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Pullout capacity of ladder-type metal reinforcements in tire shred-sand mixtures
Construction and Building Materials 113 (2016) 544–552
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Pullout capacity of ladder-type metal reinforcements in tire shred-sand mixtures Varenya Kumar D. Mohan a, Hobi Kim b,⇑, Umashankar Balunaini c, Monica Prezzi d a
Golder Associates Inc., 500 Century Plaza Drive, Suite 190, Houston, TX 77073, USA Fugro Consultants, Inc., 6100 Hillcroft Ave, Houston, TX 77081, USA c Department of Civil Engineering, IIT Hyderabad, Yeddumailaram, Telangana 502205, India d School of Civil Engineering, Purdue University, West Lafayette, IN 47907-1284, USA b
h i g h l i g h t s Experimental investigation of pullout capacity of ladder-type metal reinforecments backfilled in tire shred-sand mixtures. Higher pullout capacity of reinforcement embedded in the tire shred-sand mixtures than in sand alone by about 26–92% for the normal stresses
considered in this study. Preliminary guidelines on the pullout capacity of ladder-type reinforcement backfilled with different tire-shred sand mixtures.
a r t i c l e
i n f o
Article history: Received 6 August 2015 Received in revised form 10 February 2016 Accepted 23 February 2016 Available online 24 March 2016 Keywords: Tire shreds Lightweight backfill Reinforcement Pullout capacity MSE walls
a b s t r a c t Tire shreds have gained wide acceptance as an engineered fill in the last two decades. Ladder-type metal reinforcement can be used to reinforce MSE walls with tire shred-sand mixtures as a backfill material. This paper reports the results of laboratory pullout testing performed on ladder-type metal reinforcement embedded in tire shred-sand mixtures. The ladder-type metal reinforcement consists of two parallel longitudinal steel bars welded to a series of cross bars forming rectangular apertures. Mixtures of Ottawa sand with 50–100 mm size tire shreds were prepared at different mixing ratios (0%, 20%, 25%, and 35% by weight of tire shreds). Pullout tests were performed under three normal stresses – 40 kPa, 65 kPa, and 90 kPa. The test results show that the ladder-type metal reinforcement provides higher pullout resistance in tire shred-sand mixtures than in sand alone. Compared to metal-strip reinforcement, ladder-type metal reinforcement provides higher pullout capacity due to the passive resistance that results from the interlocking of tire shreds within the grids of the ladder-type reinforcement. The pullout resistance increased with increasing tire shred content up to 35% (by weight of tire shreds), beyond which segregation of the mixtures was observed. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction About 233 million scrap tires were generated in the U.S. in 2013, while an additional 75 million scrap tires remained in stockpiles [30]. In addition, there has been a steady rise in scrap tire production in the U.S. and in other parts of the world [28,25]. In the tire shredding process, two methods are commonly used to reduce the size of whole tires: (a) cryogenic processing and (b) mechanical grinding [8]. The mechanical grinding process, which is much cheaper than the cryogenic process, reduces scrap tires to different sizes. ASTM D 6270 [6] defines tire shreds as pieces ⇑ Corresponding author. E-mail address: [email protected] (H. Kim). http://dx.doi.org/10.1016/j.conbuildmat.2016.02.160 0950-0618/Ó 2016 Elsevier Ltd. All rights reserved.
of scrap tires with sizes between 50 mm and 305 mm. Reduction of whole scrap tires to ground or particulate rubber size (0.425 mm to 2 mm) requires many cycles through the shredder unit. Hence, it is more economical to use large-size tire shreds in civil engineering applications. Large-volume utilization of tire shreds as a fill material can help reduce the amount of scrap tires stockpiled every year. The advantages of using tire shreds as a fill material include availability and low cost [24,29], light weight [27], good stiffness and shear strength [8,23,40,7,38,19,26,10,36], high hydraulic conductivity [35], and ease of placement and compaction in the field [16,32]. Resistance to utilization of tire shreds in civil engineering projects is often attributed to unavailability of tire shreds near construction sites and lack of design standards and detailed construction guidelines [21]. However, extensive
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List of notations GTS GS Pult F⁄
a
is the specific gravity of tire shreds is the specific gravity of Ottawa sand is the pullout capacity obtained from the pullout test is the pullout resistance factor is the scale effect correction factor for nonlinear stress reduction over the embedded length
studies conducted by many researchers in the last two decades have led to better understanding of the behavior of this engineered material. Tire shred-sand mixtures have gained wide acceptance in the last twenty years, leading to their utilization in construction of geotechnical structures. One such application is the use of tire shred-soil mixtures as a backfill material for mechanically stabilized earth (MSE) walls. Tire shred-soil mixtures can be reinforced with extensible (geotextiles or geogrids) or inextensible (metal strips or metal grids) components. An important design consideration for such reinforcements embedded in tire shred-soil mixtures is their pullout resistance. Pullout resistance is mobilized through the interaction between the mixture and the reinforcement, as the pullout force is applied on the reinforcement. According to Federal Highway Administration (FHWA) guidelines [12], the pullout resistance of reinforcement can be calculated as:
Pult ¼ F ar0v Le BC
ð1Þ
where Pult is the pullout resistance of the reinforcement, F⁄ is the pullout resistance factor, a is the scale effect correction factor for nonlinear stress reduction over the embedded length, rv0 is the vertical effective stress at the depth of the reinforcement-soil interface, Le is the embedment length of the reinforcement, B is the width of the reinforcement and C is the effective unit perimeter of the reinforcement. Le C is the total surface area per unit width of the reinforcement in the zone of resistance beyond the slip surface. Berg et al. [12] prescribes values of a = 1.0 (for metallic reinforcement) and C = 2.0 (for strips and grid reinforcement). Full-scale field and laboratory studies have been performed to understand the interaction of geogrids with tire shred-sand mixtures [33,13,14,9,37,31,11]. These studies indicate that the pullout capacities of geogrids in tire shred-sand mixtures are similar or higher than that of geogrids embedded in sand alone. Youwai et al. [39] studied the pullout behavior of hexagonal wire reinforcement and suggested that the required embedment length in the resisting zone is similar for sand and tire shred-sand mixtures. Balunaini and Prezzi [9] performed laboratory pullout tests on ribbed-metal-strip reinforcement, showing that the pullout capacity of ribbed-metal strips in tire shred-sand mixtures lies between the capacities obtained in sand alone (upper boundary) and tire shreds alone (lower boundary). Also, the pullout capacities of ribbed-metal-strips decreased with increasing tire shred content of the mixtures [9]. Ladder-type steel reinforcement, because of their geometry, has a potential for increased pullout capacity due to the additional passive resistance when tire shreds get interlocked within the gridlike structure of the ladder. The main sources of pullout resistance for the ladder-type metal reinforcement are: (1) shear resistance developed due to friction at the mixture-reinforcement interface (along the longitudinal and transverse bars) through the embedded length of the ladder [see Fig. 1(a)], and (2) passive resistance generated by the sand particles and tire shreds wedged against the
rv0 Le B C
is the vertical effective stress at depth of reinforcementsoil interface is the embedment length of the reinforcement is the width of the reinforcement is the effective unit perimeter of the reinforcement
transverse bars of the ladder [see Fig. 1(b)]. The sources of pullout resistance are the same for both ladder-type metal reinforcement and geogrid reinforcement while the pullout resistance is due mainly to interface shear resistance for metal strips. The objective of this paper is to investigate the pullout capacity of ladder-type steel reinforcement embedded in tire shred-sand mixtures by performing a series of pullout tests. The size of the voids within the tire shred matrix increases with increases in the tire shred sizes. As the size of tire shreds increases, segregation of sand becomes predominant in mixtures of tire shreds and sand with large tire shred content. Hence, considering the balance between the economics of the tire shredding process and the potential segregation of sand, tire shreds with sizes between 50 and 100 mm were used in the present study. Samples were prepared in a pullout box at mixing ratios of 0%, 20%, 25%, and 35% by weight of tire shreds. At each mixing ratio, pullout tests were performed at three normal stresses – 40, 65 and 90 kPa. Pullout resistance factors F⁄ were calculated from the pullout test results for ladder-type metal reinforcement. A few tests were performed on ladder-type reinforcement embedded in tire chip-sand mixtures (25% and 35% by weight of tire chips) to study the effect of size of tire shred on the pullout resistance of the reinforcement. In addition, the pullout capacity and resistance factor values from ladder-type metal reinforcement were compared with those obtained from ribbed-metal-strip reinforcement provided in Balunaini and Prezzi [9].
Fig. 1. Mechanism of development of pullout resistance for ladder-type metal reinforcement (a) interface friction, and (b) passive resistance from the interlocking of particles within the reinforcement grid – not to scale (after D. Mohan [15]).
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2. Laboratory pullout testing 2.1. Materials used 2.1.1. Ottawa sand The sand used to prepare the tire shred-sand mixtures was obtained from the U. S. Silica Company, Ottawa, Illinois. This sand conforms to ASTM C 778 [4] and is commercially known as Ottawa sand. Fig. 2 shows the grain-size distribution of Ottawa sand. The specific gravity GS of this sand was determined to be 2.65, according to ASTM D 854-10 [5]. 2.1.2. Tire shreds The tire shreds used in the experimental testing were supplied by Entech Inc., White Pigeon, Michigan. Fig. 3(a) shows a photograph of the tire shreds used in the study. Most of the metal wires and fibers used to reinforce vehicular tires were manually trimmed off the edges of the tire shreds. However, some protruding wires were present at their edges (Fig. 3). The wires were trimmed to prevent testers from being stabbed. The role of these wires associated with pullout capacity is conceptually insignificant but protruding wires may provide little increase in pullout capacity of ladder-type reinforcement due to geometry of the reinforcement. Fig. 3(b) shows a photograph of the tire chips used in the study. The tire chips used in this study were 9.5 mm in nominal size (approximately equidimensional shape). The geometric characterization of the tire shreds was done by measuring the dimensions of randomly selected tire shreds. The largest distance between any two points of a shred is taken as its length [17]. Measurements were made on about 100 tire shred pieces, and Fig. 4 shows the distribution of length and width of the tire shreds. The average length of the tire shreds was 76 mm, the average width was 49 mm and the average thickness was 10 mm, with the average aspect ratio (length/width) equal to 1.6. The tire shreds used in this study are the same as the ones used in previous studies at Purdue University [37,9,10,11]. The specific gravity, GTS, of tire chips and tire shreds was measured in accordance with ASTM C 127-07 [3]. Values of the GTS of tire chips and tire shreds were equal to 1.16 and 1.26, respectively. The presence of steel wires in the tire shreds might have led to their higher GTS values [1,18]. 2.1.3. Ladder-type metal reinforcement The ladder-type metal reinforcements used in this study were manufactured by The Reinforced Earth Company (http://www.reinforcedearth.com/). The laddertype metal reinforcement is commercially known as ‘‘High Adherence Reinforcement Ladders” or ‘‘HA Ladders.” As seen in Fig. 5, the ladder-type metal reinforcement consists of two parallel, cylindrical steel bars welded to a series of cross bars. The total length of the reinforcement used for this research is 1 m. The center-to-center distance between the two longitudinal bars is 0.05 m, and the length of the cross bars is 0.1 m. The diameter of the circular steel bars (longitudinal and transverse) is 0.01 m. At one end, a flat connection plate is welded between the longitudinal bars. This connection plate has a hole, through which a bolted connection can be established to the facing panels of MSE walls. 2.2. Mixing ratio of tire shred-sand mixtures The mixing ratio of tire shreds and sand influences the mechanical behavior of the mixtures. The voids between the tire shreds are very large compared with the size of the sand particles. Hence, during compaction of the mixtures, the sand particles tend to get into the voids present in the mixtures. Segregation of sand particles in the mixtures is a concern, especially when high tire shred contents are used. Balunaini et al. [11] conducted segregation studies and concluded that segregation of sand within the tire shred matrix is predominant for tire shred contents higher
Percent Finer (%)
100 80
Ottawa Sand
60 40 20 0 10
1
Grain Size (mm) Fig. 2. Grain-size distribution of Ottawa sand.
0.1
Fig. 3. Tire pieces used in the study (a) tire shreds used ranging from length 50 to 100 mm, and (b) tire chips. than 35% (by weight of tire shreds in the mixture). Thus, based on the results from the above study, three mixing ratios of 20:80, 25:75 and 35:65 (i.e., tire shred-tosand weight ratio) were adopted in the present study to determine the pullout characteristics of ladder-type metal reinforcement embedded in mixtures. Pullout tests for ladder-type metal reinforcement embedded in sand alone samples were also performed. 2.3. Experimental set-up The main components of the pullout test set-up are: (1) reinforcement clamping system, (2) pullout system, (3) monitoring system, and (4) data acquisition system. Fig. 6 shows a schematic of cross-sectional and plan views of the pullout test set-up. The test set-up has two chambers – the load cell chamber and the tire shredsand mixture chamber. The samples are prepared inside the tire shred-sand mixture chamber, which has inner dimensions of 1.0 m 0.38 m 0.47 m (length width height). The inner sides of the walls of this chamber were covered with smooth-vinyl sheets to minimize friction between the walls and the sample. A sleeve is fitted at the front face within the sample chamber (see Fig. 6) to transfer the point of application of the pullout load beyond the rigid front wall. In the absence of this sleeve, the pullout resistance of the reinforcement is overestimated due to increased lateral pressure against the front wall [22]. The laddertype metal reinforcement was tightly gripped in the load cell chamber between metal wedges using bolts to prevent slippage during testing. Out of the 1.0 m length of the ladder-type metal reinforcement, 0.74 m was embedded inside the tire shred-sand mixture chamber. The normal pressure on the sample was applied by inflating an airbag placed on the top of the sample. Further details on the test equipment can be found in Balunaini and Prezzi [9]. 2.4. Sample preparation and testing Depending on the target mixing ratio of the tire-shred sand mixture, predetermined weights of tire shreds and sand were mixed in buckets and poured into the chamber from small heights to avoid segregation. No water was added during
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25
above the reinforcement, the distance between the top edge of the box and the top of the sample was measured at ten different locations along the four edges of the chamber and averaged to get a representative sample height based on the internal height of the sample in the chamber. A protective sand layer 3-to-4 cm thick was placed on the prepared sample to prevent potential puncture of the rubber airbag by the protruding wires present in the tire shreds. The rubber airbag was placed on top of the protective sand layer to apply the required normal stress to the sample. A steel plate bolted on the top of the chamber provided the reaction to the applied air pressure. Once the required normal stress stabilized, the ladder-type metal reinforcement was pulled out using stepper motors at a rate of 1 mm/min (ASTM D 6706 [2]), while the pullout load and frontal displacement were measured using a load cell and a linear variable differential transformer (LVDT). The test was stopped at a frontal displacement of the reinforcement of about 40 mm or at a displacement at which the pullout load did not change with further displacement. According to the Federal Highway Administration (FHWA) guidelines [20,12], the pullout capacity of an inextensible reinforcement is taken as the peak load obtained within a frontal displacement of 20 mm, except if the reinforcement is ruptured within this frontal displacement range. In the case of absence of both a peak load and reinforcement rupture, the pullout load at 20 mm displacement is considered to be the pullout capacity. Table 1 lists the details of all the tests performed. In this table, the sand-matrix unit weight of the samples prepared for each test is also provided. The sand-matrix unit weight, which gives an indication of packing of sand particles in the voids, is defined as the ratio of the weight of sand to the difference of the volume occupied by the mixture and the volume occupied by tire shreds [23].
20
3. Pullout test results and discussion
15
3.1. Pullout load vs. frontal displacement
10
The results of the pullout tests conducted on Ottawa sand and tire shred mixtures are plotted in Fig. 8. For ladder-type metal reinforcement embedded in Ottawa sand, the test results indicate that the pullout load increases linearly with increasing frontal displacement and then becomes relatively constant at a frontal displacement ranging from 6 mm to 8 mm. However, for the reinforcement embedded in tire shred-sand mixtures, the pullout load increases continuously with displacement until the end of the test. This is possibly due to increasing mobilization of passive resistance by the interlocking of tire shreds within the grids of the ladder-type reinforcement (refer to Fig. 1). In a few tests, a tendency for a decrease in pullout capacity at large frontal displacements, towards the end of the test, was observed.
30
Frequency (%)
25 20 15 10 5 0 50
60
70
80
90
100
Length (mm)
(a) 30
Frequency (%)
547
5 0 20
30
40
50
60
70
80
90
100
Width (mm)
(b) Fig. 4. Geometry of tire shreds of 50–100 mm in length (after Balunaini and Prezzi [9]): (a) distribution according to length, and (b) distribution according to width.
3.2. Effect of normal stress on pullout resistance
Fig. 5. Ladder-type metal reinforcement used in this study, manufactured by the Reinforced Earth CompanyÓ (after D. Mohan [15]).
preparation of mixtures. Then, the mixture was placed in the tire shred-sand mixture chamber in four layers of thicknesses of 0.1 m each. Each layer was compacted using a vibratory compactor according to the procedure discussed in Balunaini and Prezzi [9]. Fig. 7 shows the vibratory compactor used in the sample preparation. After placement of two layers, the ladder-type metal reinforcement was placed on top of the sample (fitting it through the gap between the sleeves on the pullout box). The connection plate on the reinforcement was clamped within the load-cell chamber using bolted connections. After the placement of two additional layers
Table 2 provides the pullout capacity obtained at different normal stresses for the mixing ratios tested. The trend of showing the pullout capacity according to normal stresses in graphical form is shown in Fig. 9. For all mixing ratios, the pullout capacity increases with increasing normal stress. The percent increase in pullout capacity for the two steps of increase in normal stress – 40 to 65 kPa and 65 to 90 kPa – are shown in Table 2. For the sand alone samples, the same stress increase produces a larger increase in pullout capacity at higher stresses (i.e., the percent increase in pullout capacity is higher for normal stresses increasing from 65 to 90 kPa than from 40 to 65 kPa). This trend is reversed as more tire shreds are added to the sand. The test results show that the effect of the normal stress on the pullout capacity is more pronounced for the sand alone samples at higher normal stresses than for the tire shred-sand mixture samples. The pullout capacity of reinforcement embedded in the tire shred-sand mixtures is found to be higher than that in sand alone by about 26–92% for the normal stresses considered in this study. Fig. 9 indicates that there is a trend of increasing the pullout capacity with increase in tire shred content by weight as well as with increase in confinement, except at 40 kPa confinement. At 40 kPa confinement, the reversal in trend that appeared between the 20:80 mixing ratio and the 25:75 mixing ratio can be attributed to the decreased effect of the tire-shred content on the pullout capacity at lower confinements and to experi-
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Air valve Sleeve
Protective sand layer
PVC foam Bolts
Airbag
~ 0.05 m
Reinforcement
0.47 m
0.5 m
Pullout Force Tire shred-sand mixture
Load cell
Metal wedges (shown in orange)
Effective Length = 0.74 m 1m
0.5 m
(a)
0.38 m
Pullout Force
0.4 m
Tire shred-sand mixture
Effective Length = 0.74 m
(b) Fig. 6. Schematic representation of the pullout test set-up (a) longitudinal cross section of the pullout box, and (b) plan view of the pullout box – NOT TO SCALE (after Balunaini [8]).
Inlet valve
Piston vibrator
Air supply (pressure line)
Steel plate (35.5 cm by 35.5 cm, 0.95-cm-thick) Fig. 7. Impact piston-type compactor bolted to a steel plate (compactor model 502LS-EM, manufactured by VIBCO).
mental variability. The increase in pullout capacity with increase in tire shred content is more pronounced for the other tests, performed at higher confining pressures, reflecting the greater strength mobilization by the tire shreds in the mixtures. 3.3. Effect of mixing ratio on pullout resistance Fig. 10 presents the pullout capacity obtained for various tire shred-to-sand mixing ratios. The pullout capacity of ladder-type metal reinforcement embedded in tire shred-sand mixtures is higher than that in Ottawa sand for the normal stresses considered.
The pullout capacity of the reinforcement increases with increase in the tire shred content when compared to that of the reinforcement in sand alone, as shown in Table 2. This contrasts with the findings from pullout studies on metal-strip reinforcement in mixtures in which the pullout capacity of the metal-strip reinforcement was highest when embedded in sand alone and decreased with increasing tire shred content [9]. It should be emphasized that the pullout resistance of metal-strip reinforcement is due to the frictional resistance between the reinforcement and the mixtures (no interlocking effect present). For ladder-type reinforcement embedded in tire shred-sand mixtures, considerable pullout resistance is obtained from the interlocking of the tire shreds in the grids of the ladder-type metal reinforcement in addition to interface frictional resistance. Also, for a given normal stress, the pullout capacity of ladder-type metal reinforcement embedded in tire shred-sand mixtures typically increases with increases in the tire shred content in the mixture. For instance, the pullout capacity of reinforcement embedded in 35:65 mixtures was about 92% higher than that when embedded in sand alone for a normal stress of 65 kPa. This can be attributed to the higher contribution of the interlocking effect of tire shreds in the reinforcement grids. However, the only exception to this was in the case of a normal stress of 40 kPa, for which the mixture of 20:80 tire shred-to-sand weight ratio showed 11% higher pullout capacity than the mixture with 25:75 tire shred-to-sand weight ratio. This exception may be due to the fact that the 20:80 mixing ratio has a 6% higher sandmatrix unit weight than that of the 25:75 mixing ratio. For the tests at higher normal stresses of 65 and 90 kPa, the test results may appear to suggest that the effect of the sand-matrix unit weight may have been outweighed by the higher tire shred interlocking effect produced for reinforcement embedded in the
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V.K. D. Mohan et al. / Construction and Building Materials 113 (2016) 544–552 Table 1 Summary of pullout testing program for ladder-type metal reinforcements embedded in tire shred-sand mixtures. Tire shred-Sand ratio (by weight)
à
Maximum dry unit weight (kN/m3) [37]
Normal stress (kPa)
Average dry unit weight in pullout box (kN/m3)
Average sand-matrix unit weight (kN/m3)
Average sand-matrix unit weight (kN/m3) for tests on ribbed-metal-strip reinforcementà
0:100
40 65 90
18.0
17.2
17.2
17.2
20:80
40 65 90
17.1
15.6
16.7
–
25:75
40 65 90
16.6
14.8
15.8
15.8
35:65
40 65 90
14.2
12.3
12.3
–
Pullout test results for ribbed-metal-strips were obtained from Balunaini and Prezzi [9].
14
14 Ottawa Sand: 40 kPa Ottawa Sand: 65 kPa Ottawa Sand: 90 kPa
25% tire shred: 40 kPa 25% tire shred: 65 kPa 25% tire shred: 90 kPa
12
10
Pullout Load (kN/m)
Pullout Load (kN/m)
12
8
6
10
8
6
4
4
2
2
0
0 0
10
20
30
0
40
Frontal Displacement (mm)
10
(a)
30
40
(c) 14
14 20% tire shred: 40 kPa 20% tire shred: 65 kPa 20% tire shred: 90 kPa
12
35% tire shred: 40 kPa 35% tire shred: 65 kPa 35% tire shred: 90 kPa
12
10
Pullout Load (kN/m)
Pullout Load (kN/m)
20
Frontal Displacement (mm)
8
6
10
8
6
4
4
2
2
0
0 0
10
20
30
Frontal Displacement (mm)
(b)
40
0
10
20
30
40
Frontal Displacement (mm)
(d)
Fig. 8. Measured pullout load vs. frontal displacement for ladder-type metal reinforcements embedded in mixtures of tire shreds and sand with (a) 0% tire shred content by weight, (b) 20% tire shred content by weight, (c) 25% tire shred content by weight, and (d) 35% tire shred content by weight.
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Table 2 Pullout capacities and pullout resistance factor F⁄ for ladder-type and ribbed-metal-strip reinforcements embedded in tire shred-sand mixtures. Normal stress (kPa)
Ladder-type metal reinforcement pullout capacity (kN)y
Increase in pullout capacity from previous normal stress (%)
Increase in pullout capacity compared to sand-alone samples (%)
Pullout resistance factor F⁄ Ladder-type metal reinforcement
Ribbed-metalstrip reinforcementà
0:100
40 65 90
4.2 4.9 6.6
– 17 35
–
1.31 0.97 0.95
1.49 1.08 0.87
20:80
40 65 90
6.1 7.2 8.3
– 18 15
45 47 26
1.91 1.43 1.20
–
25:75
40 65 90
5.5 8.0 9.5
– 46 19
31 63 44
1.73 1.59 1.38
1.05 0.77 0.66
35:65
40 65 90
6.4 9.4 9.7
– 47 3
52 92 47
2.04 1.88 1.42
–
Tire shred-sand ratio (by weight)
y à
The pullout capacity is associated with pullout load at 20 mm displacement. Pullout test results for ribbed-metal-strips were obtained from Balunaini and Prezzi [9].
12
12
10
10
8
8
6
6
4
4
2
2
0 30
0 40
50
60
70
80
90
100
Fig. 9. Measured pullout capacity vs. normal stress for different mixing ratios.
mixture with 25:75 tire shred-to-sand weight ratio. Nevertheless, more testing is required to confirm this trend and isolate the physical phenomena responsible for it.
0
5
10
15
20
25
30
35
40
Fig. 10. Measured pullout capacity vs. tire shred content at different normal pressures.
ments caused by the larger particle size, and/or (2) the interference of the zone of influence with the box boundaries resulting from the large-size tire shreds.
3.4. Effect of tire shred size on pullout resistance 3.5. Pullout resistance factor F⁄ The effect of tire shred size on the pullout resistance of laddertype reinforcement was studied by performing tests on two tire shred sizes – 50-to-100 mm tire shreds and 9.5 mm nominal size tire chips prepared at mixing ratios of 35:65 and 25:75 (by weight of tire shreds to sand). Table 3 presents the comparison of results from pullout capacity on tire chip- and tire shred-sand mixtures. The pullout resistance of reinforcement ladders in tire shredsand mixtures showed 0.5 kN to 0.9 kN higher values for all the three mixing ratios. This higher pullout capacity of 0.5 kN to 0.9 kN is associated with 106–116%. This higher pullout capacity is possibly due to (1) the difference in generation of passive resistance in the case of tire shred-sand mixtures at large displace-
In this study, the effective length Le of the ladder-type metal reinforcement is 0.74 m inside the pullout box. The perpendicular distance between two longitudinal bars of the ladder-type metal reinforcement, equal to 0.05 m, is taken as the width B of the reinforcement. Based on the pullout capacities obtained from the tests, the F⁄ values were calculated. Fig. 11 shows the pullout resistance factor versus normal stress for the different mixing ratios tested. Table 2 summarizes the F⁄ values for each of the pullout tests performed in this study. Fig. 12 shows the pullout resistance factor F⁄ varies with the tire shred content in the mixture for different normal stresses.
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V.K. D. Mohan et al. / Construction and Building Materials 113 (2016) 544–552 Table 3 Comparison of results from pullout tests on tire chip- and tire shred-sand mixtures. Mixing ratio by weight of tires to sand
Normal stress (kPa)
Size of tires
Maximum dry Unit Weight (kN/m3) [37]
Average dry unit weight in pullout box (kN/m3)
Relative compaction (%)
Average sandmatrix unit weight (kN/m3)
Pullout capacity of ladder-type metal reinforcement at 20 mm displacement (kN)
25:75
90
9.5 mm nominal size chips 50–100 mm shreds
15.2
13.7
90.0
14.2
9.0
16.6
14.8
89.1
15.8
9.5
9.5 mm nominal size chips 50–100 mm size shreds
13.6
11.9
87.3
11.6
5.5
14.2
12.3
86.5
12.3
6.4
9.5 mm nominal size chips 50–100 mm size shreds
13.6
11.9
87.3
11.6
8.9
14.2
12.3
86.5
12.3
9.7
35:65
40
35:65
90
2.4 0.8
1.2
1.6
2
2.4
20
Pullout Resitance Factor, F*
30 40 50 60
2
1.6
1.2
70
40 kPa 65 kPa
0.8
80
90 kPa 0
90
5
10
15
20
25
30
35
40
Percentage of re shreds by weight (%) 100 ⁄
Fig. 11. Normal stress vs. pullout resistance factor F for different mixing ratios.
3.6. Comparison of F⁄ values for ladder-type metal and ribbed-metal strip reinforcements Balunaini and Prezzi [9] reported the results of pullout tests performed on ribbed-metal strip reinforcement embedded in tire shred-sand mixtures for mixing ratios of 0:100, 12:88, 25:75, and 100:0 (by weight of tire shreds to sand). Table 2 also provides a comparison of the pullout resistance factors obtained for the ladder-type metal reinforcement from this study with those for ribbed-metal-strips for 0:100 and 25:75 mixing ratios. Table 1 lists the sand-matrix unit weights for relevant tests from Balunaini and Prezzi [9]. The pullout resistance factors for ribbed-metal-strips embedded in sand alone samples are similar (8–13% variation for different normal stresses) to those for ladder-type metal reinforcement. Note that for sand alone samples, the contribution from passive resistance to the pullout capacity is minimal for both types of reinforcement when compared to that for tire shred-sand mixtures due to the tire shred interlocking contribution. However, for the mixtures (i.e., tire shred-sand mixtures of 25:75 by weight) under the same testing conditions, the pullout
Fig. 12. Pullout resistance factor, F⁄ vs. tire shred content at different normal pressures.
capacity of the ribbed-metal strip was 40–52% lower than that of the ladder-type metal reinforcement. This means that passive resistance in the case of ladder-type reinforcement contributes significantly to its pullout capacity. Hence, use of ladder-type metal reinforcement proves more advantageous than ribbed-metal strip reinforcement when tire shred-sand mixtures are used as backfill in the construction of MSE walls. 4. Summary and conclusions Pullout tests were performed on mixtures consisting of tire shreds of 50–100 mm nominal size and Ottawa sand. The tests were performed at mixing ratios of 0:100, 20:80, 25:75 and 35:65 by weight of tire shreds to sand. At each mixing ratio, pullout tests were performed at 40, 65 and 90 kPa normal stresses. From the results of the pullout tests the following conclusions were made: 1. The pullout load vs. frontal displacement curve shows a constant increase up to a displacement of about between 6 mm and 8 mm and then it becomes relatively constant at a frontal
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displacement of approximately 15 mm for ladder-type metal reinforcement embedded in Ottawa sand (0:100 mixing ratio by weight). 2. In the case of the reinforcement embedded in tire shred-sand mixtures, the pullout capacity continues to increase through the test. This may result from the increase in the mobilization of the interlocking resistance of tire shreds within the grids of the ladder-type reinforcement. 3. The pullout capacity of ladder-type metal reinforcements embedded in tire shred-sand mixtures is higher than in sand alone by about 26–92% based on the mixing ratios and the normal stresses considered in this study. This can be attributed to the contribution of the passive resistance resulting from the tire shreds interlocking in the grids of the ladder-type metal reinforcement, in addition to the resistance due to interface friction. 4. For a given normal stress, the pullout capacity of ladder-type metal reinforcements embedded in tire shred-sand mixtures typically increases with increases in the tire shred content in the mixture. The pullout capacity of reinforcement embedded in 20:80, 25:75, and 35:65 mixtures increased by 26–47%, 31– 63%, 52–92% compared to sand alone for the range of normal stresses considered in the study. A systematic approach was followed in this experimental investigation on the pullout capacity of ladder-type metal reinforcement in tire shred-sand mixtures. Results from this study can be used to guide the development of ‘‘optimal” mixtures of tire shreds and sand. In addition, the pullout resistance factors proposed in the study can be referred to in the design of MSE walls reinforced with ladder-type reinforcements with tire-shred sand mixtures as the backfill material. It is important to note that the laboratory results reported in this research are subject to limitations: (a) inevitable variation of the unit weight of the tire-shred sand mixtures with sample preparation in the pullout test box using different sizes of tire shreds and (b) chamber boundary effects. Also, the behavior of the mixtures depends on the properties of both the tire shreds and sand. Thus, the applicability of the results reported in this paper depends on whether the materials selected and used for construction at the project sites are similar to those used in this study. This paper provides preliminary guidelines on the pullout capacity of ladder-type reinforcement backfilled with different tire-shred sand mixtures based on laboratory test results. Further testing is recommended to investigate the pullout capacity of the ladder-type reinforcements embedded in tire shred-sand mixtures for field applications. Acknowledgement The authors would like to acknowledge the financial support provided by the Indiana Department of Transportation (INDOT) via the Joint Transport Research Program (JTRP) for this study. References [1] I. Ahmed, Laboratory study on properties of rubber-soils. Report FHWA/IN/ JHRP-93/4, Indiana Department of Transportation, Purdue University, West Lafayette, Indiana, 1993. [2] ASTM D 6706-01, Standard test method for measuring geosynthetic pullout resistance in soil, ASTM International, West Conshohocken, PA, USA, 2001. [3] ASTM C127-07, Standard test method for density, relative density (specific gravity), and absorption of coarse aggregate, ASTM International, West Conshohocken, PA, USA, 2007. [4] ASTM C 778-06, Standard specification for standard sand, ASTM C 778-06, West Conshohocken, PA, USA, 2007. [5] ASTM D854-10, Standard test methods for specific gravity of soil solids by water pycnometer, ASTM International, West Conshohoken, PA, USA, 2010. [6] ASTM D 6270-08, Standard practice for use of scrap tires in Civil Engineering applications, ASTM International, West Conshohoken, PA, USA, 2012.
[7] M.F. Attom, The use of shredded waste tyres to improve the geotechnical engineering properties of sands, Environ. Geol. 49 (4) (2006) 497–503. [8] U. Balunaini, 2009, Experimental study of the use of mixtures of sand and tire shreds as a geotechnical material, Ph.D. thesis submitted to the faculty of Purdue University, West Lafayette, Indiana. [9] U. Balunaini, M. Prezzi, Interaction of ribbed-metal strip reinforcement with tyre shred-sand mixtures, Geotech. Geol. Eng. 28 (2) (2010) 147–163. [10] U. Balunaini, Kumar.D. Varenya, M. Prezzi, R. Salgado, Shear strength of tyre chip-Sand and tyre shred-sand mixtures, Proc. Geotech. Eng. Inst. Civ. Eng. 167 (6) (2014) 585–595. [11] U. Balunaini, S. Yoon, M. Prezzi, R. Salgado, Pullout response of uniaxial geogrid in tire shred-sand mixture, Geotech. Geol. Eng. 32 (2) (2014) 505–523. [12] R.R. Berg, C.R. Barry, N.C. Samtani, 2010, Design of mechanically stabilized earth walls and reinforced soils slopes – Volume I and II. Publication No. FHWA-NHI-10-024, Federal Highway Administration Report. [13] A. Bernal, C.W. Lovell, R. Salgado, 1996, Laboratory study on the use of tyre shreds and rubber-sand in backfills and reinforced soil applications, FHWA/IN/ JHRP-96/12, Purdue University, West Lafayette, Indiana. [14] A. Bernal, R. Salgado, R.H. Swan Jr., C.W. Lovell, Interaction between tire shreds, rubber-sand and geosynthetics, Geosynth. Int. 4 (6) (1997) 623–643. [15] D. Mohan, Varenya Kumar, 2011, Use of tire shred-sand mixtures as backfill material for mechanically stabilized earth retaining walls, Masters thesis submitted to the faculty of Purdue University, West Lafayette, Indiana. [16] T.H. Dickson, D.F. Dwyer, D.N. Humphrey, Prototype tyre-shred embankment construction, Transp. Res. Rec. 1755 (2001) 160–167. [17] A. Drescher, D. Newcomb, T. Heimdahl, Deformability of shredded tires Final Report, Minnesota Department of Transportation, 1999. [18] T.B. Edil, P.J. Bosscher, Engineering properties of tyre chips and soil mixtures, Geotech. Test. J. 17 (4) (1994) 453–464. [19] A. Edinçliler, V. Ayhan, Influence of tyre fiber inclusions on shear strength of sand, Geosynth. Int. 17 (4) (2010) 183–192. [20] V. Elias, B.R. Christopher, R.R. Berg, 2001, Mechanically stabilized earth walls and reinforced soil slopes, design, and construction guidelines, Publication No. FHWA-NHI-00-043, Federal Highway Administration Report. [21] G. Engstrom, R. Lamb, 1994, Using shredded waste tires as a lightweight fill material for road subgrades, Minnesota Department of Transportation Report No. 94–10 Materials Research & Engineering, Maplewood, MN. [22] K. Farrag, B.A. Yalcin, I. Juran, Pull-out resistance of geogrids reinforcements, Geotext. Geomembr. 12 (1993) 133–159. [23] G.J. Foose, C.H. Benson, P.J. Bosscher, Sand reinforced with shredded waste tyres, J. Geotech. Eng. ASCE 122 (9) (1996) 760–767. [24] R.D. Holtz, 1989, Treatment of problem foundations for highway embankments, National Cooperative Highway Research Program Report No. 147, Washington D.C. [25] Japan Automobile Tyre Manufacturers Association, Tyre Recycling Handbook, Japan Automobile Tyre Manufacturers, 2014. [26] D. Meles, A. Bayat, H. Soleymani, Compression behavior of large-sized tirederived aggregate for embankment application, J. Mater. Civ. Eng. 25 (9) (2013) 1285–1290. [27] D.E. Newcomb, A. Drescher, Engineering properties of shredded tires in lightweight fill applications, Transp. Res. Rec. 1437 (1994) 1–7. [28] G.V. Rao, R.K. Dutta, Compressibility and strength behavior of sand-tyre chip mixtures, J. Geotech. Geol. Eng. 24 (3) (2006) 711–724. [29] K.R. Rowe, Geotechnical and Geoenvironmental Handbook, Springer, Berlin, 2001. [30] Rubber Manufacturers Association, 2014, 2013 U.S. Scrap tire management summary, Washington DC. [31] T. Tanchaisawat, D.T. Bergado, P. Voottipruex, K. Shehzad, Interaction between geogrid reinforcement and tire chip-sand lightweight backfill, Geotext. Geomembr. 28 (2010) 119–127. [32] V. Tandon, D. Velazco, S. Nazarian, M. Picornell, Performance monitoring of embankments containing tire chips: case study, J. Perform. Constr. Facil. 21 (3) (2007) 207–214. [33] N. Tatlisoz, T.B. Edil, C.H. Benson, Interaction between reinforcing geosynthetics and soil-tyre chip mixtures, J. Geotech. Geoenviron. Eng. ASCE 124 (11) (1998) 1109–1119. [34] J.J. Tweedie, D.N. Humphrey, T.C. Sandford, Full scale field trials of tire shreds as lightweight retaining wall backfill, at-rest condition, Transp. Res. Rec. 1619 (1998) 64–71. [35] M.A. Warith, E. Evgin, P.A.S. Benson, S.M. Rao, Evaluation of permeability of tire shreds under vertical loading, J. Test. Eval. 33 (1) (2005) 51–54. [36] M. Xiao, M. Ledezma, C. Hartman, Shear resistance of tire-derived aggregate using large-scale direct shear tests, J. Mater. Civ. Eng. 27 (1) (2015) 04014110. [37] S. Yoon, Mechanical response of tire shred–sand mixtures and applications to geotechnical structures Ph.D. thesis, School of Civil Engineering, Purdue University, West Lafayette, Indiana, 2006. [38] S. Yoon, M. Prezzi, N.Z. Siddiki, B. Kim, Construction of a test embankment using a sand-tyre shred mixture as fill material, Waste Manage. 26 (9) (2006) 1033–1044. [39] S. Youwai, D.T. Bergado, N. Supawiwat, Interaction between hexagonal wire reinforcement and rubber tyre chips with and without sand mixture, Geotech. Test. J. 27 (3) (2004) 1–9. [40] J.G. Zornberg, Y.D. Costa, B. Vollenweider, Mechanical performance of a prototype embankment backfill built with tire shreds and cohesive soil, in: Proceedings of the 83rd Annual Meeting, Transportation Research Board, January 11–15, Washington, D.C., 2004.
Contents lists available at ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Pullout capacity of ladder-type metal reinforcements in tire shred-sand mixtures Varenya Kumar D. Mohan a, Hobi Kim b,⇑, Umashankar Balunaini c, Monica Prezzi d a
Golder Associates Inc., 500 Century Plaza Drive, Suite 190, Houston, TX 77073, USA Fugro Consultants, Inc., 6100 Hillcroft Ave, Houston, TX 77081, USA c Department of Civil Engineering, IIT Hyderabad, Yeddumailaram, Telangana 502205, India d School of Civil Engineering, Purdue University, West Lafayette, IN 47907-1284, USA b
h i g h l i g h t s Experimental investigation of pullout capacity of ladder-type metal reinforecments backfilled in tire shred-sand mixtures. Higher pullout capacity of reinforcement embedded in the tire shred-sand mixtures than in sand alone by about 26–92% for the normal stresses
considered in this study. Preliminary guidelines on the pullout capacity of ladder-type reinforcement backfilled with different tire-shred sand mixtures.
a r t i c l e
i n f o
Article history: Received 6 August 2015 Received in revised form 10 February 2016 Accepted 23 February 2016 Available online 24 March 2016 Keywords: Tire shreds Lightweight backfill Reinforcement Pullout capacity MSE walls
a b s t r a c t Tire shreds have gained wide acceptance as an engineered fill in the last two decades. Ladder-type metal reinforcement can be used to reinforce MSE walls with tire shred-sand mixtures as a backfill material. This paper reports the results of laboratory pullout testing performed on ladder-type metal reinforcement embedded in tire shred-sand mixtures. The ladder-type metal reinforcement consists of two parallel longitudinal steel bars welded to a series of cross bars forming rectangular apertures. Mixtures of Ottawa sand with 50–100 mm size tire shreds were prepared at different mixing ratios (0%, 20%, 25%, and 35% by weight of tire shreds). Pullout tests were performed under three normal stresses – 40 kPa, 65 kPa, and 90 kPa. The test results show that the ladder-type metal reinforcement provides higher pullout resistance in tire shred-sand mixtures than in sand alone. Compared to metal-strip reinforcement, ladder-type metal reinforcement provides higher pullout capacity due to the passive resistance that results from the interlocking of tire shreds within the grids of the ladder-type reinforcement. The pullout resistance increased with increasing tire shred content up to 35% (by weight of tire shreds), beyond which segregation of the mixtures was observed. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction About 233 million scrap tires were generated in the U.S. in 2013, while an additional 75 million scrap tires remained in stockpiles [30]. In addition, there has been a steady rise in scrap tire production in the U.S. and in other parts of the world [28,25]. In the tire shredding process, two methods are commonly used to reduce the size of whole tires: (a) cryogenic processing and (b) mechanical grinding [8]. The mechanical grinding process, which is much cheaper than the cryogenic process, reduces scrap tires to different sizes. ASTM D 6270 [6] defines tire shreds as pieces ⇑ Corresponding author. E-mail address: [email protected] (H. Kim). http://dx.doi.org/10.1016/j.conbuildmat.2016.02.160 0950-0618/Ó 2016 Elsevier Ltd. All rights reserved.
of scrap tires with sizes between 50 mm and 305 mm. Reduction of whole scrap tires to ground or particulate rubber size (0.425 mm to 2 mm) requires many cycles through the shredder unit. Hence, it is more economical to use large-size tire shreds in civil engineering applications. Large-volume utilization of tire shreds as a fill material can help reduce the amount of scrap tires stockpiled every year. The advantages of using tire shreds as a fill material include availability and low cost [24,29], light weight [27], good stiffness and shear strength [8,23,40,7,38,19,26,10,36], high hydraulic conductivity [35], and ease of placement and compaction in the field [16,32]. Resistance to utilization of tire shreds in civil engineering projects is often attributed to unavailability of tire shreds near construction sites and lack of design standards and detailed construction guidelines [21]. However, extensive
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List of notations GTS GS Pult F⁄
a
is the specific gravity of tire shreds is the specific gravity of Ottawa sand is the pullout capacity obtained from the pullout test is the pullout resistance factor is the scale effect correction factor for nonlinear stress reduction over the embedded length
studies conducted by many researchers in the last two decades have led to better understanding of the behavior of this engineered material. Tire shred-sand mixtures have gained wide acceptance in the last twenty years, leading to their utilization in construction of geotechnical structures. One such application is the use of tire shred-soil mixtures as a backfill material for mechanically stabilized earth (MSE) walls. Tire shred-soil mixtures can be reinforced with extensible (geotextiles or geogrids) or inextensible (metal strips or metal grids) components. An important design consideration for such reinforcements embedded in tire shred-soil mixtures is their pullout resistance. Pullout resistance is mobilized through the interaction between the mixture and the reinforcement, as the pullout force is applied on the reinforcement. According to Federal Highway Administration (FHWA) guidelines [12], the pullout resistance of reinforcement can be calculated as:
Pult ¼ F ar0v Le BC
ð1Þ
where Pult is the pullout resistance of the reinforcement, F⁄ is the pullout resistance factor, a is the scale effect correction factor for nonlinear stress reduction over the embedded length, rv0 is the vertical effective stress at the depth of the reinforcement-soil interface, Le is the embedment length of the reinforcement, B is the width of the reinforcement and C is the effective unit perimeter of the reinforcement. Le C is the total surface area per unit width of the reinforcement in the zone of resistance beyond the slip surface. Berg et al. [12] prescribes values of a = 1.0 (for metallic reinforcement) and C = 2.0 (for strips and grid reinforcement). Full-scale field and laboratory studies have been performed to understand the interaction of geogrids with tire shred-sand mixtures [33,13,14,9,37,31,11]. These studies indicate that the pullout capacities of geogrids in tire shred-sand mixtures are similar or higher than that of geogrids embedded in sand alone. Youwai et al. [39] studied the pullout behavior of hexagonal wire reinforcement and suggested that the required embedment length in the resisting zone is similar for sand and tire shred-sand mixtures. Balunaini and Prezzi [9] performed laboratory pullout tests on ribbed-metal-strip reinforcement, showing that the pullout capacity of ribbed-metal strips in tire shred-sand mixtures lies between the capacities obtained in sand alone (upper boundary) and tire shreds alone (lower boundary). Also, the pullout capacities of ribbed-metal-strips decreased with increasing tire shred content of the mixtures [9]. Ladder-type steel reinforcement, because of their geometry, has a potential for increased pullout capacity due to the additional passive resistance when tire shreds get interlocked within the gridlike structure of the ladder. The main sources of pullout resistance for the ladder-type metal reinforcement are: (1) shear resistance developed due to friction at the mixture-reinforcement interface (along the longitudinal and transverse bars) through the embedded length of the ladder [see Fig. 1(a)], and (2) passive resistance generated by the sand particles and tire shreds wedged against the
rv0 Le B C
is the vertical effective stress at depth of reinforcementsoil interface is the embedment length of the reinforcement is the width of the reinforcement is the effective unit perimeter of the reinforcement
transverse bars of the ladder [see Fig. 1(b)]. The sources of pullout resistance are the same for both ladder-type metal reinforcement and geogrid reinforcement while the pullout resistance is due mainly to interface shear resistance for metal strips. The objective of this paper is to investigate the pullout capacity of ladder-type steel reinforcement embedded in tire shred-sand mixtures by performing a series of pullout tests. The size of the voids within the tire shred matrix increases with increases in the tire shred sizes. As the size of tire shreds increases, segregation of sand becomes predominant in mixtures of tire shreds and sand with large tire shred content. Hence, considering the balance between the economics of the tire shredding process and the potential segregation of sand, tire shreds with sizes between 50 and 100 mm were used in the present study. Samples were prepared in a pullout box at mixing ratios of 0%, 20%, 25%, and 35% by weight of tire shreds. At each mixing ratio, pullout tests were performed at three normal stresses – 40, 65 and 90 kPa. Pullout resistance factors F⁄ were calculated from the pullout test results for ladder-type metal reinforcement. A few tests were performed on ladder-type reinforcement embedded in tire chip-sand mixtures (25% and 35% by weight of tire chips) to study the effect of size of tire shred on the pullout resistance of the reinforcement. In addition, the pullout capacity and resistance factor values from ladder-type metal reinforcement were compared with those obtained from ribbed-metal-strip reinforcement provided in Balunaini and Prezzi [9].
Fig. 1. Mechanism of development of pullout resistance for ladder-type metal reinforcement (a) interface friction, and (b) passive resistance from the interlocking of particles within the reinforcement grid – not to scale (after D. Mohan [15]).
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2. Laboratory pullout testing 2.1. Materials used 2.1.1. Ottawa sand The sand used to prepare the tire shred-sand mixtures was obtained from the U. S. Silica Company, Ottawa, Illinois. This sand conforms to ASTM C 778 [4] and is commercially known as Ottawa sand. Fig. 2 shows the grain-size distribution of Ottawa sand. The specific gravity GS of this sand was determined to be 2.65, according to ASTM D 854-10 [5]. 2.1.2. Tire shreds The tire shreds used in the experimental testing were supplied by Entech Inc., White Pigeon, Michigan. Fig. 3(a) shows a photograph of the tire shreds used in the study. Most of the metal wires and fibers used to reinforce vehicular tires were manually trimmed off the edges of the tire shreds. However, some protruding wires were present at their edges (Fig. 3). The wires were trimmed to prevent testers from being stabbed. The role of these wires associated with pullout capacity is conceptually insignificant but protruding wires may provide little increase in pullout capacity of ladder-type reinforcement due to geometry of the reinforcement. Fig. 3(b) shows a photograph of the tire chips used in the study. The tire chips used in this study were 9.5 mm in nominal size (approximately equidimensional shape). The geometric characterization of the tire shreds was done by measuring the dimensions of randomly selected tire shreds. The largest distance between any two points of a shred is taken as its length [17]. Measurements were made on about 100 tire shred pieces, and Fig. 4 shows the distribution of length and width of the tire shreds. The average length of the tire shreds was 76 mm, the average width was 49 mm and the average thickness was 10 mm, with the average aspect ratio (length/width) equal to 1.6. The tire shreds used in this study are the same as the ones used in previous studies at Purdue University [37,9,10,11]. The specific gravity, GTS, of tire chips and tire shreds was measured in accordance with ASTM C 127-07 [3]. Values of the GTS of tire chips and tire shreds were equal to 1.16 and 1.26, respectively. The presence of steel wires in the tire shreds might have led to their higher GTS values [1,18]. 2.1.3. Ladder-type metal reinforcement The ladder-type metal reinforcements used in this study were manufactured by The Reinforced Earth Company (http://www.reinforcedearth.com/). The laddertype metal reinforcement is commercially known as ‘‘High Adherence Reinforcement Ladders” or ‘‘HA Ladders.” As seen in Fig. 5, the ladder-type metal reinforcement consists of two parallel, cylindrical steel bars welded to a series of cross bars. The total length of the reinforcement used for this research is 1 m. The center-to-center distance between the two longitudinal bars is 0.05 m, and the length of the cross bars is 0.1 m. The diameter of the circular steel bars (longitudinal and transverse) is 0.01 m. At one end, a flat connection plate is welded between the longitudinal bars. This connection plate has a hole, through which a bolted connection can be established to the facing panels of MSE walls. 2.2. Mixing ratio of tire shred-sand mixtures The mixing ratio of tire shreds and sand influences the mechanical behavior of the mixtures. The voids between the tire shreds are very large compared with the size of the sand particles. Hence, during compaction of the mixtures, the sand particles tend to get into the voids present in the mixtures. Segregation of sand particles in the mixtures is a concern, especially when high tire shred contents are used. Balunaini et al. [11] conducted segregation studies and concluded that segregation of sand within the tire shred matrix is predominant for tire shred contents higher
Percent Finer (%)
100 80
Ottawa Sand
60 40 20 0 10
1
Grain Size (mm) Fig. 2. Grain-size distribution of Ottawa sand.
0.1
Fig. 3. Tire pieces used in the study (a) tire shreds used ranging from length 50 to 100 mm, and (b) tire chips. than 35% (by weight of tire shreds in the mixture). Thus, based on the results from the above study, three mixing ratios of 20:80, 25:75 and 35:65 (i.e., tire shred-tosand weight ratio) were adopted in the present study to determine the pullout characteristics of ladder-type metal reinforcement embedded in mixtures. Pullout tests for ladder-type metal reinforcement embedded in sand alone samples were also performed. 2.3. Experimental set-up The main components of the pullout test set-up are: (1) reinforcement clamping system, (2) pullout system, (3) monitoring system, and (4) data acquisition system. Fig. 6 shows a schematic of cross-sectional and plan views of the pullout test set-up. The test set-up has two chambers – the load cell chamber and the tire shredsand mixture chamber. The samples are prepared inside the tire shred-sand mixture chamber, which has inner dimensions of 1.0 m 0.38 m 0.47 m (length width height). The inner sides of the walls of this chamber were covered with smooth-vinyl sheets to minimize friction between the walls and the sample. A sleeve is fitted at the front face within the sample chamber (see Fig. 6) to transfer the point of application of the pullout load beyond the rigid front wall. In the absence of this sleeve, the pullout resistance of the reinforcement is overestimated due to increased lateral pressure against the front wall [22]. The laddertype metal reinforcement was tightly gripped in the load cell chamber between metal wedges using bolts to prevent slippage during testing. Out of the 1.0 m length of the ladder-type metal reinforcement, 0.74 m was embedded inside the tire shred-sand mixture chamber. The normal pressure on the sample was applied by inflating an airbag placed on the top of the sample. Further details on the test equipment can be found in Balunaini and Prezzi [9]. 2.4. Sample preparation and testing Depending on the target mixing ratio of the tire-shred sand mixture, predetermined weights of tire shreds and sand were mixed in buckets and poured into the chamber from small heights to avoid segregation. No water was added during
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25
above the reinforcement, the distance between the top edge of the box and the top of the sample was measured at ten different locations along the four edges of the chamber and averaged to get a representative sample height based on the internal height of the sample in the chamber. A protective sand layer 3-to-4 cm thick was placed on the prepared sample to prevent potential puncture of the rubber airbag by the protruding wires present in the tire shreds. The rubber airbag was placed on top of the protective sand layer to apply the required normal stress to the sample. A steel plate bolted on the top of the chamber provided the reaction to the applied air pressure. Once the required normal stress stabilized, the ladder-type metal reinforcement was pulled out using stepper motors at a rate of 1 mm/min (ASTM D 6706 [2]), while the pullout load and frontal displacement were measured using a load cell and a linear variable differential transformer (LVDT). The test was stopped at a frontal displacement of the reinforcement of about 40 mm or at a displacement at which the pullout load did not change with further displacement. According to the Federal Highway Administration (FHWA) guidelines [20,12], the pullout capacity of an inextensible reinforcement is taken as the peak load obtained within a frontal displacement of 20 mm, except if the reinforcement is ruptured within this frontal displacement range. In the case of absence of both a peak load and reinforcement rupture, the pullout load at 20 mm displacement is considered to be the pullout capacity. Table 1 lists the details of all the tests performed. In this table, the sand-matrix unit weight of the samples prepared for each test is also provided. The sand-matrix unit weight, which gives an indication of packing of sand particles in the voids, is defined as the ratio of the weight of sand to the difference of the volume occupied by the mixture and the volume occupied by tire shreds [23].
20
3. Pullout test results and discussion
15
3.1. Pullout load vs. frontal displacement
10
The results of the pullout tests conducted on Ottawa sand and tire shred mixtures are plotted in Fig. 8. For ladder-type metal reinforcement embedded in Ottawa sand, the test results indicate that the pullout load increases linearly with increasing frontal displacement and then becomes relatively constant at a frontal displacement ranging from 6 mm to 8 mm. However, for the reinforcement embedded in tire shred-sand mixtures, the pullout load increases continuously with displacement until the end of the test. This is possibly due to increasing mobilization of passive resistance by the interlocking of tire shreds within the grids of the ladder-type reinforcement (refer to Fig. 1). In a few tests, a tendency for a decrease in pullout capacity at large frontal displacements, towards the end of the test, was observed.
30
Frequency (%)
25 20 15 10 5 0 50
60
70
80
90
100
Length (mm)
(a) 30
Frequency (%)
547
5 0 20
30
40
50
60
70
80
90
100
Width (mm)
(b) Fig. 4. Geometry of tire shreds of 50–100 mm in length (after Balunaini and Prezzi [9]): (a) distribution according to length, and (b) distribution according to width.
3.2. Effect of normal stress on pullout resistance
Fig. 5. Ladder-type metal reinforcement used in this study, manufactured by the Reinforced Earth CompanyÓ (after D. Mohan [15]).
preparation of mixtures. Then, the mixture was placed in the tire shred-sand mixture chamber in four layers of thicknesses of 0.1 m each. Each layer was compacted using a vibratory compactor according to the procedure discussed in Balunaini and Prezzi [9]. Fig. 7 shows the vibratory compactor used in the sample preparation. After placement of two layers, the ladder-type metal reinforcement was placed on top of the sample (fitting it through the gap between the sleeves on the pullout box). The connection plate on the reinforcement was clamped within the load-cell chamber using bolted connections. After the placement of two additional layers
Table 2 provides the pullout capacity obtained at different normal stresses for the mixing ratios tested. The trend of showing the pullout capacity according to normal stresses in graphical form is shown in Fig. 9. For all mixing ratios, the pullout capacity increases with increasing normal stress. The percent increase in pullout capacity for the two steps of increase in normal stress – 40 to 65 kPa and 65 to 90 kPa – are shown in Table 2. For the sand alone samples, the same stress increase produces a larger increase in pullout capacity at higher stresses (i.e., the percent increase in pullout capacity is higher for normal stresses increasing from 65 to 90 kPa than from 40 to 65 kPa). This trend is reversed as more tire shreds are added to the sand. The test results show that the effect of the normal stress on the pullout capacity is more pronounced for the sand alone samples at higher normal stresses than for the tire shred-sand mixture samples. The pullout capacity of reinforcement embedded in the tire shred-sand mixtures is found to be higher than that in sand alone by about 26–92% for the normal stresses considered in this study. Fig. 9 indicates that there is a trend of increasing the pullout capacity with increase in tire shred content by weight as well as with increase in confinement, except at 40 kPa confinement. At 40 kPa confinement, the reversal in trend that appeared between the 20:80 mixing ratio and the 25:75 mixing ratio can be attributed to the decreased effect of the tire-shred content on the pullout capacity at lower confinements and to experi-
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Air valve Sleeve
Protective sand layer
PVC foam Bolts
Airbag
~ 0.05 m
Reinforcement
0.47 m
0.5 m
Pullout Force Tire shred-sand mixture
Load cell
Metal wedges (shown in orange)
Effective Length = 0.74 m 1m
0.5 m
(a)
0.38 m
Pullout Force
0.4 m
Tire shred-sand mixture
Effective Length = 0.74 m
(b) Fig. 6. Schematic representation of the pullout test set-up (a) longitudinal cross section of the pullout box, and (b) plan view of the pullout box – NOT TO SCALE (after Balunaini [8]).
Inlet valve
Piston vibrator
Air supply (pressure line)
Steel plate (35.5 cm by 35.5 cm, 0.95-cm-thick) Fig. 7. Impact piston-type compactor bolted to a steel plate (compactor model 502LS-EM, manufactured by VIBCO).
mental variability. The increase in pullout capacity with increase in tire shred content is more pronounced for the other tests, performed at higher confining pressures, reflecting the greater strength mobilization by the tire shreds in the mixtures. 3.3. Effect of mixing ratio on pullout resistance Fig. 10 presents the pullout capacity obtained for various tire shred-to-sand mixing ratios. The pullout capacity of ladder-type metal reinforcement embedded in tire shred-sand mixtures is higher than that in Ottawa sand for the normal stresses considered.
The pullout capacity of the reinforcement increases with increase in the tire shred content when compared to that of the reinforcement in sand alone, as shown in Table 2. This contrasts with the findings from pullout studies on metal-strip reinforcement in mixtures in which the pullout capacity of the metal-strip reinforcement was highest when embedded in sand alone and decreased with increasing tire shred content [9]. It should be emphasized that the pullout resistance of metal-strip reinforcement is due to the frictional resistance between the reinforcement and the mixtures (no interlocking effect present). For ladder-type reinforcement embedded in tire shred-sand mixtures, considerable pullout resistance is obtained from the interlocking of the tire shreds in the grids of the ladder-type metal reinforcement in addition to interface frictional resistance. Also, for a given normal stress, the pullout capacity of ladder-type metal reinforcement embedded in tire shred-sand mixtures typically increases with increases in the tire shred content in the mixture. For instance, the pullout capacity of reinforcement embedded in 35:65 mixtures was about 92% higher than that when embedded in sand alone for a normal stress of 65 kPa. This can be attributed to the higher contribution of the interlocking effect of tire shreds in the reinforcement grids. However, the only exception to this was in the case of a normal stress of 40 kPa, for which the mixture of 20:80 tire shred-to-sand weight ratio showed 11% higher pullout capacity than the mixture with 25:75 tire shred-to-sand weight ratio. This exception may be due to the fact that the 20:80 mixing ratio has a 6% higher sandmatrix unit weight than that of the 25:75 mixing ratio. For the tests at higher normal stresses of 65 and 90 kPa, the test results may appear to suggest that the effect of the sand-matrix unit weight may have been outweighed by the higher tire shred interlocking effect produced for reinforcement embedded in the
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V.K. D. Mohan et al. / Construction and Building Materials 113 (2016) 544–552 Table 1 Summary of pullout testing program for ladder-type metal reinforcements embedded in tire shred-sand mixtures. Tire shred-Sand ratio (by weight)
à
Maximum dry unit weight (kN/m3) [37]
Normal stress (kPa)
Average dry unit weight in pullout box (kN/m3)
Average sand-matrix unit weight (kN/m3)
Average sand-matrix unit weight (kN/m3) for tests on ribbed-metal-strip reinforcementà
0:100
40 65 90
18.0
17.2
17.2
17.2
20:80
40 65 90
17.1
15.6
16.7
–
25:75
40 65 90
16.6
14.8
15.8
15.8
35:65
40 65 90
14.2
12.3
12.3
–
Pullout test results for ribbed-metal-strips were obtained from Balunaini and Prezzi [9].
14
14 Ottawa Sand: 40 kPa Ottawa Sand: 65 kPa Ottawa Sand: 90 kPa
25% tire shred: 40 kPa 25% tire shred: 65 kPa 25% tire shred: 90 kPa
12
10
Pullout Load (kN/m)
Pullout Load (kN/m)
12
8
6
10
8
6
4
4
2
2
0
0 0
10
20
30
0
40
Frontal Displacement (mm)
10
(a)
30
40
(c) 14
14 20% tire shred: 40 kPa 20% tire shred: 65 kPa 20% tire shred: 90 kPa
12
35% tire shred: 40 kPa 35% tire shred: 65 kPa 35% tire shred: 90 kPa
12
10
Pullout Load (kN/m)
Pullout Load (kN/m)
20
Frontal Displacement (mm)
8
6
10
8
6
4
4
2
2
0
0 0
10
20
30
Frontal Displacement (mm)
(b)
40
0
10
20
30
40
Frontal Displacement (mm)
(d)
Fig. 8. Measured pullout load vs. frontal displacement for ladder-type metal reinforcements embedded in mixtures of tire shreds and sand with (a) 0% tire shred content by weight, (b) 20% tire shred content by weight, (c) 25% tire shred content by weight, and (d) 35% tire shred content by weight.
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Table 2 Pullout capacities and pullout resistance factor F⁄ for ladder-type and ribbed-metal-strip reinforcements embedded in tire shred-sand mixtures. Normal stress (kPa)
Ladder-type metal reinforcement pullout capacity (kN)y
Increase in pullout capacity from previous normal stress (%)
Increase in pullout capacity compared to sand-alone samples (%)
Pullout resistance factor F⁄ Ladder-type metal reinforcement
Ribbed-metalstrip reinforcementà
0:100
40 65 90
4.2 4.9 6.6
– 17 35
–
1.31 0.97 0.95
1.49 1.08 0.87
20:80
40 65 90
6.1 7.2 8.3
– 18 15
45 47 26
1.91 1.43 1.20
–
25:75
40 65 90
5.5 8.0 9.5
– 46 19
31 63 44
1.73 1.59 1.38
1.05 0.77 0.66
35:65
40 65 90
6.4 9.4 9.7
– 47 3
52 92 47
2.04 1.88 1.42
–
Tire shred-sand ratio (by weight)
y à
The pullout capacity is associated with pullout load at 20 mm displacement. Pullout test results for ribbed-metal-strips were obtained from Balunaini and Prezzi [9].
12
12
10
10
8
8
6
6
4
4
2
2
0 30
0 40
50
60
70
80
90
100
Fig. 9. Measured pullout capacity vs. normal stress for different mixing ratios.
mixture with 25:75 tire shred-to-sand weight ratio. Nevertheless, more testing is required to confirm this trend and isolate the physical phenomena responsible for it.
0
5
10
15
20
25
30
35
40
Fig. 10. Measured pullout capacity vs. tire shred content at different normal pressures.
ments caused by the larger particle size, and/or (2) the interference of the zone of influence with the box boundaries resulting from the large-size tire shreds.
3.4. Effect of tire shred size on pullout resistance 3.5. Pullout resistance factor F⁄ The effect of tire shred size on the pullout resistance of laddertype reinforcement was studied by performing tests on two tire shred sizes – 50-to-100 mm tire shreds and 9.5 mm nominal size tire chips prepared at mixing ratios of 35:65 and 25:75 (by weight of tire shreds to sand). Table 3 presents the comparison of results from pullout capacity on tire chip- and tire shred-sand mixtures. The pullout resistance of reinforcement ladders in tire shredsand mixtures showed 0.5 kN to 0.9 kN higher values for all the three mixing ratios. This higher pullout capacity of 0.5 kN to 0.9 kN is associated with 106–116%. This higher pullout capacity is possibly due to (1) the difference in generation of passive resistance in the case of tire shred-sand mixtures at large displace-
In this study, the effective length Le of the ladder-type metal reinforcement is 0.74 m inside the pullout box. The perpendicular distance between two longitudinal bars of the ladder-type metal reinforcement, equal to 0.05 m, is taken as the width B of the reinforcement. Based on the pullout capacities obtained from the tests, the F⁄ values were calculated. Fig. 11 shows the pullout resistance factor versus normal stress for the different mixing ratios tested. Table 2 summarizes the F⁄ values for each of the pullout tests performed in this study. Fig. 12 shows the pullout resistance factor F⁄ varies with the tire shred content in the mixture for different normal stresses.
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V.K. D. Mohan et al. / Construction and Building Materials 113 (2016) 544–552 Table 3 Comparison of results from pullout tests on tire chip- and tire shred-sand mixtures. Mixing ratio by weight of tires to sand
Normal stress (kPa)
Size of tires
Maximum dry Unit Weight (kN/m3) [37]
Average dry unit weight in pullout box (kN/m3)
Relative compaction (%)
Average sandmatrix unit weight (kN/m3)
Pullout capacity of ladder-type metal reinforcement at 20 mm displacement (kN)
25:75
90
9.5 mm nominal size chips 50–100 mm shreds
15.2
13.7
90.0
14.2
9.0
16.6
14.8
89.1
15.8
9.5
9.5 mm nominal size chips 50–100 mm size shreds
13.6
11.9
87.3
11.6
5.5
14.2
12.3
86.5
12.3
6.4
9.5 mm nominal size chips 50–100 mm size shreds
13.6
11.9
87.3
11.6
8.9
14.2
12.3
86.5
12.3
9.7
35:65
40
35:65
90
2.4 0.8
1.2
1.6
2
2.4
20
Pullout Resitance Factor, F*
30 40 50 60
2
1.6
1.2
70
40 kPa 65 kPa
0.8
80
90 kPa 0
90
5
10
15
20
25
30
35
40
Percentage of re shreds by weight (%) 100 ⁄
Fig. 11. Normal stress vs. pullout resistance factor F for different mixing ratios.
3.6. Comparison of F⁄ values for ladder-type metal and ribbed-metal strip reinforcements Balunaini and Prezzi [9] reported the results of pullout tests performed on ribbed-metal strip reinforcement embedded in tire shred-sand mixtures for mixing ratios of 0:100, 12:88, 25:75, and 100:0 (by weight of tire shreds to sand). Table 2 also provides a comparison of the pullout resistance factors obtained for the ladder-type metal reinforcement from this study with those for ribbed-metal-strips for 0:100 and 25:75 mixing ratios. Table 1 lists the sand-matrix unit weights for relevant tests from Balunaini and Prezzi [9]. The pullout resistance factors for ribbed-metal-strips embedded in sand alone samples are similar (8–13% variation for different normal stresses) to those for ladder-type metal reinforcement. Note that for sand alone samples, the contribution from passive resistance to the pullout capacity is minimal for both types of reinforcement when compared to that for tire shred-sand mixtures due to the tire shred interlocking contribution. However, for the mixtures (i.e., tire shred-sand mixtures of 25:75 by weight) under the same testing conditions, the pullout
Fig. 12. Pullout resistance factor, F⁄ vs. tire shred content at different normal pressures.
capacity of the ribbed-metal strip was 40–52% lower than that of the ladder-type metal reinforcement. This means that passive resistance in the case of ladder-type reinforcement contributes significantly to its pullout capacity. Hence, use of ladder-type metal reinforcement proves more advantageous than ribbed-metal strip reinforcement when tire shred-sand mixtures are used as backfill in the construction of MSE walls. 4. Summary and conclusions Pullout tests were performed on mixtures consisting of tire shreds of 50–100 mm nominal size and Ottawa sand. The tests were performed at mixing ratios of 0:100, 20:80, 25:75 and 35:65 by weight of tire shreds to sand. At each mixing ratio, pullout tests were performed at 40, 65 and 90 kPa normal stresses. From the results of the pullout tests the following conclusions were made: 1. The pullout load vs. frontal displacement curve shows a constant increase up to a displacement of about between 6 mm and 8 mm and then it becomes relatively constant at a frontal
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displacement of approximately 15 mm for ladder-type metal reinforcement embedded in Ottawa sand (0:100 mixing ratio by weight). 2. In the case of the reinforcement embedded in tire shred-sand mixtures, the pullout capacity continues to increase through the test. This may result from the increase in the mobilization of the interlocking resistance of tire shreds within the grids of the ladder-type reinforcement. 3. The pullout capacity of ladder-type metal reinforcements embedded in tire shred-sand mixtures is higher than in sand alone by about 26–92% based on the mixing ratios and the normal stresses considered in this study. This can be attributed to the contribution of the passive resistance resulting from the tire shreds interlocking in the grids of the ladder-type metal reinforcement, in addition to the resistance due to interface friction. 4. For a given normal stress, the pullout capacity of ladder-type metal reinforcements embedded in tire shred-sand mixtures typically increases with increases in the tire shred content in the mixture. The pullout capacity of reinforcement embedded in 20:80, 25:75, and 35:65 mixtures increased by 26–47%, 31– 63%, 52–92% compared to sand alone for the range of normal stresses considered in the study. A systematic approach was followed in this experimental investigation on the pullout capacity of ladder-type metal reinforcement in tire shred-sand mixtures. Results from this study can be used to guide the development of ‘‘optimal” mixtures of tire shreds and sand. In addition, the pullout resistance factors proposed in the study can be referred to in the design of MSE walls reinforced with ladder-type reinforcements with tire-shred sand mixtures as the backfill material. It is important to note that the laboratory results reported in this research are subject to limitations: (a) inevitable variation of the unit weight of the tire-shred sand mixtures with sample preparation in the pullout test box using different sizes of tire shreds and (b) chamber boundary effects. Also, the behavior of the mixtures depends on the properties of both the tire shreds and sand. Thus, the applicability of the results reported in this paper depends on whether the materials selected and used for construction at the project sites are similar to those used in this study. This paper provides preliminary guidelines on the pullout capacity of ladder-type reinforcement backfilled with different tire-shred sand mixtures based on laboratory test results. Further testing is recommended to investigate the pullout capacity of the ladder-type reinforcements embedded in tire shred-sand mixtures for field applications. Acknowledgement The authors would like to acknowledge the financial support provided by the Indiana Department of Transportation (INDOT) via the Joint Transport Research Program (JTRP) for this study. References [1] I. Ahmed, Laboratory study on properties of rubber-soils. Report FHWA/IN/ JHRP-93/4, Indiana Department of Transportation, Purdue University, West Lafayette, Indiana, 1993. [2] ASTM D 6706-01, Standard test method for measuring geosynthetic pullout resistance in soil, ASTM International, West Conshohocken, PA, USA, 2001. [3] ASTM C127-07, Standard test method for density, relative density (specific gravity), and absorption of coarse aggregate, ASTM International, West Conshohocken, PA, USA, 2007. [4] ASTM C 778-06, Standard specification for standard sand, ASTM C 778-06, West Conshohocken, PA, USA, 2007. [5] ASTM D854-10, Standard test methods for specific gravity of soil solids by water pycnometer, ASTM International, West Conshohoken, PA, USA, 2010. [6] ASTM D 6270-08, Standard practice for use of scrap tires in Civil Engineering applications, ASTM International, West Conshohoken, PA, USA, 2012.
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