Experiences of Developing and Validating a New Mix … of Developing and Validating a New Mix Design Procedure ... A new mix design procedure for CIR-foam was developed and validated

Proceedings of the 2007 Mid-Continent Transportation Research Symposium, Ames, Iowa, August 2007. © 2007 by Iowa State University. The contents of this paper reflect the views of the author(s), who are responsible for the facts and accuracy of the information presented herein.

Experiences of Developing and Validating a New Mix Design Procedure for Cold In-Place Recycling Using Foamed Asphalt Yongjoo Kim Public Policy Center University of Iowa 227 South Quadrangle Iowa City, IA 52241 [email protected] Hosin “David” Lee Public Policy Center University of Iowa 227 South Quadrangle Iowa City, IA 52241 [email protected] Michael Heitzman Office of Materials Iowa Department of Transportation 800 Lincoln Way Ames, IA 50010 [email protected] ABSTRACT

Asphalt pavement recycling has grown dramatically over the last few years as the preferred way to rehabilitate existing asphalt pavements. Rehabilitation of existing asphalt pavements has employed different techniques; one of them, cold in-place recycling with foamed asphalt (CIR-foam), has been effectively applied in Iowa. However, Iowa’s current cold in-place recycling (CIR) practice utilizes a generic recipe specification to define the characteristics of the CIR mixture. The contractor is given latitude to adjust the proportions of stabilizing agent to achieve a specified level of density. As CIR continues to evolve, the desire to place CIR mixture with specific engineering properties requires the use of a mix design process. First, some strengths and weaknesses of the mix design parameters were identified, and the laboratory test procedure was modified to improve the consistency of the mix design process of CIR-foam. Both Marshall and indirect tensile strength test procedures were evaluated as foamed asphalt mix design procedures using reclaimed asphalt pavement (RAP) materials. Based upon the critical mixture parameters identified, a new mix design procedure using indirect tensile testing and vacuum-saturated wet specimens was developed. The second phase was then launched to validate the developed laboratory mix design process against various RAP materials in consideration of its predicted field performance. The optimum foamed asphalt contents, for all RAP materials, were consistently found at values between 1.5% and 2.5%. The dynamic modulus values were affected by both foamed asphalt contents and RAP aggregate structure. The flow number is affected dominantly by the RAP aggregate structure.

Key words: cold in-place recycling—dynamic modulus—flow number—foamed asphalt—indirect tensile strength—mix designs procedure—reclaimed asphalt pavement

Kim, Lee, Heitzman 2

INTRODUCTION

A desire to maintain a safe, efficient, and cost-effective roadway system has led to a significant increase in the demand to rehabilitate the existing pavement. Asphalt recycling has grown dramatically over the last few years as the preferred way to rehabilitate existing pavements. Rehabilitation of existing asphalt pavements has employed different techniques, one of them being cold in-place recycling (CIR).

Recently in Iowa, cold in-place recycling with foamed asphalt (CIR-foam) has become more common for rehabilitating existing asphalt pavements due to its cost-effectiveness, the conservation of paving materials, and its environmental friendliness. Iowa’s current CIR practice utilizes a generic recipe specification to define the characteristics of the CIR mixture. The contractor is given latitude to adjust the proportions of stabilizing agent to achieve a specified level of density. As CIR continues to evolve, the desire to place CIR mixture with specific engineering properties requires the use of a mix design process. However, there is no design procedure available for CIR-foam.

A new mix design procedure for CIR-foam was developed and validated. First, some strengths and weaknesses of the mix design parameters were identified, and the laboratory test procedure was modified to improve the consistency of the mix design process of CIR-foam. The phase two study was conducted to validate the developed laboratory mix design process against various RAP materials across the state of the Iowa. As part of the validation effort to evaluate the consistency of the new CIR-foam mix design process, simple performance tests, which included the dynamic modulus test, dynamic creep test, and raveling test, were conducted over a wide range of traffic and climatic conditions.

DEVELOPMENT OF CIR-FOAM MIX DESIGN PROCESS

Various foamed asphalt mix design parameters produced from numerous past studies for full-depth reclamation (FDR) and CIR were reviewed, and detailed laboratory test results are documented in Lee and Kim (2003). To conduct laboratory experiments for the CIR-foam mix design process, RAP materials were collected from the CIR-foam project site on US-20, which is located at about four miles west of the intersection of US-20 and Highway 13 near the city of Manchester. The existing asphalt pavement was milled throughout the day, and, to identify the possible variation in RAP gradations, the temperatures of the milled RAP materials were measured throughout the day. Based on the limited study of RAP materials, the time of the milling and the temperature of the pavement during the milling process did significantly affect the RAP gradation. To identify the impact of the RAP gradation on the CIR-foam mix design, three different RAP gradations were designed as “fine,” “field,” and “coarse.”

The PG 52-34 asphalt binder was used as the stabilizing agent for the laboratory foamed asphalt mix design. Using the laboratory foaming equipment from Wirtgen, shown in Figure 1, the foaming water content of 1.3% created the optimum foaming characteristics in terms of an expansion ratio of 10-12.5 and a half-life of 12-15 at 170°C. As illustrated in Figure 2, the flowchart of the laboratory mix design process of CIR-foam was developed to identify the critical mix design parameters, which included laboratory test procedures, RAP gradation, and the optimum moisture content of RAP.


Foamed Asphalt binder

Air (4 bar)

Cold Water(5 bar)

HotAsphalt(1.5 bar)

2.5mm

6mm

Figure 1. Laboratory foaming equipment (left) and production of foamed asphalt (right)

Collect RAP Materials from the Job Site

Dry RAP Materials in the Air

Analyze RAP Gradation

Sort RAP Materials as Five Different Sizes

(12.5mm, 9.5mm, 4.75mm, 1.18mm, passing 1.18mm)

Select Asphalt Binder Grade

Determine Optimum Foaming Water Content (OFWC)

Determine Optimum Moisture Content (OMC) of RAP Materials

Laboratory Mix Design (Combinations of FAC and MC)

- Using Gyratory compactor at 30 gyrations

- Curing at 40°C oven for two days

Measure Volumetric Characteristics

Conduct Indirect Tensile Strength Test

- Using vacuum saturated CIR-foam Specimen

Determine Optimum FAC and MC

Collect RAP Materials from the Job Site

Dry RAP Materials in the Air

Analyze RAP Gradation

Sort RAP Materials as Five Different Sizes

(12.5mm, 9.5mm, 4.75mm, 1.18mm, passing 1.18mm)

Select Asphalt Binder Grade

Determine Optimum Foaming Water Content (OFWC)

Determine Optimum Moisture Content (OMC) of RAP Materials

Laboratory Mix Design (Combinations of FAC and MC)

- Using Gyratory compactor at 30 gyrations

- Curing at 40°C oven for two days

Measure Volumetric Characteristics

Conduct Indirect Tensile Strength Test

- Using vacuum saturated CIR-foam Specimen

Determine Optimum FAC and MC

Figure 2. Laboratory foamed asphalt mix design flowchart


The laboratory mix design process for CIR-foam is described below to understand how to perform the CIR-foam mix design process in the laboratory:

• Step 1. RAP materials collected from the field should be dried in the air until the moisture content exhibits between 0.3% and 0.1%.

• Step 2. RAP materials collected from the field should be analyzed to determine gradation. First, for gradation analysis, RAP materials larger than 25 mm were discarded. To produce the laboratory CIR-foam mixtures of the field gradation, the remaining RAP materials were then divided into four or five stockpiles retained on each of the following sieves, 19.0 mm, 9.5 mm, 4.75 mm, 1.18 mm, as well as the materials that passed the 1.18 mm sieve.

• Step 3. Optimum foaming water content should be determined by achieving the maximum expansion ratio and half-life for a given asphalt binder. Expansion ratio is defined as the maximum volume over its original volume, and half-life is defined as the time in seconds for foam to become a half of its maximum volume.

• Step 4. The optimum moisture content during mixing and compaction is considered one of the most important mix design criteria for CIR-foam mixtures. The modified proctor test should be conducted in accordance with ASTM D 1557, “Standard Test Methods for Laboratory Compaction Characteristics of Soil Using Modified Effort (2,700 kN-m/m3).”

• Step 5. Thirty gyrations of the Superpave gyratory compactor should be used to produce CIR-foam test specimens with a diameter of 100 mm. The compacted foamed asphalt specimens should be cured in the oven for two days at 60 °C (Figure 3).

Figure 3. Gyratory compaction and curing process

• Step 6. The cured CIR-foam specimen should be placed under water at 25 ºC for 30 minutes and, to achieve the 100% saturation level, a vacuum of 20 mmHg should be applied for 30 minutes. The saturated sample is then placed under water for additional 30 minutes (Figure 4).


Figure 4. Vacuum saturation process

• Step 7. The indirect tensile strength test should be performed on three saturated CIR-foam mixtures for each of five foamed asphalt contents ranging from 1.0% to 3.0% at 0.5 increments (Figure 5).

Figure 5. Indirect tensile strength test

• Step 8. The optimum foamed asphalt content (FAC) and optimum moisture content should be determined where the maximum indirect tensile strength is obtained.

COLLECTION AND EVALUATION OF RAP MATERIALS

In order to validate the mix design process developed during the first research phase, RAP materials were collected from seven different CIR project sites: three CIR-foam and four CIR-ReFlex sites. Figure 6 (left) shows the selected the CIR project sites across the state of the Iowa: Muscatine County, Webster County, Hardin County, Montgomery County, Bremer County, Lee County, and Wapello County.

First, RAP materials were divided into six stockpiles that were retained on the following sieves, 25 mm, 19 mm, 9.5 mm, 4.75 mm, 1.18 mm, as well as those passing through the 1.18 mm sieve. The sorted RAP materials were then weighed, and their relative proportions were computed. As shown in Figure 6 (right), gradation analyses for seven RAP sources were conducted, and the RAP materials from Muscatine County were the coarsest, followed by Montgomery, Webster, and Wapello Counties. Those from Hardin, Bremer, and Lee Counties were finer. All RAP materials were considered from fine to coarse with a very small amount of fine aggregates passing through the 0.075 mm (No. 200) sieve.


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#200 #50 #30 #16 #8 #4 9.5mm 12.5mm 19mm 25mm#100

Figure 6. Location of selected CIR job sites (left) and gradation plots (right)

As summarized in Table 1, the characteristics of the RAP materials from seven different RAP sources were analyzed in terms of (1) residual asphalt content, (2) residual asphalt stiffness in penetration index, and (3) amount of fines passing No.8 sieve. The extracted asphalt content ranged from 4.6% for RAP materials collected from Wapello County to 6.1% from Hardin County. The extracted asphalt of RAP material from Montgomery County exhibited the highest penetration of 28, whereas that of Hardin and Lee Counties showed the lowest penetration of 15.

Table 1. Characteristics of seven different RAP materials

RAP Characteristics RAP Source Residual AC (%) Stiffness (Pen.) % Passing No.8 Sieve

Lee County Middle (5.4%) Hard (15) Fine (36.5%) Webster County High (6.0%) Hard (17) Middle (28.6%) Hardin County High (6.1%) Hard (15) Fine (32.0%) Wapello County Low (4.6%) Soft (21) Coarse (26.0%) Bremer County Middle (5.0%) Hard (17) Fine (34.4%) Montgomery County High (5.7%) Soft (28) Coarse (25.8%) Muscatine County Low (4.7%) Middle (19) Coarse (21.9%)

VALIDATION OF A NEW MIX DESIGN PROCESS

To determine the consistency of a new CIR-foam mix design procedure, the newly developed CIR-foam mix design procedure was validated against seven different sources of RAP materials in Iowa. As shown in Figure 7, the indirect tensile strength test of the vacuum-saturated specimens was conducted using seven different RAP materials at five foamed asphalt contents, 1.0%, 1.5%, 2.0%, 2.5%, and 3.0%, given a fixed moisture content of 4.0%. The specimens were compacted by gyratory compactor at 30 gyrations or by a Marshall hammer at 75 blows and were cured at 40°C oven for three days or 60°C for two days (Lee and Kim 2007).


Five Stockpiles: 19mm, 12.5mm, 4.75mm, 1.18mm, passing 1.18mm

Wet Condition:Vacuum-Saturation at 20 mmHg for 30 min

Moisture Content:

Fine (4.5%), Middle (4.0%), Coarse (3.5%)

Foamed Asphalt Content:

1.0 %. 1.5 %, 2.0 %, 2.5 %, 3.0 %

Curing Condition: 40 °C for 3 days

60 °C for 2 days

Compaction Method: 75 blows (Marshall)

30 Gyrations (SGC)

• Extracted Aggregate Gradation• Extracted Asphalt Content• RAP Gradation Analysis• Penetration Test• Dynamic Shear Rheometer Test• Flat and Elongation Ratio

Collection of RAP Materials from job site

Evaluation of RAP Characteristics

Separation of RAP Materials

Selection of RAP Gradation, Asphalt Binder

Determination of Moisture Content

Mix Design

Measurement of Volumetric Characteristics

Indirect Tensile Strength Test

Determination of Optimum Foamed Asphalt Content

and Moisture Content

• Bulk Specific Gravity • Maximum Theoretical Specific Gravity • Air Void

Five Stockpiles: 19mm, 12.5mm, 4.75mm, 1.18mm, passing 1.18mm

Wet Condition:Vacuum-Saturation at 20 mmHg for 30 min

Moisture Content:

Fine (4.5%), Middle (4.0%), Coarse (3.5%)

Foamed Asphalt Content:

1.0 %. 1.5 %, 2.0 %, 2.5 %, 3.0 %

Curing Condition: 40 °C for 3 days

60 °C for 2 days

Compaction Method: 75 blows (Marshall)

30 Gyrations (SGC)

• Extracted Aggregate Gradation• Extracted Asphalt Content• RAP Gradation Analysis• Penetration Test• Dynamic Shear Rheometer Test• Flat and Elongation Ratio

Collection of RAP Materials from job site

Evaluation of RAP Characteristics

Separation of RAP Materials

Selection of RAP Gradation, Asphalt Binder

Determination of Moisture Content

Mix Design

Measurement of Volumetric Characteristics

Indirect Tensile Strength Test

Determination of Optimum Foamed Asphalt Content

and Moisture Content

• Bulk Specific Gravity • Maximum Theoretical Specific Gravity • Air Void

Figure 7. Laboratory mix design procedure used for validation study

The indirect tensile strength of the gyratory-compacted and vacuum-saturated specimens was more sensitive to foamed asphalt contents than that of Marshall hammer-compacted and vacuum-saturated specimens. The indirect tensile strength of CIR-foam specimens cured for two days in the 60°C oven was significantly higher than that of CIR-foam specimens cured for three days in the 40°C oven.

The optimum foamed asphalt content (OFAC) was determined when the highest indirect tensile strength of vacuum-saturated specimens was obtained. The indirect tensile strength of the gyratory-compacted specimens is higher than that of Marshall hammer-compacted specimens. The indirect tensile strength of foamed asphalt specimens cured at 60°C for two days is significantly higher than that of foamed asphalt specimens cured at 40°C for three days.

Due to its high moisture sensitivity, it is recommended that the indirect tensile strength test be performed on the vacuum-saturated CIR-foam mixtures. Further, the indirect tensile strength test is recommended for the CIR-foam mix design because it is a relatively simple procedure using the standard equipment available in a typical asphalt laboratory. As a result, the Iowa Department of Transportation and most contractors in Iowa can now easily perform the proposed CIR-foam mix design procedure.


Correlation between OFAC and RAP Characteristics

Attempts were made to discover a correlation between FAC and RAP characteristics, such as residual asphalt stiffness and residual asphalt content. The OFAC was determined based on a polynomial regression equation. A higher OFAC value was obtained from the RAP materials containing large amounts of hard residual asphalt. Figure 8 shows the correlation plots of residual asphalt stiffness values measured as a penetration index against OFAC at four different mix design conditions. As shown in Figure 8, a good correlation between OFAC and residual asphalt stiffness is exhibited, but no correlation between OFAC and residual asphalt contents was observed. The stiffer residual asphalt required more foamed asphalt, whereas the higher residual asphalt content did not require less foamed asphalt.

WapelloMuscatine

Hardin

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y = -13.766x + 49.438

R2 = 0.5754

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Figure 8. Correlations between optimum foamed asphalt content and residual asphalt stiffness

PERFORMANCE TEST RESULTS

The performance tests, which included the dynamic modulus test, dynamic creep test, and raveling test, were conducted to evaluate the consistency of a new CIR-foam mix design process to ensure reliable mixture performance over a wide range of traffic and climatic conditions.


Dynamic Modulus Test

For dynamic modulus and dynamic creeps tests, CIR-foam specimens with a 100 mm diameter and a 150 mm height were prepared at three foamed asphalt contents, 1.0%, 2.0%, and 3.0%, and at a 4.0% moisture content. CIR-foam specimens were compacted using the gyratory compactor at 30 gyrations, and the compacted CIR-foam specimens were cured in the oven at 40°C for three days.

The dynamic modulus test is used to determine the stiffness of asphalt mixtures on the response to traffic loading and various climate conditions. The dynamic modulus tests were performed on CIR-foam mixtures at six different loading frequencies, 0.1, 0.5, 1, 5, 10, and 25 Hz, and three different test temperatures, 4.4°C, 21.1°C, and 37.8°C, using simple performance test equipment, as shown in Figure 9.

Figure 9. Simple performance test equipment

Within each source of RAP materials, the dynamic moduli of RAP materials were not affected by loading frequencies but were significantly affected by the test temperatures. The rankings of RAP materials changed when the foamed asphalt was increased from 1.0% to 3.0%, which indicates that the dynamic modulus values are affected by both FAC and RAP aggregate structure. Based on the dynamic modulus test results performed at 4.4˚C, the coarser RAP materials with a small amount of residual asphalt content were more resistant to fatigue cracking. Based on the dynamic modulus test results performed at 37.8˚C, the finer RAP materials with the harder binder and with a higher amount were more resistant to rutting.

Dynamic Creep Test

With increasing truck traffic and tire pressure, rutting is one of the most critical types of load-associated distresses occurring in asphalt pavements. Therefore, it is important to characterize the permanent deformation behavior of asphalt mixtures in order to identify problematic mixes before they are placed in roadways. Dynamic creep tests were performed on CIR-foam mixtures under a loading stress level of 138 kPa at 40˚C using simple performance test equipment.


Based on the dynamic creep test, RAP materials from Muscatine County exhibited the lowest flow number of all foamed asphalt contents, whereas those from Lee and Webster Counties reached the highest flow numbers. The lower the foamed asphalt contents, the higher the flow number, which indicates that the foamed asphalt content with 1.0% is more resistant to rutting than 2.0% and 3.0%.

Characteristics of seven RAP materials are summarized in Table 2, along with the rankings in terms of flow number. RAP materials from seven different sources were ranked by flow number. Overall, the rankings of RAP materials did not change when the foamed asphalt was increased from 1.0% to 3.0%, which indicates that flow number is affected more dominantly by the RAP aggregate structure than by the foamed asphalt content. The finer RAP materials with a higher amount of the harder binder were more resistant to rutting. This result is consistent with the findings based on the dynamic modulus testing performed at 37.8˚C.

Table 2. Rankings of flow number from seven different RAP sources

Ranking of Flow Number

RAP Source Stiffness

(Pen.) Residual AC (%)

% Passing No.8 Sieve FAC=1.0% FAC=2.0% FAC=3.0%

Lee County 15 5.4% 36.5% 1 1 1

Webster County 17 6.0% 28.6% 2 2 2

Hardin County 15 6.1% 32.0% 3 3 3

Wapello County 21 4.6% 26.0% 4 4 6

Bremer County 17 5.0% 34.4% 5 5 5

Montgomery County 28 5.7% 25.8% 6 6 4

Muscatine County 19 4.7% 21.9% 7 7 7

Based on both the dynamic modulus and dynamic creep test results, it can be postulated that RAP materials from Wapello and Webster Counties would be more resistant to both fatigue and rutting. RAP materials from Muscatine, Bremer, and Montgomery Counties would be more resistant to fatigue cracking but less resistant to rutting. RAP materials from Hardin and Lee Counties would be more resistant to rutting but less resistant to fatigue cracking.

Raveling Test

A CIR-foam layer is normally covered by a hot mix asphalt (HMA) overlay or chip seal in order to protect it from water ingress and traffic abrasion and to obtain the required pavement structure and texture. During curing in the field, some raveling occurred from the surface of the CIR pavement before HMA overlay was placed. The raveling test was performed at room temperature using a gyratory-compacted 150 mm CIR-foam specimen to evaluate resistance to raveling right after construction.

The percent mass loss of the foamed asphalt specimens at 1.5% FAC and 2.5% FAC for two different cuing time periods is plotted in Figure 10. Based on the raveling test results, the foamed asphalt specimens at 2.5% foamed asphalt content showed less raveling loss than those of 1.5% foamed asphalt


content. It was found that the raveling test was very sensitive to the curing period and foamed asphalt content of the CIR-foam specimens. To increase cohesive strength quickly, it is necessary to use the higher foamed asphalt content of 2.5% instead of 1.5%.

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Figure 10. Percent raveling losses of CIR-foam specimens from seven different RAP sources

CONCLUSIONS AND RECOMMENDATIONS

Asphalt pavement recycling has grown dramatically over the last few years as a viable technology to rehabilitate existing asphalt pavements. Rehabilitation of existing asphalt pavements has employed different techniques; one of them, CIR-foam, has been effectively applied in Iowa. This research developed and validated the mix design procedure for CIR-foam. It was also demonstrated that the field performance of various CIR-foam mixtures could be predicted based on the test results from newly purchased performance testing equipment. Based on the extensive laboratory experiments on CIR-foam, the following conclusions, recommendations, and suggestions for future studies are made:

Conclusions

• The indirect tensile strength of gyratory-compacted specimens is higher than that of Marshall hammer-compacted specimens. The indirect tensile strength of CIR-foam specimens cured in the oven at 60˚C for two days is significantly higher than that of CIR-foam specimens cured in the oven at 40˚C for three days.

• The OFAC is influenced by residual asphalt stiffness more than the residual asphalt content. The stiffer residual asphalt would require more foamed asphalt

• Dynamic modulus of CIR-foam is affected by a combination of the RAP sources and foamed asphalt contents. Coarse RAP materials with a small amount of residual asphalt content may be more resistant to fatigue cracking but less resistant to rutting.

• Based on the dynamic creep tests performed at 40˚C, CIR-foam with 1.0% foamed asphalt is more resistant to rutting than CIR-foam with 2.0% or 3.0% foamed asphalt. RAP aggregate structure has a predominant impact on its resistance to rutting. The finer RAP materials with the more and harder residual asphalt were more resistant to rutting.

• CIR-foam specimens with 2.5% foamed asphalt content are more resistant to raveling than ones with 1.5%.


Recommendations

• To determine the OFAC, indirect tensile strength testing should be performed on vacuum-saturated specimens, which should be placed in 25˚C water for 20 minutes, vacuumed saturated at 20 mmHg for 30 minutes, and left under water for an additional 30 minutes without vacuuming.

• The OFAC should be increased from 1.5% to 2.5% if the penetration index of the residual asphalt from RAP materials increases from 28 to 15.

• The proposed mix design procedure should be implemented to assure the optimum performance of CIR-foam pavements in the field.

Suggestions for Future Studies

• CIR-foam pavements should be constructed following the new mix design process, and their long-term field performance should be monitored and verified against the laboratory performance test results.

• New mix design and laboratory simple performance tests should be performed on the CIR-foam mixtures using stiffer asphalt binder grade, i.e., PG 58-28 or 64-22.

• The CIR-foam mix design should be adapted for CIR-emulsion mixtures. • A comprehensive database of mix design, dynamic modulus, flow number, and raveling for both

CIR-foam and CIR-emulsion should be developed to allow for an input into the Mechanistic-Empirical Pavement Design Guide.


ACKNOWLEDGMENTS

The authors would like to thank the financial support provided by the Iowa Highway Research Board and the members of the steering committee for their guidance throughout the project.

REFERENCES

Lee, H. and Y. Kim. 2003. Development of a Mix Design Process for Cold In-place Rehabilitation Using Foamed Asphalt. Phase I Final Report. IHRB TR-474. Ames, IA: Iowa Highway Research Board, Iowa Department of Transportation.

Lee, H. and Y. Kim. 2007. Validation of the Mix Design Process for Cold In-place Rehabilitation Using Foamed Asphalt. Phase II Final Report. IHRB TR-474. Ames, IA: Iowa Highway Research Board, Iowa Department of Transportation.

Documents

Experiences of Developing and Validating a New Mix … of Developing and Validating a New Mix Design Procedure ... A new mix design procedure for CIR-foam was developed and validated