Tian Xiaoge, Wang Xiaofei, Yuan Huitong, "Standard of Crushing Value of Coarse Aggregates for Permeable Asphalt Mixture Based on Contact Stress between Aggregates", Journal of Engineering, vol. 2020, Article ID 1261368, 7 pages, 2020. https://doi.org/10.1155/2020/1261368
Crushing resistance of coarse aggregate is the key to the stability and durability of the skeleton structure of permeable asphalt (PA) mixture. To determine the technical requirements of crushing value of coarse aggregate used in PA mixture, step-loading compression tests were conducted on the mixtures of PA-13 and a control asphalt mixture AC-13, respectively. Virtual compression tests under the same loading conditions were simulated on the corresponding digital specimens with PFC2D. By comparing the load-deformation curves obtained from the actual tests and virtual simulation, the values of the microscopic parameters of the two graded mixtures were obtained through trial calculation and adjustment. Then, the states of contact stress between aggregates in PA-13 and AC-13 mixtures under the standard crushing pressure (400kN) were analyzed with PFC2D. It was found that the average normal contact stress and the maximum normal contact stress between aggregates in PA-13 were 1.71 times and 1.28 times larger than those in AC-13, respectively. The crushing values of two different lithologic coarse aggregates were measured under different pressures, 400kN or 600kN, respectively. The crushing value criterion of coarse aggregates used in the PA mixture was suggested to be no greater than 16% after comparative analysis.
Permeable asphalt (PA) mixture is a kind of skeleton-void mixture, and the vehicle load is mainly borne by the stone-stone contacting skeleton structure formed by coarse aggregates. So, the stability and durability of the skeleton are very important for PA mixture. If the compressive capacity of coarse aggregate is poor, some of the coarse aggregates will be crushed under the action of vehicle load, which will destroy the stability of the skeleton structure. As a result, the deformation resistance of PA mixture will be weakened, the voids in pavement will be reduced, and the drainage capacity will be gradually weakened. A stable skeleton structure is the key to ensure durable drainage of the PA pavement. The crushing of coarse aggregate in PA mixture is caused by the excessive contact pressure between aggregates, so the coarse aggregates are more easily crushed by vehicle load. So, the crushing value of coarse aggregate is very important for the durability of PA mixture .
Yan et al. proposed that the requirement for the crushing value should be no more than 20% . Huang proposed that the crushing value of coarse aggregates for OGFC mixture should be no more than 25% . Wu et al. proposed that the aggregate used for OGFC should meet the standard of SMA, and the crushing value of coarse aggregates should not be more than 24% . Guo believed that the skeleton-void structure formed in the PA mixture reduced the contact area between the aggregates by about 25% compared with the ordinary dense graded asphalt mixture, resulting in an increase in stress magnitude between the contacting aggregates. Therefore, the properties of aggregates have a great influence on the properties of PA mixtures. It was proposed that the crushing value of the coarse aggregate used in PA mixture should be no more than 20% . Liu et al. suggested that the crushing value of coarse aggregate for the OGFC-13 mixture should not exceed 15% . Dang et al. proposed that the crushing value of coarse aggregate was not more than 15% . Cao et al. suggested that it should be no more than 16% . Wang et al. studied the effect of the change of the passing rates of aggregates with different particle sizes on the stability of the skeleton structure of PA mixture . It can be seen that the researchers had realized the importance of the skeleton structure and stability of the PA mixture and proposed improving the crushing value technical requirements of the coarse aggregate for PA mixture. However, the proposed crushing values differed greatly, from 15% to 25%. This may be due to the lack of theoretical analysis and further research.
The authors suggested that the technical requirements for the crushing value of the coarse aggregate should be corresponding to the maximum value of contact pressure between the coarse aggregates in the asphalt mixture. So, the difference of the contact stress between coarse aggregate particles in the mixtures of PA-13 and AC-13 was analyzed through the discrete element method program, PFC2D. Then, the crushing value tests were conducted on two kinds of coarse aggregates under two different pressures to obtain the effect of pressure to the crushing values. Finally, the technical requirement for the crushing value of coarse aggregate in the PA mixture was proposed according to the relationship between the crushing value of different lithologic aggregates and pressure.
The coarse aggregates used in this paper were crushed from two different rocks, basalt aggregates produced by Shucheng Stone factory, and diabase aggregates produced by Dawang stone factory. Their technical indexes are shown in Table 1.
Asphalt concrete AC-13 is commonly used for surface course of asphalt pavement. PA mixture used for surface course of permeable asphalt pavement is usually PA-13. And, in Chinese technical specifications of asphalt pavement construction, the criterion for crushing value of coarse aggregate in PA mixture is the same as that in AC mixtures, no greater than 26%. So, the state of contact stress between coarse aggregates in PA-13 mixture and that in AC-13 mixture was compared. Both of their gradations are the median gradations recommended by the Chinese specification , as shown in Table 4.
Because both of the molds used in step-loading compression test and crushing value test are cylindrical steel models, a closed rectangle generated by the built-in command wall in PFC2D was used to simulate the mold. The width of the mold, D, is 150mm and its height, h, is 100mm. Four walls were used to constrain the particle elements in the mold, and the external pressure is simulated by moving the wall 3 down. Wall 1, wall 2, and wall 4 were fixed constraints , as shown in Figure 1.
According to the gradations of the mixtures (Table 4) and the apparent densities of different sizes of aggregates (Table 3), the mass ratios of aggregates were converted into area ratios under the two-dimensional plane state.
The numbers of aggregates at every sieve size can be calculated through equation (1) assuming that aggregates of particle size i were evenly distributed between the maximum particle diameter rh and the minimum particle diameter rl:where ni is the number of aggregates remains on the sieve size i, Ai is the total area of aggregates with size i, and rh and rl are the maximum and minimum radii of aggregates with size i.
The expansion coefficient method  was utilized to generate the digital specimens of graded mixtures. To avoid excessive overlap between the randomly generated particles, the expected final particle sizes were firstly divided by the specified constant coefficient, m, to reduce their sizes (equation (2)). After the specified number of particles was generated, all of the particles were expanded with the same expansion factor, m, to meet the desired void ratio:where n0 is the void ratio after particles radii were decreased and n is the void ratio after particles radii were expanded. (m=1.6 was taken in this paper).
PFC2D is a system based on the discrete element method, and some microscopic parameters such as particle element and contact between particle elements are needed to simulate the flow and deformation of particles aggregation model under external load, for example, normal stiffness of particles (Kn), tangential stiffness of particle (Ks), friction coefficient (), normal stiffness of the loaded walls (Kjn), and normal stiffness of restraint wall (Kyn). The values of these microscopic parameters have a great influence on the simulation results of PFC . However, there is no suitable test method to determine the values of these microparameters so far. In this paper, the method and steps to determine the values of these related microscopic parameters are as follows.
Determining the values of every microscopic parameters of the mixture is elaborate for virtual simulation with PFC2D . The method applied to obtain the values of every microscopic parameters of graded mixture was as follows:(a)A set of initial values of the microscopic parameters was assumed, and then PFC2D was used to simulate the step-loading compression process on the digital specimen (Figure 2) to obtain the simulated load-displacement relationship.(b)Graded aggregates specimen was prepared according to the gradation and step-loading compression was exerted on it, and the actual load-displacement curve was measured.(c)The simulated load-displacement curve was compared with the measured curve. If the deviation between the two curves exceeded allowance, the assumed values of the microscopic parameters should be adjusted and then simulated again until the deviation met the allowance.
In this way, the final fine-tuned values of the microscopic parameters can be taken as the actual values of the microscopic parameters of the gradation mixture. The specific process is shown in Figure 3.
Cylindrical steel mold, whose diameter is 150mm and height is 100mm, were filled with graded aggregates for the step-loading compression test. The aggregates at different sizes were weighed, respectively, according to its gradation (Table 1), mixed uniformly and poured into the mold by loose packing and stacking method. Then, the step-loading compression test  was carried out through the hydraulic servo material testing machine, MTS-810. The pressure was stepwise increased by 50kN until it reached to 400kN. The measured vertical deformation curves to pressure of the two graded mixtures are shown in Figure 4.
The digital specimens of PA-13 and AC-13 mixtures (Figure 2) with the values of their microscopic parameters (Table 5) were step-loading compressed to 400kN in PFC2D system, respectively, which is called thevirtual crushing value test. The maximum pressure applied to the wall 3 is 2.26107Pa, which is equivalent to 400kN exerted on the actual cylindrical specimen. The state of microscopic contact stress between aggregates in the digital specimens was obtained, as shown in Figure 5.
Figure 5 shows the transfer path of the maximum pressure (400kN) in the two different graded mixtures under lateral restraint conditions. The thickness of the black mesh lines indicates the magnitude of the contact stress between aggregates or the contact stress between aggregate and inner wall of steel mold. The thicker the mesh line, the greater the contact stress. It can be clearly seen that the contact stresses in the PA-13 mixture is significantly greater than those in the AC-13 mixture.
Virtual crushing values tests were simulated three times as above with different aggregates distribution in specimens to take into account the effect of random distribution of aggregates in the actual specimens. The simulation results are shown in Table 6.
It can be seen from Table 6 that the normal contact stress between aggregates in the PA-13 mixture is significantly greater than that in the AC-13 mixture. The average value of the normal contact stress in the PA-13 mixture is 5.340/3.127=1.71 times greater than that in AC-13 mixture. The maximum value of the normal contact stress in the PA-13 mixture is 4.183/3.260=1.28 times greater than that in the AC-13 mixture.
The greater the pressure on aggregate, the easier it will be crushed. When the crushing test of coarse aggregates is carried out, the contact stress between coarse aggregates should be consistent with the actual contact stress between coarse aggregates in actual pavement. Therefore, the maximum pressure applied on the coarse aggregate specimen should be varying with the gradation of the mixture. The value of maximum pressure in the crushing test of coarse aggregate should not be a fixed value, 400kN, for all kinds of asphalt mixtures. According to the results of PFC2D analysis, the contact stress between coarse aggregates in PA-13 mixture is higher than that in AC-13 mixture to reflect the difference of contact stress between coarse aggregates.
It can be seen from Table 6 that, under the same external pressure, the average contact stress and the maximum contact stress between coarse aggregates in the PA-13 mixture were 1.71 times and 1.28 times greater than those in the AC-13 mixture, respectively. A multiple factor of 1.5 was yielded through averaging these two ratios. So, it was proposed that the maximum pressure of the crushing value test for coarse aggregates used in PA mixture should be increased to 1.5 times of the maximum pressure in the standard crushing value test. That is, the maximum pressure should be increased from 400kN to 600kN. Meanwhile, if the loading rate remains unchanged, the loading time will be extended from 10 minutes to 15 minutes.
Coarse aggregates of two different lithologies, basalt and diabase, that can be used for PA mixture, were subjected to the crushing value test with different maximum pressures, 400kN and 600kN, respectively . The results are shown in Table 7.
It can be seen from Table 7 that, when the maximum pressure was increased to 600kN, the crushing values of the two lithologic aggregates were larger than those under the standard pressure, 400kN. But, the increased ratio is different from the lithologic type of aggregates. For basalt aggregate, it increased by 1.48 times and it increased by 1.63 times for diabase aggregate. So, it can be derived that diabase aggregate is more sensitive to the pressure. Therefore, in order to ensure stability and durability of skeleton-void structure in PA mixture, the criteria of crushing value of coarse aggregate used in PA mixture should be improved.
In current Chinese technical specification, the crushing value of the coarse aggregate for surface layer of asphalt pavement is no greater than 26% . So, the crushing value standard of basalt coarse aggregate used in PA mixture should be increased to not more than 26%/1.48=17.6%. As for diabase aggregate, it should be no more than 26%/1.63=16.0%.
Aggregates of different lithologies produced from different places have different mechanical properties, and their crushing values vary with pressure. Although only two kinds of aggregate were compared in this paper, considering the importance of improving the crushing value standard of coarse aggregate for improving the durability of PA pavement, it is suggested that the crushing value should be no greater than 16% to ensure the stability and durability of its skeleton-void structure in PA mixture.
In this paper, microscopic contact stresses between aggregates in PA-13 mixture and those in AC-13 mixture were compared through simulation with PFC2D. The crushing value tests were physically conducted on two kinds of coarse aggregates under two different maximum pressures of 400kN and 600kN, respectively. Finally, the crushing value standard of coarse aggregates for PA mixtures was proposed. The following conclusions were obtained.(1)The average and maximum values of contacting stresses between aggregates in the PA-13 mixture are 1.71 times and 1.28 times greater than those in the AC-13 mixture, respectively. The average of these two values is 1.5.(2)It was proposed that the maximum pressure in the crushing value test for coarse aggregate used in PA mixture should be increased 1.5 times from 400kN to 600kN, according to the relationship between the contact stress of aggregates in the PA-13 and that in the AC-13 mixtures.(3)It was found that the crushing value of coarse aggregate increased with the increases of the maximum pressure. The larger the maximum pressure was, the more the amount of aggregates was crushed. The increased multiple of crushing values of basalt and diabase aggregates were 1.48 and 1.63 times, respectively, when the maximum crushing load was increased from 400kN to 600kN.(4)It was proposed that the crushing value of coarse aggregate used in PA mixture should be increased to no more than 16% according to current Chinese technical specifications.
Copyright 2020 Tian Xiaoge et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
This paper presents a fundamental study on the effect of the relative humidity on the rockfill crushing strength. This aspect plays an important role in the mechanical behaviour of rockfill, and it is known that certain characteristics of the granular materials, such as compressibility and shear strength, depend on the confining stress, which is a function of the particles crushing. An increased interest has been observed regarding the effect of the relative humidity in the mechanical behaviour of rockfill. Unfortunately, limited research has been conducted until now regarding the study of individual particle crushing. Therefore, this paper thoroughly investigated particle crushing, by performing single-particle crushing tests on rockfill particles divided into four size ranges, under different relative humidity conditions. The experimental results reveal a considerable influence of the relative humidity in the studied rockfill particles, whose strength of the particles with the greatest dimensions in saturated conditions was reduced by half. Consistent macro-mechanical evidence demonstrates that particles size and relative humidity conditions depict the most important factors that influence particle crushing strength.
B Standard BS EN ISO 483:2005 (2005) Plastics. Small enclosures for conditioning and testing using aqueous solutions to maintain the humidity at constant value. International Organization for Standardization, vol 483, Geneva, Switzerland, p 12
Deng B, Jiang D, Gong J (2018) Is a three-parameter Weibull function really necessary for the characterization of the statistical variation of the strength of brittle ceramics? J Eur Ceram Soc 38(4):22342242
Ovalle C, Frossard E, Dano C, Hu W, Maiolino S, Hicher PY (2014) The effect of size on the strength of coarse rock aggregates and large rockfill samples through experimental data. Acta Mech 225(8):21992216
Zhou C, Xu C, Karakus M, Shen J (2019) A particle mechanics approach for the dynamic strength model of the jointed rock mass considering the joint orientation. Int J Numer Anal Methods Geomech 43(18):27972815
Manso, J., Marcelino, J. & Caldeira, L. Single-particle crushing strength under different relative humidity conditions. Acta Geotech. 16, 749761 (2021). https://doi.org/10.1007/s11440-020-01065-w
Sizing a crusher can be done reliably calculated thanks to the Impact/Crushing Work Index and the testwork research done by Fred Chester Bond and his 1952 paper. According toBonds Third Theory of Comminution, the work/energy input is proportional to the new crack tip length created during particle breakage and equivalent to the work represented by the product the feed.
A crude test procedure description of this Crushability test follows in that rock samples smaller than 75 mm but greater than 50 mm are placed 2 hammers of 13.6 kg each and mounted to swing on bicycle wheels. Both hammers of the pendulum will impact the smallest measured side on the rock. The hammers are set to fall again from a higher drop position to input enough energy to break the rock. This is done 10 times to obtain 10 breaks and the WI (kWh/t) = 53.49 x Impact Crushing Strength / Specific Gravity of the sample. **jktech.com.au
The test determines the Bond Impact Work Index which is used with Bonds Third Theory of Comminution to calculate net power requirements when sizing crushers*. It is also used to determine the required open-side settings (jaw crushers and gyratory crushers) or closed-side settings (cone crushers) for a given product size.
Where Oss = Open-side setting in inches Css = Closed-side settings in inches Ecc = Eccentric throw in inches P80 = Aperture through which 80% of the product will pass. Wi = Work Index
The impact apparatus consists of two pendulum-mounted hammers, mounted on two bicycle wheels so as to strike equal blows simultaneously on opposite sides of each rock specimen. The height that the pendulum is raised is increased until the energy is sufficient to break the specimen.
The inferior quality of recycled concrete aggregate (RCA) caused by the adhered mortar (AM) can be principally enhanced through either removing or strengthening the AM. This paper reviews the effect of various pretreatment methods (removal, polymer impregnation, pozzolanic slurry immersion, accelerated carbonation curing and bio-deposition) on the inherent defects of RCA, and compares their modification efficiency based on statistical analyses of results in published papers. The pretreatment methods can significantly reduce the mercury intrusion porosity (an average of 41.3% drop) and water absorption (an average of 24.5% drop) of RCA and concurrently augment the mechanical properties considerably. The modification efficiency of removal methods is mainly governed by treatment approaches and parameters. The improved efficiency of polymer impregnation and pozzolanic slurry immersion is primarily determined by the quality of integument and cladding related to the type of treated materials and pretreatment parameters. The carbonation and bio-deposition efficiency of RCA is closely related to aggregate characteristic/the type of microorganism and pretreatment conditions. Overall, paraffin soaking, presoak-accelerated carbonation and combined removal of AM are the most efficient methods to improve the performance of RCA. A better understanding of the characteristics of the treated RCA can provide a theoretical base for future research and applications of RCA in construction industry.
Choosing the right size and grade of aggregates impact the overall strength of concrete. The aggregates are taken out from the natural resources and classified into different grades. Grading of aggregates helps to identify the right aggregates for any construction works.
The aggregates are used as an important ingredient in the preparation of concrete. The fine aggregates are used in concrete as a filler material, and the coarse aggregates give the compressive strength to the concrete.
The classification of aggregates depends on the grain size, density, shape, and geographical origin. So the properties of aggregates will differ based on the classification, and it may influence the mix of the concrete and strength.
When the moisture content is present in the sand, it forms a thin film around each sand particle that makes the adjusted particles push a little away from them. This change makes the overall volume of the sand increase.
The bulking of sand is in the range between 20% to 30% and for the coarse aggregate is minimal. Bulk density is the ratio between the dry weight of the aggregates to the saturated weight of aggregates in kg/litre.
The aggregates may contain different chemical particles that react with the cement and form cracks on the concrete surface. So the aggregates must be tested to ensure that such kinds of particles are not present in the aggregates.
Moreover, the incorrect size of aggregates such as flaky, rounded, angular, and irregular aggregates increase the voids and reduce the bondage of other ingredients in concrete. It may increase the material cost and indirectly reduce the strength of the concrete.
The surface texture represents whether the surface of the aggregates is smooth, polished, or rough. The rough surface texture is good for developing higher bondage between other ingredients of concrete which increases the strength of the concrete.
We can see some tiny holes on the surface of the aggregates called pores and such kinds of rocks called porous rocks. The pores happen on aggregates due to the air bubbles formed on the surface when the molten magma solidification.
The surface area of fine aggregate is higher than the coarse aggregate. The identification of the surface area of the aggregate is another parameter to grade the aggregate. The specific surface area is nothing but the surface area per unit weight of the aggregate.
Deleterious particles affect the bondage of the concrete ingredients, thus resulting in reduced strength and durability of the concrete. The aggregates which may be used in concrete must be free from silt, clay, and other marine impurities.
Determination of the crushing value of an aggregate is helpful to find the compressive strength of the aggregate, whether it is suitable for the concrete work or not. One of the important properties of aggregates affects the overall strength of the structure.
When making comparisons of the efficiencies of different grinding and crushing machines it is desirable to be able to estimate the work actually done in crushing the ore from a given size of feed to a given size of product, the screen analysis of both feed and product being determined. Messrs. Klug and Taylor, in a paper on this subject, published in the monthly journal of the Chamber of Mines, have described a method adopted by them in calculations made in connection with a series of trials of grinding pans. Their method is based upon the assumption that if a quantity Q of material consisting of particles of average diameter x be crushed down until the average diameter of the particles is y then the work done in grinding is proportional to Q x/y. Thereis no theoretical basis for this assumption that the writer is aware of, and it does not appear to satisfy the fundamental conditions necessary. If the material be further crushed until the diameter become z, the additional work done is measured by Q y/z. Now the work done in crushing from diameter x to diameter z in one operation is proportional to Q x/z, and therefore we ought to have:
But these expressions are not in general equal to one another. In other words, according to the theory underlying this method, if we crush from diameter x to diameter y, and then from diameter y to diameter z, the total work done will be different to what it will be if we crush direct from diameter x to diameter z. This cannot be right. Further, if we make y = x, the expression for the work done still has the value Q, although in reality no work is done at all.
The method proposed in this paper enables such calculations to be made more simply, and rests upon the principle that if a quantity Q of material that will just pass through a screen of m meshes to the lineal inch be crushed until it will just pass through a screen of m meshes to the inch, then the work done in crushing is proportional to Q (n-m). This is founded upon the reasons following, and is also in agreement with the results of practical experiment.
Suppose that we have a quantity Q of material made up of particles of average diameter x, then the number of particles is proportional to the quantity divided by the volume of each particles i.e. to Q/x.Now, let these be crushed to some smallerdiameter y. If we suppose one of the original particles is a cube, whose side is x, and that this is divided up into cubes each of side y, the division may be affected by shearing along three sets of parallel planes at right angles as shown in Fig. 1. The number of such shearing planes parallel to one side is x/y 1 and these are each of area x. In order to divide up the original cube, then, along one set of these parallel planes the work required to be done is proportional to (x/y 1)x, and since the shearing has to be effected along three such sets of planes, the total work done is measured by 3(x/y 1)x. But these are Q/xparticles, and therefore the total work done in reducing a quantity Q from average diameter x to diameter y is measured byQ/x3(x/y 1)x, or leaving out the constant, Q (1/y 1/x). According to this, if we now further crush down to diameter z the additional work done is Q (1/z 1/y),and the total work done in crushing from diameter x to diameter z = Q(1/y 1/x) + Q(1/z 1/y), which = Q(1/z 1/x), thesame as if the crushing were all done in one operation.
To make the computation, then, it is necessary to determine the reciprocal of the diameter of the particle. But this is very nearly double the number of meshes per lineal inch of the screen through which the particle will just pass, as is seen from the following table for the I.M.M. standard laboratory screens:
It consequently follows that the crushing of a quantity Q of ore, consisting of particles of uniform size that will just pass through a screen of m meshes per lineal inch down to another uniform size, such that they will just pass through a screen of n meshes per lineal inch, requires an amount of work proportional to Q (n m).
a, per cent. of material just passing through p meshes to the inch b, per cent. of material just passing through q meshes to the inch c, per cent. of material just passing through r meshes to the inch and this be ground down to a product consisting of d, per cent. of material just passing through s meshes to the inch e,per cent. of material just passing through t meshes to the inch
the work done in grinding may be found by first of all computing the work required to reduce the feed down to some very small uniform size of, say, m meshes to the inch, next computing the work required to reduce the product of the machine down to the same size of m mesh, and then substracting the two. That is to say, the total work done in the machine is proportional to:
This gives us, then, a very simple rule for finding the work done in a grinding machine, which we may best describe by a numerical example. Suppose that we wish to compare the efficiencies of the grinding effected by two machines, the feed and resultant products of which grade as follows :
We have then to multiply each percentage by the number of meshes to which it corresponds. Thus, the material passing through 20 mesh, and caught on 40, will be considered to correspond to an average size of 30 mesh, and the percentage of this class of material is accordingly multiplied by 30. Regarding the material that passes through 160 mesh as equivalent to an average size of 180 mesh, we may arrange the computation as follows:
Thus the total work done in crushing by machine A is proportional to 5,808 x 20 = 116,160, and that by machine B to 6,103 x 17 = 103,751, and the crushing efficiencies of the two machines are in the proportion of 116 : 104 nearly.
The road aggregate is generally manufactured to a specified grading and stock piled at a sight. Before being used in the pavement it undergoes certain operations as loading and transporting in trucks, tipping, spreading and compacting which may result in a change in grading and or the production of excessive and undesirable fines. Significant degradation may take place when the aggregate is weak. Thus, an aggregate complying with a specification at the production site may fail to do so when it is in the pavement.
The aggregate crushing value is a numerical index of the resistance of an aggregate to crushing under a gradually applied compressive load. It is expressed as the percentage by mass of the crushed (or finer) material obtained when the test sample is subjected to a specified load under standard conditions. Aggregates with lower crushing value indicate a lower crushed fraction under load or higher strength and would provide a longer service life to the structure. Weaker aggregates if used in road pavements would get crushed under traffic loads and produce smaller pieces not coated with binder and these would be easily loosened out resulting in loss of the surface or layer. The aggregate crushing test is conducted as per IS : 2386-1963 (Part IV)
2. Case Hardened Test Mould consists of a 152 mm diameter open-ended steel cylinder with square base plate plunger having a piston of diameter 150 mm, with a hole provided across the stem of the plunger so that a rod could be inserted for lifting or placing the plunger in the cylinder.
The material for the standard test consists of aggregates of size 10 mm to 12.5 mm. The aggregates should be in surface dry condition before testing. The aggregates may be dried by heating at 100 degree to 110 degree Celsius for more than 4 hours and cooled to room temperature before testing, if necessary.
Step-2 : Take about 2 kg of aggregate prepared in Step-1 and fill the measuring cylinder with aggregate in three layers, each layer being tamped with 25 strokes of the rounded end of the tamping rod, distributing the strokes gently and evenly over the surface. The measuring cylinder is finally filled to overflow.
Step-6 : Transfer the whole of this weighed specimen to the test mould by filling it in three layers as explained in Step-2. The total depth of the sample is then about 100 mm and the surface a little below the top of mould.
Step-11 : Sieve the crushed aggregate through 2.36 mm IS sieve and weigh the fraction passing through it to an accuracy of 0.1 gram. This fraction is a measure of loss of material due to crushing. The mean value of two samples, rounded to nearest whole number is taken as the aggregate crushing value.
The values of aggregate crushing test agree in general with that of aggregate impact test. The crushing strength test is most significant as far as the conditions to which the aggregates is likely to be subjected in practice are concerned. The suitability of aggregate in the pavement layers is dependent upon the type of road construction. The specified limits of aggregate crushing value for different types of road construction are given in the table below.
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With the increasing production of construction and demolition waste (CDW), the use of CDW as a road base has become a sustainable and eco-friendly disposal approach. The aggregate crushing value is an essential index in the construction of road bases and is defined as the mass percentage of fine particles after a confined compression test of recycled CDW. In this study, experiments and numerical simulations were conducted to evaluate the crushing characteristics of the CDW, consisting of three main components, i.e., gravel, brick, and mortar. From the single-particle crushing tests, the gravel was strongest, and the mortar was weakest. Multi-particle confined compression tests showed that the loaddisplacement curves of the recycled aggregates exhibited an increasing trend. The fluctuations during loading were attributed to the particle breakage. A three-dimensional discrete element method (DEM) with a laser scanning and reconstruction technique was proposed to model irregular-shaped aggregate particles using a cluster of bonded particles. A comparison of the experimental and numerical results showed that the bond breakage ratio linearly increased with the aggregate crushing value, providing a quantitative linkage between the micro-scale numerical model and the macro-scale test. It was found that the volume proportion of the gravel significantly affected the mechanical behavior of the mixed aggregates, and 75% was the critical threshold of the gravel component. As gravel with a volume proportion from zero to 75% was added to the brick or mortar, the aggregate crushing value increased by only 1015%. In the mixed aggregates with brick and gravel, more of the brick particles were crushed when gravel with a proportion of 5075% was added due to inconsistent deformation and stress concentration. If the gravel proportion exceeded 75%, the aggregate crushing value was reduced by 30%. Based on the numerical and experimental investigations of the crushing characteristics of the recycled aggregates, a ternary diagram was proposed to design the proportion of the three components in recycled CDW for road bases.Get in Touch with Mechanic