Keywords

1 Introduction

The fine aggregates (i.e. 0.075 mm to 2.36 mm) used in the paving asphalt mixtures are usually consisted of stone chips, natural sand, or crushed sand. It is typically considered that the characteristics of the fines aggregate are influential to the performance of asphalt mixtures, especially on the rutting performance of the materials. Around the world, different researches drew different conclusions on this which leads to different emphasis on the material specifications. For example, (1) The specification of JTJ 032-94 (China) regulates that the fine aggregates of hot mix asphalt should be consisted of superior quality natural sands or crushed sands, of which the dosage of stone chips used in topping course or wearing course of the asphalt concrete of the expressway, first-grade highway should not exceed that of natural sands (2). The Asphalt Pavement Guide of Japan also rules that the dosage of stone chips should not exceed the dosage of natural sands in the design of mixtures (3). The Federal Highway Administration of America (FHWA) regulates that the dosage of natural sands in the total fine aggregates is 20 % approximately and also many highway authorities in the US require minimum values of FAA (43–45 %) to be used in bituminous mixtures on high traffic roads (4). In addition, the Superpave asphalt mixtures design method indicates the fine aggregates angularity (FAA) as the criteria for controlling the quality of fine aggregates (5). However, some scholars considered that, under different testing content or testing conditions, the value of FAA is not always the important factor that affects the performance of asphalt mixtures. (6) However, the Hong Kong specification does not put emphases on the properties of the fine aggregates other than specifying the requirements on gradations.

On the basis of the different viewpoints mentioned above, this study reviews the effects on the property of the asphalt mixture (i.e. moisture susceptibility) by varying the component proportion of fine aggregates (thus changing the value of FAA) by the adoption of the freeze-thaw splitting test of asphalt mixtures.

2 Experiment

2.1 Materials

The bitumen used in the study is SK-AH-70 obtained from China and the technical specifications of the material are shown in Table 1. The crushed gritsand was used as the coarse aggregates and its bulk specific gravity of coarse aggregate is 2.692. The fine aggregates composed of limestone chips and natural river sand. The two types of fine aggregates were mixed together according to different mass percentages to form different natural sand to total fine aggregates ratio (N/FA). The corresponding specific gravities of the different fine aggregates are shown and the associated FAAs (6) as tested based on ASTM C1252 are shown in Table 2. The mineral filler used in the mixtures was limestone powder and its apparent specific gravity was 2.720. A dense-grade aggregates gradation was selected for mixture design, and the associated gradation of the material is shown in Table 3.

Table 1 Main technical specification of asphalt
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Table 2 Bulk specification gravity and FAA value of fine aggregates
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Table 3 Aggregate gradation of asphalt concrete mixtures
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2.2 Determination of Asphalt Content of Mixtures

Based on the aggregates gradation shown in Table 3, the bitumen content of mixtures were determined according to the composed proportion of the two fine aggregates, with the two boundary levels of 10 and 50 % of N/FA ratio (being the lower and upper limits respectively). The mixtures samples were formed by the Marshall compaction method. The Marshall test results (stability, percent air voids, flow value and percent of the voids in mineral aggregate that are filled with asphalt of bituminous mixtures) indicate that the optimum asphalt aggregate ratio of 4.7 % was achieved at the level of 10 % N/FA, and 4.6 % was obtained at the level of 50 % N/FA. Finally the asphalt aggregate ratio of 4.7 % was selected for all experiments under this study.

2.3 Testing of Volume Specification

By keeping the composition of the coarse aggregates (sizes from 4.75 to 13.2 mm) the same, the proportions of the fine aggregates were adjusted in five levels, being from 10 to 50 % (i.e. with 10 % interval). The ratios of mineral aggregates are shown in Table 4. Two batches of Marshall samples were made at each level, of which the asphalt aggregate ratio of 4.7 % was adopted for all the samples of the mixtures. The bulk specific gravity of samples was obtained according to ASTM D 1188-96 (7), the percentage air voids (VV), percentage of the voids in mineral aggregate (VMA) were also pursued and the results are shown in Table 5.

Table 4 Mineral aggregate composing ratios
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Table 5 Volume specification of marshall samples
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2.4 Freeze and Thaw Split Tests

A series of freeze-thaw split tests were conducted (3 nos. samples were adopted for each test). One of the two batches samples was pre-conditioned according to ASTM D4867/D4867 M-96 (8), of which the samples were frozen under the condition of −18 °C for 16 h ± 1 h and thawed at 60 °C ± 0.5 °C for 24 h. Another batch of samples (control samples) was placed under the condition of room temperature. The moisture susceptibility was evaluated based on the ratio of freeze-thaw split strength (TSR). The splitting tests were conducted at the temperature of 25 °C and the loading rate was set as 50 mm/min. The results of freeze-thaw split tests are shown in Table 6.

Table 6 Testing results of freeze-thaw split
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2.5 Test Results

Based on the test results as shown in Table 5, the effects of N/FA on the volume properties are plotted in Figs. 1 and 2. The data indicates that, by varying the values of N/FA, the two volume properties of VV and VMA tend to vary considerably and when N/FA value is approximately at the level of 20 %, VV and VMA of samples appear to be the highest.

Fig. 1
figure 1

N/FA versus the percent air voids in asphalt mixtures (VV)

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Fig. 2
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N/FA versus the total voids in mineral aggregate (VMA)

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Based on the results of the freeze-thaw split tests, the ratio of split strength of the samples obtained under the freeze-thaw conditioning against the split strength of the control samples are demonstrated in Table 6 according to different N/FA ratios. Figures 3, 4, 5 and 6, respectively, illustrate how the FAA values, split strengths (RT), and TSR(s) were affected by varying the N/FA ratio. In the meanwhile, the effects of FAA on RT and TSR are shown in Figs. 6 and 7 respectively.

Fig. 3
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N/FA versus FAA FAA

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Fig. 4
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N/FA versus RT

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Fig. 5
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N/FA versus TSR

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Fig. 6
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FAA versus RT

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Fig. 7
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FAA versus TSR

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Several important trends from the results are observed, firstly, the FAA of the fine aggregates reduces considerably when the N/FA ratio increases and this trend is well expected as the increasing content of sand (with more rounded shapes) would reduce the overall angularity of the mixed fine aggregates. Secondly the split strengths (RT1 and RT2) and TSRs of the test samples varied significantly with the change in N/FA ratio, and it is shown that the optimum TSR (74.5 %) tends to be at the N/FA ratio of approximately 30 %, which corresponds to the ratio of natural sand to total mineral aggregates of about 13.2 % and the associated FAA value is 44.1. Thirdly, the increase in FAA seems to enhance the split strengths (in a rather vague extent) but it shows the inability of enhancing the TSRs and subsequently the moisture susceptibility. Hence it is opined that the FAA is only a general indicator used to qualify the macro physical characteristic of fine aggregates, as there may be many other factors that may contribute to the behavior of the bituminous mixtures. The story is further complicated if different components are present (i.e. more angular aggregates and sand which is less angular).

2.6 Interpretation

The shape of natural sand is generally rounder and the surface is usually smoother than the fine aggregates that are all composed of stone chips, so by varying the content of natural sand within the fine aggregates, the workability of the asphalt mixtures are expected to change accordingly. The most important factor of influencing the moisture susceptibility of asphalt mixtures, have been well recognised to be the (1) percent air voids in bituminous mixture which is affected by the workability of fresh-mix asphalt mixtures and (2) the adhering capacity between the bitumen and the aggregates.

A lower N/FA ratio is expected to produce a higher FAA, and the compaction of the bituminous mixtures would become difficult because of the poor workability, and the associated air void content is rather high and hence the TSR is rather low. With the increase in the value of N/FA ratio, the workability of the mixture would be higher and the compaction of the mixture could be correspondingly improved and thus the density of mixture would increase. Under such a condition, the volume of voids of the mixture would decrease and thus TSRs would increase correspondingly. When the N/FA ratio continually to increase, the workability of mixture would become higher and higher and finally it reaches a point that further increase in workability does not significantly reduce the Volume of Voids (i.e. change in VV (%) becomes steady and small) of the bituminous mixtures. At that point the TSR value would achieve the maximum value. Further decrease in air voids (i.e. due to the increase in workability) may not significantly affect the TSRs. Within such a range, the effect of varying air voids of mixture on changing the TSR value of mixture would not be significant. Whereas because of the increased content of the natural sand, and due to the relatively high overall roundness and smoothness of fine aggregates increase and thus it causes a reduction in the sorption of asphalt to the aggregates (last column of Table 5 refers). In the consequence of this, the proportion of adsorbed asphalt is reduced and in the meantime, the proportion of the free asphalt is increased correspondingly. As the bounding capacity of asphalt binder reduces, thus the moisture susceptibility becomes higher and hence the TSR decreases.

3 Conclusions and Recommendations

Several conclusions can be drawn after this study:

  1. 1.

    The composition of fine aggregates has apparent influence on the compaction characteristic of the bituminous mixture. The compacted characteristic and cohering characteristic of mixture that result from the variety of fine aggregate composition reduced the change of volume performance.

  2. 2.

    The existence of natural sand in the fine aggregates obviously cause variations on the FAA. However, it is considered that by viewing the FAA alone is not sufficient to justify or conclude on the possible performances of the subsequent bituminous mixtures formed (the moisture susceptibility is one example of the performance criteria under the study). Moreover, FAA may have different influences on the different performance criterion (rutting/fatigue, etc.) of the bituminous mixtures.

  3. 3.

    The composition of fine aggregate affects the moisture susceptibility of asphalt mixture in some degree. In a determined range, corresponding to the ratio of split strength of bituminous mixture, the N/FA ratio has an optimal value. This value is propitious to improve the workability of asphalt mixture and enhance the compacting degree of mixture, thus decrease the percent of air voids in asphalt mixture and reduce the moisture susceptibility. Further increase in N/FA ratio may cause a lower adhering ability between the bitumen and aggregate. For this study, the optimal value of N/FA is 30 %, approximately reach the 13 % in the total mineral aggregate.

The authors opine more detailed investigations are needed and worthwhile for further recommendations on the additional requirements of the fine aggregates (apart from gradation) used within the bituminous mixtures for construction of road pavement in Hong Kong.