Construction and Building Materials, Vol. 10, No. 2, pp. 141-146, 1996 Copyright Q 1996 Elsevier Science Ltd

ELSEVIER 095&0618(95)OOM2-9

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Tensile reinforcement polymer coating

of asphalt concrete using

Kwang W. Kim*, Yong-Churl Park and Kyu-Seok Yeon

Department of Agricultural Engineering, Kangwon National University, Chun Cheon, 200-707 South Korea

Received 7 March 1995; revised 16 May 7995; accepted 20 May 1995

This study investigates the possibility of utilizing a polyester resin for reinforcing flexible pave- ments. The application of a thin-layer coating with a polymer, unsaturated polyester resin (UPR) on the surface of a laboratory-prepared unmodified asphalt concrete mixture was studied as a tensile reinforcement method for such a material. Selected laboratory performance tests were conducted and the results are compared with those of a normal (uncoated) asphalt concrete mixture and a modified asphalt mixture, both mixtures being widely used in Korea. The polymer coating was found to be effective in improving Marshall stability, tensile strength and flexural strength of asphalt concrete. These improvements can be explained as the effect of reinforcement by a thin polymer layer which is fully bonded to the specimen faces. The reinforcement was also effective in reducing the stiffness of the mixture whilst improving load-carrying capacity. This improvement in strength and reduction in stiffness resulted in a retardation of crack initiation resulting from cyclic load application and a significantly improved resistance to crack propagation. The study has shown that there is a possibility of using the polymer coating as a method of tensile reinforcement with flexible pavements.

Keywords: asphalt concrete; tensile reinforcement; polymer coating

Tensile strength is an important property for flexible pavements to provide a satisfactory service perfor- mance. Different types of reinforcement have been used to improve the tensile properties of flexible pavements in an effort to extend the service life of pavements. Researchers have investigated the use of wire meshesi,z, geotextiles3.4 and their related productss.6. Brown et al.7 and Kennepohl et al.8 introduced the concept of poly- mer grid reinforcement. Many performance evaluation methods and data with tensile reinforcement were pre- sented at a RILEM conferences. These reinforcement systems were known to provide improved load-carrying capacity through interlock of the material and signif- icant retardation against reflective cracking with overlays. Recently, various polymer-modified asphalt binders have also been widely used, not only to improve tensile strength, but also to overcome many other weaknesses common with asphalt binder+). How- ever, each particular option dictates where the rein- forcement should be placed, and its relative technical and cost effectiveness.

In this regard, the road maintenance authorities want to use the method or material for tensile reinforcement

*Correspondence to K. W. Kim

with the highest cost/benefit ratio. This study was devised to investigate the possibility of utilizing the superior characteristics of a polymer material for rein- forcing flexible pavement layers. An application of a thin-layer coating with a polymer resin was studied as a simple tensile reinforcement method for asphalt concrete. Since typical low-cost polymer resins are available, which can generate a significant strength after curing in a short time, the simple coating of the tension side at a pavement layer with such a resin would provide an economic tensile reinforcement to the layer.

The objective of this study is to evaluate the effectiveness of a polymer coating for the purpose of tensile reinforcement of an asphalt concrete pavement material in the laboratory. The flat surfaces of laboratory-prepared unmodified asphalt concrete speci- mens were coated with a very thin layer of polymer, and the significance of reinforcement was evaluated. Selected laboratory performance tests were conducted and the results were compared with those of a normal (uncoated) asphalt concrete mixture and a modified asphalt concrete mixture, both of which are widely used in Korea. A brief description of the experimental procedures used and results of the laboratory investi- gation on performance of the mixtures are presented in this paper.

141

142 Polymer reinforcement for asphalt concrete: K, W. Kim et al.

Test programme

Muteriuls

Penetration grade 85/100 asphalt cement, the most widely used in South Korea, was used as a binder with a normal asphalt mixture. A coarse granite aggregate and a river sand were used to make specimens of all three asphalt concrete mixtures. The gradation of the combined aggregate along with the specification limit are presented in Tuble 1.

A normal hot dense graded asphalt concrete mixture (complying with WC-3) was designed based on the Korean Highway Corps specificationtz. The optimum asphalt concrete for a job-mix formula was decided at 5.5%. An unsaturated polyester resin (UPR), which is an economic material being widely used in polymer concrete manufacturets~tt, was selected as a polymer- coating material. A polymer-coated asphalt mixture was produced by applying the polymer in a thin layer (film) on the flat surface of laboratory specimens and the tensile face of the beam of the normal mixture for tensile reinforcement. A modified asphalt binder, a polymer modifier (styrene-butadien polymer) plus the asphalt cement (PMA), which is commercially available in Korea, was used to produce a modified asphalt mixture. 7% of the modifier by weight of asphalt was added to asphalt cement at 150C and stirred for an hour before using. The properties of the polymer and the modifier are shown in Tuble 2.

Specimens

Figure 2 shows a beam designed for flexural strength, static strain and crack propagation tests. Details of the procedure used for making the beam specimen are

Marshall specimensts were made for Marshall stability and indirect tensile strength (ITS) tests using 50 blows per side with the Marshall compaction hammer. To make a polymer-coated Marshall specimen, the polymer was evenly sprayed on both flat sides of the normal mix- ture specimen, resulting in a thin coating on the surfaces after curing. A unit quantity of 262.1 g/m2 was applied for a layer coating as determined from preliminary trials to make it as thin as possible whilst covering the surface completely. The specimen with a polymer coating is schematically illustrated in Figure I. Three replicates were prepared with each mixture for each test.

Absorbed polymer Polymer coat c: 0.27mm

Figure 1 Schematic illustration of polymer coating

S = 320mm +

Figure 2 Three-point bending test setup

given elsewheret6. To make a polymer coated beam, a thin layer of polymer was applied to the bottom face of the beam by the method used with the material samples. Three beams were prepared with each mixture for each test.

Test procedure

To examine the basic properties of the three types of

To examine the effectiveness of the polymer coating

asphalt mixtures, Marshall and ITS tests were conducted on Marshall specimens. The specimens for Marshall

for reinforcement, flexural strength tests, strain

stability and ITS were allowed to age for 36 h at room temperature (25C) before testing. Both tests were

measurement tests and crack propagation tests were

carried out at a loading rate of 50 mmimin at room temperature.

Table 1 Passing percentage of combined aggregate and specification for WC-3

Sieve size (mm) 19 13 4.76 2.38

Specification 955100 78-90 48-65 38-50

Combined agg. 99.8 85.8 58.6 42.8

0.59 0.297 0.149 0.074

20-30 12-21 7-16 48

25.3 16.3 10.3 4.6

Table 2 Selected properties of UPR and polymer modifier

Material S.G. Viscosity Acid value Styrene PH Total Appearance (25C) (ps) content (%) solid (/)

UPR 1.138 3.00 20.0 40 _ Clear liquid Modifier 0.980 2.25 10.5 50 Milky-white liquid

Polymer reinforcement for asphalt concrete: K. W, Kim et al.

conducted on the beam specimens. Asphalt concrete beams for the three tests were also allowed to age for 36 h at room temperature before handling for test setup instrumentation. To examine flexural strength a three-point bending test was conducted on the beam at 25C. Since the test was done at room temperature, a relatively fast loading speed, 50 mm/min, was selec- ted to prevent self flow during the test. Two LVDTs were placed near each side of the bottom centre of the beam to measure beam mid-span deflection until failure (Figure 2).

To simulate an asphalt pavement, a beam was over- laid on a discontinued ridged pavement. The beam was bonded using an epoxy on top of concrete blocks that were placed 10 mm apart on a rubber mat, as shown in Figure 3. To examine the strain at the gap due to a vertical load from the top of the beam in that particu- lar situation, a static strain test was performed on the beam. An electronic strain gauge was placed in the longitudinal direction at the bottom centre of the beam to measure the strain across the gap between the concrete blocks under the beam. A static load with a loading rate of 10 mm/min was applied through a load- ing plate, as shown in Figure 3. The strain at failure at the gap for each mixture was measured at 25C.

To evaluate the effectiveness of the polymer coating against crack initiation and propagation, a cyclic vertical load was applied to the beam through a 60 cm2 loading plate at 10 Hz. The load was cycled between 0.2 and 5 kN, using an MTS machine, to induce a cyclic ten- sile strain across the gap. The minimum load of 0.2 kN

Static load

Loading plate Interface Bonded

-/ AsphaltConcrete

beam

1 15mm -rl it-

Gap(lomm) Interface without Strain gage bonding

Figure 3 Specimen and test setup for static strain measure

Dvnamic load

Loading plate

Measure crack growth up to this line Asphalt *. ~~~~~~~~ ~_~~F&___

4cm beam

Support 4 - Gap

LVDT

Figure 4 Specimens and test setup for crack propagation test

, irl Fixed cross bar

~ Servo hydraulic power

Figure 5 Loading frame and test setup for crack propagation test

(approximately 20 kgf) was used to avoid separation of the loading head from the top of the beam during cyclic loading. All tests were performed at 25C and the crack growth was monitored visually on the sides of the beam which were painted white. An LVDT was installed one side of the two concrete blocks to measure the horizon- tal deformation of the beam, as shown in Figure 4. Horizontal deformation by the number of cycles was measured until the crack tip reached half the depth of the beam height. The experimental setup for the crack propagation test in the MTS machine is shown in Figure 5.

Results and analysis

Properties of the mixture

Air void, specific gravity, Marshall stability, flow and ITS of the three mixtures are shown in Tuble 3. The modified asphalt mixture showed a better Marshall stability and ITS, compared with those of the normal mixture. However, the improvement was not as great as those of the polymer coated mixture. The mixture with polymer coating showed the best value with both properties.

The UPR specimen is the one where the polymer resin was simply coated on both sides of the normal mixture specimen. With this coating, an approximate 32% and

Table 3 Physical properties, Marshall values and ITS of three mixtures

Mixtures

Property Normal PMA UPR

Bulk S.G. 2.33 2.31 2.33 Air void (%) 4.21 4.73 4.21 Marshall stability (kN) 14.89 17.07 19.79 Marshall flow (0.1 mm) 28.73 30.46 32.02 ITS (kPa) 790.70 974.41 1351.49

144 Polymer reinforcement for asphalt concrete: K. W. Kim et al.

Table 4 Average value of flexural strength test result for three mixtures

Mixture Maximum load Deflection at maximum load

W) (mm) Flexural strength

(kPa) -

Normal 2.169 0.81 2711.0 RMA 2.417 0.89 3036.3 UPR 2.587 1.03 3234.0

70% improvement in Marshall stability and ITS was achieved, with respect to the unmodified normal mixture. These improvements can be explained as the reinforcing effect of the thin polymer layer that is com- pletely bonded onto the specimen faces. The coating appeared to be particularly effective in improving the tensile strength of the asphalt concrete. Improved Marshall stability caused an increased flow, but the value was within specification limits, 20-401s.

Flexural strength und strain at the gup

Test results of flexural strength are presented in Tuble 4. The UPR mixture showed the highest flexural strength and its difference was approximately 20% compared with the normal mixture.

Figure 6 shows three load-deflection curves for each asphalt concrete mixture tested in three-point bending. In spite of this viscous nature of the asphaltic material at ambient temperature the load-deflection curves were

Normal mixture

0.0 I 0.0 0.5 1.0 1.5 2.0 2.5

Deflection

(4

similar to those of quasi-elastic materials. This could be the result of using a fast loading rate, 50 mm/min. Normal asphalt concrete was the stiffest amongst the three mixtures. This is shown from the initial stiffness of the load-deflection curves. Using modified asphalt and a polymer coating created reduced stiffnesses and improved ultimate strengths. This improved load- carrying capacity with lower stiffness was expected from the results of the Marshall test (higher stability and larger flow) given in Table 3. Since the modified binder was, in general, developed for the purpose of reducing stiffness and improving strength, there was no difficulty in expecting this kind of result. However, it is significant to find that the simple polymer coating on the normal asphalt mixture surfaces could provide a better result than the modified binder mixture.

Figure 7 shows the strain developed across the gap at the bottom of the beam face of asphalt concrete due to continuously increasing the static load to failure. The

Polymer Modified Mixture

0.0 0.5 1.0 1.5 2.0 2.5

Deflection

0))

Polymer-coated mixture

0.0 0.5 1.0 1.5 2.0 2.5

Deflection

(cl

Figure 6 Load-deflection curves for each mixture

Polymer reinforcement for asphalt concrete: K. W. Kim et al. 145

-J

0 2 4 6 8 10 12 14 16

Strain(E-3)

Figure 7 Strain of asphalt concrete beam at the gap under static loading

normal mixture showed the highest strain initially, and immediate failure with a small increase in strain. The modified mixture showed the lowest strain initially and a somewhat improved ultimate. load. The polymer coated mixture showed much higher strain sustenance with a continuous load increase, which is evidence of a clearly reduced stiffness whilst at the same time sustain- ing the highest load at failure.

Near the end of each curve, there is a point where the load rapidly increases almost without any strain incre- ment. This point is interpreted as the crack initiation point. From this test it was found that the polymer coated mixture sustained continuous load much longer before crack initiation. In other words, the high strength of the thin polymer layer can make a specimen sustain high load and strain values before crack initiation.

Crack propagation

Figure 8 shows curves for crack growth with number of load cycles in the crack propagation test. Each lime was drawn by power regression on the average value of three replicate samples. Crack length was measured as the vertical linear distance from the bottom of the beam every 5000 cycles. The most significant feature of the result is that when the polymer was coated at the bottom of the beam, no visible crack developed within 15 000 cycles, whilst cracks did develop in earlier cycles in other beams. Since the crack developing pattern in the polymer coated specimen was noticeable, a desdip- tion of it is worthy of note.

50[ I I I I I I I I I

P 10 20 30 40 50 60 70 80 90

No. of Cycle (x 103)

Figure 8 Crack propagation by number of load cycles

Figure 9 Photograph showing profiles of crack after crack propagation tests for the three mixture beams

z-2.0

g c 1.6 .g

E 1.2 5 2 0.6 1 E 0 .N 0.4

5 I0.d.

0 10 20 30 40 50 60 70 60

No. of Cycle (X 105)

Figure 10 Horizontal deformation by number of cycles

1

At around 15 000 cycles, a thin crack was created just above the polymer layer of the specimen. Immediately after the polymer layer failed at the bottom, the crack was connected to the opening of the polymer layer and it grew up to 10 mm. The propagation rate of the crack was stabilized thereafter.

146 Polymer reinforcement for asphalt concrete: K. W. Kim et al.

With the thin layer of polymer coating, a minor crack effective in reducing the stiffness of the mixture whilst developed, but it took approximately 2.5 times as long, improving load-carrying capacity. This improvement in on average, to reach half the beam height (4 cm) as for strength and reduction in stiffness resulted in a the normal mixture beam. Although the two lines for retardation of crack initiation due to cyclic loading. the modified mixture and the polymer coated mixture The effect of the polymer coating was extended to a are similar in Figure 8, it also took more cycles than significant improvement in the resistance to crack that of the PMA mixture for the crack tip in the polymer propagation, approximately 2.5 times the load cycle of coated mixture to reach the given length. a normal mixture without reinforcement.

Furthermore, the crack was much thinner in the polymer coated specimen than the normal mixture specimen, or with the modified mixture specimen, as shown in Figure 9. This was evidenced by the horizon- tal deformations measured by an LVDT and plotted against number of load cycles, as shown in Figure 10. Each line in this figure was also drawn by power regression with an average value of three replicates. The deformation is the accumulated results of hori- zontal beam expansion due to vertical load and crack width expansion.

The polymer coating used in this study appeared to be a viable tensile reinforcement method for flexible pavement layers. However, since the beam using the modified mixture showed the lowest horizontal deformation rate, using the polymer coating on the modified asphalt mixture surface will be the more effective method for tensile reinforcement.

Further study to compare this method with others, such as, polymer grid and geotextiles, will give a clearer idea of how much this polymer coating will be effective in improving the tensile strength of flexible pavement.

Initially, the polymer coated specimen showed the lowest deformation, but the rate of deformation (slope) was somewhat higher than with the modified mixture. Once the polymer film (layer) failed by crack initiation, the crack width expanded relatively fast because the material above the polymer layer is just a normal mixture. This caused the deformation rate of the polymer coated specimen to be somewhat higher than the modified mixture. However, the visually observed crack width of the polymer coated beam was very thin and almost invisible, as shown in Figure 9, compared with the other two mixtures. This may be due to the confining effect of the material at the bottom of the polymer coat. Therefore, in the polymer coated beam, overall expansion of the beam, rather than crack width expansion, seemed to be a major cause of the horizontal deformation.

References

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Since crack initiation was retarded significantly in the polymer coated beam, the polymer coating shown in this study appeared to be an acceptable reinforcement method for flexible pavement materials. However, since the modified mixture showed the lowest horizontal deformation rate, combined use of the modified mixture and the polymer coating at the bottom will be the better method not only for tensile reinforcement, but also for cracking retardation.

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Conclusions

A polymer coating of laboratory prepared asphalt concrete specimens improved Marshall stability and indirect tensile strength significantly. These improve- ments can be explained as the effect of reinforcement by a thin polymer layer that is completely bonded to the faces of a specimen. This coating seemed to be particularly effective in improving the tensile strength of asphalt concrete. Flexural strength was also improved when the polymer was coated on the face of the tensile side of a beam in a very thin layer. The coating was also

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