climaco e regan_evaluation of bond strength between old and new concrete in structural repairs

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Evaluation of bond strength between old and

new concrete in structural repairsJ. C. T. S. ClõÂmacoÃ and P. E. Regan{

University of Brasilia; University of Westminster

Good bond between old and new concrete is indispensable for many repairs. Following an examination of the

factors involved and the methods of testing bond, the results of an experimental programme of slant shear tests are

presented. The programme comprised 223 tests designed to study test methods and the effects of bond coats, surface

preparation and ageing of the base concrete. The results are analysed in terms of a Coulomb criterion and evidence

is produced that good bond can be achieved by casting the repair against mature concrete with no bonding aids,

provided the base surface is dry and reasonably rough.

Notation

c apparent cohesion

f c compressive strength of composite prism (slant

shear strength)

f cc concrete cylinder compressive strength

f cu concrete cube compressive strength

f ct concrete uniaxial tensile strengthá angle between a joint and the normal to the force

applied to a composite prism

ì coefficient of friction

ô shear stress

ö angle of friction

Introduction

Neither joining fresh concrete to old nor the search

for a reliable method of testing bond is a new problem,

as can be seen from Perry's work.

1

The slant shear testillustrated by Fig. 1 appears to be a generally satisfac-

tory method in terms of being representative of realistic

stress states at the joint and also in regard to simplicity,

reproducibility and sensitivity. It has been adopted in

many standards but test procedures are not agreed and

there are significant differences in specimen dimen-

sions, joint angles and surface preparations.

The French standard NFP18-8722

uses a prism

(100 3 100 3 300 mm) sawn at an angle of 608 to the

cross section and repaired by casting fresh concrete or

bonding on another half-prism of hardened concrete.

The Italian standard 3

adopts composite prisms meas-uring 70 3 70 3 200 mm with a joint angle of 738.

According to the ASTM Standard C882-83,4

the

evaluation of bond strength of epoxy systems should be

carried out on 76 3 152 mm cylinders of Portland ce-

ment mortar made in two halves with the epoxy coat

applied to sand-blasted or wire-brushed surfaces at 608

to the cross section.

BS 6319: Part 45

uses a fractured surface obtained

by splitting a 150 3 150 3 55 mm concrete plaque.

The plaque is placed between a pair of steel plates cut

into matching trapezia and the sandwich is mounted

between rubber pads in a press. When compression isapplied the dilation of the rubber splits the concrete

along the line defined by the cuts in the plates. The

half-plaques are then repaired and sawn into 150 3

55 3 55 mm test specimens with a complete joint at

608 to the cross section. The BS offers alternative tests

using off-the-mould to top cast surfaces but the frac-

tured joint is suggested as being the most relevant for

repair situations.

The variety of approaches to the evaluation of bond

shows that the subject is still one for discussion. In this

article the bond strength in concrete repairs is exam-

ined by means of an experimental programme adopting

the slant shear method to study test procedures and the

Magazine of Concrete Research, 2001, 53, No. 6, December, 377±390

377

0024-9831 # 2001 Thomas Telford Ltd

Ã Department of Civil Engineering, University of Brasilia, Campus

UnB, Brasilia, D.F. 70910-900 Brazil

{ School of Architecture and Engineering, University of Westminster,

35 Marylebone Road, London NW1 5LS

(MCR 870) Paper received 5 June 2000; last revised 8 March 2001;

accepted 16 July 2001



influence of factors such as surface preparation, joint

angles (608, 63´38, 708 and 908 to the cross section),

bond coats (eight bonding agents), specimen dimen-

sions (three versions), and ageing of the base concrete

(2±3 weeks old at repair: short-term repairs; 10 months

old at repair: long-term repairs).6

Results of this pro-

gramme and an analysis in terms of a Coulomb failure

criterion are presented.

Analysis of the slant shear method

Stress analysis

When a composite concrete prism containing a jointat an acute angle to its longitudinal axis is subjected to

axial compression, the stress state at failure depends

primarily on the efficiency of the bond. If the bond is

effective the failure is characterised by a rupture in the

concrete, either of the `double pyramid' type common

in tests of monolithic specimens or on a diagonal line

near the joint. If the bond is insufficient a diagonal

failure occurs along the joint line.

In the case of effective bond the joint is submitted to

an uneven distribution of shear, compression and ten-

sile stresses. The situation is complex and highly influ-

enced by the thickness of any bond coat.

7,8

In the case of a joint failure the global stresses at the

interface are compression and shear and the Coulomb

failure criterion can be used to describe the ultimate

strength

ô c ìó (1)

where ô is the shear stress at the joint; ó is the normal

stress; c is an apparent cohesion; ì is a coefficient of

friction.

This expression is represented by the solid straight

line in Fig. 1, with the slope defined by what is usually

called the `angle of friction' of the joint: ö arc tan ì.

The friction ì is a function of the roughness of the

parent concrete surface and experimental work indi-

cates that it is influenced by bonding agents. The cohe-

sion c, represented in Fig. 1 by the intercept of the

straight line with the shear stress axis, depends on the

concrete compressive and tensile strengths, and seems

to be affected by environmental conditions, surface

preparation, duration of loading and the use of bond

coats.6±12

From the equilibrium of forces at the composite

prism of Fig. 1, the normal and shear stresses at a joint

with an angle á to the longitudinal or uniaxial strength,

f c, can be written as

ó f c cos2 á (2)

ô f c sin á cos á (3)

From equations (1), (2) and (3), the compressive

strength of the composite prism governed by the weak

plane is

f c c1 tan2 á

tan á À ì(4)

Assuming a constant c value for a given concrete

and conditions, the most critical joint angle correspond-

ing to a minimum f c from equation (4) is given by

ácrit arc tan( ì ì2 1

p ) (5)

With the ácrit from equation (5), the minimum com-

pressive strength of the composite prism from equation

(4) will be

f cmin c1 ( ì

ì2 1

p )2

ì2 1

p (6)

Table 1 gives the values of ácrit and f cmin from equa-

tions (5) and (6), for values of c and ì presented by

Regan.10

Fig. 2 shows the variation of the composite

prism compressive strength, related to the factor f 2a3cc ,

proportional to the tensile strength of concrete, as a

function of á, according to equation (4), for different

values of the friction coefficient ì.

Joint failure

Joint

0

Monolithic

failure

envelope

f c

f ct f cmin f c

c

σ

σ

τ

τ

α

φ

φ = arc tan µ

Fig. 1. Failure criterion for concrete composite prisms

ClõÂ maco and Regan

378 Magazine of Concrete Research, 2001, 53, No. 6



By means of trigonometrical relationships and mak-

ing ì tan ö, equations (4) and (6) can take the fol-

lowing forms, common in the plasticity theory13

f c c cos ö

cos á sin(á À ö)(7)

f cmin 2c cos ö

1 À sin ö(8)

According to the Coulomb criterion, sliding failure

at the joint takes place on a section subjected to combi-

nation of shear and normal stresses, described by equa-

tion (1), represented graphically by the straight line in

Fig. 1. If a limited concrete tensile strength is assumed,

the failure envelope in the tension zone becomes a part-circle tangent to the straight line, shown broken in

Fig. 1, with its centre on the ó -axis and containing the

point with coordinates ( f ct, 0).12

Since the straight line

has an inclination ö and intercept c, it can be proved

that this line is tangential to the Mohr's circle with

diameter f cmin, the minimum compressive strength

given by equations (6) or (8) and shown dotted in

Fig. 1.

If the concrete tensile strength is neglected, which is

usual in design, the modified Coulomb criterion for

joint failures is simplified to a straight line of inclina-

tion ö and intercept c and an initial arc of circle

tangential to both the straight line and the vertical axis

and having a diameter f cmin.13

This envelope is repre-

sented by a solid line in Fig. 1. Test results of this

research show that the assumption of concrete zero

tensile strength is conservative for rough joint surfaces

and a modification on the initial curved part of the

failure envelope is to be proposed.

If bond is effective at the joint, failure occurs by

monolithic concrete crushing. The ultimate normal and

shear stresses at the interface can be expressed by

equations (2) and (3) in terms of the prism compressive

strength f c,12

and represented graphically by the part-

circle shown in Fig. 1 as a chain-dotted line, defining

the region where monolithic crushing failure should

prevail. The part-circle diameter is the compressive

strength of monolithic concrete prisms, f c, for which

an usual value, if end restraints are negligible, is10

f c 0X95 f cc 9 0X75 f cu (9)

Surface preparation

To assess repair techniques the interface should simu-late the most usual texture, i.e. rough with aggregate

exposed. The cracked surface specified by BS 6319:

Part 4 is extremely rough and not representative of

most repair substrates. Proposals of using sawn sur-

faces2,9

seem unsuitable as their exaggerated smooth-

ness is not realistic for concrete repairs or even for

most precast concrete. Moreover, it was consistently

demonstrated by Paulay, Park and Phillips14

and

Daschner 15

that bond strengths of rough joints are

undoubtely greater than those of smooth joints but the

different methods to obtain rough surfaces do not pro-

duce very different results. FoureÂ

16

also showed atlaboratory conditions that effective concrete repairs are

obtained even by leaving the concrete surface in its

natural condition after vibration.

At the same time it does not seem to be difficult to

define standard conditions for a rough surface. Stan-

dard classifications of surface treatments are given in

reference 17 and the depth of treatment defined as

`medium' for sandblasting (sufficient to generally ex-

pose coarse aggregate with slight revealÐmaximum

reveal 6 mm) seems realistic for repair situations and

not difficult to obtain with simple manual or mechani-

cal scarification. The maximum aggregate size can be

specified for any particular situation.

Table 1. Critical joint angles and minimum compressive strength of prisms10

Interface conditions Cohesion c Friction ì ácrit f cminÃ (Namm2)

Roughy 0X25 f 2a3cc 1´4 72´28 1X56 f 2a3

cc

Medium 0X25 f 2a3cc 0´9 66´38 1X12 f 2a3

cc

Smooth 0X5 Namm2 0´7 62´58 1X92 Namm2

Ã Composite prism minimum compressive strength f cc (Namm2)y Implies that aggregate is exposed at the interface

3·0

3·5

2·0

2·5

1·0

1·5

0

0 1 2 3 4 5 6 7 8 9 10 11 12

0·5

R a t i o ,

f c

/ f c c

2 / 3

tan α

55 60 65 70 75 80 α: degrees

Drawn for c = 0·25 f cc

2/3 :N/mm2

µ = 1·4µ = 1·0µ = 0·7

Fig. 2. Composite prism compressive strength3 joint angle

Bond strength between old and new concrete

Magazine of Concrete Research, 2001, 53, No. 6 379



Joint angle

Equation (5) defines the critical joint angle if failure

is governed by the Coulomb criterion. In spite of the

great influence of this angle on the slant shear strength,

several works neglect this parameter. The critical angle

depends upon the friction, ì, and different authors give

different values. Regan10

proposes ì 1X4 for rough

surfaces with exposed aggregates, making ácrit 72X28.While Franke

11gives ì 0X7 for sandblasted surfaces.

For this ácrit 62X58 but he adopted 608 in his own

tests.

ACI 318M-9518

stipulates ì 1X0 for concrete

placed against hardened concrete with the surface

roughened to a full amplitude of about 6 mm. In this

case, ácrit 67X58.Johnson's

19research showed ácrit to be about 708. He

made tests at 508, 608 and 708 and found the joints to

be stronger than the parent concrete for the first two

angles while for 708 the two strengths were roughly

equal.

Test programme

Initial tests to the BS 6319: Part 4 carried out in this

research showed the standard specification to be rather

inappropriate, probably due to the extreme roughness

of the cracked concrete surface and the adoption of an

unsuitable joint angle. The high bond strengths ob-

tained for cracked and dry-jointed specimens and even

for concrete prisms with waxed interfaces raise serious

doubts as to the ability of the test method to evaluate

bond.20

Series 1, 2, 3 and 4: slant shear tests of large prisms

Following the unsatisfactory results from the BS

tests, alterations to the slant shear test method were

introduced aiming for a better assessment of the factors

involved.

Specimen dimensions. To allow better simulation

of real repair surfaces the prism size was increased to

1000 mm height and 150 3 150 mm test cross section.

The geometry of the base concrete part was made as

in Fig. 3(a) to avoid the surface preparation damaging

an acute angle wedge.

Joint angle. The angle between the joint and the

normal to the load was changed to 708 (Series 1) to

be closer to the theoretically critical angle. Additional

tests were made for the angles 63´38 (series 2) and

908 (series 3).

Surface preparation. The surface of the base con-

crete was roughened with a pneumatic needle gun to

expose coarse aggregate and provide a rough surface

with a depth of treatment that could be defined as

medium according to reference 17. This preparation

seems more representative of repair situations than

either splitting or sawing.

Ageing of the base concrete. For test series 1, 2

and 3 the repair concrete was placed when the base

Series 1 and 4

150 150

102

150 150

100 100

Series 3

Series 5 to 7

'Large' specimens

'Small' specimens

2 9 4

3 0 5

6 7 5

3 0 5

3 0 5

2 0

2 0

2 5

2 5

2 9 4

7 0 6

1 0 0 0

1 0 0 0

200

200

150 150

102

Rollers

Rollers

(a) (b) (c)

Old concrete(base)

New concrete(repair)

Joint

70˚ 70˚

6 mm

steel plate

92Repair

Base

Fig. 3. Shear tests of concrete composite prisms (measurements in mm)





material was about two weeks old. The age of the

base at testing ranged from three to 26 weeks. To

explore the effects of differential shrinkage, compo-

site prisms with 708 joints were made with the same

procedures, except that the base concrete was about

eight months old when the repair material was added

and the specimens were not tested until the repair

concrete was two months old (series 4).The concrete mix design was as proposed in BS

63195

(1´0: 2´0: 1´6, cement: aggregate: sand with a

w/c ratio of 0´4), to give a 28 day average cube strength

of 65 Namm2, except that the aggregate was a mixture

of 10 mm and 20 mm natural gravel in the ratio 1:2.

The base concrete half-specimens were cast in steel

moulds with oiled wooden inserts to form the slant

surfaces. After curing, scarification and cleaning with

compressed air, the bases were replaced in the same

moulds (minus inserts) with their joint faces upward to

receive a specific bond treatment and the repair con-

crete. Rapid-hardening Portland cement was used, con-

crete was compacted on a vibrating table and the

moulds were covered in plastic immediately after cast-

ing. Specimens were demoulded after 24 h and covered

with wet hessian to cure for a minimum of three days.

In all series, each set comprised three test specimens

together with three control cubes of the base and repair

concretes.

In series 3, 6 mm-thick steel plates were attached,

with epoxy resin and expanding bolts, to the 150 mm

wide opposite faces of the specimens to prevent tension

failures originating from the eccentric compression

(Fig. 3(b)).

In series 4 additional joint treatments were included and some sets were repeated to confirm the increase in

the strength of the specimens without bond coat (abbre-

viation `R' for the repeating sets).

The slant shear tests and compressive tests of control

prisms were performed in a 10 000 kN testing machine

and the control cubes of the base and repair concrete

were tested on the same days as the composite prisms.

The influence of platen restraint was studied and roller

pads were used sandwiched between steel plates below

the specimen to reduce this restraint.12,15

Series 5, 6 and 7: slant shear tests of small prisms

These series aimed to investigate the size influence

and to define a specimen of small dimensions, quicker

and easier to test, and able to duplicate the results

obtained from real scale prototypes (Fig. 3(c)). Addi-

tionally, some extra bonding systems available on the

market were tested.

Test specimens were produced in 4 0 3 40 3 200

(102 3 102 3 508 mm) metal moulds, standard form-

work for concrete flexure tests, available in most

laboratories, with joint angles of 708 (series 5 and 7)

and 608 (series 6).

Procedures were similar to those for the large com-

posite prisms. Half-specimens were cast in steel beam

moulds with oiled wooden inserts in their bottoms to

form the slant surfaces and the repair concrete was cast

against the previously treated surface.

Series 7 was designed to study the effects of the

ageing of the base concrete in small composite prisms.

The procedures were as above except that the base

concrete was about 8 months old when the repair was

cast and the specimens were tested when the repair concrete was two months old. Four joint treatments

were used in this series.

Compressive tests were performed in a 2000 kN test-

ing machine, with roller pads below the specimens, and

the control cubes of the base and repair concretes were

tested on the same days of the slant specimens.

Joint surface treatments

The test programme comprised 15 different joint

treatments, the characteristics of which are described

below.

(a) No bond coat, joint surface wetted and free water

removed before casting the repair (sets 1´1, 4´1, 5´1

and 6´1).

(b) No bond coat, joint surface dry (sets 1´2, 2´1, 3´1,

4´2, 5´2, 6´2, and 7´1).

(c) Portland cement mortar bond coat of equal parts

by weight of rapid-hardening cement and fine dry

sand (passing no. 14 sieve), w/c ratio about 0´4 to

give a creamy consistency (sets 1´3, 2´2, 3´2, 4´3,

5´3, 6´3 and 7´2).

(d ) One bond coat of epoxy resin (Sikadur 32 supplied

by SIKA) a two component resin applied by brush

to the substrate in a thin layer 1 to 2 mm (sets 1´4,2´3, 3´3, 4´4, 5´4, 6´4 and 7´3).

(e) One bond coat of a prepacked SBR system (Resi-

bond SBR and Resipatch Primer Powder, supplied

by Celtite±Selfix) applied by brush to a saturated

substrate after removal of free water. New concrete

cast while the bond coat was still wet (sets 1´5, 4´5,

5´5 and 6´5).

( f ) Two bond coats of SBR, the first applied as for set

1´5 and the second applied after the first was dry

(set 1´6).

( g ) Two bond coats of epoxy resin, the first applied as

for set 1´4 and the second applied after the firstwas dry (set 1´7).

(h) No bond coat, surface dry and no roller pads below

the specimens at test (set 4´6).

(i) One epoxy mortar bond coat (Ep.m.A, proposed by

Johnson19

with proportions of 1:4 (resin: sand

silica flour) and 1:2´3 (silica : sand passing sieve

14) applied by trowel in a layer about 3 mm thick,

with no primer coat of plain resin (set 4´7).

( j ) One pre-packed epoxy mortar bond coat (Sikadur

41 supplied by SIKA, a two component resin

added to a pre-mixed powder, Ep.m.B) applied by

trowel in a layer about 3 mm thick with no primer

coat (sets 4´8 and 7´4).





(k ) Joint surface left smooth as cast (sets 5´6 and 6´6).

(l ) One bond coat of an elastic 60% solid pure acrylic

emulsion (VDM758, supplied by Harlow Chemi-

cals Co Ltd) mixed with water at a ratio of 1:4 by

volume (sets 5´7 and 6´7).

(m) One bond coat of a 55% plasticiser free dispersion of

vinyl acetate/ethylene co-polymer (Vinnapas-Dis-

persion EP400, supplied by Wacker Chemicals Ltd)mixed with water at a ratio of 1:1 (sets 5´8 and 6´8).

(n) One bond coat of a pure acrylic ester dispersion

(Mowiton M370, supplied by Harlow Chemicals

Ltd) mixed with sand, cement and co-polymer in

water (1:4 co-polymer: water) (sets 5´9 and 6´9).

(o) Monolithic control specimens (sets 1´8 and 5´10).

Results

Series 1, 2 and 3: large prismsÐshort-term repairs

Table 2 presents the test results of series 1, 2 and 3,

including cube strengths of base and repair concretes

and failure modes. Fig. 4 compares the efficacy of

different bond systems in terms of the ratio f cma f cumin

(average slant shear strength of composite prisms/lower Ðbase or repairÐcube strength) for series 1 and 2, as

a proportion of the control specimens ratio f cma f cu

(set 1´8).

From Table 2 and Fig. 4 it can be seen that.

(a) The modified slant shear test is sensitive to the

Table 2. Slant shear testsÐseries 1, 2 and 3: large prismsÐshort-term repairs

Series Set no. Joint system Test ages (days) f cu (Namm2)

concrete

Prism strength

f c (Namm2)

Average

strength

Failure

modeÃStandard

deviation

f cm (Namm2)

Base Repair Base Repair 1 1´1 No b. coat 21 7 46´6 56´5 17´0 18´1 (s) 1´6

wet joint 17´5 (s)

Joint 19´9 (s)

angle 1´2 No b. coat 82 68 64´3 66´0 26´1 26´4 (s) 0´8

708 dry joint 25´8 (s)

27´3 (s)

1´3 Portland cement 80 65 67´7 57´9 32´9 31´6 (s) 1´7

mortar b. coat 32´2 (s)

29´7 (s)

1´4 Epoxy resin 78 62 63´8 73´2 37´0 37´4 (c) 1´2

b. coat 38´8 (c)

36´5 (c)

1´5 SBR latex 74 57 68´3 65´4 27´1 27´7 (s) 1´0

b. coat 28´8 (s)

27´2 (s)

1´6 2 SBR 78 58 69´9 67´9 15´8 17´9 (s) 1´9

b. coats 19´3 (s)

18´6 (s)

1´7 2 epoxy resin 80 48 63´2 62´6 35´3 35´3 (s/c) 0´1

b. coats 35´2 (s/c)

1´8 Monolithic control 50 ± 60´5 ± 34´9 34´6 (m) 0´4

prisms 34´8 (m)

34´2 (m)

2 2´1 No b. coat 50 35 60´6 62´7 32´4 32´4 (m) 0´1

dry joint 32´5 (m)

Joint 32´3 (s)

angle 2´2 Portland cement 47 33 67´3 65´2 30´7 32´1 (s) 1´5

63´38 mortar b. coat 33´6 (m)

32´0 (s)2´3 Epoxy resin 129 115 68´7 68´5 31´0 31´7 (s) 1´0

b. coat 31´3 (s)

32´9 (s)

3 3´1 No b. coat 136 122 63´7 65´6 7´3 7´1 (s) 0´3

Joint dry joint 201 187 6´8 (s)


908 mortar b. coat 8´1 (s)

6´1 (s)

3´3 Epoxy resin 194 179 71´0 67´8 13´5 13´4 (c) 0´7

b. coat 12´7 (c)

14´1 (c)

Failure modes: (s) shear failure along the joint; (c) failure of concrete adjacent to the joint; (s/c) shear failure along the joint with

simultaneous concrete failure adjacent to the joint; (m) monolithic failure with concrete crushing in the upper part of the specimen (section

1503 150 mm).





different bond treatments and consistent in terms

of the low standard deviations of the strengths in

each set.

(b) The strengths of joints made without bond coats

increased as the joint angle changed from 708 to

63´38 (72% of set 1´2 compared to 93% of set 2´1).

The predominantly monolithic failures for the

63´38 joint also suggests the 708 angle to be critical

for rough joints without bond coat.(c) The best adhesion was in set 1´4 with one bond

coat of epoxy resin giving a ratio of 103% with no

bond failures. The results for set 1´7 with two bond

coats of epoxy were also good, reaching a 99%

ratio, although showing partial bond failures. The

95% strength of set 1´3, specimens with PC mortar

bond coats, should also be noted although all the

specimens failed at the joints.

(d ) The use of one bond coat of SBR mortar did not

produce a significant improvement over results for

specimens without bond coats. Set 1´6 with two

bond coats of SBR mortar gave the lowest bond

strength (46%), in agreement with previous

work.21

It should be mentioned that the manufac-

turer's instructions advise that if, for any reason,

the slurry coat is allowed to dry it must be re-

moved and a new application made.

(e) The reductions of bond strengths as the joint angle

changed from 708 to 63´38 are noticeable for the

PC mortar (reduction of 9% on the slant shear

ratio) and particularly for the epoxy resin bond

coats (reduction of 22%). This suggests that these

joints under higher normal stresses develop differ-

ential interface strains, which break down the bond

sooner than when no bond coat is used. These

results agree with Eyre and Domone's9

conclusion

that bonding systems can show different perfor-

mances as the joint angle changes.

( f ) The compressive strength of the monolithic prisms

is low ( f cm 0X57 f cumin of set 1´8) compared to

the relationship f c 0X75 f cu, commonly accepted

when end restraints are negligible. The only appar-

ent reason for this was the geometry of the test

specimens, with the lower part width 50 mm larger than the upper one. This fact can induce uneven

stress distributions on the lower prism reducing

compressive strengths.

( g ) In the direct shear tests (series 3) the strengths of

prisms without bond coat and with PC mortar bond

coat are virtually the same. Epoxy bond coats

provided a major improvement of about 90% upon

the other two systems with concrete failures adja-

cent to the joint instead of pure bond failures.

Series 5 and 6: small prismsÐshort-term repairs

Tests results are shown in Table 3. Fig. 5 comparesthe different bond systems for the 708 and 608 joint

angles, in terms of the ratios f cma f cumin as a proportion

of f cma f cu for the solid prisms. In sets 5´1, 5´4 and 6´4

the value of f cumin was replaced by the average for the

two concretes, as the low strength of the repair concrete

would artificially produce high values for the slant

shear ratios.

Results from Table 3 and Fig. 5 show that.

(a) Strengths and failure modes of large and small

composite prisms compare favourably, indicating

that the results from the smaller version may well

be as realistic as those produced with the larger

120

100

Series 1 (joint angle 70̊ ) Series 2 (joint angle 63·3˚)

80

60

40

20

0

R a t i o ,

f c m

/ f c u m i n :

% o

f c o n t r o l

68%72% 74%

46%

99%

93%95%

103%100%

86%81%

1·81·71·61·51·41·3

2·1 2·2 2·3

1·21·1Set

No bond

coat – wet

joint

No bond

coat – dry

joint

Portland

cement mortar

bond coat

Epoxy

resin

bond coat

2 epoxy

resin

bond coats

Monolithic

control

prisms

SBR

latex

bond coat

2 SBR

latex

bond coats

Joint systems

Fig. 4. Slant shear ratiosÐlarge prismsÐshort-term repairs





Table 3. Slant shear testsÐseries 5 and 6: small prismsÐshort term repairs


concrete

Prism strength

f c (Namm2)

Average

strength

Failure

modeÃStandard

deviation

f cm (Namm2)

Base Repair Base Repair

5 5´1 No b. coat 81 47 57´5 48´0 23´9 23´8 (s) 3´0

wet joint 27´4 (s)

Joint 20´1 (s)

angle 5´2 No b. coat 77 43 72´0 71´0 29´3 28´4 (s) 2´4


25´0 (s)



32´1 (s)

5´4 Epoxy resin 85 42 64´4 48´1 38´7 36´4 (s/c) 1´8

b. coat 34´4 (s/c)

36´0 (s/c)

5´5 SBR latex 84 42 60´3 54´0 22´9 23´5 (s) 1´7

b. coat 25´8 (s)

21´8 (s)

5´6 Smooth as cast 80 43 68´4 68´7 1´1 2´6 (s) 1´4

joint ± no b. coat 2´2 (s)

4´4 (s)

5´7 Acrylic emulsion 76 42 52´0 82´3 11´1 11´2 (s) 0´7

b. coat 12´1 (s)

10´3 (s)

5´8 Vynil acetate/ 75 40 74´1 70´8 17´6 19´3 (s) 1´6

ethylene b. coat 18´9 (s)

21´4 (s)

5´9 Acrylic ester 74 41 54´8 60´5 21´0 16´4 (s) 3´3

b. coat 14´8 (s)

13´4 (s)

5´10 Monolithic control 42 ± 60´2 ± 49´0 45´7 (m) 4´1

prisms 47´0 (m)

41´0 (m)

6 6´1 No b. coat 80 37 60´6 62´8 24´6 28´1 (s) 5´9wet joint 24´4 (s)

Joint 35´4 (s)

angle 6´2 No b. coat 76 39 74´1 61´3 34´2 30´0 (s) 3´2


26´5 (s)



38´0 (s)

6´4 Epoxy resin 85 42 64´4 48´1 39´9 40´7 (c) 1´1

b. coat 42´2 (c)

40´0 (c)

6´5 SBR latex 84 42 60´5 54´0 26.7 24´8 (s) 2´7

b. coat 21´0 (s)

26´6 (s)

6´6 Smooth as cast 81 38 60´6 62´8 10´3 14´0 (s) 2´8 joint ± no b. coat 17´1 (s)

14´5 (s)

6´7 Acrylic emulsion 79 42 53´6 82´3 9´6 9´8 (s) 0´5

b. coat 10´5 (s)

9´4 (s)

6´8 Vynil acetate/ 76 40 52´0 70´8 22´2 20´1 (s) 1´5

ethylene b. coat 18´9 (s)

19´2 (s)

6´9 Acrylic ester 75 41 74´1 60´5 19´2 17´7 (s) 1´7

b. coat 18´6 (s)

15´2 (s)

Ã See notes to Table 2





specimens. The variability of results from the small

prisms (maximum standard deviation 5X9 Namm2)

is higher than that obtained with large prisms

(maximum standard deviation 1X9 Namm2).

(b) Epoxy resin and PC mortar exhibited the best

adhesion capabilities out of all the bond coats

examined, with very similar slant shear ratios

although all the prisms with PC mortar failed at

their joints.

(c) Results of sets 7 to 9 (pure acrylic emulsion, vynilacetate/ethylene copolymer and pure acrylic ester

dispersion bond coats) typify a weaker bond group

and the use of SBR bond coat did not produce any

significant improvement over the results without

bond coats.

(d ) Test results of smooth joints highlight the need for

adequate surface preparation. Removing the ce-

ment latence from the joint to a depth of around

6 mm increased the strength ratios from set 5´6 to

5´2 (708 joints) by 48% and from set 6´6 to 6´2

(608 joints) by 35%.

(e) The strengths of the 708 jointed prisms are consis-

tently lower than the 608 prisms, the only exception

being for the pure acrylic VDM758 set of very

poor bond strengths.

( f ) The f cma f cu ratio was 0´57 for the larger solid

prisms and 0´75 for the small prisms. This differ-

ence significantly decreased the slant shear ratios

of the small prisms which suggests that a better

comparison of results between small and large

prisms should be given by their absolute strengths.

( g ) The reduction of 19% in the average strengths

shows the effect of wetting the parent concrete

prior to casting the new material to be detrimental

to bond strengths as in series 1. This fact can not

be noticed in Fig. 5 because of the low strength of

the repair concrete of set 5´1.

Series 4 and 7: long-term repairs

Results are summarised in Table 4. Fig. 6 presents a

comparison between the short-term and long-term data

for large prisms with the same angles and surface treat-

ments. It can be seen that:

(a) In absolute terms the strengths of joints made with-out bond coats increased with age (average ratio of

72% in series 1 to 99´5% in series 4). Monolithic

failures occurred in all the specimens of set 4´9.

This high strength reduces the range of possible

improvements due to the use of a bond coat.

(b) Test results confirm the detrimental effect of wet-

ting the parent concrete prior to casting the new

material.

(c) The tests without rollers below the specimens (set

4´6) shows a 10% increase in slant shear strength

as compared with those of set 4´2. Previous works

which used such rollers to encourage joint failuresinclude the tests by Daschner 15

and those by Clark

and Gill.12

(d ) The low strengths of set 4´7 (79%) can be attribu-

ted to the dryness of the epoxy mortar Ep.m.A.

The performance of the prepacked epoxy mortar

Ep.m.B (91%Ðset 6´8) was inferior to that of

specimens without any bond coat (dry joint). Pro-

blems with the workability of these epoxy mortar

formulations can mar their use as bond coats with-

out the application of a primer coat of pure resin.

(e) The average increase of 52% in the ratios

f cma f cumin from the short-term (series 5) to

long-term repairs (series 7) ratifies in the small

120

100

Series 5 (joint angle 70̊ ) Series 6 (joint angle 60̊ )

80

60

40

20

0

R a t i o ,

f c m

/ f c u m i n :

% o

f c o n t r o l

59%61%65%

53%

30% 28%24%

5%

51%

36%39%39%

86%

94% 95%100%

85%

60%57%

6 · 1

5 · 2

6 · 2

5 · 3

6 · 3

5 · 4

6 · 4

5 · 5

6 · 5

5 · 6

6 · 6

5 · 7

6 · 7

5 · 8

6 · 8

5 · 9

5 · 1 0

6 · 9

S e t 5 · 1

No bond

coat – wet

joint

No bond

coat – dry

joint

Portland

cement mortar

bond coat

Epoxy

resin

bond coat

Monolithic

control

prisms

SBR

latex

bond coat

Acrylic

emulsion

bond coat

Acrylic

ester

bond coat

Vynil acetate

ethylene

bond coat

Smooth

as cast

joint

Joint systems

Fig. 5. Slant shear ratiosÐsmall prismsÐshort-term repairs





prisms tests the finding that the strengths of joints

made without bond coat increase significantly with

age.

Discussion

Ultimate stresses at the interfaces of composite prisms

Test results for short-term repairs are presented in

Fig. 7 in terms of the ultimate stresses at the interface,

ô and ó , calculated by equations (1) and (2). Stresses

are related to the factor f 2a3cc , f cc being the minimum

value in each set calculated from the cube strengths by

f cc 0X8 f cu. In the previous sections, systems were

compared in terms of the `slant shear ratios', defined

as the ratio f cma f cumin (average strength of composite

prisms/lowerÐbase or repairÐcube strength) related to

the control specimens ratio f cma f cu. Bond systems

with slant shear ratios lower than 50% were excluded

in Fig. 7, as they typify `slip joints'.

In Fig. 7(a), the data for the large prisms of series 1

and 2, including the results affected by the detrimental

Table 4. Slant shear testsÐseries 4 and 7: long-term repairs


concrete

Prism strength

f c (Namm2)

Average

strength

Failure

modeÃStandard

deviation

f cm (Namm2)

Base Repair Base Repair

4 4´1 No b. coat 314 57 58´9 63.6 25´5 27´1 (s) 2´4

wet joint 26´0 (s)

Large 29´9 (s)

prisms 4´2 No b. coat 334 64 62´2 61´0 33´2 32´7 (s) 1´3

dry joint 33´7 (s)

Joint 31´2 (s)


708 mortar b. coat 24´3 (m)

25´9 (m)

4´4 Epoxy resin 314 59 56´6 73´4 32´3 33´2 (c) 1´1

b. coat 34´4 (s/c)

33´0 (s/c)

4´5 SBR latex 320 61 56´2 70´2 26´1 31´3 (s/c) 4´6

b. coat 33´1 (s)

34´8 (s)

4´6 No b. coatÐdry joint 323 63 58´1 63´4 35´2 34´9 (s) 2´1

no rollers below 32´7 (s)

prisms at tests 36´9 (s)4´7 Epoxy mortar 313 54 55´9 64´3 25´8 25´1 (s) 2´6

b. coatÐA 22´2 (s)

27´2 (s)

4´8 Epoxy mortar 313 55 85´1 58´5 33´4 30´6 (s) 2´5

b. coatÐB 27´3 (s)

31´2 (s)

4´9 No b. coat 321 85 73´0 62´7 36´3 37´6 (m) 0´9

dry joint (R y) 38´0 (m)

38´5 (m)

4´10 Portland cement 315 76 71´7 65´0 37´4 34´9 (m) 2´4

mortar b. coatÐ(R y) 31´7 (m)

35´5 (m)

4´11 Epoxy resin 317 82 61´5 69´6 41´4 39´6 (c) 1´3

b. coatÐ(R y) 38´4 (m)

39´1 (m)7 7´1 No b. coat 321 85 73´0 62´7 38´6 38´1 (s) 2´9

dry joint 34´2 (s)

Small 41´3 (s)

prisms 7´2 Portland cement 315 76 71´7 65´0 40´8 40´6 (m) 1´0


Joint 39´3 (s)

angle 7´3 Epoxy resin 317 82 61´5 69´6 37´4 34´8 (s/c) 1´9

708 b. coat 33´1 (s/c)

33´9 (s/c)

7´4 Epoxy mortar 313 55 85´1 58´5 32´5 31´9 (c) 0´9

b. coatÐB 32´5 (c)

30´6 (c)

Ã See notes to Table 2y

Repeating sets





effect of pre-wetting the joint without bond coat, show

good agreement with the straight line of the joint fail-

ure envelope, that can be expressed by

ô 0X25 f 2a3cc ó (Namm2) (10)

This straight line assumed the values of c and ì

presented by Regan10

for rough interface conditions,

presented in Table 1. As can be seen in Fig. 7, Regan's

proposal ì 1X4 for the line inclination showed a good

agreement with experimental results. Franke's11

value,

ì 0X7, for sandblasted surfaces and the ACI 318M-

95,18 ì 1X0, for concrete placed against hardened

concrete with the surface roughened to a full amplitude

of about 6 mm, are rather conservative.

The value of the line intercept in equation (10),

c 0X25 f 2a3cc (Namm2), was compared with results

from the direct shear tests (series 3Ðpoints on the

vertical axis) and showed to need correction. Only the

epoxy bonded prisms presented ultimate shear stresses

in accordance with that value and results of six prisms

with no bond coat or PC mortar coat are virtually

identical and consistently below that value. Results

from these tests will be used again in this section to

propose a different line intercept.

In Fig. 7(b) it can be seen that the data for the 708

120

100

Series 1: Short-term repairs Series 4: Long-term Joint angles: 70̊

80

60

40

20

0

R a t i o ,

f c m

/ f c u m i n :

% o

f c o n t r o l

68%

80%

108%

74%

97%

72%

99%103%

100%

94%95%

1·81·5 4·54·41·41·3 4·31·2 4·24·21·1Set

No bond

coat – dry

joint

No bond

coat – wet

joint

Portland

cement mortar

bond coat

Epoxy

resin

bond coat

Monolithic

control

prisms

SBR

latex

bond coat

Joint systems

Fig. 6. Large prismsÐshort- and long-term repairs ( joint angle 708 )

3

Series 2 - joint 70̊

Series 4 - direct shear

Control

arctan µ = 1·4arctan µ = 1·4

Series 3 - 63·3˚Series 6 - joint 70̊

Series 7 - joint 60̊

Control

2

1

0

0 1 2

(a)

3

τ / f c c

2 / 3

σ /f cc2/3

0 1 2

(b)

3 4

σ /f cc2/3

f cc = 49 N/mm2f cc = 50 N/mm2

Fig. 7. Ultimate stresses at jointÐshort-term repairs: (a) series 1, 2 and 3Ðlarge prisms; (b) series 5 and 6Ðsmall prisms





small prisms also satisfy the joint failure line. However,

some results of 608 prisms are slightly below the

straight line although their strengths were higher than

the 708 prisms, suggesting that analysis only in terms

of absolute strengths may be insufficient, agreeing with

Eyre and Domone's conclusion.9

A comparison of the control prism data, points on

the horizontal axes in Fig. 7, again shows the signifi-cantly lower strengths of the larger solid prisms.

Figure 8 shows the test data from short- and long-

term repair series, comprising tests of large and small

composite prisms, for three relevant bond systems of

this research: `no bond coat' with the parent concrete

surface kept dry before pouring the new concrete; PC

mortar; and epoxy resin bond coats.

The initial curved part of the failure envelope de-

scribed in Fig. 1, based on the concrete having zero

tensile strength, was reported to be adequate for the

prediction of the strength of smooth concrete joints.12

However, for roughened joint surfaces it appears to be

conservative as indicated by the average ultimate

strengths of the direct shear tests (Table 2Ðseries 3).

In Fig. 8 the initial arc is replaced by a straight line

between the point where the arc was tangential to the

joint failure envelope line, corresponding approxi-

mately to the vertical coordinate 0´4, and the point on

the vertical axis given by ô 0X10 f 2a3cc (Namm2). This

proposal is assumed to be safe for any system pro-

ducing a bond strength equal or superior to that of the

new concrete cast directly against the old without a

bond coat. Regan10

proposed an identical expression

for the cohesion at crack surfaces (width 1 mm) in

monolithic gravel concrete, which is probably moreunfavourable.

The analysis of Fig. 8 shows the data for the three

bond systems in good agreement with the failure envel-

ope expressed by

ô 0X25 f 2a3cc 1X4ó for ó > 0X1 f 2a3

cc (Namm2) (11)

ô 0X10 f 2a3cc 3X0ó for 0 < ó < 0X1 f 2a3

cc (Namm2)

(12)

Daschner 15

carried out an extensive experimental programme concerning the influence of the parent con-

crete surface preparation, concrete strength and joint

angle on the compressive strength of composite prisms.

Tests were made with precast saw-tooth joint surfaces,

no bond coats and 150 3 150 3 600 mm prisms. A

wide range of angles were used, from 58´58 to 908

between the joint and the normal to the load. Zelger

and RuÈsch22

tested 803 250 3 700 mm prisms also

with saw-tooth joints and no bond coat, with angles

59´48, 48´98, 36´98, 24´28, 13´18 and 08. Fig. 9(a) pre-

sents these test results, the ultimate stresses ô and ó

being related to the factor f 2a3

cc

. The data show good

agreement with the proposed failure criteria.

Franke11

collated a reasonable amount of data of

slant shear and direct shear tests on epoxy jointed con-

crete specimens with the surfaces roughened by sand-

blasting and with variable joint angles. Results from

Franke and other authors' data are displayed in Fig.

9(b) (symbols are mean values) and a satisfactory

agreement with the criteria is achieved. The analysis

also suggests that a better correlation for epoxy joints

might be achieved by using a higher value of c and a

lower value of ì, as represented by the broken line

corresponding to cohesion and friction values

c 0X50 f 2a3cc and ì 1X1. Although a similar valuewas found for the friction in this research, for the

cohesion it was about half this value, as can be seen in

arctan µ = 1·4 arctan µ = 1·4arctan µ = 1·4

Epoxy bond coatPC mortar bond coatNo bond coat

0·1

0·4

0·8

1·2

1·6

2·0

0 0·5

τ / f c c

2 / 3

σ /f cc2/3

0 0·5

σ /f cc2/3

0 0·5 1·0

σ /f cc2/3

Fig. 8. Proposed failure criterion for composite prismsÐshort- and long-term repairs





Fig. 8, suggesting that the adoption of a correlation for

steep joints (á 9 908) is still needed.

Conclusions

An experimental programme of slant and direct shear

tests was developed to evaluate the strength of bond

between old and new concrete with different bonding

systems and surface preparations. The tests provided

evidence that a sound bond can be achieved by castingthe repair against mature concrete without the use of

bonding aids, provided the base surface is dry and

roughened to an extent that the aggregate is exposed

and no damage is caused to the concrete near the joint.

Casting new concrete directly against the old without a

bond coat produced very high slant strengths in the

long-term tests, with very little or no improvement left

to be achieved by bonding agents. The very dry surface

of the aged concrete probably improves the adhesion at

the interface by migration and penetration of hydrated

cement crystals into the scabbled surface of the old

concrete probably in combination with a decrease inthe w/c ratio of the concrete near the joint. Efficient

curing is certainly essential to guarantee a good hydra-

tion of the new concrete and to minimise early shrink-

age.

For rough surfaces without any bond coat the angle

of 708 between the joint line and the normal to the load

was shown to be more critical than the 608 recom-

mended by American and British standards.4,5

The influence of bond coats was found to be greater

for steeper joints and repairs cast against relatively

new base concrete. In the slant shear tests with 708

joints, epoxy resin coats substantially increased bond

strengths. In the direct shear tests the epoxy coats even

doubled the shear strengths compared with joints with-

out bond coat. However, an opposite tendency was

manifested for prisms with 63´38 angles: joints made

with epoxy bond coats exhibited a considerable reduc-

tion in the slant shear strengths.

The use of a Portland cement mortar bond coat im-

proved adhesion by about 30% in comparison with

specimens with no bond coat in the short-term tests,

although it did not prevent bond failures. As for epoxy

joints, PC mortar bond coats exhibited poorer perform-

ance when the angle between the joint and the crosssection decreased in relation to the modified angle of

708. No improvement was achieved in the direct shear

tests of PC mortar joints in relation to the no bond coat

joints.

Some materials marketed as having `bonding proper-

ties' showed in the present work detrimental effects on

bond strengths (formulations of pure acrylic emulsion,

vinyl-acetate/ethylene co-polymer and pure acrylic es-

ter dispersion). SBR latex bond coats did not improve

bond strengths.

The small specimens developed and tested in this

investigation generally reproduced the main results

from the large prism tests. The preparation required is

minimal and this type of specimen can be useful to

evaluate bond strength performance in quicker and

easier tests.

A modified Coulomb criterion was shown to provide

a satisfactory prediction for the ultimate stresses at the

interface for any joint system producing a bond

strength equal or superior to that of the new concrete

cast directly against the old without a bond coat. The

failure criteria adopted in this analysis exhibited good

agreement with experimental data from this and other

work. However, it should be remarked that test results

indicated that the constants in the equation may be

3

Daschner:

Zelger & Rusch:

arctan µ = 1·4arctan µ = 1·4

2

1

0·1

0·4

0 1 2

(a)

3

τ / f c c

2 / 3

σ /f cc2/3

0 1 2

(b)

3 4

σ /f cc2/3

f cc = 30 N/mm2

f cc = 60 N/mm2

f cc = 14 N/mm2

f cc = 32 N/mm2

f ccmin = 14 N/mm2

f ccmin = 25 N/mm2

f ccmin = 32 N/mm2

f ccmin = 25 N/mm2

Diaz:

Buchting & Moosbrugger:Eibl et al .:

Rehm & Franke:

Ladner & Flueler:

f cc = 57 N/mm2

f cc = 46 N/mm2

f cc = 32 – 54 N/mm2

f cc = 39 – 80 N/mm2

f cc = 18 – 26 N/mm2

Fig. 9. Proposed failure criterionÐcomparison with other authors: (a) no bond coat jointsÐdata collated by Daschner;15

(b)

epoxy bond coat jointsÐdata collated by Franke11





affected by the bond coat material. For epoxy joints,

for example, lower values of ì should be considered

possibly in combinations with higher values of c.

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Discussion contributions on this paper should reach the editor by

1 May 2002.



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