shear behaviour of reinforced phyllite concrete beams

Materials & Design, 43, 438-446

1

Shear behaviour of reinforced phyllite concrete beams

Mark Adom-Asamoah , Russell Owusu Afrifa Department of Civil Engineering, Kwame Nkrumah University of Science and Technology, College of

Engineering, Kumasi, Ghana.

* Corresponding author. Tel: +233-51-60226 Fax: +233-51-60226

Email: [email protected]; [email protected]

(Dr Mark Adom-Asamoah)

ABSTRACT

The shear behaviour of concrete beams made from phyllite aggregates subjected to

monotonic and cyclic loading is reported. First diagonal shear crack load of beams with and

without shear reinforcement was between 42–58% and 42–92% of the failure loads

respectively. The phyllite concrete beams without shear links had lower post-diagonal

cracking shear resistance compared to corresponding phyllite beams with shear links. As a

result of hysteretic energy dissipation, limited cyclic loading affected the stiffness, strength

and deformation of the phyllite beams with shear reinforcement. Generally, beams with and

without shear reinforcement showed anchorage bond failure in addition to the shear failure

due to high stress concentration near the supports. The ACI, BS and EC codes are

conservative for the prediction of phyllite concrete beams without shear reinforcement but

they all overestimate the shear strength of phyllite concrete beams with shear reinforcement.

It is recommended that the predicted shear capacity of phyllite beams reinforced with steel

stirrups be modified by a reduction factor of 0.7 in order to specify a high enough safety

factor on their ultimate strength. It is also recommended that susceptibility of phyllite

concrete beams to undergo anchorage bond failure is averted in design by the provision of

greater anchorage lengths than usually permitted.

1. Introduction

As the population of the world increases and urban centres expand, the levels of construction

are also expected to increase. The construction industry in most countries makes use of

aggregates extracted from natural sources for the production of concrete. In the UK alone,

nearly 208 million tonnes of sand, gravel and crushed rock aggregates are mined for

construction annually [1]. The use of aggregates in concrete is primarily dependent on their

mailto:[email protected]

mailto:[email protected]


2

availability. The depletion and over-exploitation of sources of conventional aggregates (such

as granite and sandstone) used in the production of conventional (normal) concrete has led to

the use of other available aggregates. One of such aggregates, phyllites; are foliated rocks

with preferential cleavage that has the property of easily breaking up into thin slabs [2]. The

presence of phyllites has been reported in several parts of the world such as Europe [3,4],

South America [5], the Himalayan region [6] and Africa [7]. In Ghana (especially the gold

mining areas), phyllite aggregates have become convenient in their use as a construction

material as a result of its availability in addition the scarcity of material coupled with the high

cost of normal aggregates. The lack of information regarding the properties and structural

performance of concrete members made from phyllite aggregates is a hindrance to the use of

the material by engineers both during the design and construction phases of structures.

Reports of collapsed reinforced concrete (RC) buildings made of phyllite aggregates are

documented [7,8].

Earlier work [7] has expressed concerns about the low compression and flexural strengths

of plain phyllite concrete. This was attributed to the flakiness and elongation properties

coupled with the reactive materials in phyllite aggregates that affect the absorption and bond

characteristics of its concrete. A study [8] of the flexural behaviour of reinforced concrete

phyllite beams adequately reinforced with shear stirrups indicated concerns about their shear

capacity as a result of the formation of diagonal tension cracks at failure. It was concluded

that the BS8110 [9] theoretical shear strength of the concrete made from phyllite aggregates

be reduced as a result of the early formation of diagonal tension cracks observed during the

testing of the beams.

Shear mechanism is an important aspect of structural concrete behaviour, yet this is an area

lacking in-depth understanding and consensus amongst researchers [10]. It is therefore not

surprising that neither the distribution of shear stress across a flexurally cracked beam nor the

determination of shear stress magnitude have been fully understood. The complex nature of

this phenomenon-non has been attributed to the many different parameters that are identified

to influence shear strength. However, it is generally accepted [11,12] that the shear capacity

of reinforced normal concrete beams without shear reinforcement derive its contributions

from compression shear zone (ranging between 20% and 40%), aggregate interlock

mechanism (ranging between 35% and 50%) and the dowel action of longitudinal

reinforcement (ranging between 15% and 25%). Walraven [13] reported that shear failure is


3

usually caused by a large amount of shear force that is transferred along the cracked surface

via aggregate interlock which provides resistance against slip. The effect of aggregate

interlock on concrete shear resistance reduces as the depth of beam increases. This is because

an increase in crack widths at points above the main reinforcement weakens the aggregate

interlock capacity and reduces concrete shear capacity [14–17]. It is also known that

aggregate interlock is dependent on longitudinal reinforcement ratio. Higher percentages of

longitudinal reinforcement results in smaller crack widths which in turn enhance aggregate

interlock and shear capacity. For beams with very small shear-span to depth ratio, Kani [18]

explained that the shear resistance may be affected because the applied shear force may be

transmitted directly to the supports by arch action (compressive struts) of the concrete.

Table 1 Properties of aggregates.

Physical Properties Phyllite Aggregates

Unit weight (relative) 2.72

Water Absorption (%) 1.80

Aggregate Impact (%) 9.80

Aggregate Crushing (%) 18.64

Ten-percent Fines (KN) 255.75

Flakiness Index (%) 28.00

Elongation Index (%) 25.00

Los Angeles Abrasion 17.50

As mentioned earlier, reports on the mechanical characteristics of phyllite aggregates,

characterisation of compressive and flexural strengths of plain phyllite aggregates, and the

flexural behaviour of phyllite concrete beams adequately reinforced in shear are available in

literature [7, 8]. However, information on the shear strength of reinforced concrete beams

made from phyllite aggregates is not available. This research focuses on the experimental

investigation into the shear strength of reinforced concrete beams made from phyllite

aggregates with and without web reinforcement. It is important to study the shear failure

mechanism of large scale reinforced concrete beams without web reinforcement to better

understand the failure mechanisms in RC beams [12,19,20]. Furthermore, current design

procedures for RC beams with shear links are largely based on the truss analogy method

developed over a century ago [21]. Experience has revealed that the truss analogy method is

over conservative in calculating the shear strength of RC beams with shear reinforcement.

This occurs as a result of the fact that the compressive force carried by the concrete in the


4

compression zone (top chord of the truss) is ignored [20, 22].

The aim of the study is to investigate the shear strength of rein-forced concrete beams

made from phyllite aggregates. Sixteen (16) beams were manufactured and tested in the

laboratory. Ten (10) of the beams were without stirrups whilst the other six (6) were made

with different levels of shear reinforcement. The 10 beams without shear reinforcement in

addition to 3 of those with shear stirrups were tested under monotonic four point bending

tests whilst the rest were tested under limited cyclic tests of 20 loading–unloading cycles. The

experimental shear capacity results

Were compared with shear capacity prediction obtained from ACI 308-02 [23], BS 8110 [9]

and EC 2 [24] codes of practice.

(a) Phyllite aggregates of diverse sizes

(b) Particle size distribution of fine and coarse aggregates

Fig. 1 Phyllite aggregates of diverse sizes-particle distribution.

0

20

40

60

80

100

120

0.01 0.1 1 10 100

Per

cen

tag

e p

ass

ing

(%

)

Seive size (mm)

Fine aggregate Phyllite coarse aggregate


5

2. Experimental and test program 2.1. Materials

Ordinary Portland Cement (OPC) to the specifications of BS 12:1989 [25] was used for

concrete manufacture. Grading of fine and coarse aggregates were done to the specifications

of appropriate codes [26]. River sand was used as fine aggregates whilst crushed phyllite

stones were used as coarse aggregates in phyllite concrete beams. Table 1 shows the physical

properties of the phyllite aggregates performed according to specifications [27]. Fig 1a shows

phyllite aggregates of diverse sizes whilst Fig 1b shows particle size distribution curves for

fine and coarse aggregates. The average yield strength of the tensile reinforcing bars was 375

N/ mm2 with nominal diameter (size) of 12 mm. Steel bar size of 6 mm was used as nominal

top reinforcement having average yield strength of 250 N/mm2.

2.2. Beam specimens and testing

Sixteen (16) beams were cast and tested in the laboratory. Ten (10) of these beams were

without shear reinforcement whilst the remaining had shear reinforcement. All the 16 beams

were of design mix ratio 1:1.5:3 (cement: sand: coarse aggregates) and water-cement ratio of

0.5 with a target 28-day compressive strength of 25 MPa. Five different beam dimensions

with different cross sectional areas (140 mm X310 mm, 140 mm X 265 mm, 110 mm X 225

mm, 110 mm X184 mm and 90 mm X 150 mm) and respective lengths (2400 mm, 2000 mm,

1700 mm, 1500 mm and 1000 mm) were considered for the beams without shear

reinforcement (P1–P10). Each type of beam dimension was reinforced with 1% and 2%

amount of longitudinal steel reinforcement. There was no shear reinforcement, within the

effective span of the beams (P1–P10). The 6 beams with shear reinforcement (P11–P16) had

dimensions 145 mm X 265 mm X 2000 mm as shown in Table 2. The beams with stirrup

reinforcement were made of 3 stirrup spacing types; R6@250mm (P11 and P12),

R6@150mm (P13 and P14) and R6@100mm (P15 and P16). The shear span to depth ratio

was kept between 2.35 and 2.5 to ensure that shear rather than bending failure occurred in all

beams during testing. The reinforcement details of the beams are shown in Fig 2.


6

Table 2 Beam description

BEAM

No.

B X D

(mm x mm)

Length

(mm)

Shear

span/eff.

depth ratio

(av/d)

Shear

reinforcement

& spacing

Percentage

Longitudinal

reinforcement

ρ (%)

28days

Concrete

compressive

strength,

fcu (N/mm2)

28days

Concrete

flexural

strength,

fcr(N/mm2)

P1 140 X 310 2400 2.45 - 1 23.50 3.40

P2 140 X 310 2400 2.45 - 2 23.50 3.40

P3 140 X 265 2000 2.45 - 1 23.50 3.40

P4 140 X 265 2000 2.45 - 2 23.50 3.40

P5 110 X 225 1700 2.45 - 1 23.00 3.38

P6 110 X 225 1700 2.45 - 2 23.00 3.38

P7 110 X 184 1500 2.45 - 1 23.00 3.38

P8 110 X 184 1500 2.45 - 2 23.00 3.38

P9 90 X 150 1000 2.35 - 1 23.00 3.38

P10 90 X 150 1000 2.35 - 2 23.00 3.38

PS4A 140 X 265 2000 2.45 R6@250mm 2 23.50 3.40

PS4B 140 X 265 2000 2.45 R6@250mm 2 23.50 4.00

PS4C 140 X 265 2000 2.45 R6@150mm 2 23.50 2.50

PS4D 140 X 265 2000 2.45 R6@150mm 2 23.50 3.40

PS4E 140 X 265 2000 2.45 R6@100mm 2 23.50 4.00

PS4F 140 X 265 2000 2.45 R6@100mm 2 23.50 2.50

All the beams were cast, placed in wooden moulds and compacted with a shutter vibrator.

Beams were left to cure under damp mats for 28 days after which they were tested.

Companion concrete cubes (100 X 100 X 100 mm) and modulus of rupture beams (100 X

100 X 500 mm) were cast and cured in water for 28 days to determine the compressive and

bending strengths of the plain concrete respectively. The average compressive strength of

phyllite concrete was 23.2 N/mm2. The beams were tested by four point bend test as

described in Fig 3. Beam deflections at mid-span for a steady loading rate of 0.2 kN/s were

taken with the aid of a dial gauge fixed at the bottom of each beam. Crack development on

the concrete beam surface was closely monitored to ascertain first flexural and shear cracks,

and crack width at tension steel level. Crack widths were measured at the load increments

using a crack microscope of optical magnification 10X and reading to 0.02 mm. Crack


7

patterns were marked on the beams after failure.

Fig.2 Beam details

a) Schematic diagram of experimental set-up

(b) Test set-up and instrumentation

Fig. 3 Schematic diagram of experiment set

265mm

2000 mm

1800 mm 145mm

D

B

(a) Beam without shear reinforcement (P1-P10)

900 – 2100 mm

1000 – 2400 mm

(b) Beam with shear reinforcement (P11-P16)

Link spacing varies (R6@250, R6@150 ,R6@100)


8

3. Test results and discussions

Results obtained from testing both phyllite aggregate concrete beams with and without

shear reinforcement are discussed in this section. Experimental first flexural crack, first shear

crack and failure loads were identified. Load–deflection behaviour, failure behaviour, number

of cracks and maximum crack width of RC beams are also discussed using Fig. 4–7 and

Tables 3–5. Further analysis and comparisons are made with respect to shear strength

prediction made by other design codes of practice.

3.1. Load–deflection behaviour

Typical load versus mid-span deflection curves of the beams without stirrups are compared

for phyllite concrete beams in Fig 4. The initial linear portion of the beams without links

subjected to monotonic loading (P1–P10) before first flexural cracking was similar. The

initial stiffness of the beams was independent of the percentage of tension reinforcement (1%

or 2%) as shown in Fig 4 for corresponding beam pairs (P1–P2, P3–P4 and P7–P8). In all

cases, the beams with lower tension reinforcement (P1, P3 and P7) failed at lower ultimate

loads but higher mid-span deflections when compared with those of higher reinforcement

(P2, P4 and P8). This observation about beams with lower tension reinforcement exhibiting

higher displacement ductility at failure has been made by other researchers who worked on

reinforced concrete beams made from other coarse aggregates [28–30]. It is worthy of note

that even though RC phyllite concrete beams without shear reinforcement (P1–P10) has

exhibited this behaviour, RC beams made from phyllite aggregates with adequate shear

reinforcement in an earlier work [8] showed counter behaviour. The ultimate loads of the

beams with same percent tension reinforcement de-creased with decreasing beam cross-

section capacity. Very low ultimate displacements of 3–6 mm were measured for the beams

without shear reinforcement (P1–P10).

The load–deflection characteristics of 4 beams with same dimensions and tension

reinforcement (2%) but different shear reinforcement; P4 (no links), P11 (R6@250), P13

(R6@150) and P15 (R6@100) are shown in Fig 5. The beams which had similar concrete

compressive strength but slightly different flexural strengths as shown in Table 2 were

subjected to monotonic static loading. The initial stiffness, ultimate loads and ultimate

displacements of the curves generally increased with increasing shear reinforcement (or link


9

spacing). This implies that confinement of concrete by shear reinforcement and increasing

effect of shear reinforcement per unit volume influences the strength and ductility

characteristics of phyllite concrete beams with same longitudinal tension reinforcement.

However, the ultimate load and displacement of P11 (R6@250 link spacing) was higher than

that of P13 (R6@150 link spacing) which is counterintuitive. The only logical explanation

could be due to the higher flexural strength of concrete in P11 as compared to P13. The

ultimate displacements of the beams with shear reinforcement were also very low ranging

from about 4–6 mm. From Fig. 4 and 5, the RC phyllite beams showed poor ductility

behaviour. All the beams exhibited low amount of deflection coupled with brittle failure

modes that occurred without providing ample warning to the imminence of failure.

3.2. Experimental load characteristics of phyllite concrete beams

In most of the beams, vertical flexural cracks appeared first in middle portion of the beams.

As the load was increased, new flexural cracks occurred outside the mid-span which

eventually resulted in diagonal cracks that formed towards the loading point. The

experimental first flexural crack loads, first shear crack loads and failure loads are shown in

Table 3. First flexural crack loads varied from 28% to 40% of the failure load for phyllite

concrete beams without links (P1–P10) and beams with stirrups (P11– P16). The first flexural

crack load for P4 (no shear links) was 10 kN as compared to values ranging from 2.8 to 4.0

times for corresponding beams (P11-P16) with various shear stirrup spacing. This implies

that the presence of shear links influenced the formation of first flexural cracks in phyllite

aggregate beams when only corresponding 2% tension reinforcement beams (P4, P11–P16)

are considered.

First diagonal shear cracks were developed between 50% and 92% of the failure load for

phyllite concrete beams without shear links (P1–P10). The first diagonal shear load for beam

P4 (66 kN) was higher than or equal to those of corresponding beams (P11– P16) with shear

reinforcement tested either by monotonic or limited cyclic loading and unloading. This

indicates that first diagonal shear crack is independent of the amount of shear links

(contribution of shear links) and mode of application of the load (monotonic or cyclic). The

first shear crack load of beam P4 occurred at 83% of the failure load whilst those of beams

with shear links (P11–P16) ranged from 42% to 58% of the failure loads whether subjected to

monotonic or limited cyclic loading. It is evidenced from Table 3 that phyllite concrete beam

P4 without shear links had a lower post-diagonal cracking shear resistance (P0) of 14 kN


10

compared to corresponding phyllite beams (P11–P16) with shear link values ranging from 38

to 64 kN. As expected, post-diagonal cracking shear resistance increased with decreasing

stirrup spacing.

Fig. 4. Typical load–deflection curves for beams without shear reinforcement.

3.3. Experimental and predicted shear capacities

The experimental failure loads were compared with the shear resistance of the beams

predicted using ACI 318-02 [23], BS 8110 [9] and EC 2 [24]. In most design codes, the shear

capacity of rein-forced concrete beams with shear reinforcement (V) is derived from two

sources; contribution from shear resistance of concrete (Vc) and shear resistance of shear

reinforcement (Vs).

The ACI 318-02 [23] equations for shear resistance of concrete and steel shear

reinforcement are given by:

Vc =

(1)

kN (2)

Where Vu is factored shear force at section; M is factored moment at section; b is the beam

width; d is the effective depth of beam cross-section; fcu is compressive strength of concrete;

fyv is characteristic yield strength of reinforcement; q is the percentage of tension

P1

P2

0

10

20

30

40

50

60

70

80

90

0 1 2 3 4 5 6

Load

(K

N)

Midspan Deflection

(mm)

P3

P4

0

10

20

30

40

50

60

70

80

90

0 1 2 3 4 5 6

Load

(K

N)

Midspan Deflection

(mm)

P7

P8

0

10

20

30

40

50

60

70

80

90

0 1 2 3 4

Lo

ad

(K

N)

Midspan Deflection

(mm)


11

reinforcement; Av is area of shear reinforcement and sv is spacing of shear reinforcement.

Fig. 5. Comparison of curves for beams P4, P11, P13 and P15.

BS 8110: Part 1: 1997 [9] equations used in the calculation of shear resistance of concrete

and shear reinforcement are given by:

(3)

kN (4)

As per EC 2 specification [ ], shear resistance of concrete and shear reinforcement are given by:

kN (5)

kN (6)

where a is the shear span and is the diagonal concrete strut angle which is set between 22o

and 45o.

In this research, a value of 45o was used to give the lowest possible shear resistance due to shear

reinforcement in equation 6.

In BS code, cube compressive strength is used as compared to cylinder compressive

strength used in both ACI and EC2 codes. A reduction factor of 0.8 was used to convert cube

compressive strength into cylinder compressive strength.

P11

P4

P13

P15

0

20

40

60

80

100

120

140

0 1 2 3 4 5 6

Lo

ad

(k

N)

Midspan Deflection (mm)


12

(a) Comparison of P11 (monotonic loading) with P12 (cyclic loading)

(b) Comparison of P13 (monotonic loading) with P14 (cyclic loading)

P12

P11

0

20

40

60

80

100

120

0 1 2 3 4 5 6 7 8

Load

(kN

)

Midspan deflection (mm)

P14

P13

0

20

40

60

80

100

120

0 1 2 3 4 5 6 7 8

Load

(kN

)



13

(c) Comparison of P15 (monotonic loading) with P16 (cyclic loading)

Fig.6. Comparison of monotonic and cyclic loading of beams.

Table 4 shows the experimental shear force in KN compared with those based on the

recommendations of the ACI, BS and EC codes. The shear strength ratios (Vexp/Vcode) for the

beams without shear reinforcement (P1–P10) vary from 1.20 to 2.84. Therefore, all the three

codes underestimate the actual ultimate shear strength of the beams without shear

reinforcement and are therefore conservative in their estimations. Others [20] have shown

that the shear prediction of normal concrete beams without shear reinforcement using ACI,

BS and EC codes was conservative with shear strength ratios in the range 1.98–2.17. This

suggests when beams without shear reinforcement are considered, there could be a wide

variation in the code prediction of phyllite concrete shear as com-pared to that of normal

concrete. In the current study, the ACI code was the most conservative whilst EC 2 was the

least conservative in terms of shear prediction for beams which has only bottom tension

reinforcement. As expected in beams P1–P10, the beams with 1% longitudinal reinforcement

ratio recorded lower shear strengths than those of corresponding beams with 2% longitudinal

reinforcement ratio. This is because for beams with same geometrical dimensions and

concrete compressive strength, dowel action is controlled by the amount of longitudinal

tension reinforcement provided. BS 8110 and EC 2 codes have very conservative equations

for the deeper beams but the ACI code proved most conservative for the smaller beams

predicting low shear strength as compared to the rest. Hassan et al. [12] also indicated in an

P16

P15

0

20

40

60

80

100

120

140

0 1 2 3 4 5 6

Load

(kN

)



14

experimental study on normal concrete and self-consolidating concrete beams in shear that

the ACI code is conservative for smaller beams. The unconservative prediction of the ACI

code for larger beams with low reinforcement ratios has also been recognized by others

[12,32].

As expected, the experimental shear capacities (Vexp) for the beam without shear

reinforcement (P4) is lower than those of cor-responding beams with stirrups (P11–P16).

Shear capacities in beams with stirrups, generally increased with decreasing stirrup spacing.

The experimental shear strength for beams with shear reinforcement (P11–P16) was

generally lower than those derived from the code prediction. The strength ratios for P11–P16

for all the 3 codes were generally less than one (1) for all the 3 codes. In-fact, 75% of the

shear strength ratios had values from 0.8 to 0.98. These results are in contrast to tests done on

both normal concrete and palm kernel shell concrete beams with shear reinforcement by

Alengaram et al. [20]. In that experimental study, the experimental to predicted shear

capacities varied from 1.57 to 2.83 for both type of beams using the ACI, BS and EC codes.

Higher experimental shear capacities obtained for beams made of normal concrete and palm

kernel shell concrete beams could be attributed to good aggregate interlock [20,33].

A reduction factor of 0.8 recommended for the modification of the BS 8110 code shear

capacity prediction against early formation of shear cracks in an earlier study on flexural

behaviour of phyllite beams [8] may still not be safe because of the limits of the unsafe ratios.

Structural concrete codes of practice (e.g. ACI, BS and EC) al-low for strength adjustments

to be made for shear strength calculation of non-conventional aggregates. The ACI code

permits the use of a reduction factor for shear strength calculation of light weight concrete

(LWC). The ACI code allows for respective reduction factors of 0.85 and 0.75 for sand-

lightweight concrete and all-lightweight concrete. BS code makes provision for a reduction

factor of 0.8 for LWC whilst EC 2 also provides for a reduction factor varying from 0.6 to

0.94 for LWC depending on the dry density of the concrete. Based on the unsafe ratio range

obtained in this limited study, it is recommendation that a reduction factor of 0.7 must be

applied to code prediction of shear capacity when designing RC beams with shear

reinforcement made from phyllite aggregates. This will ensure the specification of a high

enough safety factor on ultimate strength. It must be emphasised that whilst all the 3 codes

are conservative for the prediction of phyllite concrete beams without shear reinforcement,

they over predict the shear strength of beams with shear reinforcement. It can therefore be

inferred that the shear contribution of the steel stirrups in the phyllite concrete beams could


15

not be effectively mobilised. This could be attributed to the flaky nature of the aggregates

which indirectly affects the load transfer mechanism between the concrete and steel rods.

3.4. Effect of cyclic loading

The effect of limited cyclic loading on the behaviour of phyllite beams with stirrups (P11–

P16) was studied. Three (3) of the beams with stirrups (P12, P14 and P16) were subjected to

20 loading– unloading cycles at service loads and their load–deflection behaviour studied.

This was to investigate the effect of limited cyclic loading on the stiffness and strength of the

beams. The individual load–deflection curves of the corresponding monotonically loaded

beams with stirrups (P11, P13 and P15) are expected to envelope the corresponding cyclically

loaded beams (P12, P14 and P16) to indicate either loss of structural integrity or not. For

beams with low shear reinforcement, in this case beams with stirrup specifications of

R6@250mm and R6@150mm (P11–P14), initial stiffness of the beams remained constant

beyond cracking of concrete and up to yielding of reinforcement for both monotonic and

cyclic loading (Fig 6a and b). Beyond the yielding of reinforcement, the stiffness of the

beams degraded and their strengths deteriorated as a result of the lower shear failure capacity

of the shear reinforcement. However, there was an increase in ultimate deflection probably as

a result of the strain hardening and ductility of the steel reinforcement. For the corresponding

beams of high shear reinforcement (P15 and P16), subjected to respective monotonic and

cyclic loads, the stiffness and strengths did not change up to the point of failure of P16 (Fig

6c). As a result of hysteretic energy dissipation, the beam subjected to cyclic load (P16)

failed at a lower ultimate load and deflection compared to P15 which was subjected to

monotonic loading. It was therefore observed that cyclic loading affected the stiffness,

strength and deformation of the phyllite beams with shear reinforcement.


16

Fig.7. Typical crack pattern of beams after failure.


17

Table 3 Experimental loads of tested beams in shear

Beam

No.

Experimental load,

First

flexural

crack,

Pcr

First

shear

crack

Ps

Failure

load,

Pult

P’s

P1 14 40 68 0.29 0.59 28

P2 12 60 78 0.28 0.77 18

P3 10 44 66 0.30 0.67 22

P4 10 66 80 0.30 0.83 14

P5 14 28 50 0.32 0.77 22

P6 14 46 50 0.32 0.92 4

P7 16 24 40 0.40 0.60 16

P8 14 28 46 0.39 0.61 18

P9 12 18 28 0.33 0.50 10

P10 14 24 40 0.40 0.60 16

P11 28 44 106 0.26 0.42 62

P12*

26 48 86 0.30 0.56 38

P13 38 56 96 0.40 0.58 40

P14*

36 58 114 0.31 0.51 56

P15 36 64 128 0.28 0.50 64

P16* 40 68 118 0.34 0.58 50

* Beams subjected to 20 cycles of loading and unloading

3.5. Cracking and failure behaviour Generally, the failure mode observed in the phyllite concrete beams was shear (Table 5). All

the beams failed in shear failure modes as expected in beams with shear span/effective depth

ratio of between 1.0 and 2.5. This failure mode was independent of flexural crack formation.

Generally, most of the beams with or without shear links (P1–P4, P6, P7, P9, P10 and P11–

P16) showed anchor-age bond failure in addition to the shear failure due to high stress

concentration near the supports (Fig 7). Structural codes of practice [9,23,24] give

recommendation for hook or bend at supports to fulfill anchorage requirements. Anchorage

bond stress and the con-tact area of the bar are employed in prescribing anchorage lengths. In

the event of very high concentration of anchorage stress in the anchorage zone, the mutual

bond between the concrete and reinforcing bar interfaces is destroyed. This adverse condition

is of-ten averted in design by the provision of greater anchorage lengths than usually

permitted [29]. It is therefore recommended that in phyllite concrete beams, greater


18

anchorage length than the calculated length based on anchorage bond stress and contact area

of the bar be used as done in other non-conventional aggregate concrete beams where

anchorage failure occurred [30]. In view of durability problems which cause concrete components to fail prematurely,

cracking characteristic of phyllite concrete beams were investigated. During loading, flexural

cracks propagated first in the midspan and the number of cracks also in-crease and become

wider with increase in loading. With further in-crease in loads, cracks develop in the shear

region extending diagonally toward the loading point. Other new diagonal cracks be-gin to

form separately from locations closer to the supports. The number of cracks at failure varied

from 2 to 10 in phyllite concrete beams without stirrups (P1–P10). There was a notable trend

in the distribution of cracks and crack widths. More number of cracks (11–28) and wider

crack widths (3.93–9.82 mm) were observed in the beams with shear reinforcement (P11–

P16). As observed by other researchers [12,28–31], a high number of cracks resulted in

smaller crack widths. Beams subjected to cyclic loads (P12, P14 and P16), exhibited wider

crack widths as compared to corresponding beams subjected to monotonic loading (P11, P13

and P15). Table 4 Experimental and predicted shear capacity

BEAM Vexp

(kN) Predicted shear force, (kN) Shear strength ratios, Vexp/Vcode

BS8110 ACI EC2 BS8110 ACI EC2

P1 68 51.77 51.12 53.16

1.31 1.33 1.28

P2 78 65.23 56.77 68.46 1.20 1.37 1.14

P3 66 45.55 43.08 47.74 1.45 1.53 1.38

P4 80 57.42 47.87 61.53 1.39 1.67 1.30

P5 50 30.01 26.61 30.65 1.67 1.88 1.63

P6 50 37.81 29.80 39.97 1.32 1.68 1.25

P7 40 25.82 21.35 25.48 1.55 1.87 1.57

P8 46 32.53 24.05 33.56 1.41 1.91 1.37

P9 28 15.80 12.69 17.84 1.77 2.21 1.57

P10 40 19.91 14.10 22.96 2.01 2.84 1.74

P11 106 91.60 80.70 89.08 1.16 1.31 1.19

P12* 86 91.60 80.70 89.08 0.94 1.07 0.97

P13 96 116.38 104.17 111.38 0.82 0.92 0.86

P14* 114 116.38 104.17 111.38 0.98 1.09 1.02

P15 128 147.36 133.52 139.26 0.87 0.96 0.92

P16* 118 147.36 133.52 139.26 0.80 0.88 0.85

a Beams subjected to 20 cycles of loading and unloading.


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Table 5 Failure and crack behavior

Beam

No.

Failure mode

Number

of cracks

at failure

Maximum

crack width

at failure

P1 Shear/anchorage 10 0.14




P5 Shear 6 0.66

P6 Shear 4 2.00

P7 Shear 6 0.96

P8 Shear 2 0.40

P9 Shear 4 0.80

P10 Shear 6 1.00

PS4A Shear 23 3.93

PS4A* Shear 18 5.82

PS4B Shear/anchorage 19 4.34

PS4B* Shear/anchorage 18 6.78

PS4C Shear/anchorage 16 4.52

PS4C* Shear/anchorage 11 9.82

a Beams subjected to 20 cycles of loading and unloading.

Fig.8. Typical variation of crack width with load for beams P4, P11, P13 and P15.


20

Fig. 8 shows graphs of load against crack width for corresponding beams P4 (no links), P11

(R6@250mm), P13 (R6@150mm) and P15 (R6@100mm) all subject to monotonic loading.

The graphs reveal that the phyllite aggregate developed wider cracks as steel stirrup spacing

increased. This is because aggregate interlock which plays a major role in the shear transfer

mechanism is influenced by frictional forces that develop across diagonal cracks. These

frictional forces are at a minimum in phyllite concrete as a result of the physical

characteristics of the aggregates thus leading to wider cracks. The presence of wider cracks

also affects ultimate shear capacity of concrete beams. An increase in crack widths at points

above the main reinforcement weakens the aggregate inter-lock capacity and reduces

concrete shear capacity [14–17].

4. Conclusions

The shear resistance of phyllite concrete beams with and with-out shear reinforcement is

described based on test results of experimental beams. The crack load, crack width, number

of cracks, failure modes and overall shear resistance at failure are critically analyzed to study

the effect of longitudinal tension reinforcement, shear reinforcement, monotonic loading and

cyclic loading on RC beams made from phyllite aggregates. Based on the results presented in

this paper, the following conclusions are made:

All the RC phyllite beams showed typical structural behaviour in shear. Most of the beams

(14 out 16) with or with-out shear reinforcement showed anchorage bond failure in addition

to the diagonal shear failure due to high stress concentration near the supports.

The RC phyllite beams showed poor ductility behaviour. All the beams exhibited low amount

of deflections coupled with brittle failure modes that occurred without providing ample

warning to the imminence of failure.

The shear capacity of phyllite concrete beams without shear reinforcement predicted using

ACI 318-02, BS 8110 and EC 2 are conservative for the prediction of phyllite concrete beams

without shear reinforcement, with average experimental shear capacities of approximately

80%, 50% and 40% respectively higher compared to predicted.

For beams with shear reinforcement, the codes (ACI 318-02, BS 8110 and EC 2)

underestimate the ultimate shear capacity of RC phyllite beams by between 4% and 20%.

It is recommended that the predicted shear capacity of phyllite beams reinforced with steel

stirrups be modified by a reduction factor of 0.7 in order to specify a high enough safety

factor on their ultimate strength.


21

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