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Evaluation of Reclaimed Asphalt Pavement Based Portland Cement Concrete for Pavement ApplicationsAuthor 1: Xijun Shi, Postdoctoral Researcher, Center for Infrastructure Renewal, Texas A&M University, College Station, TX, USA
Author 2: Anol Mukhopadhyay, Research Scientist, Texas A&M Transportation Institute, College Station, TX, USA
Author 3: Dan Zollinger, Professor, Zachry Department of Civil and Environmental Engineering, Texas A&M University, College Station, TX, USA
For the corresponding author: [email protected]
KEYWORDS: Reclaimed asphalt pavement; portland cement concrete; pavement behavior
Conflict of Interest: None
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ABSTRACT
Aggregate is an indispensable ingredient of concrete materials. The enormous demand for concrete in new construction requires an increasingly higher amount of aggregate materials. Using reclaimed asphalt pavement (RAP) in portland cement concrete (PCC) as a coarse aggregate replacement is considered a potential solution to solving the nationwide aggregate shortage. In this study, two different Texas coarse RAPs were collected and characterized; they partially replaced the virgin coarse aggregate at 20 vol.% and 40 vol.% to formulate RAP-PCC mixtures. An extensive experimental program to test some pavement related PCC properties including compressive strength, modulus of rupture, modulus of elasticity, Poisson’s ratio, coefficient of thermal expansion, and thermal properties was carried out. Potential impacts of the addition of RAP on concrete pavement behavior and any recommendations for this application are discussed.
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1. INTRODUCTION
Aggregate is an indispensable ingredient of concrete materials, accounting for nearly 70-80% of the total volume of concrete. The enormous demand for concrete in new construction requires an increasingly higher amount of aggregate materials. The depleting trend of many good aggregate sources leads to a continuous increase in aggregate cost caused by higher energy consumption during aggregate production and transportation along with higher expense to regulate environment related issues (1). The increasing maintenance and rehabilitation of asphalt concrete pavement has generated excess reclaimed asphalt pavement (RAP) in some states. According to a survey, the possession of excess RAP has been reported by 91% of U.S contractors (2). The expanding RAP stockpiling not only requires higher fees for space and regulation but also poses a threat to both environment and public safety. Although the use of RAP to make hot mix asphalt (HMA) is a common practice, the replacement levels of RAP (20-25% (2)) in this application is not adequate to solve the RAP stockpiling issue. The use of RAP in portland cement concrete (PCC) was found to be an effective way to promote greater consumption of RAP and mitigate RAP stockpiling issues (3). Contractors have been motivated to use RAP as aggregate replacements to produce PCC wherever aggregate shortage is a critical issue. A significant amount of lab-scale research has been conducted worldwide on utilization of RAP to make PCC with few field-level case studies. Lab based research on the use of RAP in PCC probably dates to the 1970s, after which a considerable amount of efforts has been continuously made around the world (4-9). The rapidly growing interest in exploring alternative ways to use RAP has motivated several state Departments of Transportation (DOTs) and Toll Highway Authorities to support research projects focusing on using RAP in PCC (10-14).
2. RESEARCH SIGNIFICANCE
This paper aims at investigating the effect of the RAP addition on pavement related PCC properties and subsequently the effect on pavement performance. Two different Texas coarse RAPs were collected and characterized; they partially replaced the virgin coarse aggregate at 20 vol.% and 40 vol.% to formulate RAP-PCC mixtures. An extensive experimental program to test some PCC properties of the studied RAP-PCC mixtures including compressive strength, modulus of rupture (MOR), modulus of elasticity (MOE), Poisson’s ratio, coefficient of thermal expansion (CoTE), and thermal properties was carried out. Potential impacts of the addition of RAP on concrete pavement behavior and any recommendations for this application are discussed.
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3. METHODOLOGY
3.1 Materials and mix design
The concrete ingredients include a virgin limestone coarse aggregate (#57 gradation in accordance with ASTM C 33), a natural siliceous concrete sand, a locally available Type I/II cement, a class F fly ash, a typical mid-range water reducer and an air entraining agent. Two types of RAP were collected from the Houston district and the Bryan district and are labeled as HOU and BRY, respectively. The gradation of the virgin coarse aggregate, natural siliceous sand along with RAP coarse aggregates are presented in Figure 1. The gradation of the HOU RAP is similar to that of the virgin coarse aggregate used in this study. The BRY RAP is an un-fractionated RAP that contains coarse (retained on 9.5-mm sieve), intermediate (passing 9.5-mm size and retained on 2.36-mm sieve), and fine particles (passing 2.36-mm sieve). So, BRY RAP is in general finer than the HOU RAP. In this study, only the coarse and intermediate sized particles were sieved out from as received un-fractioned BRY RAP and used for producing RAP-PCC with BRY RAP. The results for additional aggregate material characterization are tabulated in Table 1. A detailed discussion of the material characterization results can be found in the authors’ previous publications (13; 15).
0102030405060708090
100
0.01 0.1 1 10 100
%Pa
ssin
g
Sieve Size (mm)
FA upper limit
FA lower limit
CA upper limit
CA lower limit
Sand
BRY
HOU
Virgin CA
Figure 1 Gradation of the aggregate materials.
Table 1 Aggregate materials characterization. RAP/
Aggregate ID
Stone type Asphalt content (%)
Dry rodded unit weight
(kg/m3)
Oven dry specific gravity
Absorption (%)
Abrasion loss by micro-deval (%)
Virgin CA Limestone with minor chert particles … 1551 2.51 2.79 22.84
Sand Siliceous river sand … … 2.58 2.06 na
HOU
Gravel made of mostly limestone
with some siliceous particles
4.00 1335 2.41 2.61 26.54
BRYLimestone with few siliceous particles
(minor phase)6.19 1373 2.36 1.78 17.54
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In general, RAP coarse aggregates show lower dry rodded unit weight, specific gravity, and absorption compared to virgin coarse aggregate due to the presence of aged asphalt in RAP. Although HOU RAP shows higher abrasion loss compared to virgin CA, the BRY RAP shows lower loss than that for virgin CA. The HOU RAP contains more agglomerated particles (than the BRY RAP and this possibly is the main reason for HOU RAP showing more loss (16). The BRY RAP contains well-separated particles with higher asphalt content than HOU RAP. It is indicated that BRY RAP is smoother, less angular, and more spherical compared to other two aggregate materials (15); this may have caused BRY RAP to have the lowest abrasion loss.
The PCC mixtures were designed based on the TxDOT standard specifications for a typical paving concrete (class P). The mix design for all the studied mixtures is shown in Table 2. The w/cm ratio was selected as 0.40 and the cementitious content was 309 kg/m3 with 20 percent of the cement replaced by the fly ash. An air content of 5.0% was designed using an appropriate amount of air entraining agent. The coarse RAP was incorporated in the mixtures by replacing 20% and 40% of the virgin coarse aggregate on the volumetric basis.
Table 2 Mix design. Materials 0.40_520_
REF0.40_520_
20HOU0.40_520_
40HOU0.40_520_
20BRY0.40_520_
40BRYCement (kg/m3) 247 247 247 247 247Fly ash (kg/m3) 62 62 62 62 62
Virgin coarse aggregate (kg/m3) 1058 825 604 830 611
RAP (kg/m3) 0 206 403 208 408FA (kg/m3) 769 787 804 776 783
Water reducer (ml/m3) 402 402 402 402 402Air entraining agent
(ml/m3) 60 60 60 60 60
Water (kg/m3) 123 123 123 123 123TAVF (%) 0 1.080 2.127 1.653 3.285
Note: The mix ID in this study was assigned with the following format: w/cm_cementitious content*_replacement level+RAP type
*Since all the mixes were designed according to the TxDOT specifications, which use the U.S customary units, the cementitious content in the mix ID was assigned as 520 (lb/cy, equal to 309 kg/m3).As an example: 0.40_520_40HOU represents a mix has 0.40 w/cm ratio, 520 lb/cy cementitious content and with HOU replacing 40% of virgin coarse aggregate. Specially, 0.40_520_REF represents the plain concrete mix containing 100% virgin aggregate that has 0.40 w/cm ratio and 520 lb/cy cementitious content
It was observed that the asphalt film around RAP particles is the primary weak zone in the RAP-PCC as crack easily propagates through the asphalt film under force during strength testing (13). It is also shown that the asphalt volumetric content in the concrete mixture is highly relevant to RAP-PCC properties. Accordingly, a term called total asphalt volumetric fraction (TAVF) was used to represent asphalt content contributed by the RAP used to make concrete. TAVF is defined as the volume fraction of the asphalt in the total aggregate mixture (virgin aggregates plus RAP aggregates). The TAVFs for the studied mixtures are presented in the last row of Table2.
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3.2 Experimental program
The material properties relevant to concrete performance were directly determined from an extensive experimental program. The tested properties include compressive strength, modulus of rupture, modulus of elasticity, Poisson’s ratio, coefficient of thermal expansion, and thermal properties. These materials properties are the major inputs for several widely used pavement design tools (17; 18). A detailed description of each test is presented in this section.
3.2.1 Compressive strength testCompressive strength is the most widely used concrete property. It can be directly correlated with other concrete properties such as MOE and MOR. The compression test in this study was carried out with a 1000-kN MTS machine using a force control mode at 36 kN/min. The testing procedure followed ASTM C39, and 100×200 mm cylindrical specimens were tested.
3.2.2 Modulus of rupture testConcrete is a material that is strong in compression but weak in tension. The characterization of RAP-PCC tensile property is of great importance for pavement applications since pavement is primarily subjected to flexure. A uniaxial direct tension test may be the ideal test to evaluate concrete tensile property, however, such test is not easy to perform due to the challenges of grasping the specimens. Therefore, modulus of rupture (or flexural strength) is widely used to infer the tensile property of concrete in an indirect way. The flexural test conducted in this study used a simple beam (150×150×500 mm) with third-point loading method in accordance with ASTM C78. The testing machine was an MTS machine with a 100-kN loading capacity.
3.2.3 Modulus of elasticity testConcrete MOE is directly related to the stress and deflection in concrete slab. The test to characterize the MOE of the studied mixtures followed ASTM C469. A 1000-kN MTS machine was used, and the test was performed on 100×200 mm cylindrical specimens at a constant displacement rate of 1.3 mm/min. A ring attachment holding two axial linear variable differential transformers (LVDTs) was used to record the axial displacement during the test.
3.2.4 Poisson’s ratio testPoisson’s ratio of the studied mixtures was tested in accordance with ASTM C469. The test was carried out using an 1800-kN Tinius Olsen machine at a constant displacement rate of 1.3 mm/min. The specimens were cylindrical specimens with 150 mm in diameter and 300 mm in height. A similar ring attachment for testing MOE was used. The ring fixture was equipped with three radial LVDTs and three axial LVDTs in order to measure the displacement in both directions.
3.2.5 Coefficient of thermal expansion testCoTE of PCC controls concrete slab expansion and contraction behavior. It is also an input for predicting slab curling and warping. The measurement made in this study followed the AASHTO T336 standard. Before the test, 100×200 mm cylindrical specimens with a moist cured age of 28 days were shortened to 178 mm in length. The specimens were then submerged into a lime-saturated water storage tank at 23°C for 2 days. During the CoTE test, the specimens were placed in a water bath that was able to control temperature between 10°C and 50°C. An LVDT was
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mounted at the top of the testing specimen to record the length change of the specimen due to the temperature change.
3.2.6 Thermal properties testConcrete thermal properties (i.e., thermal conductivity and heat capacity) control the heat transfer within pavement (19). Thermal conductivity measures how fast a material can conduct heat, while heat capacity quantifies the amount of heat needed to raise a unit of material temperature. Both thermal conductivity and heat capacity are important inputs in calculating pavement temperature profile, which has a direct impact on the stress and deflection of the pavement slab. In this study, the thermal properties of the studied mixtures were determined using a hot disk thermal constants analyzer (model TPS-2500S) (Figure 2) according to the procedures described in the previously published works (20; 21). Each cylindrical specimen with 100 mm in diameter and 200 mm in length was sliced into four pieces in the transverse direction. During the test, the TPS 2500S sensor was sandwiched by two disk shaped specimens to measure the temperature changes of the specimens. With the recorded temperature change and the amount of heat generated, the thermal properties of the specimens can be automatically calculated by the software. For each type of the mixture, three data points were obtained using four disk samples.
Figure 2 Thermal properties test
4. RESULTS
4.1 Compressive strength
Figure 3 presents the compressive strength results. It is clearly indicated that adding RAP reduced concrete compressive strength. The strength reduction is 20% and 19% when 20% HOU RAP and BRY RAP was incorporated, respectively. When 40% RAP was added, the reduction reaches 37% for HOU RAP and 31% for BRY RAP. The results also show the BRY RAP had a lower negative impact on concrete compressive strength compared to HOU RAP. This is because BRY RAP is well graded with relatively finer particles while HOU RAP contains bigger
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particles with coarser gradation. It is evident that bigger sized RAP is usually agglomerated RAP particle, and the RAP agglomeration is considered one of the major weak zones in the RAP-PCC.
33.61
26.91
21.30
27.15
23.12
0.00
10.00
20.00
30.00
40.00
0.40_520_REF 0.40_520_20HOU 0.40_520_40HOU 0.40_520_20BRY 0.40_520_40BRY
Com
pres
sive
stre
ngth
(MPa
)
Figure 3 Compressive strength test results
4.2 Modulus of rupture
Figure 4 presents the MOR results for the studied mixtures at age of 28 days. From Figure 4, the addition of RAP invariably reduces PCC’s MOR. The flexural strength reduction is around 10% for the 20% HOU RAP-PCC mixture and 11 for the 20% BRY RAP-PCC mixtures. For the 40% replacement level, the reduction is 24% and 17% for HOU RAP and BRY RAP, respectively. Comparing to the compressive strength reduction, the flexural strength reduction is significantly lower.
4.464.03
3.38
3.953.68
0.00
1.00
2.00
3.00
4.00
5.00
6.00
0.40_520_REF 0.40_520_20HOU 0.40_520_40HOU 0.40_520_20BRY 0.40_520_40BRY
MO
R (M
Pa)
Figure 4 Modulus of rupture test results
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4.3 Modulus of elasticity
Figure 5 shows the calculated chord MOE of the studied mixtures. The results suggest that RAP-PCC has reduced MOE. In general, the RAP-PCC containing BRY has a lower MOE compared to the RAP-PCC mixture made with HOU at the same RAP replacement level because of the higher total asphalt content in the mixture.
32.95
28.95
24.51
28.71
24.06
0.00
10.00
20.00
30.00
40.00
0.40_520_REF 0.40_520_20HOU 0.40_520_40HOU 0.40_520_20BRY 0.40_520_40BRY
MO
E (G
Pa)
Figure 5 Modulus of elasticity test results
4.4 Poisson’s ratio
Figure 6 shows the Poisson’s ratio results for the studied mixtures at 28 days. It is shown that adding RAP into concrete increases the Poisson’s ratio. It is found that the BRY RAP-PCC mixtures have higher Poisson’s ratio than the HOU-RAP mixtures. Since the BRY RAP-PCC has a higher TAVF, the higher Poisson’s ratio might be due to the higher amount of asphalt in the system.
0.1510.162
0.176 0.1800.190
0.000
0.050
0.100
0.150
0.200
0.250
0.40_520_REF 0.40_520_20HOU 0.40_520_40HOU 0.40_520_20BRY 0.40_520_40BRY
Pois
son'
s rat
io
Figure 6 Poisson’s ratio test results
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4.5 Coefficient of thermal expansion
The CoTE test results are shown in Figure 7. The all the CoTE values are within the normal range for PCC. The RAP-PCC specimens invariably exhibited higher CoTE than the control specimen. The higher the RAP replacement level, the higher the CoTE is. Table 1 indicates that the all the coarse aggregates (virgin aggregates and RAP aggregates) are primarily limestones; given that the aggregate minerology is similar, the change of CoTE is largely attributed to the asphalt binder in the RAP because asphalt itself has a higher CoTE compared to aggregate or cement paste. Because of the higher TAVF, the BRY RAP-PCC has higher CoTE values than the HOU RAP-PCC with the same RAP replacement level.
8.0348.725 8.911 9.154
10.206
0.000
2.000
4.000
6.000
8.000
10.000
12.000
0.40_520_REF 0.40_520_20HOU 0.40_520_40HOU 0.40_520_20BRY 0.40_520_40BRY
Coe
ffic
ient
of t
herm
al e
xpan
sion
(î10
-6/ƕC
)
Figure 7 Coefficient of thermal expansion test results
4.6 Thermal properties
The thermal conductivity and the heat capacity of the studied mixtures at 28 age are presented in Figure 8. From Figure 8(a), the thermal conductivity of RAP-PCC mixtures was lower than the control mixture. This is because asphalt is a more insulating material compared to aggregate or cement paste. Figure 8(b) suggests that adding RAP into PCC reduces the heat capacity, but the trend is not clear.
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2.7172.481 2.381
2.526 2.442
0.000
0.500
1.000
1.500
2.000
2.500
3.000
3.500
0.40_520_REF 0.40_520_20HOU 0.40_520_40HOU 0.40_520_20BRY 0.40_520_40BRY
Ther
mal
con
duct
ivity
(W/m
îK)
(a) Thermal conductivity
1.8881.751 1.800
1.646 1.6833
0.000
0.500
1.000
1.500
2.000
2.500
0.40_520_REF 0.40_520_20HOU 0.40_520_40HOU 0.40_520_20BRY 0.40_520_40BRY
Hea
t cap
acity
(MJ/
m3 î
K)
(b) Heat capacity
Figure 8 Thermal properties test results
4.7 Other properties
Although not directly tested in this study, it is reported that adding RAP as a coarse aggregate replacement in concrete increases drying shrinkage (22). It is also indicated that the RAP incorporation helps improve concrete fracture properties (23; 24).
5. DISCUSSION
As shown in Section 4, the incorporation of RAP causes considerable changes on concrete properties, which will lead to significant impacts on pavement behavior. One immediate concern
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of using RAP in concrete pavement is the reduction in strengths. More specifically, concrete tensile strength is one of the most important inputs for pavement slab thickness design, and the use of RAP leads to reduction in flexural strength of RAP-PCC. Fortunately, it has been reported that the tensile strength reduction is not significant as the compressive strength reduction, and the tensile strength reduction can be controlled within 25 % if not more than 40% coarse RAP is used to replace the same volume of virgin coarse aggregate in concrete. To compensate the reduced tensile strength, a thicker pavement slab may need to be considered in the design for RAP-PCC pavement.
The asphalt in RAP (even though aged) is less stiff than cement matrix and virgin aggregates. Therefore, the produced RAP-PCC invariably has reduced MOE. Concrete pavement slab with reduced MOE is anticipated to have a better cracking resistance due to the lower stress level (17). In addition, the higher viscoelasticity of RAP based concrete could potentially lead to higher creep in the concrete structures, which further relaxes stress (22). On the other hand, the low MOE could cause higher slab deflections, though. The higher differential energy caused by the deflection differential between the unloaded and loaded slabs results in a higher amount of base erosion, which eventually leads to higher slab faulting (17). This finding has been validated in a field study in Oklahoma. The field evaluation findings based on a recycled concrete aggregate (RCA) pavement section in Oklahoma can be applicable for the RAP-PCC pavement case because RAP-PCC and RCA-PCC behavior similarly in terms of having reduced MOE and MOR and increased CoTE. Falling weight deflectometer results from the RCA-pavement field study showed that slabs built with the RCA exhibited higher slab deflection differential compared to control slabs. Distress survey data confirmed that there existed higher joint faulting in the RCA concrete pavement section relative to the control section (25). To account for the higher base erosion caused by the softer slab, the use of stronger base materials is highly recommended for pavements built with recycled aggregate based concrete slabs.
The increased coefficient of thermal expansion and drying shrinkage of the RAP-PCC could lead to lower concrete pavement performance. According to the simulation studies by the authors (17; 18), the increased CoTE causes higher tensile stress levels in concrete labs, leading to a higher chance of fatigue cracking. Reducing joint spacing turned out to be effective in reducing the slab stress. Blending recycled aggregates with aggregate having a low CoTE (such as limestone, granite or basalt) and using shrinkage reducing admixture can also be useful to mitigate the problem.
The decrease in thermal conductivity turns out to be marginal for the RAP-PCC mixtures in this study. Based on a previous study, the change of the thermal properties will not cause a significant change on the temperature profile in the pavement for the studied RAP-PCC mixtures (17). Therefore, the effect of RAP on pavement performance due to the pavement temperature change is minimal. However, it should be noted that pavement slab built with a low thermal conductivity material will develop an increased temperature gradient, causing higher thermal stress and deflection in the slab. It is also shown that pavement with lower thermal conductivity could have a hotter pavement surface during summer and a colder surface during winter, and this may lead to higher urban heat island effect and extra snow and ice formation, respectively (26).
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In addition, the RAP-PCC exhibits improved fracture properties (23). The size effect theory suggests that, compared to tensile strength, the concrete fracture properties are more relevant to the performance of intermediate and large sized structures such as pavements (27). Therefore, RAP-PCC pavement is likely to have an improved performance.
6. CONCLUSIONS
In this paper, two different Texas coarse RAPs were collected and characterized; they partially replaced the virgin coarse aggregate at 20 vol.% and 40 vol.% to formulate RAP-PCC mixtures. An extensive experimental program to test the PCC properties of the studied RAP-PCC mixtures was carried out. Potential impacts of the addition of RAP on concrete pavement behavior and any recommendations for this application are discussed. The following conclusions are made:
Adding RAP into PCC to partially replace up to 40% volume of virgin coarse aggregate yields lower compressive strength, modulus of elasticity, modulus of rupture and higher Poisson’s ratio and coefficient of thermal expansion.
The effect of RAP on concrete flexural strength is not as significant as the compressive strength, so RAP-PCC may still meet the flexural strength criterion for pavement applications. A thicker slab is needed to compensate the strength reduction.
The reduced MOE can cause lower stress levels in the slab, which can increase slab fatigue resistance. However, the lower MOE may cause higher slab deflection, leading to higher base erosion and slab faulting. A stronger base is required for RAP-PCC pavement.
The higher CoTE of the RAP-PCC increases the stress level in the slab, causing higher fatigue damage. Blending RAP with low CoTE aggregate and reducing the joint spacing will mitigate this issue.
ACKNOWLEDGEMENTS
The research presented in this paper was partially supported by the Texas Department of Transportation (TxDOT). Any opinions, findings, conclusions, and recommendations expressed in this paper are those of the authors alone and do not necessarily reflect the views of the sponsoring agencies.
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