Shear Failure in Plain Concrete as Applied to Concrete Pavement Overlays

SHEAR FAILURE IN PLAIN CONCRETE AS

APPLIED TO CONCRETE PAVEMENT OVERLAYS

Yi Xu1 and John N Karadelis

2

1, 2

Department of Civil Engineering, Architecture and Building, Coventry

University, CV1 5FB, UK

Abstract. This study applied the modified Iosipescu loading configuration on

beams and direct shear loads on cylinders to investigate the concrete behaviour

under minimal flexural and prominent shear stress conditions, particularly in plain

concrete tests. It aims to make a contribution in understanding the behaviour of a

concrete pavement under shear loading and failure and ultimately design an

adequate overlay system. A finite element model corresponding to the modified

Iosipescu beam test was set up to assist with the recognition and study of the

complex stress patterns developing at high stress concentration regions and the

evaluation of principal stresses. Both, normal and high strength concrete were

used, to imitate the performance of the existing (old) pavement and the prospect

overlay under shear. On the basis of the laboratory results and the preliminary

finite element analysis, it was found that all specimens failed abruptly without

warning in a typical brittle-material / shear-failure manner. Considerably higher

shear strengths were achieved compared to those specified in the standards.

Unexpectedly, the high strength concrete developed slightly lower shear strength

than the normal concrete. This is probably attributed to insufficient compaction

and relatively less effective aggregate interlock in high shear regions. The research

is in progress.

Introduction and Literature Survey

Shear failure at cracks/joints is a major cause of degradation of concrete

pavements. It leads to serviceability problems and introduces reflective cracks and

becomes an issue of structural integrity, durability, riding quality and safety of the

deteriorated pavement.

To address the problem, a Rapid Pavement Repair and Strengthening Management

System (RPRSMS) is under development at the Department of Civil Engineering,

Yi Xu and John N Karadelis

2

Coventry University. It aims to bring to light the structural and functional

deficiencies and extend the life of the pavement and at the same time introduce

substantial benefits, such as saving construction materials, time and labour costs,

by bonding a layer of special-quality concrete on top of the existing damaged

pavement. Therefore, all the aforementioned contribute to a more sustainable

pavement repair system.

In this study, the modified Iosipescu loading configuration [1] was adopted for

testing the beams. Direct shear loads were applied on cylindrical specimens to

investigate the concrete behaviour under high shear and low moment conditions.

Cylinder torsion tests were carried out to assist with the investigation of the

concrete shear performance. A finite element model simulated the behaviour of the

specimens and assisted with the recognition and study of the loading configuration

that otherwise would not be easily identifiable (complex stress patterns developing

at high stress concentration regions, evaluation of stresses and stress intensity

factors). Both, normal and high strength concrete were used, to imitate the

performance of the existing/old pavement and the prospect overlay under shear.

Glass fibre was added to the high strength concrete mix in an effort to enhance its

shear resistance further. The influence of coarse aggregate, mixing procedure,

compaction and curing method on the shear resistance was also assessed

experimentally. Emphasis was given to the experience built up so far, so essential

for outlining future similar type of work.

Shear Problems in Pavement Overlays

Three possible main failure modes were identified:

Flexure failure: The crack propagates upwards into the new overlay due to

exceeding its flexural strength.

Shear failure: The crack propagates upwards into the new overlay mainly due

to insufficient shear resistance.

Delamination: The crack develops along the interface.

In a real pavement scenario, failure takes place under the combined effects of

bending moment, shear force and possible involvement of axial force due to

boundary conditions. This article isolates and focuses on the problem of shear

failure. Parallel studies on flexural and delamination problems are currently under

scrutiny by other members of our research group at Coventry University. The

findings will be combined to formulate optimum design guidelines for a “Green”

concrete overlay.

When the load is located either side of an existing, old crack, high shear stress

becomes dominant and a relative vertical displacement is expected at crack edges,

which may lead to reflective cracking into the overlay under repetitive traffic

loads, as shown in Figure 1.

Shear Failure in Plain Concrete as Applied to Concrete Pavement Overlays

3

Figure 1. Reflective shear cracking under repetitive traffic loading (Left)

Figure 2. Correlation of ACI 318-02 with experimental data [2] (Right)

Depending on the nature of crack edge movement, it can be categorized into mode

I openning, mode II shearing/sliding and mode III tearing. Mode II corresponds to

the shearing problem but it usually coexists with mode I. The Paris’ law [3], as

shown below is usually deployed to describe the crack propagation under repetitive

loading.

(1)

where, Keq = equivalent stress intensity factor; A, n= fracture mechanics factors; c=

crack length; N = number of loading cycles; dc/dN = increase of crack length per

load cycle.

A fracture mechanics approach may be needed to highlight the crack initiation and

propagation process. The stress intensity factor, K, due to bending and shear is one

of the fundamental input parameters. Therefore, it was investigated in the

following experimental study, assisted by numerical modelling.

Design Codes and Standards on Shear

A selection of design codes and published formulae from previous researchers are

summarized in Table I. The following assumptions are made to derive

representative shear strengths using each code: A relatively high compressive

strength of fc’=fck=50MPa is assumed. The shear span-to-depth ratio

is employed based on the geometry of the single notch shear beam test as shown in

Figure 3. Since the steel ratio, ρ, is deemed necessary in some of the codes, to

execute the calculation and return a non-zero result, a minimal value of

is assumed. It is understood that errors may be introduced by the latter

and some empirical formulae may become invalid in the case of plain concrete.


4

Design

Codes and

Published

Formulae

Shear Strength

(MPa)

Shear

strength

(MPa)

Comments

Eurocode 2

[4]

0.48

0.72 If the safety factor

is omitted.

ACI 318-02

[5]

1.13 for

, i.e. relatively

large shear span-to-depth

ratio

17 ≤ 0.29√

2.05 for

i.e. relatively

small shear span-to-depth

ratio

. 1.17 General

≤ √ 5.87 Deep beams ( ≤ )

A23.3-94 [6] . 1.414 No consideration for a/d

Zsutty

Eqn [7, 8]

1.057 a/d > 2.5 or a/d < 2.5 under

indirect load, as provided

by side flanges

3.204 1.5 < a/d < 2.5 with direct

loading (top load & bottom

supports), lower bound

strength predictor

Table I. Summary of Design Codes and Published Shear Design Formulae [4-8] 1) Not all standards/codes take a/d into account, commonly acknowledged as one of

the most influential parameters in evaluation of the shear strength. 2) steel ratio, , not

applicable in plain concrete. 3) Generally, ≤ 68.9 MPa in ACI 318-02 [5]. 4) A

wide scatter was presented in the correlation of ACI 318-02 with experimental data, as

shown in Figure 2.

Experimental, Pilot Study

Materials

Stage I. Three concrete mixes were tested in the preliminary experimental work.

They were Ordinary Portland Cement Concrete (OPC1) as control specimens, plain

Polymer Modified Cement Concrete (PMC1) and Polymer Modified Cement

Concrete with 4kg/m3 glass fibre (PMC2). Following previous studies [9, 10], the


5

PMC mix developed is of relatively high strength and high modulus of rapture and

suitable for roller compaction. A specially modified Kango hummer was used to

consolidate the specimens. 10mm granite was employed for coarse aggregate.

Stage II. In addition to the mixes in stage I, three more concrete mixes were

produced and tested in cylinder direct shear mode. All six mixes are summarized in

Table II. Gritstone replaced granite in OPC2, PMC3 and PMC4 as its aggregate

crushing value (ACV), tested according to BS 812-110:1990 [11], indicated that

the former has a higher resistance to crashing than the latter (11.40% and 19.98%).

Also, PMC4 was cast following a new mixing and curing method in accordance

with ASTM 1439 [12] (4-day moist curing at 23±2oC and relative humidity of

approx. 95%, followed by 24-days air curing at room temperature environment). A

new compaction device simulated the hammering and vibrating actions.

Coarse

Aggregate

Fibre

Used

Mixing

Method

Curing

Method

Compaction

Equipment

OPC1 Granite N/A Traditional Traditional Kango

OPC2 Gritstone N/A Traditional Traditional Kango

PMC1 Granite N/A Traditional Traditional Kango

PMC2 Granite Glass Traditional Traditional Kango

PMC3 Gritstone N/A Traditional Traditional Kango

PMC4 Gritstone N/A New New Vib. Comp/tor

Table II Summary of Concrete Mixes

Single Notch Shear Beam Test (SNSBT)

Figure 3. Specimen Geometry and Loading and Crack Trajectory

This paper adopts the concept of single edge notched beams employed by

Iosipescu [1] and Arrea and Ingraffea [13]. This single notch short beam shear test

created a concentrated shear zone in the (near) absence of bending moments at the

notch and evaluated the shear strength level of OPC1, PMC1 and PMC2. Twelve


6

beams were tested comprising four beams per mix. The geometry and loading

arrangements are shown in Figure 3. All the beams failed abruptly with the crack

propagating from the tip of the notch in an inclined 45o direction, reaching the

surface of the specimen to the opposite side of the applied force P (Figure 3). The

results were summarized in Table III. Despite the high compressive strengths of

PMC, the latter failed earlier under shear than ordinary concrete. This is in

harmony with existing literature [14-16].

Figure 4 shows the variation of shear stress versus shear strain for a magnitude of

shear stress up to 2.5MPa, before failure occurs. Four beams were tested for each

mix. A wide range of scatter points (hatched areas) was present in the results. The

same scatter was reported in Figure 2. Both PMC1 and PMC2 mixes show steeper

slopes, i.e. higher shear modulus than the OPC1. Comparing plain PMC1 with

glass-fibre reinforced PMC2 it is observed that the shear stress develops faster in

PMC2. This indicates that the introduction of glass fibre did enhance the shear

resistance of PMC but not the ductility.

Overall, PMC has high compressive strength but lower shear strength, possibly due

to insufficient compaction. The addition of glass fibre enhanced the shear

properties of the PMC. A good consistency exists between the average saturated

densities and the shear strengths. In descending order, both rank as: OPC1 > PMC2

> PMC1, as listed in Table III. As compaction during PMC casting is critical,

insufficient compaction may induce voids (low density). When subject to shear,

these voids become weak points and eventually lead to failure at a “lower-than-

expected” load. However, in compression, the microstructures developed by

superior ingredients still allow for good performance.

Figure 4. Shear Stress/ Strain at the Initial Stage

For a single edge crack subject to in-plane shear, a relationship has been developed

by Tada [17] for KII, the stress intensity factor, as shown below.


7

(2)

Where: Q = applied shear at the notch at the onset of slow crack growth; a = crack

depth, 27mm in the testing geometry; b = height of the beam, 100mm; B = width

of the beam, 100mm.

Substituting the above values into Eqn. (2), the latter is reduced to:

in (3)

All beams failed abruptly. Owing to the rapid crack development, the load at the

onset of slow crack growth was not detected. If the failure loads were to be used in

the assessment, the average KII for OPC1, PMC1 and PMC2 would be 2435, 2136

and 2390 respectively. As failure occurs at a higher load than that

needed for initial crack growth, the actual K-values are likely to be smaller than

those calculated. Although the definitive estimates of KII are not available at the

current stage, the PMC returned with smaller toughness than the OPC. Further

investigation into the aggregate interlock and friction is underway. Additional tests

conducted to increase confidence are cylinder torsion tests and direct shear tests as

explained in the following paragraphs.

Cylinder Torsion Test

A total of eight concrete cylinders, four OPC1 and four plain PMC1 with 100mm

diameter and 200mm length were cast. Table III provides all necessary properties

and results. Studying failed specimens of OPC and PMC showed that the fractured

plane in PMC was smoother than that of OPC. Hence, the aggregate failed to

demonstrate good interlocking abilities.

Cylinder Direct Shear Test

The Iowa Testing Method 406-C [18] on four 150mm diameter by 300mm long

OPC1 cylinders and four PMC2 (glass fibre) cylinders was adopted. To enhance

the shear strength performance of PMC, the coarse aggregate as well as the mixing,

compacting and curing methods were revised in stage II. Gritstone replaced granite

in OPC2, PMC3 and PMC4. The new mixing, compaction and curing methods

were deployed for PMC4, as described earlier. The test arrangement is pictured in

Figure 5. All cylinders were sheared off in a vertical plane, as shown in Figure 6.

PMC2 (with glass fibre) delivered higher shear strength than plain PMC1, similar

to OPC1. The results were summarized in Figure 7. Unexpectedly, they failed to

demonstrate a clear enhancement in shear strength, beside the use of gritstone. It is

likely that the water absorption value for gritstone was used in the granite mix

(PMC1), by mistake. As the water absorption of granite is higher than that of

gritstone, the w/c ratio of PMC1 was found to be 0.33, while all other PMC mixes


8

revealed 0.37. Had the correct value been used, PMC1 should have a lower shear

strength than the current value, or even lower than that of PMC3 and PMC4. An

investigation is currently under way. Nevertheless, the new mixing /compaction

/curing methods did exhibit a great improvement to the strength and density,

proving that effective compaction is essential in strength enhancement. This good

practice shall be continued in the next phase of experiments.

Summary

The results in all three tests are summarized in the same Table III. Results

involving gritstone and the new mixing/compaction/curing methods have been

discussed in the previous chapter and therefore are not repeated herein.

Figure 5. Cylinder Dir Shear Test Figure 6. Failed Section (PMC2)

It is noted that the tests carried out have excellent consistency on the evaluation of

shear performance. Despite high compressive strength, plain PMC failed to

provide an equivalent shear resistance as the OPC. This is attributed to the high

strength hydrated cement paste (HCP) in PMC and the relatively weak coarse

aggregate, which led to less efficient interlock at the failure planes. The OPC

failure face exposes a great amount of aggregates and most of the failure takes

place at the HCP-aggregate interface. In contrast, the PMC shows a much

smoother face through crushed aggregates. Hence, the replacement of granite with

gritstone.

Figure 7. Summary of Results

5.26 5.55 4.59 5.28 4.26 4.53

38

64 62 54.5

67 74.5

0

10

20

30

40

50

60

70

80

0

1

2

3

4

5

6

OPC1 OPC2 PMC1 PMC2 PMC3 PMC4

Co

mp

ress

ive

Str

ength

(MP

a)

Shea

r S

tren

gth

(M

Pa)

Concrete Mixes Shear Strength (Mpa) Compressive Strength (Mpa)


9

Compaction was also blamed for the “lower than expected” shear resistance of

PMC. Different from high/normal-workability concrete, potent compaction in

PMC is essential to attain the desired strength. After examining the cube densities,

it was deduced that there were voids in the mix due to insufficient compaction.

Hence, the investment in the new vibrating-compactor.

Mat.

Prop.

Compr.

Stren. (MPa)

Density

(kg/m3)

Mean Shear Strength (MPa)

Estimated Mean

Shear Modulus

G (GPa)

Tests

Cube Compre.

Test SNSBT Torsion

Direct

Shear SNSBT

Tors

ion

OPC1 38.0 2361.1 3.59 3.59 5.26 15.5 15.9

PMC1 60.5 2298.7 2.90 2.79 - 24.7 17.2

PMC2 54.5 2323.6 3.52 - 5.28 26.2 -

Table III. Summary of Test Results

PMC2 with a glass fibre input did improve the shear properties of plain PMC to a

similar level of OPC, but failed to provide the required ductility. Hence, glass

fibres were replaced with steel fibres and later with synthetic fibres. A “cock-tail”

of reinforcement has been introduced in the mix to enhance the shear performance

and to provide the required resistance to reflective cracking. This is currently under

investigation.

A quick comparison of Table III with Table I reveals that all experimentally

obtained shear strengths are much higher than most of the values obtained by the

codes. The following reasoning is stated:

All tests had very small shear span-to-depth (a/d) ratio. The a/d ratio was

0.067 and 0.46 respectively in cylinder direct shear test and SNSBT. It is

generally agreed that the influence of a/d is not insignificant [5]. A low a/d

implies small bending moment, in which case, most of the cross section is

available to resist shear. Hence, a high shear property is expected. Also, a low

a/d ratio corresponds to a strong “strut” (arch) action. Hence, the high shear

resistance could be attributed to the high compressive strength in concrete.

It is noted that in some of the formulae in the codes, the shear strength, , is

derived from . The correlation between

and is summarized from

diagonal – tension failure, a form of indirect shear failure, whereas the tests

conducted were direct shear. Additionally, even in the case of diagonal

tension, a wide scatter is usually present in experimental investigations as

shown in Figure 2. Therefore, in this case, the real concrete shear capacity is

under-estimated by the empirical formulae.


10

Finite Element (FE) Analysis

A 2D FE-model corresponding to the single notch shear beam test was setup using

ANSYS software [19] to study the stress distribution and the associated

parameters. The material properties obtained in the experiment (Table III) were

used in the model. The 6-node triangular PLANE183 element, with two degrees of

freedom (DOF) per node, supporting plasticity, large deflection and strain

behaviour was used. This is defined by a quadratic shape function, considered to be

well suited to model irregular meshes occurring at crack regions. All usual output

is supported by the analysis, plus the displacement extrapolation method in

calculating stress intensity factors. A “singularity” point was created at the crack

tip with skewed nodes at quarter positions. The failure loads from the experimental

tests were introduced. A non-linear, large displacement analysis was performed,

converging after a few iterations. It is stressed here that this is a preliminary model

designed to help with experimentation rather than the other way round.

Figure 8 portrays the high stress concentration at the crack tip. The maximum

shear strengths and the mode II stress intensity factor derived at the crack tip are

listed in Table IV with the corresponding experimental results for comparison. It

was found that shear stress at the crack tip is nearly twice the average shear

strength of the reduced cross section above the crack in OPC1 (6.63 corresponding

to 3.59). The predicted KII results are lower than the measured values calculated

using Tada’s formula [17] in OPC1 (1216 compared to 2435). Swartz conducted a

similar single notch shear test configuration both experimentally and numerically

on concrete beams [20], noticing a similar discrepancy between KII values. This

leads to suggestions that Tada’s formula may need modification to reflect the real

geometry and loading conditions. Note that to calculate K using FEM, the crack

width has to be zero. However, a 2mm width crack was created in the test beams.

This should be allowed for. A 3D model is currently under development.

Predicted

Shear Stress

at Crack Tip

(MPa)

Measured

Average Shear

Strength

(MPa)

Predicted

KII

@ Crack Tip

(

Measured

KII

@ Crack Tip

( OPC1 6.63 3.59 1216 2435

PMC1 5.82 2.90 1047 2136

PMC2 6.51 3.52 1172 2390

Table IV. Numerical Modelling Results

Figure 8. SNSBT Model Shear Stress, Sxy - Plot


11

Conclusions

All specimens failed abruptly in a typical shear failure and brittle material manner.

The introduction of glass fibre did not provide acceptable results. Other forms of

reinforcement are currently under scrutiny. Steel fibres seemed to be a successful

alternative to conventional shear reinforcement.

On the basis of the laboratory results, it was found that considerably higher shear

strengths were achieved than those specified in the standards. This is attributed to

the nature of the tests with relatively low a/d ratio. For reasons explained earlier, it

is considered inappropriate to apply the indirect and empirical shear design

procedure on direct shear tests involving minimal bending, especially in the case of

plain concrete.

The high strength concrete developed slightly lower shear strength than the normal

concrete and this is attributed to the relatively less effective aggregate interlock in

high shear regions and possibly insufficient compaction. A replacement coarse

aggregate was introduced contributing to the shear capacity but requires further

confirmation. The new mixing, compaction and curing method was also tested and

proved to be effective in increasing the shear strength. This successful practice

shall be continued and developed further to achieve better results. This is an on-

going research programme funded by an EPSRC grant. The final results should be

published next year.

Acknowledgements

The financial support of the Engineering and Physical Science Research Council

(EPSRC), UK and Aggregate Industries (AI) is gratefully acknowledged. The

authors would like to express their gratitude to Dr. Salah Zoorob and all the

colleagues and technical staff at Coventry University for their valuable suggestions

and comments. Special mention should also be made to Tarmac, Everbuild

Products, AGS Mineraux and Power Minerals for providing the research materials.

References

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1995 ACI Codes, McGraw-Hill, New York.

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12

[4] Beeby, A. W. and Narayanan, R. S. (1995), Designers' Handbook to

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[17] Tada, H., Paris, P. C. and Irwin, G. R. (2000), The Stress Analysis of Cracks

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[20] Swartz, S. E., Lu, L. W., Tang, L. D. and Refai, T. M. E. (1988), Exp. Mech.,

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Documents

Shear Failure in Plain Concrete as Applied to Concrete Pavement Overlays