effect of steel pins on interface shear behavior of segmental concrete units

Effect of Steel Pins on Interface Shear Behavior of Segmental Concrete

Units

Md. Zahidul Islam Bhuiyan

*

Postgraduate student

Department of Civil Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia

Email: [email protected]

Faisal Hj Ali

Professor

Department of Civil Engineering, National Defense University of Malaysia, 57000 Kuala Lumpur, Malaysia


Firas A. Salman

Senior lecturer

Department of Civil Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia


Abstract - This study mainly focuses on shear strength behavior of newly and locally produced modular block units

with and without steel shear pins. A series of direct shear tests was executed to find out the effectiveness of steel

shear pins under different normal loading conditions. Test results were outlined in the form of shear force-

displacement relationship to compare the influence of shear pins on shear strength behavior. Test results revealed

that the presence rigid shear pins reduce the shear strength capacity than a purely frictional condition.

Key words: interface shear, rigid pin, segmental block, shear strength, shear connector

1. INTRODUCTION

Geosynthetic reinforced segmental retaining walls (GR-

SRWs) consisting of polymeric reinforcements and precast

modular block units have achieved popularity worldwide in the

last three decades because of their many fold advantages. They

are frequently used in many geotechnical applications. In

Malaysia, the use of dry-stacked column of segmental units as a

facing column in retaining wall constructions has been

extensively practicing for last decades (Lee, 2000).

Today, facing stability is an important issue in the current

design guidelines (NCMA, 1997; Elias et al., 2001) and it

mainly depends on interface shear and connection failures. Past

research works (Soong & Koerner, 1997; Bathurst & Simac,

1993; Buttry et al., 1993) reported that facing instability

basically occurs due to poor connection strength and inadequate

connection systems.

To develop interlocking mechanism between successive

vertical courses of units, two different types of shear

connections are mainly used in retaining wall constructions.

One is built-in mechanical interlock in the form of concrete

shear keys or leading/trailing lips and another one is the

mechanical connector consisting of pins, clips, or wedges.

Mechanical connectors are mainly used to help out unit

alignment and control the wall facing batter. Bathurst and

Simac (1997) reported that shear connectors (mechanical) or

shear keys provide additional interface shear capacity of

segmental concrete units.

As a shear connector, steel pins (rigid) were used in this

research to investigate its effect upon interface shear behavior

of infilled units. To evaluate interface shear behaviors or

performance parameters, a series of full scale laboratory tests

was conducted with and without steel pins (NCMA SRWU-2,

1997; ASTM D 6916-03). Shear force-shear displacement

graphs were drawn to compare performance of the infilled

concrete units with and without shear pins. Shear capacity

envelope graphs were also plotted by using Morh-Coulomb

failure criteria under peak and service state criteria.

*Corresponding Author: Md. Zahidul Islam Bhuiyan


mailto:[email protected]



2. MATERIALS

2.1 Segmental Concrete Unit

In this investigation “I” blocks were used as segmental

concrete units. “I” blocks are machined mold wet cast concrete

units (G 30), which have one center web and the tail/rear flange

is extended beyond the web (Fig. 1). The rear flange is tapered

that allows the blocks to form curve walls. The maximum

tapered angle of the “I” block is 11.3 deg. “I” blocks are double

open-ended units and make an equivalent hole in conjunction

with two units, and the equivalent dimensions are around 450

mm in length, 280 mm in width and 300 mm in height. The

infill weight is approximately 93 to 94 kg with the aggregate of

bulk density of 1527 kg/m3.The physical and mechanical

properties of the used block are outlined in Table 1.

Figure 1: Schematic of used “I” block.

Table 1: Physical and mechanical properties of segmental

concrete units.

Property Value

Dimensions (WxHxL)* in mm 370x300x500

Weight (kg) 41-42

Oven dry density (kg/m3) 2166

Water absorption capacity % 7.1

kg/m3

155

Moisture content (%) 3.7

Net compressive strength (MPa) 8.0

* W = Width (Toe to heel), H= Height, L= Length (Parallel to

the wall face)

2.2 Granular Infill

The hollow cores between the blocks were infilled with

100% crushed limestone aggregate and lightly compacted. The

maximum and nominal maximum sizes of the aggregate were

25 and 19 mm, respectively. The particle size distribution of the

granular infill meets the lower limit of the NCMA (1997)

gradation requirements. The physical properties of infill are

given in Table 2.

2.3 Steel Bar

Galvanized mild steel round bars were used in the study as

mechanical connectors that are generally known as shear

connectors. According to the hole dimensions of the segmental

concrete units, 12 mm dia bars were selected, and the bars were

cut into 125 mm in length. The physical and mechanical

properties of the used round steel bars are illustrated in Table 3.

Table 2: Physical properties of granular infill.

Table 3: Physical and mechanical properties of steel bar

(Courtesy of AMSteel Mills Sdn Bhd, Malaysia).

Property Value

Yield strength (MPa) 347

Modulus of elasticity (MPa) 210000

Rolling mass (Kg/m) 0.859

Cross section area (mm2) 113.10

3. EXPERIMENTAL DESIGNS

3.1 Test Apparatus

A specially designed and modified large-scale apparatus

originally reported by Bathurst and Simac (1993) was used to

carry out the performance tests of the “I” blocks. A photograph

of the modified test apparatus is shown in Fig. 2. It is seen that

the apparatus was mainly consisting of loading frame, hydraulic

actuators, and a fabricated electric hydraulic pump. The vertical

actuator was mounted with the loading frame with rollers to

allow block movement during the shear test, but in ASTM D

6916-06c test protocol the vertical actuator was kept fixing. The

vertical and horizontal actuators were capable of applying

around 45 tons of surcharge load and 130 tons of push/pull out

force respectively and simultaneously. The electric hydraulic

pump was connected to the actuators with pressure hoses, and

the pump was capable of delivering flow rate 3 cc per minute.

A geosynthetic loading clamp was set with horizontal actuator

to apply the tensile load as well as shear load.

Property Value

Bulk density (kg/m3) 1527

Specific gravity 2.63

Void content (%) 42

Coefficient of gradation, Cc 1.15

Fineness Modulus (FM) 7.16

Two (2) pressure transducers of 0 to 3,625 psi capacity were

mounted over each hydraulic actuator of 150 mm stroke, and

the actuators were calibrated by using load cell against the

pressure transducers. Two (2) flow regulators (Atos QV06160)

were attached with the pump to control the rate of displacement

of horizontal (shear) and vertical actuators.

The shear displacements were measured using of two 50

mm linear variable displacement transducers (LVDTs) with an

accuracy of 0.001mm. Pressure transducers and LVDTs reading

were continuously measured and recorded during the test by a

data logger. The data were recorded at every 10 second interval.

3.2 Interface Shear Tests

Two layers of modular block units were used for interface

shear test. The bottom layer/course consisting of two (2) “I”

blocks was installed and braced laterally at the front of loading

frame. A single “I” block was placed centrally over the running

joint formed by the two underlying units to simulate the

staggered construction procedure used in the field. Two layers

of segmental units were connected with shear pins and setback

was kept as zero. The hollow sections between the blocks were

filled with 19 mm crushed stone aggregate and lightly

compacted using a steel rod. To hold the infilled aggregate of

top block, two (2) steel plates were used (Fig. 3).

Surcharge/normal load was imposed only over the top

block through stiff rubber mat and simulated an equivalent

height of stacked blocks. The shear load was applied against the

top block and immediately above the shear interface to

minimize the moment loading at a constant rate of 1 mm/min of

the horizontal actuator (ASTM D 6916-03). A steel plate with

stiff rubber mat was used with geosynthetic loading clamp to

concentrate the shearing load only over the centrally installed

top block. A horizontal seating load was applied to the top

block to ensure close fitting of the shear pins and after that the

load and displacement devices were set to zero. The imposed

seating load was 10% of maximum shear strength.

Mohr-Coulomb failure criteria were used to find out

interface shear capacity at ultimate and service state strength

criteria.

(1)

Where:

= Interface shear capacity (kN)

= Normal load (kN)

= angle of friction (deg.)

= interception

Figure 2: Photograph of test apparatus.

Figure 3: Photograph of interface shear test arrangement.

4. TEST RESULTS AND DISCUSSIONS

A series of interface shear tests was performed under a

range of normal forces. Shear force - displacement graphs were

plotted to evaluate the effects of rigid shear pins on the interface

frictional behavior of the tested blocks. Shear capacity

envelopes were also drawn to compare the ultimate and service

state shear strength of “I” blocks.

Figs. 4(a) and 4(b) compare the magnitude and distribution

of shear force with displacement of infilled concrete units. Fig.4

(a) shows the gradually increment of shear force without any

pick points that results from the absence of steel pins at joints.

In this case, shear force reaches the steady state condition

(around 10 kN) after a significant amount of shear

displacement. On the other hand, a sudden pick shear force can

be seen in Fig. 4(b) prior to serviceability displacement limit

and blocks fail quickly at joints (Fig. 6).

Horizontal

actuator

Vertical

actuator

Electric pump

Pressure transducer

The serviceability displacement is identified by vertical dashed

line in the graphs, which is around 7 mm according to the block

Geometry (2% of the block width). It is also seen that after

immediate failure at joints, the shear interface behaves like

purely frictional surface and there is no significant rise and fall

of shear force with displacement increment rather than near to

straight line (Fig. 4(b)).

Fig. 4(c) also demonstrates the gradual increment of shear

force against shear displacement without any pick points in the

curve like Fig. 4(a). Fig. 4(d) reports three (3) repeated tests at

almost same surcharge loads, which were controlled using

analog pressure controlling valve. Three (3) nominally identical

curves also show the accuracy of the performed laboratory tests

and the peak shear forces of these tests are less than 10%

from the mean of the three tests. The most interesting thing of

Fig. 4(d), it is showing two (2) pick points; before and after

serviceability line. It happens due to the failure mechanism of

concrete blocks at the pin joints. At the time of the experiment,

it was observed that one shear pin joint fails first than other

because of block irregularity and set up alignments. As a result,

shear force increases up to completed failure of both pin joints

and drops permanently or becomes a steady state due to

aggregate frictions.

The test numbers 1 & 2 of Fig. 4(e) illustrate abrupt drops

of shear strengths that happen due to sudden relief of frictional

contact area of blocks’ interface like the behavior of tectonic

plates. From the Fig. 4(f), it is seen that the steady state shear

strength of pins connected infilled blocks reduce to about 15 kN

that is less than the purely frictional condition (Fig. 4(e)). It

occurs due to the stress concentration at the connection joints,

which accelerate the failures of blocks at the flange area (Fig.

6(b)). As a result, the failure interface areas of blocks become

loose enough and unable to carry shear force that causes to

reduce interface shear strength at the high surcharge load

conditions. The same behaviors are also observed in Fig. 4 (h)

with respect to Fig. 4(g). The curve of Fig. 4(g) is wavier than

others purely frictional graphs that result from a high surcharge

load. The increment of normal load makes stress concentrations

at different contact points of interface, which leads to rise and

fall of shear force with the mobilization of blocks.

The data presented in the Figs. 5(a) & 5(b) illustrates the

influence of steel shear pins on shear capacity envelopes. It is

clear from the Fig. 5(b) that shear pins provide more apparent

cohesion (normal force-independent strength) than purely

frictional block systems (Fig. 5(a)) at the ultimate condition,

although the angle of internal friction is less. Bathurst and

Simac (1994), and Bathurst et al. (2008) reported the same

behaviors with different types of block geometries and shear

connectors. The frictional interface area is a complex surface

consisting of block-block, block-infill, and infill-infill contact

areas. The presence of steel shear pins in running bond causes

immediately failures (spalling/cracks) in the block-block

contact areas, which results in the reduction of concrete contact

areas (Fig. 6). As a result, angle of internal friction becomes

lower than purely frictional interface systems. Fig. 5(b) reports

no serviceability capacity envelope due to the presence of steel

pins in the connection system that breaks the blocks before

reaching the serviceability limit.

So it can be said that this connection system is not

effective for service state design of segmental retaining walls

with “I” blocks. As an alternative of steel pins, plastic pins may

be used and investigated its effect on interface behavior.

5. CONCLUSIONS

This study investigates the results of interface shear testing

executed to find out the effect of steel shear pins on frictional

behavior of newly fabricated and modified “I” blocks. In this

research, a series of interface shear tests was performed under

several conditions. The following conclusions can be drawn

based on the results:

1. Steel shear pins initially increase shear force but after

immediate failure at the joints shear force decrease to

purely frictional shear force of infilled blocks or less

than that.

2. Purely frictional behavior of infilled blocks is very

smooth and steadier than infilled blocks with steel

pins.

3. Purely frictional systems easily govern service state

criterion but the systems with steel pins are unable to

follow that criterion.

4. Steel bar is stiffer than concrete, as a result concrete

fails easily at the connections prior to any significant

shear displacement to mobilize shear strength. So, it is

important to find out effective flexible shear

connectors that give significant shear displacement as

well as shear strength.

Shear displacement (mm)

0 5 10 15 20 25

She

ar f

orce

(kN

)

0

5

10

15

20

25

30

Normal force = 8.13 kN

Shear Displacement (mm)

0 5 10 15 20 25

She

ar f

orce

(kN

)

0

5

10

15

20

25

30


(a)


0 5 10 15 20 25

Sh

ea

r fo

rce

(kN

)

0

5

10

15

20

25

30

Normal force = 8.71kN


0 5 10 15 20 25

She

ar f

orce

(kN

)

0

5

10

15

20

25

30





0 5 10 15 20 25

She

ar f

orce

(kN

)

0

5

10

15

20

25

30



0 5 10 15 20 25

She

ar f

orce

(kN

)

0

5

10

15

20

25

30



0 5 10 15 20 25

She

ar f

orce

(kN

)

0

5

10

15

20

25

30



0 5 10 15 20 25

She

ar f

orce

(kN

)

0

5

10

15

20

25

30




(h)

(b)

(d)

(f)

(h)

Figure 4: Shear force-displacement curves with (b, d, f, and h) and without (a, c, e and g) shear pins.

(g)

(c)

(e)

Figure 6: Photographs of common failure patterns at joints (a)

Spalling at bottom blocks, (b) spalling at top block, and (c)

Triangular crack at bottom block.

REFERENCES

ASTM D 6916-03 Standard test method for determining

the shear strength between segmental concrete units, West

Conshohocken, PA, USA, ASTM International.

ASTM D 6916-06c Standard test method for determining

the shear strength between segmental concrete units, West

Conshohocken, PA, USA, ASTM International.

Buttry, K. E., Mccullough, E. S., and Wetzel, R. A. (1993)

Laboratory evaluation of connection strength of geogrid to

segmental concrete units, Washington, DC 20001 USA.

Bathurst, R. J., and Simac, M.R. (1993) Laboratory testing

of modular concrete block - geogrid facing connections.

Proceedings of ASTM Symposium on Geosynthetic Soil

Reinforcement Testing, San Antonio, Texas, USA.

Bathurst, R. J. & Simac, M. R. (1994) Geosynthetic

reinforced segmental retainingwall structures in North America.

Proceedings of the Fifth International Conference on

Geotextiles, Geomembranes and Related Products, Singapore,

1-41.

Bathurst, R. J., and Simac, M.R. (1997) Design and

performance of the facing column for geosynthetic reinforced

segmental retaining walls. In J. W. Balkema, (ed), International

symposium on mechanically stabilized backfill. Denver,

Colorado.

Bathurst, R. J., Althoff, S. and Linnenbaum, P. (2008)

Influence of test method on direct shear behavior of segmental

retaining wall units, Geotechnical Testing Journal, 31, 1-9.

Elias, V., Christopher, B. R., and Berg, R. R. (2001)

Mechanically stabilized earth walls and reinforced soil slopes

"Design & construstion guidelines", FHWA-NHI-00-043,

Washington D.C., National Highway Institute.

Lee, C. H. (2000) Design and construction of a 9.6m high

segmental wall. Proceedings of Secend Asian geosynthetics

conference, Kuala Lumpur, Malaysia.

NCMA (1997) Design manual for segmental retaining

walls, Herndon, Virginia, National Concrete Masonry

Association (NCMA).

Soong, T. Y., and Koerner, R.M. (1997) On the required

connection strength of geosynthetically reinforced walls,

Geotextiles and Geomembranes, 15, 377- 393.

Normal force (kN)

0 10 20 30 40

She

ar f

orce

(kN

)

0

10

20

30

40

47.403.43tan NVp

11.44tanNVs

Peak capacity

Capacity @ 7 mm displacement

Normal force (kN)

0 10 20 30 40

She

ar f

orce

(kN

)

0

10

20

30

40

89.720.32tan NVp

Peak capacity

Figure 5: Shear capacity envelopes of “I” blocks (a) without and (b) with steel pins.

(a) (b)

(a)

(b)

(c)

AUTHOR BIOGRAPHIES

Md. Zahidul Islam Bhuiyan is a postgraduate student at the

Department of Civil Engineering, Faculty of Engineering,

University of Malaya, and Malaysia. He graduated from the

Department of Civil Engineering at Bangladesh University of

Engineering and Technology (BUET), Bangladesh in 2009. His

research interests include geotechnical engineering. He can be

reached at [email protected].

Faisal Hj Ali is a professor at the Department of Civil

Engineering, Faculty of Engineering, National Defense, and

Malaysia. He received a doctoral degree in Geotechnical

Engineering from University of Sheffield, United Kingdom in

1984. His teaching and research interests include unsaturated

residual soils, ground improvement techniques, slope

instability, foundation engineering, and reinforced earth. He can

be reached at [email protected].

Firas A. Salman is a senior lecturer at the Department of Civil

Engineering, Faculty of Engineering, University of Malaya, and

Malaysia. He received a doctoral degree in Geotechnical

Engineering from Baghdad University, Iraq. His teaching and

research interests include foundation analysis & design, soil

investigation, and ground improvement. He can be reached at

[email protected].




effect of steel pins on interface shear behavior of segmental concrete units

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