Home Improvement in the California bearing ratio of subbase soil by recycled asphalt pavement and cement
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Improvement in the California bearing ratio of subbase soil by recycled asphalt pavement and cement

  • Rasha A. Al-Fatlawy , Tawfek Sheer Ali EMAIL logo , Mohammed K. Fakhraldin , Nibras A. Hussain and Ibtihal Y. Abd
Published/Copyright: October 6, 2023
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Open Engineering
From the journal Open Engineering Volume 13 Issue 1

Abstract

As the subbase layer is one of the important layers in road construction, it is necessary to use a granular material to resist traffic loads. Usually, granular soil with a good gradation is used, which is determined by most road design codes. Sometimes it is needed to improve the properties of this soil by adding materials such as cement, which is considered a high-cost material, so it is necessary to search for low-cost materials. One of these materials is recycled asphalt pavement (RAP). Herein, we presented an experimental study to investigate using RAP and cement in roadway subbase material improvement. The indicator of the improvement has been selected as the California bearing ratio (CBR); many laboratory tests were made for many percentages of cement and RAP. The study’s main conclusions are that the CBR value increases when the cement percentage increases and the RAP percentages increase up to 12% by the mass of the virgin subbase and then decrease. Also, using a mix of 12% RAP and 1% cement blended with the subbase material gives a CBR value of 138.6. This improvement is considered the same as using 6% (133.5) cement but it is of low cost due to the low cost of the waste material (RAP) as it saved the high cost of 5% cement.

Keywords: subbase improvement; recycled asphalt pavement; CBR improvement; soil improvement

1 Introduction

1.1 Description

Generally, in road construction, the pavement system consists of three layers: pavement layer at the top, subbase, and subgrade at the bottom. The subbase layer is an aggregate material and sometimes consists of recycled materials. The recycled materials can be mixed with the subbase soil to give it an acceptable gradation, stiffness, permeability, frost resistance, deformation resistance, and low cost. One recycled material is recycled asphalt pavement (RAP) and 20–50% of it is used. The higher percentages of RAP increase the stiffness of subbase soil, but incorporating the RAP and subbase soil limits the use of high percentages. In roadway construction, the subbase soil must be able to support loads applied on the surface. One of the strongest indicators is the California bearing ratio (CBR). Research has indicated that a CBR of 10 or more can support the traffic loads [1].

In the United States, since 1990, 80% of more than 90 million tons of asphalt pavement is converted to a usable material every year. Once the asphalt pavement is removed and processed, it becomes RAP which contains aggregate and asphalt binder. RAP is used as a granular subbase and embankment or fill material with low cost than other virgin materials [2].

Maher et al. [3] evaluated the use of RAP as a base or subbase material in the roadway of New Jersey by investigating the resilient strength of RAP in the field and laboratory. The laboratory results showed that RAP has the same strength as a dense base or subbase soil. The field testing included evaluating the elastic modulus of RAP using spectral analysis of the surface waves method (SASW). The results indicated that the stiffness and elastic modulus are higher than the dense base aggregate.

Taha et al. [4] conducted a laboratory test to study the effect of adding RAP to the base and subbase materials. In this study, the conclusions indicated that the use of 100/0, 80/20, 60/40, 40/60, 20/80, and 0/100% of RAP/subbase could be expected to use RAP instead of subbase soil. The higher CBR and dry density are noticed when virgin subbase content increases. In the Sultanate of Oman roads, RAP and a conventional subbase material can be used.

Jawad and Baqir [5] studied the effect of using bentonite for improving the dry density, cohesion, and angle of internal friction of sandy soil, 2.5–10%, with a 2.5% increment percentage of bentonite used. The results showed that the dry density improves with the increase of bentonite, and the cohesion increases significantly, while the effect on the angle of internal friction was insignificant.

Mohsen and Jawad [6] investigated the use of cement to improve CBR and Atterberg limits, and the chosen cement/soil content percentages were 2, 4, 6, and 8%. The results showed that the CBR increased more than 100% at 6% cement content. Also, the equivalent single axle load increased by about 45%.

Fakhraldin et al. [7] investigated the use of one of the waste materials in Iraq, the reclaim of tires industry, as an additive to the sandy soil. The effect of reclaim mixing with the sandy soil on CBR was studied. The percentages used were 0.5, 2, and 4% by the weight of sand under the soaked conditions. The results showed that the tire reclaim decreased CBR from 86.8 to 7.6%.

Seferoglu et al. [8] investigated the effects of RAP and cement on the soaked CBR of base aggregate. The RAP percentages were 10, 20, 30, 40, 50, 60, and 100%, and 1, 2, and 3% cement. The results showed that the high percentages of RAP decreased soaked CBR without cement but increased with cement. The value of CBR for mixing 100% RAP and 3% cement with the virgin material gives the same value of CBR of only 20% RAP, so it can be concluded that the cement is considered a good stabilizing material.

Kalpakci et al. [9] evaluated the possibility of adding sand to RAP to improve CBR; the study region was Iraq. The used RAP/sand percentages were 5, 10, 15, 20, 25, and 50% sand by mass of RAP. The results showed that the CBR increased when the sand percentage increased and could be used 20% or more sand mixed with RAP in road fill material.

Balkis and Macid [10] studied the effect of adding cement to different soil types on CBR values, and 3, 7, and 10% cement by mass of virgin samples were selected. The study concluded that cement increases the CBR value of different types of soils within a range of 22–69%. Cement also improves the strength parameters of subbase or subgrade layers, ultimately lowering the construction of road costs. According to the authors, the CBR improvement is mainly related to improving compressive strength and decreasing plastic behavior due to cementation.

Aziz et al. [11] examined the shear strength of the gypseous soil using the constant head and direct shear tests to evaluate the effect of using the proposed stabilizer additives (vehicle waste oil and asphalt powder) to produce a composite material as a novel solution. 3, 5, and 7% by dry soil of burned-oil mixed with a constant quantity of asphalt powder (10% of the sample weight) to evaluate the shear strength of the mixture. The conclusions revealed that the mixture improve the strength of the gypseous soil and the stabilizer reduces percolation and water leakage and by forming an impermeable layer with a very fine texture.

Choobbasti et al. [12] studied the effect of nanosilica and cement on the properties of sandy soil. The used percentages of cement were 5, 9, and14% by the mass sample which were mixed with 0, 5, 10 and 15% by the mass of silica, and then compacted into a cylindrical specimen. The study concluded that the addition of the cement and nanosilica improves the engineering properties. The maximum dry unit weight increase when the cement content increase and the presence of nanosilica in optimum percentages improve the mechanical properties of cement sand.

1.2 Scope of study

In the last few years, Iraq has been experiencing a huge reconstruction of highways and roadways. The pavement layer waste is a waste material due to reconstruction operations. Using RAP in the subgrade, subbase, and pavement layers is very important. Many problems occurred in the roadways due to the settlement in subbase layers because of a weak bearing resistance, so a cheap material should be found to improve the subbase soil layer. This study investigates using RAP as a waste material to improve the subbase material’s California bearing ratio (CBR). Many laboratory tests for the virgin subbase material and samples with Portland cement and RAP separately have been presented to evaluate the effects on CBR. Finally, one test was conducted using the ideal RAP percentage with 1% cement added to the subbase material to give a CBR of 6% cement to reduce the high cost of cement.

2 Used materials

2.1 Subbase soil

The soil used in this study represents a subbase granular material that was brought from one of the quarries of the Bahr Al-Najaf Sea region located in the southwest of Al-Najaf governorate in Iraq. This soil was used because most of Iraq’s highways, roads, and other construction projects use it. Table 1 shows the physical and chemical properties of this subbase soil. The size distribution of the subbase grains is shown in Figure 1. The used subbase soil was classified according to the AASHTO soil classification system [18] as A-2-6 (0.3), and according to the Standard Specifications for Roads and Bridges (SSRB) in Iraq [19] can be classified as Class C.

Table 1

Physical and chemical properties of the subbase soil

Property Test method Result
Liquid limit (LL) (%) ASTM D423 [13] 33.7
Plasticity index (%) AASHTO T90 [14] 13.0
Organic content (%) BS 1377, No. 8 [15] 1.3
SO3 (%) BS 1377, No. 9 [15] 1.3
Optimum moisture content (OMC) (%) AASHTO T180 [16] 8.2
Maximum dry density (MDD) (kN/m3) AASHTO T180 [16] 21.2
CBR AASHTO T193 [17] 19.1
Figure 1 The grain size distribution of the subbase soil.
Figure 1

The grain size distribution of the subbase soil.

2.2 Cement

The cement used in this study is an Ordinary Cement produced in the Kufa Cement Plant; the chemical composition and physical properties of plant reports are presented in Table 2.

Table 2

Chemical and physical properties of ordinary cement

Chemical composition Value (%) Iraqi specifications (%)
CaO 62.5 —
SO3 2.0 ≤2.5
MgO 2.8 ≤5
Fe2O3 3.5 —
SiO2 21.1 —
Al2O3 5.8 —
Loss of ignition 1.5 ≤4
Insoluble residue 0.8 ≤1.5
C3A 10.9 —
Physical properties Value (%) Iraqi specifications
Time of setting
Initial setting (h) 1.2 ≥45 min
Final setting (min) 3.6 ≤10 h
Compressive strength (MPa)
3 days 24.3 ≥15
7 days 32 ≥23

2.3 RAP

Iraq is witnessing reconstruction works for most highways and roadways, which require removing the old asphalt layers as waste material. To benefit from the asphalt waste resulting from these works, it was possible to investigate the ability to use these wastes to improve the properties of the weak subbase soil used in the construction of roads, and hence, the idea of this research. In this study, RAP is considered an additive material, and this material can be mixed with the granular subbase soil to investigate its effect on subbase soil. Recycled materials with a suitable grain size distribution, high stiffness, high permeability, low frost sensitivity, and high deformation resistance can be good subbases; one recycled material is the RAP [1]. The typical percentages of RAP range from 20 to 50%, and using recycled materials can reduce road construction costs. Still, incorporating RAP into the subbase soil limits the use of a high percentage of RAP, although the stiffness may be larger with a high percentage of RAP [1]. RAP was ground and mixed with the subbase soil during the laboratory tests.

3 Experimental work

The laboratory work included two stages: first the CBR of the granular material (subbase soil) without additive (reference sample [RS]) and with five cement percentages as an additive, i.e., 0, 1, 2, 4, and 6% by mass of soil was investigated. CBR is determined according to the procedure of AASHTO T193 [17]. The strength of the base, subbase, and subgrade layers of highways, roads, and other projects are affected directly by the CBR value. Therefore, it is very important to improve or increase this value. CBR, which the California Department of Transportation adopted, has been used assiduously for highways and road design purposes; CBR has been used for soil materials but it was first adopted for granular aggregates with sizes between 4.75 and 20 mm. The second stage included using RAP as an additive to the subbase soil at 1, 2, 4, 6, 10, 12, 14, 16, and 20% by mass of the granular material.

3.1 Sample preparation

The procedure of AASHTO T180 [16] was used to find the OMC and MDD values; these values were 8.2% and 21.2 kN/m3, respectively, as listed in Table 1. The sample retained on a 19 mm sieve was repaid with equal mass from sieving the same unused soil passing the 19 mm and retained on the sieve of 4.75 mm. The MDD values of all samples were found with an OMC of 8.2%. The CBR apparatus has been used to investigate the CBR of many samples without and with additives (cement and RAP).

3.2 CBR testing method

The AASHTO T193 [17] specification procedure of the CBR test was used as a main test in this study to investigate the effect of additives on the mechanical properties of soil samples (the subbase granular material). The 1, 2, 4, and 6% cement by soil mass and 1, 2, 4, 6, 10, 12, 14, 16, and 20% RAP were used. Figure 2 shows the CBR testing machine with one of the samples.

Figure 2 CBR test machine.
Figure 2

CBR test machine.

4 Results and discussion

4.1 Compaction test and CBR test of the RS

The compaction test was conducted to find the MDD and OMC of the soil because they are useful in the preparation of samples to investigate other soil characteristics parameters such as the density, porosity, CBR, and triaxial test. This study used modified proctor compaction and CBR tests on the subbase sample without cement and RAP as an RS. The relationship between the dry density and moisture content is shown in Figure 3, and it can be seen that MDD and OMC are 21.2 kN/m3 and 8.2%, respectively. Figure 4 shows the relationship between displacement and pressure of the CBR test for the RS. The higher pressure values of 2.5 and 5 mm displacement divided by mold area give the CBR value of the sample. The CBR value of the RS was 19.1, as shown in Table 1.

Figure 3 Dry density and moisture content relationship of the RS.
Figure 3

Dry density and moisture content relationship of the RS.

Figure 4 Displacement and pressure relationship of CBR test for the RS.
Figure 4

Displacement and pressure relationship of CBR test for the RS.

4.2 CBR test with cement

At OMC, the CBR test was conducted using five percentages of cement, i.e., 1, 2, 4, and 6% of the total mass of subbase soil, as an additive to study its effect. Figure 5 shows the load–pressure relationship for the tests; from the figure, it can be visualized that the increase in cement percentage increases the CBR. The CBR values were 64, 96.6, 119.5, and 133.5 for 1, 2, 4, and 6% cement, respectively.

Figure 5 Displacement and pressure relationship of CBR test using cement.
Figure 5

Displacement and pressure relationship of CBR test using cement.

4.3 CBR test with RAP

At OMC, the CBR laboratory tests were performed with RAP addition to study its effect on CBR. The percentages used were 1, 2, 4, 6, 10, 12, 14, 16, and 20% by the weight of the subbase soil–granular material. The relationships between displacement and pressure of these tests are shown in Figure 6.

Figure 6 Displacement and pressure relationship of CBR test using RAP.
Figure 6

Displacement and pressure relationship of CBR test using RAP.

4.4 Discussion

A total of 14 laboratory tests and results that are presented in previous sections are listed in Table 3.

Table 3

Summary of results

Additive percentage (%) 0 1 2 4 6 10 12 14 16 20
CBR with cement 19.1 64 96.6 119.5 133.5 — — — — —
CBR with RAW 19.1 21.5 22.1 25.5 35.8 47.6 79.8 51.6 44.2 30.8

From the results, it can be seen that the CBR value increases significantly with an increase in the cement percentage. The specification of roads limits the maximum percentage of added cement to the subbase soil up to 6%. The percentage in this study increases CBR about six times of the RS (19.1–133.5). The relationship between the cement additive percentage and CBR is shown in Figure 7. The null percentage represents the CBR value of the subbase soil without any additive (RS).

Figure 7 Relationship between the cement percentage and CBR.
Figure 7

Relationship between the cement percentage and CBR.

Figure 8 shows the relationship between RAP percentages and CBR values. As CBR increases up to 12% and then decreases, it can be said that the ideal percentage of RAW is 12% to give a maximum improvement in CBR. The value of CBR increased about three times from RS (19.1–79.8). This can be explained by increasing the voids ratio of the mix, which leads to higher compressibility and thus a decrease in the CBR value. Figure 9 shows the results of using cement and RAP together.

Figure 8 Relationship between the RAP percentage and CBR.
Figure 8

Relationship between the RAP percentage and CBR.

Figure 9 Relationship between additives and CBR.
Figure 9

Relationship between additives and CBR.

With the comparison of other studies presented in the introduction of this article, the results are compatible with previous studies [4,6,8,10].

4.5 Use of cement and RAP

From the results discussed above, it can be concluded that using 1% cement with 12% RAP to improve the CBR of subbase soil gives a CBR value of 143.8 (64 + 79.8 = 143.8). A laboratory test was performed to investigate the effect of mixing 1% cement and 12% RAP with the subbase soil on CBR, which gave a CBR value of 138.6. So, it can be concluded that using 1% cement and 12% RAP improves CBR about six times the RS and reaches the value corresponding to 6% cement, so the cost savings will be very large for saving 5% cement. All results are shown in Figure 9.

5 Conclusions

Herein, we presented an experimental study investigating RAP and cement in the improvement of roadway subbase material. The indicator was selected as CBR; many laboratory tests were conducted with different percentages of cement and RAP. The conclusions of the study can be drawn as follows:

  1. The CBR value increases when the cement percentage increases; the CBR value increases about six times from the RS of the virgin subbase at 6% cement by mass (19.1–133.5).

  2. The CBR value increases when the RAP percentages increase up to 12% by the mass of the virgin subbase and then decreases within the selected percentages (1, 2, 4, 6, 10, 12, 14, 16, and 20%).

  3. For the subbase granular material used in the study, which is classified as A-2-6 (0.3) according to the AASHTO, the ideal percentage of RAP is 12%, improving CBR by about 3.7 times (19.1–79.8).

  4. To reach the near improvement of CBR by using 6% cement (133.5), 12% RAP (CBR = 79.8), and 1% cement (CBR = 64) can be used; these percentages according to the separate tests with only 1% cement and only 12% RAP give a CBR value of 143.8.

  5. Experimentally, using a mix of 12% RAP and 1% cement blended with the subbase material gives a CBR value of 138.6. This improvement is considered the same as using 6% cement but at a low cost due to the low cost of the waste material (RAP) because it saves the high cost of 5% cement.

  1. Funding information: The authors state no funding involved.

  2. Conflict of interest: The authors state no conflict of interest.

  3. Data availability statement: Most datasets generated and analyzed in this study are in this submitted manuscript. The other datasets are available on reasonable request from the corresponding author with the attached information.

  4. Competing interest: The authors state no competing interest.

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Received: 2023-04-04
Revised: 2023-04-23
Accepted: 2023-04-26
Published Online: 2023-10-06

© 2023 the author(s), published by De Gruyter

This work is licensed under the Creative Commons Attribution 4.0 International License.

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https://doi.org/10.1515/eng-2022-0449
Keywords for this article
subbase improvement; recycled asphalt pavement; CBR improvement; soil improvement
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