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Experimental investigation on the effect of hydrated lime on mechanical properties of SMA
Construction and Building Materials 70 (2014) 379–387
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Experimental investigation on the effect of hydrated lime on mechanical properties of SMA A. Shafiei ⇑, M. Latifi Namin Tehran University, College of Engineering, School of Civil Engineering, Iran
h i g h l i g h t s Partial substitution of the filler with HL significantly improves the resistance indices of SMA. There is an optimum amount of hydrated lime in SMA mixture. Using HL as additive leads to superior performance comparing with partial substitution of HL. The HL modified SMA with 9.5 NMAS is much stronger against rutting compared to the SMA of 12.5 NMAS.
a r t i c l e
i n f o
Article history: Received 28 October 2013 Received in revised form 10 July 2014 Accepted 23 July 2014 Available online xxxx Keywords: Asphalt SMA Filler Hydrated lime Rutting Dynamic modulus Creep
a b s t r a c t One of the most important and costly damages to the pavements, especially in high traffic areas is rutting. An optimal use of lime is significantly important with regard to expensive cost of the lime and its improving properties in stone matrix asphalt (SMA). In this paper, simple performance tests (i.e. dynamic modulus, dynamic creep, and static creep) were used based on NCHRP-465 report. In which the effect of directly added different percentages of hydrated lime filler to the mixture in dry state as a partial substitute of the aggregate materials in comparison with introducing it as an additive to the mixture on performance and mechanical properties of SMA was assessed. For this purpose, the SMA samples with two different aggregate gradation and different hydrated lime contents were prepared as a part of filler. The obtained results indicate that partial substitution of the filler with hydrated lime will cause an improvement in the resistance indices like flow number, flow time as well as rutting and fatigue cracking factors of SMA, while the permanent deformations of the asphalt decrease. Obviously, it indicates existence of an optimum amount of hydrated lime in mixture. The results obtained from this contribution can be used to design and implement asphalt mixtures in accordance with the requirements of the contractors, engineers and researchers. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction The road network is usually described as the essential need of any country, so that this valuable asset consumes a noticeable amount of the national budget every year for development and maintenance. Therefore, taking into consideration the limitations of financial and technological resources, current situation of the roads must be maintained and even improved by spending much less money. Implementation of asphaltic pavements with continuous distribution of grain size would possibly lead to damages like rutting of wheel raceway and bleeding. These damages require maintenance and rehabilitation in a relatively short time after the road comes into service. This will involve spending of a lot of ⇑ Corresponding author. http://dx.doi.org/10.1016/j.conbuildmat.2014.07.084 0950-0618/Ó 2014 Elsevier Ltd. All rights reserved.
money. Some European countries, as well as US and Canada have decided to use stone matrix asphalt (SMA) in order to meet some special expectations of paving such as resistance against rutting, preventing expansion of reflective cracks. The SMA contains certain ingredients and changing material, mixing method and mix design of each component will alter mechanical properties of the mixture. In asphalt mixture with continuous graded aggregate, using filler, increase the contact points and the loading capacity, improve the compressive and shear strengths, and reduce the deflection. Filler in stone matrix asphalt plays an important role which is increasing stiffness of the mixture [1]. Different types of fillers including crushed stone, cement and lime can be used in the asphalt mixtures. The multifunctional effects of hydrated lime in open graded aggregate need to be evaluated when added as additive or part of filler aggregate in mixture.
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2. Objective Stone matrix asphalts (SMA) need a kind of open grain distribution for production and enhancement of bitumen content and contact between aggregates at the same time. In fact, a direct contact between the aggregates will cause stability. Application and development of the SMA was initially started within 1980s in some of European countries. After 1990 when the first SMA projects were started in Canada and later within some states of America in 1991, application of the SMA mixture became popular in these regions [2]. Different fillers are used in this mixture. Craus et al. showed in 1978 that the physiochemical properties are dependent on intensity of interfacial adsorption between the bitumen and the filler. Thus, an active surface of the filler was expected to create a strong composition between them [3]. Previous studies conducted by Kim in 1990, 1994, 1995 and 1999 as well as an earlier contribution of Bahia in 1999 indicates that the fatigue damage and the restorability depend heavily on the properties of the bitumen, properties of bitumen additives, interaction between bitumen and its additives, and ingredients which affect growth of the micro-cracks in the mastic [4]. According to the results obtained from the performance and volumetric tests, and also by establishing a relationship between the parameters extracted from these tests, finer filler will make the asphalt mixture stiffer and will eventually reduce the rutting phenomenon [5]. The crushed stone is a common filler material. Moreover, the hydrated lime is more effective filler in comparison with other mineral fillers because accumulation of greater micro-cracks in the sample from the initial loading until the final failure will increase fatigue life of the asphalt. The physiochemical interactions between the filler and the bitumen in terms of the fine particles and their surface features affect characteristics of the fatigue failure [6]. It was 2005 that the effect of hydrate lime on dynamic modulus and stiffness of the HMA mixture was reported in some studies [7], while some researches were conducted in 2008 and 2009 on the effect of hydrated lime on stripping and moisture damages due to the bitumen-lime filler interaction [8,9]. Meanwhile, some research works were directed in 2010 using hydrated lime in the HMA asphalt to examine the effect it has on the rutting phenomenon in addition to predict the cracking behavior in asphalt [10,11]. Also the hydrated lime is added by different techniques to the hot mix asphalt. The commonest methods for adding the lime to the mixture include dry, wet and slurry method [12]. Lime can be proportioned and mixed in HMA in both batch and drum mixers in the plant. Dry lime can be added to dry aggregate and to wet aggregate. Moisture levels in wet aggregate are typically about two to three percent above the saturated surface dried condition of the aggregate. Lime slurries made from hydrated lime or quicklime have also been used. Lime-slurried aggregates are conveyed directly to the drying and mixing portion of the HMA facility or placed into stockpiles for marination. Adding dry lime to the asphalt binder and storing the lime-modified binder prior to mixing with the aggregate has not been practiced widely in the field [13]. The effect of hydrated lime and mixing approach on performance properties of the HMA was evaluated in the same year [14–16]. One other study investigated the effect of optimal content of hydrated lime on aging and workability at high temperatures [17]. In 2011, Lee et al. employed modern experimental techniques and new models to study the effect of modified HMA with hydrated lime on rutting and fatigues cracks [11]. The hydrated lime will reduce the cracking as a result of aging, fatigues or low temperature. The cracking usually occurs after formation of micro-cracks. Fine particles of the hydrated lime are expected to prevent formation of these micro-cracks [7]. The
hydrated lime as filler improves stiffness and reduces rutting. Moreover, adding the hydrated lime will not allow the asphalt to stiffen at low temperatures because the lime will act as inactive mineral filler at these low temperatures. The hydrated lime will also reduce the oxidation and ageing effects [8]. As the HMA ages due to oxidation, hydrated lime reduces not only the rate of oxidation but also the harm created by the products of oxidation. This effect keeps the asphalt from hardening excessively and from becoming highly susceptible to cracking (through fatigue and low temperature (thermal) cracking). Synergistically, the filler effect of the hydrated lime dispersed in the asphalt improves fracture resistance and further improves cracking resistance [13]. There are many observations which show the hydrated lime modified HMA has better performance of mixture against rutting, fatigue and thermal cracking [18]. In 1987, Petersen et al. evaluated two asphalt binders modified with limestone and hydrated lime at 20% by weight of binder. The results of this research indicate that lime treatment would improve the resistance of the aged pavement to thermal cracking through the reduced aging index. Since the behavior of HMA mixtures at low temperatures is mainly controlled by the properties of the aged binder, lime treatment would produce HMA pavements that are highly resistant to thermal cracking [19]. The lime-modified bitumen demonstrates a greater potential for dissipating energy through deformation (at low temperature) than the unmodified bitumen. This is a positive effect at low temperatures because it reduces fracture potential. Although the filler effect increases low temperature stiffness, fracture toughness is substantially increased. Fracture toughness is the energy expended in fracturing a material [20]. Nevertheless, base on some studies, the influence of filler on low temperature cracking was hypothesized to be independent of filler type. Unlike rutting and fatigue cracking, adding different amount of hydrated lime as filler in mixture is not so effective on the thermal cracking [21,22]. Some studies have been conducted recently in 2013 toward failure mechanics and advanced suggested models, the effect of hydrated lime on failure performance, and the mechanical properties of mixtures [23,24]. Permanent deformation is one of the most critical parameters for pavement design, which is believed to increase with traffic and tiers pressure. Most of the permanent deformations occur at the upper layers rather than the subgrade. There are different approaches to determine the plastic deformation, out of which repeated load test is used more than the others [1]. In this study simple performance tests were used in order to evaluate performance of asphalt mixtures. Based on NCHRP-465 report, simple performance test (SPT) method(s) accurately and reliably measures a mixture response characteristic or parameter that is highly correlated to the occurrence of pavement distress (e.g., cracking and rutting) over a diverse range of traffic and climatic conditions. Numerous techniques have been suggested to assess pavement performance and distresses. One of the best available solutions for this purpose is to perform three simple performance tests, namely complex modulus (E*), dynamic creep – flow number test (Fn), and static creep – flow time test (Ft). They yield the best relationship between the experimental results and the field performance [25]. The complex modulus is a way of addressing the relationship between stress and strain of the viscoelastic material. The complex modulus test is often conducted on the cylindrical samples which are exposed to a haversine compressive loading [26]. The complex modules changes by variation in the loading frequency. A Loading capacity which models the traffic load much closer than the other frequencies must be chosen for this test. The dynamic modulus obtained in this regard is equivalent to resilient modulus which is used for design purposes [27].
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The main objective of the dynamic creep test is to study asphalt concrete performance against rutting phenomenon. In other words, using the repeated dynamic load test in several thousand runs and recording the accumulative strain as a function of the cyclic loading during the test is a method to determine the permanent deformation of asphalt concrete [28]. The static creep test is not only useful for finding the plastic properties of the material, but also can be used to measure the permanent deformations of the mixture. There are many researches investigating the effect of different filler on dense grade mixtures. But there are a few studies containing the investigation on different content of hydrated lime as partial substitution of filler in SMA in which total amount of filler has considerable effect on open graded structure of stone matrix asphalts mixtures. In this paper the effect of adding different contents of hydrated lime as the effective element of open-graded asphalts is investigated on the mechanical properties of SMA through SPT examinations.
Table 3 Physical properties of the used hydrated lime. Property
Value
Specific gravity Dry brightness, G.E. Mean particle size PH BET surface area 100 Mesh 200 Mesh Apparent dry bulk density (loose) Apparent dry bulk density (packed)
2.343 92% 2 lm 12.6 22.0 m2/g 94% 90% 20 lbs./ft.3 30 lbs./ft.3
Table 4 Chemical properties of the used hydrated lime.
3. Methodology Mechanical properties of the lime-containing SMA mixtures are evaluated in this research. For this purpose, quality and type of the needed materials are described first and then, grain distribution and preparation of the samples are explained. AT last, method of performing the tests is introduced along with the reason to use the initial parameters for measurement of the performance indexes and the mixture properties. Meanwhile, some control samples are also made without hydrated lime addition to the SMA samples to examine change in the mixture properties. Two groups of samples were used based on two types of fillers and two types of grain distributions. The optimal bitumen content was extracted for each group by making the test samples and running the Rice and Marshal Tests. The main samples were made by using the optimal content of bitumen in each of the groups and were further checked via dynamic modulus, dynamic creep and static creep tests.
Property
Value
Ca(OH)2 CaO-equivalent CaO total CaCO3 Insoluble in acid CaSO4 Free H2O MgO SiO2 Al2O3 Fe2O3 SO3 P2O5 Na2O MnO
0.91 70 72 2 2 0.6 0.8 0.5 3 0.3 0.3 0.2 0.05 0.04 0.0045
The crushed stone is used as the filler for some samples which are made just for comparison with the lime-containing samples. Tables 3 and 4 list chemical and physical properties of the hydrated lime filler, respectively.
3.1. Materials For making the asphalt samples, a pea gravel of definite specifications (Table 1) was used with the required quality tests being performed (Table 2) and the 60/70 bitumen as an adhesive for the gravel aggregates. Some fibers may be used to avoid drain down of the bitumen in the SMA mixtures due to its great content. VIATOP cellulose fibers were used in this study which was composed of 90% ARBOCEL ZZ and 10% bitumen. For better mixing of them with the mixture, it is recommended to powder them first and mix them for 1 min with hot gravel of the mixture and add the bitumen at the end.
3.2. Aggregate gradation The suggested grain size distribution of NCHRP 9-8 report was used to address this parameter in making the SMA samples [2]. Taking into account the relatively large range of the applied SMA asphalt and its being practical, the nominal sizes of 9.5 and 12.5 mm were selected for the first and second gradations, respectively (Table 5). 3.3. Sample preparation
Table 1 Aggregate specific gravities (kg/m3). Grain-size fraction
Coarse aggregate (AASHTO T85) Fine aggregate (AASHTO T84) Filler aggregate (AASHTO T100) Total bulk specific gravity of aggregate
Specific gravity
Water absorption (%)
Apparent specific gravity
Bulk specific gravity
2699
2499
2.5
2716
2529
2.7
2653
–
–
2514
–
With respect to the previous studies, the SMA samples were decided to be made from 5 different contents of lime and to be tested in 5 different temperatures. In each group and after defining the optimal bitumen content, this value has been kept constant and just the hydrated lime content was changed. The lime contents were chosen from 0 to 7.5 wt% of the aggregate materials increasing in ranges of 2.5 wt%, in which the mixtures with zero (0%) percentage of hydrated lime were considered as control mix to compare the results and make conclusion in each case (Fig. 1). The hydrated lime, crushed stone filler and the other aggregates were initially heated at 180 °C in the oven and they were later mixed with the bitumen at 155 °C. This method is used in the field as was described as adding dry hydrated lime to the aggregates before blending with the bitumen that prepares aggregate surface for better bonding condition. In order to evaluate the simultaneous effect of total filler content and lime content of the mixture, a sample of 10% crushed stone filler and 2% hydrated lime as an additive (in terms of total mixture weight, and 2.1% in terms of aggregate weight) was prepared. Therefore, after heating of
Table 2 The results of tests performed on aggregate. Properties
Test method
Value (%)
Allowable value
L.A. Abrasion Coarse aggregate with one fractured faces Coarse aggregate with two fractured faces Flat and elongated particles of coarse aggregates with ratio of 1:3 Flat and elongated particles of coarse aggregates with ratio of 1:5
AASHTO T96 ASTM D5821 ASTM D5821 ASTM: D4791 ASTM: D4791
16 95 91 14 2
Max 30% 100% Min 90% Max 20% Max 5%
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Table 5 Used gradation and limits for SMA. Sieve size (mm)
3.5. Dynamic creep test
9.5 mm NMAS
19 12.5 9.5 4.75 2.36 1.18 0.6 0.3 0.075 ⁄
The dynamic creep test was done by UTM5 according to NCHRP 9-19 in unconfined condition at 50 °C. A haversine loading of 138 kPa was applied with 0.1 s and 0.9 s as loading time and rest time, respectively.
Passing (%) 12.5 mm NMAS
Used
Lower
Upper
Used
Lower
Upper
– 100 95 45 25 18 16 14 10⁄
– 100 90 26 20 13 12 12 8
– 100 100 60 28 21 18 15 10
100 95 67 28 22 19 17 14 10⁄
100 90 26 20 16 13 12 12 8
100 100 78 28 24 21 18 15 10
10% Filler for both gradation (the equal amount in gram).
CS
The dynamic modulus test was performed on the cylindrical unconfined samples under a haversine compressive loading and the loading pulse according to the NCHRP has a haversine shape. The applied frequencies are 25, 10, 1, 0.5 and 0.1 Hz, while the 25 Hz frequency is considered as the pre-loading. Number of the loading cycles associated with each frequency are 206, 106, 56, 25, 6 and 6, respectively. A rest time of 1 min long is considered between every two frequencies, so that the samples could have their recoverable strains. The static and dynamic stresses are selected for each sample regarding the loading temperature at 5, 15, 25, 35 and 45 °C, respectively and finally, the master curves was shown for dynamic modulus and phase angle.
4. Results and discussion
%filler by percentage of aggregate weight
HL
3.6. Dynamic modulus test
HL as addive
4.1. Results of complex modulus test 2.0% 2.5% 5.0% 7.5% 10% 10.0%
The complex modulus testing was used to address viscoelastic and stiffness properties of the mixture. The master curve of dynamic modulus was derived here by using the equation proposed in NCHRP-614 report [29] (Figs. 2 and 3). The master curve of phase angle was also extracted using the equation suggested by Zeng et al. in 2001 [30] for all the frequencies at 25 °C.
7.5% 5.0%
Temperature shi factor 3
2.5%
temperature degree Regression Line
Control Samples
A2, A7
A3, A8
A4, A9
A5, A10
2
the gravel aggregates and mixing of the bitumen, 2% dry hydrated lime was added to the mixture just after fully coating aggregates with bitumen. This method was used in order to easier mixing procedure, better coating aggregate surfaces and also, prevention of enveloped hydrated lime in asphalt binder. Just one sample was enough for each of the 5 testing temperatures considering the non-destructive nature of the dynamic modulus test [29]. The SMA samples were prepared for this research with two different grain size distributions and also different contents of the hydrated lime filler according to Table 6. The Rice test was done to determine the optimum binder percent to satisfy 4% air void in the mixture, while the drain down test was implemented on the samples. Samples were compacted using gyratory compactor. At last, samples of 100 mm diameter and 150 mm height were made.
Log shi factor a (T)
Fig. 1. Increase in percentage of HL (hydrated lime) filler along with CS (crushed stone) filler in mixtures.
1
0
0
5
10
15
30
35
40
45
50
y = 0.0004x2 - 0.1378x + 3.1964 R² = 1
-3
NCHRP 9-19 Superpave Models were used for unconfined samples at 50 °C. A deviator stress of 138 kPa was lasted until failure and thereafter, the sample entered the third phase.
25
-1
-2
3.4. Static creep test
20
Temperature, °c
Fig. 2. Shift factor versus temperature for SMA with9.5 mm NMAS and 2% lime as additive.
Table 6 Methodology for combined mixtures for all conditioned samples. Sample
NMAS (mm)
Crushed stone filler (%)
Substituted hydrated lime (%)
Additive% (by mixture weight)
Bitumen content (%)
A1 A2 A3 A4 A5 A6 A7 A8 A9 A10
9.5 9.5 9.5 9.5 9.5 12.5 12.5 12.5 12.5 12.5
10 7.5 5 2.5 10 10 7.5 5 2.5 10
0 2.5 5 7.5 0 0 2.5 5 7.5 0
0.03% Fiber 0.03% Fiber 0.03% Fiber 0.03% Fiber 0.03% Fiber 2% lime 0.03% Fiber 0.03% Fiber 0.03% Fiber 0.03% Fiber 0.03% Fiber 2% Lime
6.5 6.5 6.5 6.5 6.5 7 7 7 7 7
Three samples were made for each combination and average of results were used.
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Fig. 3. Master curve with shifted E* data for SMA with 9.5 mm NMAS and 2% lime as additive.
Fig. 4. Master curves of SMA mixtures with 9.5 mm NMAS and different hydrated lime contents at reference temperature of 25 °C.
Fig. 5. Master curves of SMA mixtures with NMAS 12.5 mm and different hydrated lime contents at reference temperature of 25 °C.
383
4.1.1. Dynamic modulus HL has a multifunctional effect on mixture. It can be seen from the dynamic modulus values in different temperatures and frequencies for mixtures of different contents that the effect of hydrated lime on dynamic modulus of the SMA mixture and stiffness index of the asphalt is really small at high temperatures and low frequencies. Comparison of the dynamic modulus master curves at low frequencies for mixtures of 9.5 mm nominal size (Fig. 4) indicates just a negligible rise of dynamic modulus by increasing the hydrated lime content up to 2.5% and decreasing it by further addition of the hydrated lime. This effect is rather different in the mixtures of 12.5 mm nominal size (Fig. 5) somehow; by increasing the hydrated lime up to 2.5%, the dynamic modulus reaches to its maximum value at high temperatures or low frequencies, and increasing the hydrated lime at low temperatures or high frequencies reduces the dynamic modulus. To be more exact, Presence of HL as active filler at high temperature and as inactive filler at low temperature causes physiochemical combination with bitumen which reduces the interval between maximum and minimum values of the dynamic modulus, and improve simultaneously its stiffness at high and low temperatures. This also may attributed to the fact that the specific surface area and surface energy of hydrated lime is much larger than mineral powder, resulting in the selection absorption of light molecular weight compounds of asphalt [17]. 4.1.2. Phase angle Phase angle is indicative of viscous characteristics of the mixture due to mastic properties in dense graded structures. Comparing HL contained mixtures with control samples, by replacement of HL in the mixture a slight reduction has occurred in the viscosity and phase angle of SMA (Figs. 6 and 7). Addition of the lime to the mixture as a part of aggregate or using as additive in the mixture seem to reduce slightly the degree of phase angle and viscosity of SMA. This may be due to the fact that the structure of SMA is based on direct contact of aggregate while the mastic only fills the voids between coarse aggregates and hold the structure. Therefore, when the mastic stiffens elastic property of coarse aggregate influences more effectively on viscosity of the SMA. However, increasing the temperature will increase elastic effect of gravel in the mixture, while reduce the degree of phase angle which implies reduced viscous effects of the mastic. In the SMA mixtures this effect is intensified due to the direct contact of the gravels especially at high temperatures [28]. That is why the degree of phase angle in the SMA mixtures at high temperatures is simultaneously affected by the presence of hydrated lime and interaction of the gravels on each other. Those indices involving the phase angle are thus unable to evaluate the presence of hydrated lime in the mixture properly at high temperature.
Fig. 6. Phase angle master curves for SMA mixture with NMAS 9.5 mm and different lime contents at reference temperature of 25 °C.
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4.1.3. Evaluation of rutting factor E*/sinf is the parameter obtained from the dynamic modulus test and can be used in the frequencies of 5 and 10 Hz as the index of rutting resistance. This parameter should not be smaller than the minimum requirements of Superpave. Loading at the rate of 0.1 s (10 Hz) in the laboratory, is usually indicative of the traffic speed in field [31]. The results of comparison between the rutting resistance which are obtained from the dynamic modulus test and rut depth estimation experiments like Hamburg wheel tracking device show that the parameter of E*/sinf represent correctly rutting resistance in the HMA mixture only at high temperatures [32]. This parameter was used as a criterion for evaluating rutting resistance of SMA in this research (Table 7). The results of this
Fig. 7. Phase angle master curves for SMA mixture with NMAS 12.5 mm and different lime contents at reference temperature of 25 °C.
Table 7 Superpave factors for rutting and fatigue cracking in two frequencies. % HL
Control
NMAS (mm)
9.5
12.5
9.5
12.5
9.5
12.5
9.5
12.5
9.5
12.5
411 356 251 160
354 321 309 292
458 391 307 270
441 386 240 235
443 398 246 213
435 388 244 189
420 370 177 201
432 367 137 134
420 352 276 224
366 346 217 185
|E*|/sin(f) at 45 °C |E*| sin(f) at 25 °C
10 Hz 5 Hz 10 Hz 5 Hz
2.5% HL
5% HL
7.5% HL
2% HL additive
Fig. 8. Rutting factor for SMA mixtures with 9.5 mm NMAS and different hydrated lime contents at 45 °C.
Fig. 9. Rutting factor for SMA mixtures with 12.5 mm NMAS and different hydrated lime contents at 45 °C.
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Nom 9.5 mm
350
effect of interaction of the gravels on the phase angle at 45 °C. Nevertheless, this effect becomes significant at highly elevated temperatures.
300
4.1.4. Evaluation of fatigue crack factor For fatigue cracking, a performance factor in the Superpave binder specification is |G⁄|sind. Thus, the equivalent performance factor for the mix is |E⁄|sinf. [28]. Reduction of the fatigue factor with increasing the lime content indicates that hydrated lime will enhance the resistance against fatigue cracking (Table 7). Meanwhile, the range of fatigue factor values for the mixtures with nominal size of 9.5 mm at 10 Hz frequency implies improvement of the fatigue cracking by substituting the hydrated lime for more than 5% (Fig. 10). Fig. 11 shows that the results for samples of 12.5 mm nominal size are almost the same, except that there is a growth in the resistance against fatigue cracking by increasing the hydrated lime from 0 to 7.5%. A comparison between fatigue cracking factor of two mixtures of different nominal sizes reveals that the values of |E*| sinf for the samples of 12.5 mm is smaller than those of the 9.5 mm samples. It can be explained that, the internal pores of the HL particles are filled with bitumen, and this filled particles are seen as hard spheres in the bitumen matrix, therefore prevents growth of micro crack in mastic [33]. It seems that the more content of Hl the more fatigue cracking resistance will be observed in SMA which does not show an optimum content of HL in SMA.
|E*|.sinf
250
200
150
100
50 0% lime
5% lime
2.5% lime
7.5% lime
2% lime as addive
0 0
2
4
6
8
10
12
Frequency, HZ Fig. 10. Fatigue cracking factor for SMA mixtures with 9.5 mm NMAS and different hydrated lime contents at 25 °C.
4.2. Creep test Dynamic and static creep tests were done to explore resistance of the samples against the common phenomena of rutting and permanent deformations. Two indices of flow number and flow time are the main parameters for evaluation of the rutting phenomenon. As was expected before, the results which were obtained from the both experiments represent the same trend for the effect of lime on rutting. Due to physical features of HL particles the distribution of HL throughout the binder is better, as the HL particles are much finer than most common fillers. Finer particles have more surface area for a given unit volume, thus providing more surface interactions with asphalt. Therefore, the finer the filler, the more it potentially stiffens the asphalt. The stiffening effect of filler is dependent on Rigden voids of the filler. The voids in the compacted filler combine the effects of particle density, particle shape, and particle size distribution in the filler fraction [34]. It can be seen from Table 8 that increasing the hydrated lime filler content of the SMA samples has reinforced the resistance indices of rutting Fn and Ft, also decreased the permanent deformation of these samples against the cyclic dynamic load. Increasing the filler content from 0% to 5% enhanced rutting resistance of the mixture. Stiffer asphalt binder provides more holding ability to maintain the aggregate structure leading to less permanent deformation. As is demonstrated in Figs. 12 and 13, control mixes reached a stable slope faster; this is likely attributable to less stiff asphalt mastic that allows faster densification and a steep slope during secondary phase where the rate of accumulation of permanent deformation remains constant.
Fig. 11. Fatigue cracking factor for SMA mixtures with 12.5 mm NMAS and different hydrated lime contents at 25 °C.
evaluation at 45 °C confirm improvement of the rutting resistance with increasing substituted hydrated lime filler content up to 5%. In both aggregate gradation (Figs. 8 and 9), the mixtures containing 5% lime at the frequency of 5 Hz provide a superior resistance in comparison with the 2.5% lime sample, likewise the results of the creep test. However, this value shows just a small different at the frequency of 10 Hz. Unlike the results of the creep test, adding 2% of the lime filler to the mixture as an additive has not much effect in raising the rutting resistance. It seems that much reasonable results will be generated by creep test due to destructive nature of this test which allows no recovery during the test to the sample [32]. The close similarity between the evaluation of rutting factor results (E*/sinf) and the indices of creep test implies a negligible
Table 8 Dynamic and static creep test results at 50 °C. % HL
Control
NMAS (mm)
9.5
12.5
2.5% HL 9.5
12.5
5% HL 9.5
12.5
9.5
7.5% HL 12.5
9.5
2% HL additive 12.5
Flow number Flow time Total permanent deformation (mm)
425 334 1.81
349 312 1.66
1075 631 2.10
427 400 2.34
2983 5012 1.94
2295 1259 2.48
2375 1933 2.40
1123 626 2.10
2135 1996 2.29
1527 798 3.09
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Fig. 12. Dynamic creep curves for 9.5 mm NMAS mixtures at 50 °C.
Fig. 13. Dynamic creep curves for 12.5 mm NMAS mixtures at 50 °C.
3500 nom 9.5 nom 12.5
3000 2500
flow num
Based on experimental observations adding HL more than 5% in the mixture changed the trend of permanent deformation resistance. It is possible that when more HL is added along constant amount of bitumen the filler surface absorption increases and the absorbed filler particles gather into a group that may leads to deterioration of interface bonding between aggregates and lower cohesion. On the other hand, excessive amount of HL reduces coat of coarse aggregates which affects reorientation of aggregate particles in compaction process and structure of mixtures that results in less cohesiveness and less rutting resistance. It is also clearly observed that the grain distribution of 9.5 nominal size gives a greater flow number in comparison with the grain distribution of 12.5 nominal size (Figs. 14 and 15) because Uniform coat of coarse aggregate and film thickness lead to better aggregate interaction and strong structure in SMA. By comparing the results of 2.5% HL content mixes with the mixes in which 2% HL has been added as additive, it can be concluded that the amount of total filler in mixture and the method of adding HL filler considerably affect SMA performance. This may attributed to the fact that in SMA with open graded structure, 10% crushed stone filler along 2% HL additive enhances filling up the voids between coarse aggregate and improves aggregate contacts. However, introduction of 2% dry HL to the mixture after full aggregate coating by bitumen mostly stiffens the asphalt binder, While when 2.5% HL filler is replaced in the mixture, after mixing, not all of the hydrated lime is in ‘‘direct’’ contact with the surface of the aggregate, but some becomes part of the binder itself or fills
2000 1500 1000 500 0 Control
(2.5,7.5)
(5,5)
(7.5,2.5)
(2,10)
(HL (%), CS (%)) Fig. 14. Flow number for mixtures with different percentage of HL and CS filler. ⁄ HL: hydrated lime filler, CS: crush stone filler.
up the air void between aggregate. As a consequence, lower interface bonding strength occurs and mastic stiffness decreases which leads to less rutting resistance compared with 2% HL additive in mix.
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6000 nom 9.5 nom 12.5
5000
Flow me
4000
3000
2000
1000
0
Control
(2.5,7.5)
(5,5)
(7.5,2.5)
(2,10)
(HL (%), CS (%)) Fig. 15. Flow time for mixtures with different percentage of HL and CS filler. ⁄HL: hydrated lime filler, CS: crush stone filler.
5. Conclusions Mechanical properties of SMA with regard to different content of HL (Hydrated Lime) were questioned. Simple performance tests such as creep tests and dynamic modules were conducted in order to investigate how the different percentage of hydrated lime in different HL mixing method affect performance of mixtures with open grain size distribution. The following results may be drawn. According to obtained results of creep tests, presence of hydrated lime reduced rutting and rut depth much more than control mixtures. In both aggregate gradations (9.5 NMAS, 12.5 NMAS) Rutting resistance increases as the HL percent rise up to optimum amount of HL. It was demonstrated that in SMA, increasing HL more than 5% changes the trend and reduces rutting indices. In both gradation (9.5 NMAS, 12.5 NMAS), presence of HL effect slightly on |E*| and phase angle. It is hypothesized that in SMA with direct contact of aggregate HL has negligible effect on viscosity and stiffness of SMA mixtures. The results of this research indicate that total percentage of filler and mixing method considerably impact on performance of SMA. As a result, using HL as additive leads to superior performance comparing with partial substitution of HL for filler aggregate. The obtained measures from E*/sinf do not establish properly a relationship with the results of creep test in order to evaluate the rutting potential of the SMA mixtures at high temperatures below 50 °C. It is thought that, E*/sinf and E* sinf do not represent the performance of SMA mixture as well as creep tests and related fatigue tests.
References [1] Huang YH. Pavement analysis and design. 2nd ed. Upper Saddle River, NJ: Prentice Hall; 2003. [2] Brown ER, Cooley LA. Designing stone matrix asphalt mixtures for rut-resistant pavements. Transport Res Board 1999;425. [3] Craus J, Ishai I, Sides A. Some physico-chemical aspects of the effect and the role of the filler in bituminous paving mixtures. Assoc Asphalt Paving Technol Proc 1978;47.
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Constr Build Mater May 2009;23(5):1999–2006. [9] Airey GD, Collop AC, Zoorob SE, Elliott RC. The influence of aggregate, filler and bitumen on asphalt mixture moisture damage. Constr Build Mater September 2008;22(9):2015–24. [10] Aragão FT, Lee J, Kim YR, Karki P. Material-specific effects of hydrated lime on the properties and performance behavior of asphalt mixtures and asphaltic pavements. Constr Build Mater 2010;24(4):538–44. [11] Lee S, Seo Y, Kim YR. Effect of hydrated lime on dynamic modulus of asphalt– aggregate mixtures in the state of North Carolina. KSCE J Civ Eng November 2010;14(6):829–37. [12] Button JW, Epps JA. Evaluation of methods of mixing lime in asphalt paving mixtures. Texas Transportation Institute; 1983. [13] Little DN, Epps JA, Sebaaly PE. The benefits of hydrated lime in hot mix asphalt. Natl Lime Assoc 2001. [14] Lee S, Mun S. Performance based evaluation of lime addition methods in hot mix asphalt. Can J Civ Eng February 2012;39(2):172–9. [15] Diab A, You Z, Othman A, Ahmed H. Effect of hydrated lime application method on mechanical and fatigue properties of HMA. In: CICTP, American Society of Civil Engineers; 2012. p. 3327–34. [16] Albayati AH. Mechanistic evaluation of lime-modified asphalt concrete mixtures. In: Scarpas A, Kringos N, Al-Qadi I, Loizos A, editors. 7th RILEM international conference on cracking in pavements. Netherlands: Springer; 2012. p. 921–40. [17] Chen J, Zhou L, Yan ZH. Hydrated lime content in asphalt mastic based on high temperature performances. Appl Mech Mater November 2010;40–41:669–74. [18] Huang JF, Wu SP, Ma LX, Liu ZF. Material selection and design for moisture damage of HMA pavement. Mater Sci Forum 2009;614:269–74. [19] Petersen JC, Plancher H, Harnsberger PM. Lime treatment of asphalt to reduce age hardening and improve flow properties. In: Association of asphalt paving technologists proceedings technical sessions, vol. 56, 1987, Reno, Nevada, USA, 1987. [20] Johansson LS. Bitumen ageing and hydrated lime. 1998. [21] Meininger RC. Effects of aggregates and mineral fillers on asphalt mixture performance. ASTM International; 1992. [22] Aschenbrener T. Investigation of low temperature thermal cracking in hot mix asphalt. No. CDOT-DTD-R-95–7, February 1995. [23] Vansteenkiste S, De Visscher J, Vanelstraete A. Impact of hydrated lime on the durability of SMA mixtures: laboratory and field evaluation. Road Mater Pavement Des 2013;14(Suppl.1):162–74. [24] Zou J, Isola M, Roque R, Chun S, Koh C, Lopp G. Effect of hydrated lime on fracture performance of asphalt mixture. Constr Build Mater July 2013;44:302–8. [25] Witzcak MW. Simple performance test for superpave mix design. Transport Res Board 2002;465. [26] Papazian HS. The response of linear viscoelastic materials in the frequency domain with emphasis on asphaltic concrete. In: International conference on the structural design of asphalt pavements, vol. 203, 1962. [27] Clyne TR, Li X, Marasteanu MO, Skok EL. Dynamic and resilient modulus of Mn/ DOT asphalt mixtures, 2003. [28] Kim Y. Modeling of asphalt concrete. McGraw Hill Professional; 2008. [29] Bonaquist RF. Refining the simple performance tester for use in routine practice. Transport Res Board 2008;614. [30] Zeng M, Bahia HU, Zhai H, Anderson MR, Turner P. Rheological modeling of modified asphalt binders and mixtures (with discussion). J Assoc Asphalt Paving Technol 2001;70. [31] Feng X, Hao P. Summarization on mixture design method of bituminous stabilized broken-stone base mixture. Highway 2007;6:036. [32] Chen Z, Zhang X, Cong L, Lu H, Yang J. Rutting resistance performance evaluation of superpave mixes with antirutting additives, In: Transportation Research Board 89th Annual Meeting, 2010. [33] Lesueur D, Petit J, Ritter H-J. The mechanisms of hydrated lime modification of asphalt mixtures: a state-of-the-art review. Road Mater Pavement Des 2013;14(1):1–16. [34] Zulkati A, Diew WY, Delai DS. Effects of fillers on properties of asphaltconcrete mixture. J Transport Eng 2011;138(7):902–10.
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Experimental investigation on the effect of hydrated lime on mechanical properties of SMA A. Shafiei ⇑, M. Latifi Namin Tehran University, College of Engineering, School of Civil Engineering, Iran
h i g h l i g h t s Partial substitution of the filler with HL significantly improves the resistance indices of SMA. There is an optimum amount of hydrated lime in SMA mixture. Using HL as additive leads to superior performance comparing with partial substitution of HL. The HL modified SMA with 9.5 NMAS is much stronger against rutting compared to the SMA of 12.5 NMAS.
a r t i c l e
i n f o
Article history: Received 28 October 2013 Received in revised form 10 July 2014 Accepted 23 July 2014 Available online xxxx Keywords: Asphalt SMA Filler Hydrated lime Rutting Dynamic modulus Creep
a b s t r a c t One of the most important and costly damages to the pavements, especially in high traffic areas is rutting. An optimal use of lime is significantly important with regard to expensive cost of the lime and its improving properties in stone matrix asphalt (SMA). In this paper, simple performance tests (i.e. dynamic modulus, dynamic creep, and static creep) were used based on NCHRP-465 report. In which the effect of directly added different percentages of hydrated lime filler to the mixture in dry state as a partial substitute of the aggregate materials in comparison with introducing it as an additive to the mixture on performance and mechanical properties of SMA was assessed. For this purpose, the SMA samples with two different aggregate gradation and different hydrated lime contents were prepared as a part of filler. The obtained results indicate that partial substitution of the filler with hydrated lime will cause an improvement in the resistance indices like flow number, flow time as well as rutting and fatigue cracking factors of SMA, while the permanent deformations of the asphalt decrease. Obviously, it indicates existence of an optimum amount of hydrated lime in mixture. The results obtained from this contribution can be used to design and implement asphalt mixtures in accordance with the requirements of the contractors, engineers and researchers. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction The road network is usually described as the essential need of any country, so that this valuable asset consumes a noticeable amount of the national budget every year for development and maintenance. Therefore, taking into consideration the limitations of financial and technological resources, current situation of the roads must be maintained and even improved by spending much less money. Implementation of asphaltic pavements with continuous distribution of grain size would possibly lead to damages like rutting of wheel raceway and bleeding. These damages require maintenance and rehabilitation in a relatively short time after the road comes into service. This will involve spending of a lot of ⇑ Corresponding author. http://dx.doi.org/10.1016/j.conbuildmat.2014.07.084 0950-0618/Ó 2014 Elsevier Ltd. All rights reserved.
money. Some European countries, as well as US and Canada have decided to use stone matrix asphalt (SMA) in order to meet some special expectations of paving such as resistance against rutting, preventing expansion of reflective cracks. The SMA contains certain ingredients and changing material, mixing method and mix design of each component will alter mechanical properties of the mixture. In asphalt mixture with continuous graded aggregate, using filler, increase the contact points and the loading capacity, improve the compressive and shear strengths, and reduce the deflection. Filler in stone matrix asphalt plays an important role which is increasing stiffness of the mixture [1]. Different types of fillers including crushed stone, cement and lime can be used in the asphalt mixtures. The multifunctional effects of hydrated lime in open graded aggregate need to be evaluated when added as additive or part of filler aggregate in mixture.
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2. Objective Stone matrix asphalts (SMA) need a kind of open grain distribution for production and enhancement of bitumen content and contact between aggregates at the same time. In fact, a direct contact between the aggregates will cause stability. Application and development of the SMA was initially started within 1980s in some of European countries. After 1990 when the first SMA projects were started in Canada and later within some states of America in 1991, application of the SMA mixture became popular in these regions [2]. Different fillers are used in this mixture. Craus et al. showed in 1978 that the physiochemical properties are dependent on intensity of interfacial adsorption between the bitumen and the filler. Thus, an active surface of the filler was expected to create a strong composition between them [3]. Previous studies conducted by Kim in 1990, 1994, 1995 and 1999 as well as an earlier contribution of Bahia in 1999 indicates that the fatigue damage and the restorability depend heavily on the properties of the bitumen, properties of bitumen additives, interaction between bitumen and its additives, and ingredients which affect growth of the micro-cracks in the mastic [4]. According to the results obtained from the performance and volumetric tests, and also by establishing a relationship between the parameters extracted from these tests, finer filler will make the asphalt mixture stiffer and will eventually reduce the rutting phenomenon [5]. The crushed stone is a common filler material. Moreover, the hydrated lime is more effective filler in comparison with other mineral fillers because accumulation of greater micro-cracks in the sample from the initial loading until the final failure will increase fatigue life of the asphalt. The physiochemical interactions between the filler and the bitumen in terms of the fine particles and their surface features affect characteristics of the fatigue failure [6]. It was 2005 that the effect of hydrate lime on dynamic modulus and stiffness of the HMA mixture was reported in some studies [7], while some researches were conducted in 2008 and 2009 on the effect of hydrated lime on stripping and moisture damages due to the bitumen-lime filler interaction [8,9]. Meanwhile, some research works were directed in 2010 using hydrated lime in the HMA asphalt to examine the effect it has on the rutting phenomenon in addition to predict the cracking behavior in asphalt [10,11]. Also the hydrated lime is added by different techniques to the hot mix asphalt. The commonest methods for adding the lime to the mixture include dry, wet and slurry method [12]. Lime can be proportioned and mixed in HMA in both batch and drum mixers in the plant. Dry lime can be added to dry aggregate and to wet aggregate. Moisture levels in wet aggregate are typically about two to three percent above the saturated surface dried condition of the aggregate. Lime slurries made from hydrated lime or quicklime have also been used. Lime-slurried aggregates are conveyed directly to the drying and mixing portion of the HMA facility or placed into stockpiles for marination. Adding dry lime to the asphalt binder and storing the lime-modified binder prior to mixing with the aggregate has not been practiced widely in the field [13]. The effect of hydrated lime and mixing approach on performance properties of the HMA was evaluated in the same year [14–16]. One other study investigated the effect of optimal content of hydrated lime on aging and workability at high temperatures [17]. In 2011, Lee et al. employed modern experimental techniques and new models to study the effect of modified HMA with hydrated lime on rutting and fatigues cracks [11]. The hydrated lime will reduce the cracking as a result of aging, fatigues or low temperature. The cracking usually occurs after formation of micro-cracks. Fine particles of the hydrated lime are expected to prevent formation of these micro-cracks [7]. The
hydrated lime as filler improves stiffness and reduces rutting. Moreover, adding the hydrated lime will not allow the asphalt to stiffen at low temperatures because the lime will act as inactive mineral filler at these low temperatures. The hydrated lime will also reduce the oxidation and ageing effects [8]. As the HMA ages due to oxidation, hydrated lime reduces not only the rate of oxidation but also the harm created by the products of oxidation. This effect keeps the asphalt from hardening excessively and from becoming highly susceptible to cracking (through fatigue and low temperature (thermal) cracking). Synergistically, the filler effect of the hydrated lime dispersed in the asphalt improves fracture resistance and further improves cracking resistance [13]. There are many observations which show the hydrated lime modified HMA has better performance of mixture against rutting, fatigue and thermal cracking [18]. In 1987, Petersen et al. evaluated two asphalt binders modified with limestone and hydrated lime at 20% by weight of binder. The results of this research indicate that lime treatment would improve the resistance of the aged pavement to thermal cracking through the reduced aging index. Since the behavior of HMA mixtures at low temperatures is mainly controlled by the properties of the aged binder, lime treatment would produce HMA pavements that are highly resistant to thermal cracking [19]. The lime-modified bitumen demonstrates a greater potential for dissipating energy through deformation (at low temperature) than the unmodified bitumen. This is a positive effect at low temperatures because it reduces fracture potential. Although the filler effect increases low temperature stiffness, fracture toughness is substantially increased. Fracture toughness is the energy expended in fracturing a material [20]. Nevertheless, base on some studies, the influence of filler on low temperature cracking was hypothesized to be independent of filler type. Unlike rutting and fatigue cracking, adding different amount of hydrated lime as filler in mixture is not so effective on the thermal cracking [21,22]. Some studies have been conducted recently in 2013 toward failure mechanics and advanced suggested models, the effect of hydrated lime on failure performance, and the mechanical properties of mixtures [23,24]. Permanent deformation is one of the most critical parameters for pavement design, which is believed to increase with traffic and tiers pressure. Most of the permanent deformations occur at the upper layers rather than the subgrade. There are different approaches to determine the plastic deformation, out of which repeated load test is used more than the others [1]. In this study simple performance tests were used in order to evaluate performance of asphalt mixtures. Based on NCHRP-465 report, simple performance test (SPT) method(s) accurately and reliably measures a mixture response characteristic or parameter that is highly correlated to the occurrence of pavement distress (e.g., cracking and rutting) over a diverse range of traffic and climatic conditions. Numerous techniques have been suggested to assess pavement performance and distresses. One of the best available solutions for this purpose is to perform three simple performance tests, namely complex modulus (E*), dynamic creep – flow number test (Fn), and static creep – flow time test (Ft). They yield the best relationship between the experimental results and the field performance [25]. The complex modulus is a way of addressing the relationship between stress and strain of the viscoelastic material. The complex modulus test is often conducted on the cylindrical samples which are exposed to a haversine compressive loading [26]. The complex modules changes by variation in the loading frequency. A Loading capacity which models the traffic load much closer than the other frequencies must be chosen for this test. The dynamic modulus obtained in this regard is equivalent to resilient modulus which is used for design purposes [27].
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The main objective of the dynamic creep test is to study asphalt concrete performance against rutting phenomenon. In other words, using the repeated dynamic load test in several thousand runs and recording the accumulative strain as a function of the cyclic loading during the test is a method to determine the permanent deformation of asphalt concrete [28]. The static creep test is not only useful for finding the plastic properties of the material, but also can be used to measure the permanent deformations of the mixture. There are many researches investigating the effect of different filler on dense grade mixtures. But there are a few studies containing the investigation on different content of hydrated lime as partial substitution of filler in SMA in which total amount of filler has considerable effect on open graded structure of stone matrix asphalts mixtures. In this paper the effect of adding different contents of hydrated lime as the effective element of open-graded asphalts is investigated on the mechanical properties of SMA through SPT examinations.
Table 3 Physical properties of the used hydrated lime. Property
Value
Specific gravity Dry brightness, G.E. Mean particle size PH BET surface area 100 Mesh 200 Mesh Apparent dry bulk density (loose) Apparent dry bulk density (packed)
2.343 92% 2 lm 12.6 22.0 m2/g 94% 90% 20 lbs./ft.3 30 lbs./ft.3
Table 4 Chemical properties of the used hydrated lime.
3. Methodology Mechanical properties of the lime-containing SMA mixtures are evaluated in this research. For this purpose, quality and type of the needed materials are described first and then, grain distribution and preparation of the samples are explained. AT last, method of performing the tests is introduced along with the reason to use the initial parameters for measurement of the performance indexes and the mixture properties. Meanwhile, some control samples are also made without hydrated lime addition to the SMA samples to examine change in the mixture properties. Two groups of samples were used based on two types of fillers and two types of grain distributions. The optimal bitumen content was extracted for each group by making the test samples and running the Rice and Marshal Tests. The main samples were made by using the optimal content of bitumen in each of the groups and were further checked via dynamic modulus, dynamic creep and static creep tests.
Property
Value
Ca(OH)2 CaO-equivalent CaO total CaCO3 Insoluble in acid CaSO4 Free H2O MgO SiO2 Al2O3 Fe2O3 SO3 P2O5 Na2O MnO
0.91 70 72 2 2 0.6 0.8 0.5 3 0.3 0.3 0.2 0.05 0.04 0.0045
The crushed stone is used as the filler for some samples which are made just for comparison with the lime-containing samples. Tables 3 and 4 list chemical and physical properties of the hydrated lime filler, respectively.
3.1. Materials For making the asphalt samples, a pea gravel of definite specifications (Table 1) was used with the required quality tests being performed (Table 2) and the 60/70 bitumen as an adhesive for the gravel aggregates. Some fibers may be used to avoid drain down of the bitumen in the SMA mixtures due to its great content. VIATOP cellulose fibers were used in this study which was composed of 90% ARBOCEL ZZ and 10% bitumen. For better mixing of them with the mixture, it is recommended to powder them first and mix them for 1 min with hot gravel of the mixture and add the bitumen at the end.
3.2. Aggregate gradation The suggested grain size distribution of NCHRP 9-8 report was used to address this parameter in making the SMA samples [2]. Taking into account the relatively large range of the applied SMA asphalt and its being practical, the nominal sizes of 9.5 and 12.5 mm were selected for the first and second gradations, respectively (Table 5). 3.3. Sample preparation
Table 1 Aggregate specific gravities (kg/m3). Grain-size fraction
Coarse aggregate (AASHTO T85) Fine aggregate (AASHTO T84) Filler aggregate (AASHTO T100) Total bulk specific gravity of aggregate
Specific gravity
Water absorption (%)
Apparent specific gravity
Bulk specific gravity
2699
2499
2.5
2716
2529
2.7
2653
–
–
2514
–
With respect to the previous studies, the SMA samples were decided to be made from 5 different contents of lime and to be tested in 5 different temperatures. In each group and after defining the optimal bitumen content, this value has been kept constant and just the hydrated lime content was changed. The lime contents were chosen from 0 to 7.5 wt% of the aggregate materials increasing in ranges of 2.5 wt%, in which the mixtures with zero (0%) percentage of hydrated lime were considered as control mix to compare the results and make conclusion in each case (Fig. 1). The hydrated lime, crushed stone filler and the other aggregates were initially heated at 180 °C in the oven and they were later mixed with the bitumen at 155 °C. This method is used in the field as was described as adding dry hydrated lime to the aggregates before blending with the bitumen that prepares aggregate surface for better bonding condition. In order to evaluate the simultaneous effect of total filler content and lime content of the mixture, a sample of 10% crushed stone filler and 2% hydrated lime as an additive (in terms of total mixture weight, and 2.1% in terms of aggregate weight) was prepared. Therefore, after heating of
Table 2 The results of tests performed on aggregate. Properties
Test method
Value (%)
Allowable value
L.A. Abrasion Coarse aggregate with one fractured faces Coarse aggregate with two fractured faces Flat and elongated particles of coarse aggregates with ratio of 1:3 Flat and elongated particles of coarse aggregates with ratio of 1:5
AASHTO T96 ASTM D5821 ASTM D5821 ASTM: D4791 ASTM: D4791
16 95 91 14 2
Max 30% 100% Min 90% Max 20% Max 5%
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Table 5 Used gradation and limits for SMA. Sieve size (mm)
3.5. Dynamic creep test
9.5 mm NMAS
19 12.5 9.5 4.75 2.36 1.18 0.6 0.3 0.075 ⁄
The dynamic creep test was done by UTM5 according to NCHRP 9-19 in unconfined condition at 50 °C. A haversine loading of 138 kPa was applied with 0.1 s and 0.9 s as loading time and rest time, respectively.
Passing (%) 12.5 mm NMAS
Used
Lower
Upper
Used
Lower
Upper
– 100 95 45 25 18 16 14 10⁄
– 100 90 26 20 13 12 12 8
– 100 100 60 28 21 18 15 10
100 95 67 28 22 19 17 14 10⁄
100 90 26 20 16 13 12 12 8
100 100 78 28 24 21 18 15 10
10% Filler for both gradation (the equal amount in gram).
CS
The dynamic modulus test was performed on the cylindrical unconfined samples under a haversine compressive loading and the loading pulse according to the NCHRP has a haversine shape. The applied frequencies are 25, 10, 1, 0.5 and 0.1 Hz, while the 25 Hz frequency is considered as the pre-loading. Number of the loading cycles associated with each frequency are 206, 106, 56, 25, 6 and 6, respectively. A rest time of 1 min long is considered between every two frequencies, so that the samples could have their recoverable strains. The static and dynamic stresses are selected for each sample regarding the loading temperature at 5, 15, 25, 35 and 45 °C, respectively and finally, the master curves was shown for dynamic modulus and phase angle.
4. Results and discussion
%filler by percentage of aggregate weight
HL
3.6. Dynamic modulus test
HL as addive
4.1. Results of complex modulus test 2.0% 2.5% 5.0% 7.5% 10% 10.0%
The complex modulus testing was used to address viscoelastic and stiffness properties of the mixture. The master curve of dynamic modulus was derived here by using the equation proposed in NCHRP-614 report [29] (Figs. 2 and 3). The master curve of phase angle was also extracted using the equation suggested by Zeng et al. in 2001 [30] for all the frequencies at 25 °C.
7.5% 5.0%
Temperature shi factor 3
2.5%
temperature degree Regression Line
Control Samples
A2, A7
A3, A8
A4, A9
A5, A10
2
the gravel aggregates and mixing of the bitumen, 2% dry hydrated lime was added to the mixture just after fully coating aggregates with bitumen. This method was used in order to easier mixing procedure, better coating aggregate surfaces and also, prevention of enveloped hydrated lime in asphalt binder. Just one sample was enough for each of the 5 testing temperatures considering the non-destructive nature of the dynamic modulus test [29]. The SMA samples were prepared for this research with two different grain size distributions and also different contents of the hydrated lime filler according to Table 6. The Rice test was done to determine the optimum binder percent to satisfy 4% air void in the mixture, while the drain down test was implemented on the samples. Samples were compacted using gyratory compactor. At last, samples of 100 mm diameter and 150 mm height were made.
Log shi factor a (T)
Fig. 1. Increase in percentage of HL (hydrated lime) filler along with CS (crushed stone) filler in mixtures.
1
0
0
5
10
15
30
35
40
45
50
y = 0.0004x2 - 0.1378x + 3.1964 R² = 1
-3
NCHRP 9-19 Superpave Models were used for unconfined samples at 50 °C. A deviator stress of 138 kPa was lasted until failure and thereafter, the sample entered the third phase.
25
-1
-2
3.4. Static creep test
20
Temperature, °c
Fig. 2. Shift factor versus temperature for SMA with9.5 mm NMAS and 2% lime as additive.
Table 6 Methodology for combined mixtures for all conditioned samples. Sample
NMAS (mm)
Crushed stone filler (%)
Substituted hydrated lime (%)
Additive% (by mixture weight)
Bitumen content (%)
A1 A2 A3 A4 A5 A6 A7 A8 A9 A10
9.5 9.5 9.5 9.5 9.5 12.5 12.5 12.5 12.5 12.5
10 7.5 5 2.5 10 10 7.5 5 2.5 10
0 2.5 5 7.5 0 0 2.5 5 7.5 0
0.03% Fiber 0.03% Fiber 0.03% Fiber 0.03% Fiber 0.03% Fiber 2% lime 0.03% Fiber 0.03% Fiber 0.03% Fiber 0.03% Fiber 0.03% Fiber 2% Lime
6.5 6.5 6.5 6.5 6.5 7 7 7 7 7
Three samples were made for each combination and average of results were used.
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Fig. 3. Master curve with shifted E* data for SMA with 9.5 mm NMAS and 2% lime as additive.
Fig. 4. Master curves of SMA mixtures with 9.5 mm NMAS and different hydrated lime contents at reference temperature of 25 °C.
Fig. 5. Master curves of SMA mixtures with NMAS 12.5 mm and different hydrated lime contents at reference temperature of 25 °C.
383
4.1.1. Dynamic modulus HL has a multifunctional effect on mixture. It can be seen from the dynamic modulus values in different temperatures and frequencies for mixtures of different contents that the effect of hydrated lime on dynamic modulus of the SMA mixture and stiffness index of the asphalt is really small at high temperatures and low frequencies. Comparison of the dynamic modulus master curves at low frequencies for mixtures of 9.5 mm nominal size (Fig. 4) indicates just a negligible rise of dynamic modulus by increasing the hydrated lime content up to 2.5% and decreasing it by further addition of the hydrated lime. This effect is rather different in the mixtures of 12.5 mm nominal size (Fig. 5) somehow; by increasing the hydrated lime up to 2.5%, the dynamic modulus reaches to its maximum value at high temperatures or low frequencies, and increasing the hydrated lime at low temperatures or high frequencies reduces the dynamic modulus. To be more exact, Presence of HL as active filler at high temperature and as inactive filler at low temperature causes physiochemical combination with bitumen which reduces the interval between maximum and minimum values of the dynamic modulus, and improve simultaneously its stiffness at high and low temperatures. This also may attributed to the fact that the specific surface area and surface energy of hydrated lime is much larger than mineral powder, resulting in the selection absorption of light molecular weight compounds of asphalt [17]. 4.1.2. Phase angle Phase angle is indicative of viscous characteristics of the mixture due to mastic properties in dense graded structures. Comparing HL contained mixtures with control samples, by replacement of HL in the mixture a slight reduction has occurred in the viscosity and phase angle of SMA (Figs. 6 and 7). Addition of the lime to the mixture as a part of aggregate or using as additive in the mixture seem to reduce slightly the degree of phase angle and viscosity of SMA. This may be due to the fact that the structure of SMA is based on direct contact of aggregate while the mastic only fills the voids between coarse aggregates and hold the structure. Therefore, when the mastic stiffens elastic property of coarse aggregate influences more effectively on viscosity of the SMA. However, increasing the temperature will increase elastic effect of gravel in the mixture, while reduce the degree of phase angle which implies reduced viscous effects of the mastic. In the SMA mixtures this effect is intensified due to the direct contact of the gravels especially at high temperatures [28]. That is why the degree of phase angle in the SMA mixtures at high temperatures is simultaneously affected by the presence of hydrated lime and interaction of the gravels on each other. Those indices involving the phase angle are thus unable to evaluate the presence of hydrated lime in the mixture properly at high temperature.
Fig. 6. Phase angle master curves for SMA mixture with NMAS 9.5 mm and different lime contents at reference temperature of 25 °C.
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4.1.3. Evaluation of rutting factor E*/sinf is the parameter obtained from the dynamic modulus test and can be used in the frequencies of 5 and 10 Hz as the index of rutting resistance. This parameter should not be smaller than the minimum requirements of Superpave. Loading at the rate of 0.1 s (10 Hz) in the laboratory, is usually indicative of the traffic speed in field [31]. The results of comparison between the rutting resistance which are obtained from the dynamic modulus test and rut depth estimation experiments like Hamburg wheel tracking device show that the parameter of E*/sinf represent correctly rutting resistance in the HMA mixture only at high temperatures [32]. This parameter was used as a criterion for evaluating rutting resistance of SMA in this research (Table 7). The results of this
Fig. 7. Phase angle master curves for SMA mixture with NMAS 12.5 mm and different lime contents at reference temperature of 25 °C.
Table 7 Superpave factors for rutting and fatigue cracking in two frequencies. % HL
Control
NMAS (mm)
9.5
12.5
9.5
12.5
9.5
12.5
9.5
12.5
9.5
12.5
411 356 251 160
354 321 309 292
458 391 307 270
441 386 240 235
443 398 246 213
435 388 244 189
420 370 177 201
432 367 137 134
420 352 276 224
366 346 217 185
|E*|/sin(f) at 45 °C |E*| sin(f) at 25 °C
10 Hz 5 Hz 10 Hz 5 Hz
2.5% HL
5% HL
7.5% HL
2% HL additive
Fig. 8. Rutting factor for SMA mixtures with 9.5 mm NMAS and different hydrated lime contents at 45 °C.
Fig. 9. Rutting factor for SMA mixtures with 12.5 mm NMAS and different hydrated lime contents at 45 °C.
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Nom 9.5 mm
350
effect of interaction of the gravels on the phase angle at 45 °C. Nevertheless, this effect becomes significant at highly elevated temperatures.
300
4.1.4. Evaluation of fatigue crack factor For fatigue cracking, a performance factor in the Superpave binder specification is |G⁄|sind. Thus, the equivalent performance factor for the mix is |E⁄|sinf. [28]. Reduction of the fatigue factor with increasing the lime content indicates that hydrated lime will enhance the resistance against fatigue cracking (Table 7). Meanwhile, the range of fatigue factor values for the mixtures with nominal size of 9.5 mm at 10 Hz frequency implies improvement of the fatigue cracking by substituting the hydrated lime for more than 5% (Fig. 10). Fig. 11 shows that the results for samples of 12.5 mm nominal size are almost the same, except that there is a growth in the resistance against fatigue cracking by increasing the hydrated lime from 0 to 7.5%. A comparison between fatigue cracking factor of two mixtures of different nominal sizes reveals that the values of |E*| sinf for the samples of 12.5 mm is smaller than those of the 9.5 mm samples. It can be explained that, the internal pores of the HL particles are filled with bitumen, and this filled particles are seen as hard spheres in the bitumen matrix, therefore prevents growth of micro crack in mastic [33]. It seems that the more content of Hl the more fatigue cracking resistance will be observed in SMA which does not show an optimum content of HL in SMA.
|E*|.sinf
250
200
150
100
50 0% lime
5% lime
2.5% lime
7.5% lime
2% lime as addive
0 0
2
4
6
8
10
12
Frequency, HZ Fig. 10. Fatigue cracking factor for SMA mixtures with 9.5 mm NMAS and different hydrated lime contents at 25 °C.
4.2. Creep test Dynamic and static creep tests were done to explore resistance of the samples against the common phenomena of rutting and permanent deformations. Two indices of flow number and flow time are the main parameters for evaluation of the rutting phenomenon. As was expected before, the results which were obtained from the both experiments represent the same trend for the effect of lime on rutting. Due to physical features of HL particles the distribution of HL throughout the binder is better, as the HL particles are much finer than most common fillers. Finer particles have more surface area for a given unit volume, thus providing more surface interactions with asphalt. Therefore, the finer the filler, the more it potentially stiffens the asphalt. The stiffening effect of filler is dependent on Rigden voids of the filler. The voids in the compacted filler combine the effects of particle density, particle shape, and particle size distribution in the filler fraction [34]. It can be seen from Table 8 that increasing the hydrated lime filler content of the SMA samples has reinforced the resistance indices of rutting Fn and Ft, also decreased the permanent deformation of these samples against the cyclic dynamic load. Increasing the filler content from 0% to 5% enhanced rutting resistance of the mixture. Stiffer asphalt binder provides more holding ability to maintain the aggregate structure leading to less permanent deformation. As is demonstrated in Figs. 12 and 13, control mixes reached a stable slope faster; this is likely attributable to less stiff asphalt mastic that allows faster densification and a steep slope during secondary phase where the rate of accumulation of permanent deformation remains constant.
Fig. 11. Fatigue cracking factor for SMA mixtures with 12.5 mm NMAS and different hydrated lime contents at 25 °C.
evaluation at 45 °C confirm improvement of the rutting resistance with increasing substituted hydrated lime filler content up to 5%. In both aggregate gradation (Figs. 8 and 9), the mixtures containing 5% lime at the frequency of 5 Hz provide a superior resistance in comparison with the 2.5% lime sample, likewise the results of the creep test. However, this value shows just a small different at the frequency of 10 Hz. Unlike the results of the creep test, adding 2% of the lime filler to the mixture as an additive has not much effect in raising the rutting resistance. It seems that much reasonable results will be generated by creep test due to destructive nature of this test which allows no recovery during the test to the sample [32]. The close similarity between the evaluation of rutting factor results (E*/sinf) and the indices of creep test implies a negligible
Table 8 Dynamic and static creep test results at 50 °C. % HL
Control
NMAS (mm)
9.5
12.5
2.5% HL 9.5
12.5
5% HL 9.5
12.5
9.5
7.5% HL 12.5
9.5
2% HL additive 12.5
Flow number Flow time Total permanent deformation (mm)
425 334 1.81
349 312 1.66
1075 631 2.10
427 400 2.34
2983 5012 1.94
2295 1259 2.48
2375 1933 2.40
1123 626 2.10
2135 1996 2.29
1527 798 3.09
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Fig. 12. Dynamic creep curves for 9.5 mm NMAS mixtures at 50 °C.
Fig. 13. Dynamic creep curves for 12.5 mm NMAS mixtures at 50 °C.
3500 nom 9.5 nom 12.5
3000 2500
flow num
Based on experimental observations adding HL more than 5% in the mixture changed the trend of permanent deformation resistance. It is possible that when more HL is added along constant amount of bitumen the filler surface absorption increases and the absorbed filler particles gather into a group that may leads to deterioration of interface bonding between aggregates and lower cohesion. On the other hand, excessive amount of HL reduces coat of coarse aggregates which affects reorientation of aggregate particles in compaction process and structure of mixtures that results in less cohesiveness and less rutting resistance. It is also clearly observed that the grain distribution of 9.5 nominal size gives a greater flow number in comparison with the grain distribution of 12.5 nominal size (Figs. 14 and 15) because Uniform coat of coarse aggregate and film thickness lead to better aggregate interaction and strong structure in SMA. By comparing the results of 2.5% HL content mixes with the mixes in which 2% HL has been added as additive, it can be concluded that the amount of total filler in mixture and the method of adding HL filler considerably affect SMA performance. This may attributed to the fact that in SMA with open graded structure, 10% crushed stone filler along 2% HL additive enhances filling up the voids between coarse aggregate and improves aggregate contacts. However, introduction of 2% dry HL to the mixture after full aggregate coating by bitumen mostly stiffens the asphalt binder, While when 2.5% HL filler is replaced in the mixture, after mixing, not all of the hydrated lime is in ‘‘direct’’ contact with the surface of the aggregate, but some becomes part of the binder itself or fills
2000 1500 1000 500 0 Control
(2.5,7.5)
(5,5)
(7.5,2.5)
(2,10)
(HL (%), CS (%)) Fig. 14. Flow number for mixtures with different percentage of HL and CS filler. ⁄ HL: hydrated lime filler, CS: crush stone filler.
up the air void between aggregate. As a consequence, lower interface bonding strength occurs and mastic stiffness decreases which leads to less rutting resistance compared with 2% HL additive in mix.
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6000 nom 9.5 nom 12.5
5000
Flow me
4000
3000
2000
1000
0
Control
(2.5,7.5)
(5,5)
(7.5,2.5)
(2,10)
(HL (%), CS (%)) Fig. 15. Flow time for mixtures with different percentage of HL and CS filler. ⁄HL: hydrated lime filler, CS: crush stone filler.
5. Conclusions Mechanical properties of SMA with regard to different content of HL (Hydrated Lime) were questioned. Simple performance tests such as creep tests and dynamic modules were conducted in order to investigate how the different percentage of hydrated lime in different HL mixing method affect performance of mixtures with open grain size distribution. The following results may be drawn. According to obtained results of creep tests, presence of hydrated lime reduced rutting and rut depth much more than control mixtures. In both aggregate gradations (9.5 NMAS, 12.5 NMAS) Rutting resistance increases as the HL percent rise up to optimum amount of HL. It was demonstrated that in SMA, increasing HL more than 5% changes the trend and reduces rutting indices. In both gradation (9.5 NMAS, 12.5 NMAS), presence of HL effect slightly on |E*| and phase angle. It is hypothesized that in SMA with direct contact of aggregate HL has negligible effect on viscosity and stiffness of SMA mixtures. The results of this research indicate that total percentage of filler and mixing method considerably impact on performance of SMA. As a result, using HL as additive leads to superior performance comparing with partial substitution of HL for filler aggregate. The obtained measures from E*/sinf do not establish properly a relationship with the results of creep test in order to evaluate the rutting potential of the SMA mixtures at high temperatures below 50 °C. It is thought that, E*/sinf and E* sinf do not represent the performance of SMA mixture as well as creep tests and related fatigue tests.
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