Experimental Study on Characteristics of Base …docs.trb.org/prp/13-2291.pdf2 Experimental Study on Characteristics of Base ... be used as a friction reducer. ... Study on Characteristics

Li, Tian, Niu, Sun and Zhou 1

1

Experimental Study on Characteristics of Base Friction for Concrete 2

Pavement Structure in China 3

4 5 Sili LI, Graduate Student* 6 Research Institute of Highway, Ministry of Transportation 7 Key Laboratory of Road Structure & Material, Ministry of Transportation, People’s Republic of China 8 8 Xitucheng Road, Beijing, 100088, P. R. China 9 M-Tel) +86-18701128988 Tel) +86-10-62079598 Fax) +86-10-62079597 10 [email protected] 11 12 13 Bo TIAN, Ph.D 14 Professor 15 Research Institute of Highway, Ministry of Transportation 16 Key Laboratory of Road Structure & Material, Ministry of Transportation, People’s Republic of China 17 8 Xitucheng Road, Beijing, 100088, P. R. China 18 [email protected] 19 20 21 Kaimin NIU, Ph.D 22 Professor 23 Research Institute of Highway, Ministry of Transportation 24 Key Laboratory of Road Structure & Material, Ministry of Transportation, People’s Republic of China 25 8 Xitucheng Road, Beijing, 100088, P. R. China 26 [email protected] 27 28 29 Zhengfu SUN 30 Shandong Hi-Speed Group Co., Ltd 31 8 North Long’ao Road, Jinan, Shandong Province, 250098, P. R. China 32 33 34 Wenhuan ZHOU 35 Beijing Luqiaotong International Engineering Consulting & Project Management Co., Ltd 36 8 Xitucheng Road, Beijing, 100088, P. R. China 37 38 39 Submission date: August 1, 2012 40 41 Word count: 3028 (Text) + 1,750 (7 Figures) + 500 (2 Tables) = 5278 words 42 43 *Corresponding author 44 45

TRB 2013 Annual Meeting Paper revised from original submittal.


ABSTRACT 1

The restraint between the slab and the base of a concrete pavement structure is an important factor 2

that should be taken into consideration when designing a concrete pavement. Excessive restraint can lead 3

to unexpected distresses due to the daily cycle of ambient thermal influence and the changing moisture 4

conditions that are ultimately detrimental to the performance of the pavement. Reasonable evaluation of 5

base friction contributes to configuring joint sealing, slab thickness and reinforced steel. 6

Semi-rigid base, mostly made of cement stabilized crushed stone, has been widely used as the typical 7

base for cement concrete pavement in China. Usually, a polythene sheet would be placed between the 8

concrete slab and the base to make the interface condition smooth. In certain cases, geotextile and asphalt 9

bond breaker may also be used as a friction reducer. 10

A series of push-off tests were performed under different conditions to study the characteristics of 11

base friction for a typical concrete pavement in China. Polythene sheet, geotextile, emulsified asphalt and 12

asphalt bond breakers of three different thicknesses (2cm, 4cm and 6cm) were placed between concrete 13

slabs and semi-rigid bases. Concrete slabs were also cast directly on base with no bond breaker to study 14

the influence of these factors on the characteristics of base friction. Also, potential factors affecting 15

behavior of concrete slab under frictional drag, including rate of movements, slab thickness, number of 16

movement cycles are investigated. 17

18

Keywords: Push-off Test, Base Friction, Semi-rigid Base, Horizontal Slab Movement, Concrete Pavement 19

20



INTRODUCTION 1

Frictional force between concrete slab and base is developed by horizontal slab movements 2

induced by the variation of temperature and moisture in concrete slabs. The frictional force acts 3

in the opposite direction to the horizontal slab movement and causes stress in the slab (1). 4

Rational prediction of the behavior of concrete slab under frictional drag is of significance for 5

the improvement of structural design of cement concrete pavement. 6

With respect to conventional concrete pavements, previous studies have concluded that 7

excessive restraint between the slab and the support layers of a pavement structure can lead to 8

pavement distresses that are ultimately detrimental to the performance of the pavement. In order 9

to reduce the frictional force at the interface, some materials like sand, bitumen, oil, single or 10

double layers of polyethylene sheeting are used as friction reducing medium, the role of which is 11

to reduce the tensile stresses by reducing the frictional restraint between the slab and the 12

underlying surface (2). 13

However, from a practical standpoint it would be very difficult, if not impossible, to 14

completely eliminate the frictional restraint (3). For the pavement engineering, it is not desirable 15

to totally eliminate it. This is because reducing the frictional restraint too much would result in 16

great reduction in structural capacity, thereby increasing the potential for deterioration of the slab 17

around the joints. Moreover, working and constructing on a slippery material could be difficult 18

and even hazardous. Therefore, a compromise should be sought between all these factors when 19

selecting a friction reducing medium (4). Numerous research efforts based on empirical or 20

theoretical methods were made in the past decades to better understand this mechanism and to 21

select materials. Zhang et al. successfully predicted shrinkage-induced stresses and displacement 22

in concrete pavement due to the restraint of the supporting base by developing an analytical 23

model (5). In another study, Suh et al. evaluated subbase friction for typical Korean concrete 24

pavement (6) by performing three series of push-off tests. 25

Semi-rigid base has been widely used in China as the most popular base type for cement 26

concrete pavement. Polythene sheet is usually placed between the slab and base to reduce the 27

excessive frictional force generated at the interface. The push-off test that was developed by Suh 28

et al. was employed in this study to quantify the friction force between the slab and base 29

materials. The main objective of this study is to evaluate the effectiveness of six different kinds 30



of friction-reducing media in reducing the frictional forces at the interface between the concrete 1

slab and the underlying base for typical concrete pavement in China. 2

MECHANISM OF SLAB MOVEMENT 3

In traditional physics, the classical friction model follows the Leonardo da Vinci -4

Amonton’s Law that the frictional force is a shearing force between two surfaces and is 5

proportional to the slab weight. Figure 1 shows the orientation of these relevant forces (7). 6

However, for an ideal system the classic friction model is true only if the following two 7

boundary conditions are satisfied. The first, is the elimination of adhesion between the two 8

surfaces which will not behave linearly with the nominal weight of the slab. The second, is that 9

there are no deformations in the sliding object or in the base which would change the interface 10

profile (8). According to Wesevich’s research results, base friction does not follow the classical 11

model, since it is composed of three components: an adhesion, or gluing, component between the 12

slab and the base; a bearing component that is influenced by the surface texture of the base; and a 13

shearing component which is induced by the movement of the slab across the base (9). This is 14

shown in figure 2. 15

Actually, frictional forces develop when the concrete slab contracts as a result of 16

temperature drop, moisture reduction and concrete shrinkage. As the slab contracts, the 17

movements are resisted by the friction at the interface. The resistance to movement produces a 18

direct tensile stress in the concrete. A concrete slab tends to contract or expand symmetrically 19

about its central axis (6). The movements of the slab increase from zero at the center to a 20

maximum at the edge. The stresses produced in the slab by the restraint decrease from a 21

maximum at the geometric center to zero at the free edges since the frictional resistance to the 22

movements builds up from the slab ends and the induced frictional force is affected by the 23

magnitude of movements (3, 10). The situation is graphically presented in figure 3. 24

PUSH-OFF TEST 25

Test Setups 26

Generally, the interface friction of concrete pavement is evaluated by performing a number 27

of push-off tests, which are basically measuring concrete test slab movements while applying 28

horizontal forces that induce the movements. In previous studies, the forces exerted on the slab 29

and the corresponding horizontal slab movements are recorded by use of a hydraulic jack and a 30



load cell. Because of the difficulty of recording of the friction-displacement relationship after 1

peak load, however, it was found to be very difficult to accurately control the rate of slab 2

movement by just adopting a hydraulic jack. There could be a substantial displacement after the 3

peak load and this displacement could surpass the maximum stroke of the loading device. In 4

order to simulate the real movement state of concrete slab induced by the variation of 5

temperature and moisture, a large-scale material testing system (MTS) was adopted and 6

reconfigured. The MTS was used to measure and record frictional force and horizontal 7

movement of concrete slab simultaneously at the rate of 100 data points per second. A leveling 8

instrument was set on the top of concrete slab to make sure the slab would not tilt during the test 9

procedure. Details of the test setups are depicted in figure 4. 10

Test Procedure 11

Polythene sheet, geotextile, emulsified asphalt and asphalt bond breakers of three different 12

thicknesses (2cm, 4cm and 6cm) were placed between concrete slabs (80cm×80cm×26cm) and 13

semi-rigid bases (100cm×100cm×16cm). The test conditions for different interfaces are listed in 14

table 1. The mix design for concrete slabs, semi-rigid bases and asphalt bond breakers are 15

summarized in table 2. 16

Each push-off test was conducted under different rates of movement and types of loads to 17

simulate the actual moving condition and to fully understand the working performance of 18

respective interface condition. Horizontal movement of the concrete pavement caused by 19

temperature variation is cyclic. Therefore, it is critical to investigate the effect of cyclic 20

movement on base friction. Monotonic loads are applied on concrete slabs to find out the peak 21

value of the frictional force during the push-off test while cyclic loads are applied on concrete 22

slabs to study the fatigue behavior of slab-base interaction. In order to investigate the influence 23

of slab thickness on frictional force between two layers, balancing weight was put on top of the 24

slabs to simulate thicker concrete slabs. 25

DISCUSSION OF TEST RESULTS 26

As an important indicator of frictional force between concrete slab and base, coefficient of 27

friction has been widely used in the structural design of concrete pavement. Coefficient of 28

friction is given by the relationship 29

μ= F / W (1) 30



Where μ is coefficient of friction, F is the horizontal frictional force, and W is weight of 1

slab. 2

As mentioned earlier, base friction does not follow the classical physical model, where the 3

amount of resistance is directly dependent on the nominal weight of the object when sliding 4

occurs between two materials, so the frictional force is not proportional to the slab displacement 5

during the entire test procedure. 6

A typical plot of friction force versus displacement from the push-off test is shown in figure 7

5. The whole test procedure could be divided into three continuous stages. In stage I, coefficient 8

of frictional force increases with displacement in a parabolic pattern until slab movement reaches 9

the preliminary displacement, which corresponds to the maximum frictional force. Stage II 10

follows until the friction force drops to a relatively constant level. During this state, the friction 11

force drops rapidly as soon as the displacement reaches the preliminary displacement, which 12

indicating the failure of the interface restraint. During stage III, the coefficient of frictional force 13

tends to be a constant with the increase of displacement after the failure. As mentioned in 14

previous section, frictional force between concrete slab and base is composed of three 15

components: an adhesion component, a bearing component and a shearing component. In stage I, 16

three components all exist and function as a single entity and the frictional force increases with 17

displacement. In stage II, the failure occurs at the sliding plane and the adhesion component 18

disappears. After that, the shape of friction-displacement curve changes from a parabolic pattern 19

into a horizontal straight line in stage III. 20

Effect of Cyclic Load on the Friction 21

The effect of cyclic load on the friction is also investigated. The friction displacement curve 22

is plotted for each load cycle. It was found that the variation of the friction-displacement curve 23

from the first cycle of slab movement to second cycle movement is substantial. As soon as the 24

sliding plane fails, the adhesion component does not play any role in resistance against the 25

horizontal slab movements. With the number of movement cycles increase, the interface between 26

concrete slab and base becomes smoother, which leads to the decrease of the coefficient of 27

frictional force. The experimental results are shown in figure 6. 28

Effect of Loading Rate on the Friction 29

The effect of the rate of movement and slab thickness could be evaluated in stage III of the 30

whole test where the coefficient of frictional force tends to be a constant. The adopted rate of 31



movement in the test ranges from 0.01 to 10mm/min, which can be considered to represent 1

actual slab movement rates due to daily temperature variation. No effects of rate movement on 2

the coefficient of frictional force were indicated. Balancing weight was put on top of the slabs to 3

simulate thicker concrete slabs in order to explore the effect of slab thickness on frictional force. 4

No significant change was found for the friction-movement curve with various slab thickness. It 5

was found, however, the coefficient of frictional force decreased slightly with the increase of 6

slab thickness. Both sets of experiments were conducted under the same condition that 7

polyethylene was used as the friction reducer between slab and base. The results are plotted in 8

figure 7. It should be noted that the slab thickness shown in this plot is a total equivalent 9

thickness, which is the actual slab thickness plus that computed from the balancing weight. 10

Directly Cast Slab 11

This study also includes the slabs that were cast directly on the top of the semi-rigid bases. 12

It was found, however, the slabs could not be moved without exceeding the capacity of the 13

loading equipment and therefore no friction force – displacement curve was successfully 14

obtained. This indicates that the friction between the concrete slab and semi-rigid base is 15

extremely high. This kind of bonding conditions between concrete slab and base might result in 16

quite high temperature stress in overall pavement system, which would be detrimental to the 17

pavement. It was found that for all the test samples with friction reducers inside, the failure plane 18

occurred at the interface. It was interesting, however, to note that when friction-reducing media 19

were placed between two layers, all the friction reducers stuck tightly to the bottom of the slab. 20

In the experiment, the environmental factors were not taken into consideration because of 21

the limitation of the testing facilities. The concrete slabs were thought to move just in horizontal 22

direction, the interaction between loss of slab-base friction due to slab curling and the incidence 23

of traffic loads in that conditions were not evaluated in the lab tests. 24

CONCLUSIONS AND RECOMMENDATIONS 25

(1) Most of push-off tests conducted in this study show similar shape of friction-26

displacement curve. The slab-base friction is fully characterized by dividing the friction force – 27

displacement curve into three stages. In stage I, coefficient of frictional force increases with 28

displacement in a parabolic pattern until slab movement reaches the preliminary displacement, 29

which corresponds to the maximum frictional force. Stage II follows until the friction force drops 30



to a relatively constant level. During this state, the friction force drops rapidly as soon as the 1

displacement reaches the preliminary displacement, which indicating the failure of the interface 2

restraint. During stage III, the coefficient of frictional force tends to be a constant with the 3

increase of displacement after the failure. 4

(2) Cyclic load was found to have significant effect on the friction and displacement 5

relationship. The variation of the friction-displacement curve from the first cycle of slab 6

movement to second cycle movement is substantial. As soon as the sliding plane fails, the 7

adhesion component does not play any role in resistance against the horizontal slab movements. 8

With the number of movement cycles increase, the interface between concrete slab and base 9

becomes smoother, which leads to the decrease of the coefficient of frictional force. 10

(3) The rate of movement was found to have no obvious effect on the interface restraint. 11

(4) The coefficient of frictional force was found to decrease with the increase of slab 12

thickness when plastic film placed between two layers. 13

(5) The slabs that were directly cast on the semi-rigid base could not be moved without 14

exceeding the loading capacity of the MTS and this indicates the use of semi-rigid bases without 15

using a friction reduction layer could lead to extremely high bonding stress at the slab - base 16

interface, which will be detrimental to the performance of the pavement since the temperature 17

stress will be higher. 18

(6) Polythene sheet is most effective in reducing the initial frictional force at the interface 19

between the concrete slab and the underlying base while asphalt bond-breaker and geotextile can 20

also be used as friction-reducing media. 21

(7) The location of the failure plane depends on the restraint between two layers and the 22

shear strength of the base material. The failure can actually occur at either the interface, or within 23

the base itself. If the adhesion component is high enough, the failure plane at sliding will not be 24

at the slab-base interface, but within the base. 25

(8) All the friction-reducing media employed in the test, including polythene sheet, 26

geotextile, and asphalt bond breakers of different thicknesses were observed to move along with 27

the concrete slab during test procedure. This indicates that the friction occurred between the 28

bottom surface of the friction-reducing media and the surface of base. 29



(9) For typical Chinese concrete pavement structure, semi-rigid base with high frictional 1

characteristics is widely used in highway construction. A bond breaker may be required to 2

minimize the frictional force at the interface between the concrete slab and the underlying base. 3

ACKNOWLEDGEMENTS 4

The research is based on the project “Research of Heavy-Duty Continuously Reinforced 5

Concrete Pavement Structure on Flexible Bases” which is funded by the Ministry of Science and 6

Technology of the People's Republic of China. The authors would like to extend their 7

appreciation and gratitude to all the scholars, engineers and graduate students for their sincere 8

assistance to conduct this research. Without the generous help of these individuals and 9

organization, this research would not have been possible. 10

REFERENCES 11

[1] Wimsatt, A. J., and McCullough, B. F. “Subbase friction effects on concrete pavements,” 12

Proc., 4th Int. Conf. on Concrete Pavement Design and Rehabilitation, Purdue Univ., West 13

Lafayette, Ind, 1989 14

[2] Robert Otto Rasmussen and Dan K. Rozycki, “Characterization and Modeling of Axial 15

Slab-Support Restraint,” Transportation Research Record No.1778, Transportation 16

Research Board, 2001. 17

[3] Chia, W.S., McCullough, B.F., Burns, N.H. “Field Evaluation of Subbase Friction 18

Characteristics,” Research Report 401-5. Center for Transportation Research, The 19

University of Texas at Austin, September 1986 20

[4] Seung Woo Lee, “Characteristics of Friction between Concrete Slab and Base, Vol.5, No. 21

4, 2000. 22

[5] Zhang J. and V.C. Li. Influence of Supporting Base Characteristics on Shrinkage-Induced 23

Stresses in Concrete Pavements. Journal of Transportation Engineering, ASCE, Vol. 127, 24

No. 6, pp. 455-462, 2002. 25

[6] Young Chan Suh, S.W.Lee., and M. S. Kang. Evaluation of Subbase Friction for Typical 26

Korean Concrete Pavement. Journal of Transportation Research Board, No. 1809, National 27

Research Council, Washington D.C., 2004. 28

[7] Mendoza-Diaz, Alberto, B. Frank McCullough, and Ned. H. Burns, "Behavior of Long 29

Prestressed Pavement Slabs and Design Methodology," Research Report 401-3, Center for 30

Transportation Research, The University of Texas at Austin, November 1986. 31



[8] Wimsatt, Andrew W., McCullough, B. Frank, and Burns, Ned H. “Methods of Analyzing 1

and Factors Influencing Frictional Effects of Subbases,” Research Report 459-2F, Center 2

for Transportation Research, The University of Texas at Austin, November 1987. 3

[9] Wesevich, J.W., B.F. McCullough, and N.H. Bums, "Stabilized Subbase Friction Study for 4

Concrete Pavements," Research Report 459-1, Center for Transportation Research, The 5

University of Texas at Austin, November 1987. 6

[10] Seung Woo Lee, “Behavior of Concrete Slab under Frictional Drag,” KSCE Journal of 7

Civil Engineering, Vol.5, No. 2, 2001. 8



List of Tables and Figures 1

2

Tables 3

TABLE 1 The test conditions for different cases 4

TABLE 2 The mix design for concrete slab, semi-rigid base and asphalt bond breaker 5

6

Figures 7

FIGURE 1 Classical friction model 8

FIGURE 2 Components of frictional force between concrete slab and base 9

FIGURE 3 Effects of contraction on slab movement and tensile stress 10

FIGURE 4 Details of the push-off test setups 11

FIGURE 5 A typical friction-movement curve from push-off test 12

FIGURE 6 Effect of cycle of movement on coefficient of frictional force 13

FIGURE 7 Effect of rate of movement and thickness of slab on coefficient of frictional force 14

15



TABLE 1 The test conditions for different cases 1

Test

Number Interface Condition

Rate of Movement

(mm/min)

Cycle of

Movement

Test Slab Size

L×W×T(m)

No.1 Emulsified Asphalt 0.5 Initial 0.8×0.8×0.26

No.2 Emulsified Asphalt 0.5 Cyclical 0.8×0.8×0.26

No.3 Directly Cast on 0.5 Initial 0.8×0.8×0.26

No.4 Directly Cast on 0.5 Initial 0.8×0.8×0.26

No.5 Polythene Sheet 0.01 Initial 0.8×0.8×0.26



No.8 Polythene Sheet 0.5 Cyclical 0.8×0.8×0.26

No.9 Polythene Sheet 1 Initial 0.8×0.8×0.26

No.10 Polythene Sheet 10 Initial 0.8×0.8×0.26



No.13 2cm Asphalt Bond Breaker 0.5 Initial 0.8×0.8×0.26

No.14 2cm Asphalt Bond Breaker 0.5 Cyclical 0.8×0.8×0.26





No.19 Geotextile 0.5 Initial 0.8×0.8×0.26

No.20 Geotextile 0.5 Cyclical 0.8×0.8×0.26

2



TABLE 2 The mix design for concrete slab, semi-rigid base and asphalt bond breaker 1

Concrete Slab

Mix Design

(kg/m3)

C W FA CA

360 145 733 1147

Concrete Slump (mm)

140-160

Compressive Strength (Mpa 28d)

33.5

Flexural Strength

(Mpa 28d) 5.0

Semi-rigid Base

Mix Design

(kg/m3)

C W FA CA

110 115 881 1321

Compressive Strength (Mpa 7d)

4.2

Flexural Strength

(Mpa 7d) 0.9

Asphalt Bond Breaker

Sieve

(mm)

Total Percent Pass(%) Indicator of mix in Bond breaker

Thickness

2cm 4cm

6cm 2cm

4cm

6cm

13.2 100 100 Marshall Stability (kN)

Flow Value (mm)

VV (%)

VFA (%)

VMA (%)

Asphalt aggregate ratio (%)

8.2

3.3

4.1

74.5

17.5

6.0

5.4

3.5

4.4

77.2

15.2

5.0

9.5 100 97.6

4.75 96.3 67.2

2.36 64.4 45.7

1.18 47.6 27.1

0.6 33.2 17.2

0.3 19.1 11.4

0.15 12.5 8.2

0.075 7.1 4.9

2



1

2

FIGURE 1 Classical friction model (5) 3

4



1

FIGURE 2 Components of frictional force between concrete slab and base (7) 2

3



1

FIGURE 3 Effects of contraction on slab movement and tensile stress (9) 2

3



1

2

FIGURE 4 Details of the push-off test setups 3

4



1

FIGURE 5 A typical friction-movement curve from push-off test 2

3



1

FIGURE 6 Effect of cycle of movement on maximum coefficient of frictional force in each cycle 2

3



1

FIGURE 7 Effect of rate of movement and thickness of slab on coefficient of frictional force 2

(polyethylene as the friction reducer) 3


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Experimental Study on Characteristics of Base …docs.trb.org/prp/13-2291.pdf2 Experimental Study on Characteristics of Base ... be used as a friction reducer. ... Study on Characteristics