51% for specimens ofgroup C (Table 5). A similar trend was observed foramphibolite, diabase and diorite specimens (Fig. 4).

Fig. 5. Cone Crusher Performance of the full-scale trials.

Table 6Water content of input material in the full-scale tests

Test D V1 V2 V3

Water cont. (%) 0.2 0.4 0.7 0.7

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4.2. Results of the full-scale study

Results of the full-scale trials are presented in thefollowing sections.

4.2.1. Size distribution analysisSize distribution of aggregate has a great impact on the

economyof the crushing operation since different fractionsare valued differently in the market. The produced frac-tions have beenmeasured for the different trials and shownin Fig. 5. Such an illustration is called Crusher Perfor-mance Map, CPM (Evertsson, 2000). The CPM showsthat adding water up to 1 l/s into the crusher machineincreases the undesired fractions; 0–2 and 2–4 mm anddecreases the preferred fractions including 8–11.2, 11.2–16 and 16–25 mm. But the trend is reversed when addingmore water, i.e., 2.5 l/s. Size distribution of crushedproduct in test V3 is of great interest. Here the percentageof coarse aggregate increases and that of fine fractionsdecreases. It would be worthwhile to do more trials withhigher water discharge. If the trend of the size distributioncontinues in the same manner, it will provide a largeeconomic profit.

4.2.2. Water content versus fines fractionWater content of the input material was measured for

different tests at the entrance of the crushing machine. Inthe dry test (D) no water was added to the material.However, it contained 0.2% water which is the naturalmoisture absorbed by the rock particles. The dry testwas considered as reference. For measuring water con-tent of the crushed product, the samples were taken fromthe outlet of the crusher, were wrapped in plastic bags,taped and transported to the laboratory.

Water content of input material and all fractions ofcrushed product was measured and shown in Tables 6and 7. Eight samples were analyzed for fractions 8–16and 16–25 mm, while four samples were sieved for

Table 7Water content of crushed product in the full-scale tests

Test D V1 V2 V3

Fractions(mm)0–8 0.8 4 6.4 10.68–16 0.4 1 3.1 2.5

fraction 0–8 mm per test. Water content shows a fallingtendency with increasing the size of the material. Thisimplies that water is mainly absorbed to the surface ofmaterial and since fine particles have large specificsurfaces absorb more water. Although more than 2 l/s ofwater was sprayed into the crusher in test V3, no waterwas run out of the crusher and all water was absorbed bythe new built surfaces.

4.2.3. Water content versus energy consumptionMore energy is required for crushing hard rock while

much less is needed to crush softer rocks. If water contentsoftens the rock, energy consumption should be reducedwith increasing the water content. Energy consumptionfor every trial was calculated using Eqs. (5) and (6).Results showed that energy consumption increased fortrial V1 and V2 but it decreased with increasing waterdischarge in trial V3 (Fig. 6).

Fig. 6. Water content versus the three parameters measured in the full-scale tests.

Fig. 7. Load-time and load-displacement curves for a rock specimentested in a servo-controlled Brazilian test apparatus.

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4.2.4. Water content versus production capacityProduction capacity of crusher machine was deter-

mined for every trial. It showed that with increasing watercontent in tests V1 and V2 the capacity decreases whileadding more water to the machine (test V3) the capacityincreases again.

5. Discussion

5.1. Relationship between failure load and fines fraction

A relationship between the failure load and finesfractionwas observed in the laboratory. This correlation isstill not well understood. A possible explanation could bethe following:

Fine particles are produced when failure develops andthe stress–strain curve passes the peak. The amount ofpost-failure stress on the specimen will continue toload the cracked rock pieces. Since the cracked spe-cimen has sharp edges, stress will be concentrated onthese locations and in case being high enough, it willbreak the edges. This will produce fine particles.Thus, fines production could be a function of post-failure load on the specimens which in turn is de-pendent upon both the machine stiffness and thestrength of rock. When we test the specimens ofdifferent strengths with the same machine theunloading curve of the specimens will nearly havethe same slope (Fig. 7). Therefore in harder rocks thepost-failure load on the specimenwould be higher andact a longer time than for a soft specimen. It seemsthat the strength of rock has an indirect effect onproduction of fines. Thus, more fines would be pro-duced by a harder rock.

5.2. Reduction of fines

Water content reduces the peak load and the post-failure load on the samples. Therefore, saturated speci-mens produce less fines in the laboratory. The finesfraction versus the water discharge into the crushermachine was presented in Fig. 6. When water capacitywas 0.5 l/s the amount of the fines was about the samevalue as of the reference, while adding more water, i.e.1 l/s, the amount of fines increased. This is in contrary towhat laboratory studies showed.

One of the explanations is that water is mainly ab-sorbed to the surface of the particles, binds them togetherand shortens the crushing step. This means an increase inthe number of crushing stages since the length of thecrushing machine is constant. Fines are produced during

each step. Therefore, with increasing crushing stages theamount of fines is also increased. When water capacitywas increased to 2.5 l/s, the amount of fines decreasedagain. In this case water capacity is large enough to washout the smaller particles preventing binding of blocks andshortening the crushing step. It is still hard to believe thatwater has entered the fractures of the blocks to ease thefragmentation during the very short time the material isexposed to water. Adding more water could revealwhether it is possible to reduce fines production duringshort exposure to water.

It is very hard to control all variables involved in a full-scale operation. The interaction between machine–rock–water is rather complex. This needs a comprehensivestudy of the various factors involved. Subsequently, afactorial design can be performed to reveal the influenceof different parameters on crushing operations.

The difference between the laboratory and full-scaletests was that specimens were water saturated in labo-ratory while material was only exposed to water insidethe crushing machine during seconds in the full-scale. Inorder to achieve similar results as those of the laboratorystudy, material should be immersed in water or sprayedwith water for a longer time before crushing. This couldhave some practical complications and also an impacton the total cost of the production which should be takeninto account if this topic is further investigated.

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5.3. Energy consumption

Energy consumption is dependent upon not only thestrength but also the size distribution of feed material.Sieving is not functioning well in wet conditions. Thus alot of fines accompany larger rock particles and load thecrusher. This in turn increases the energy consumption.However, when the amount of water is high enough towash the fines out of the machine mantle and of thesurface of larger particles, energy consumption decreasesas observed in test V3.

5.4. Possible sources of error

There are always sources of error in measurement ofexperimental parameters in laboratory and in full-scale.Uncertainties in the laboratory work are assumed to benegligible but there are several possible sources of errorsin full-scale studies. The two more important sources arelisted below.

1– Sampling method: the samples should be repre-sentative and the number of samples be largeenough to provide sufficient and reliable statisticaldata. More samples would have probably given usmore representative data.

2– Determination of energy consumption: we used thenominal value of the phase displacement of themotor for calculating the power consumption. Phasedisplacement could be measured during crushingoperation. Themeasured valuewould probably giveus more exact data.

6. Conclusion

• Laboratory testing of rock specimens with the Bra-zilian test showed that various rock types producedifferent amount of fines; gneiss produced the highestand amphibolite the lowest percentage. It was alsoshown that there is a relationship between the strengthof specimen and the amount of fines generated for agiven rock type. The higher the tensile strength, thehigher was the percentage of fines produced.

• Rock specimens with different degrees of saturationwere tested. Laboratory tests showed that with in-creasingwater content the percentage of fines decreasedfor all types of rocks tested. The average reduction offines for gneiss was about 50%, for diabase, diorite andamphibolite about 20%.

• Results of the full-scale tests showed that the per-centage of fines production varied with different waterdischarges to the crusher machine. The percentage offines increased with increasing water discharge up toaround 1.5 l/s while the trend reversed when addingmore water and a declining tendency in the amount offines was observed. Proper evaluation of fines gene-ration and fines reduction in full-scale requires moreexperimental studies.

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