Shear strength of High Strength Concrete

2. Shear Strength of High Performance
Concrete
Engr. Prof. Dr. Attaullah Shah

3. Bio details of the Speaker
 Engr. Prof. Dr. Attaullah Shah
 Vice Chancellor Karakorum International University Gilgit Baltistan
 Former Vice Chancellor City University of Science and IT Peshawar
 Former Director ( Planning and Projects AIOU)
 drshah965@gmail.com , www.drshahpak.weebly.com
 +92-333-5729809, +92-315-5155077
 Qualification
 PhD Civil Engineering ,M.Phil Eco ,MSc Structure Engg
 MBA, MA Eco, MSc Envir Design, BSc Civil Engg (Gold Medal),
 Post Grad Dip Comp (Gold Medal)
 Professional and Field experience:
 30+ Years
 Research Publications in refereed journals and conferences:
 6 Books published on HPC, Project Management, Disaster Management ,ESMP.
Engineering Economics and Pak China Friendship.
 Currently authoring Project Procurement & Contract Aadmn., Professional Ethics and
Academic Quality Assurance
 47 Journals publications+75Conference publications
 Areas of interests
 Academic and Research Leadership
 Financial and Productivity Assessment of Higher Education
 Structural Engineering/Sustainable built Environment
 Construction Project Procurement and Contract Management

4. Concrete as Versatile Construction
Material
 World produces 4.4 billion tons of concrete
annually,
 Expected to rise to over 5.5 billion tons by
2050 as poorer countries rapidly urbanize,
according to the Chatham House report.
 Production of one ton of Portland cement
causes the release of one ton of CO2 into
the atmosphere

5. High Performance Concrete
 High Performance Concrete (HPC) is referred to the
specialized series of concretes designed to provide several
benefits in the construction of concrete structures.
 A concrete mixture which has high workability, high strength,
high modulus of physical property, high density, high
dimensional stability, low permeability and resistance to
chemical attack is generally said to be high performance
concrete.
 The American Concrete Committee on High Performance
Concrete includes the following six
criteria:
 Ease of placement
 Long term mechanical properties
 Early-age strength
 Toughness
 Life in severe environments

6. ACI Definition of HPC
 concrete meeting special
combinations of
performance and uniformity
requirements that cannot
always be achieved
routinely using
conventional constituents
and normal mixing, placing,
and curing practices.

7. High Strength Concrete
 Definition of HSC has been changing from time to
time based on new developments.
 10 years back concrete having compressive
strength in excess of 45 MPa was HSC but today
concrete with 60 MPa is considered normal
Strength Concrete
 ACI: Concrete with a specified compressive
strength of 55 MPa (8000 psi) or higher. In
many markets today, concrete having a specified
compressive strength in excess of 69 MPa
(10,000 psi) is routinely produced on a daily
basis.

8. Shear strength of Normal Strength
Reinforced Concrete (NSRC) beams
 Beam Action and Arch Action in
NSRC beams.
 Mc = Vc.x = T.jd =
 The joint committee ASCE-ACI-426
in 1973 and later in 1998 reported
the following five mechanisms for
the shear in reinforced concrete
sections (NTRB, 2005).
 Shear in the Un-cracked Concrete
Zone
 Residual Tensile Stresses:
 Interface shear transfer
 Dowel Action
 Shear Reinforcement:
Forces acting in a beam element within the shear span
and internal arches in a RC beams (Russo et al., 2004).

9. Other parameters affecting shear
strength of Concrete members.
 Depth of member or size effect,
 Shear span to effective depth a/d or moment to shear ratio
 Axial Force,
 The tensile Strength of concrete,
 Crushing strength of the Beam web,
 Yielding of stirrups,
 The aggregates sizes leading to aggregates interlocking.
 Failure of Tension chord.
 Failure of Stirrups anchorage:
 Serviceability failure due to excessive crack width at Service
load.

10.  The ASCE-ACI Committee 426 has reported the following
equation for the concrete shear strength incorporating the
longitudinal reinforcement.
 For beams with transverse reinforcement, the basic model to explain the
mechanism for carrying the shear was proposed by Ritter (1899).
For 45 degree truss model, the capacity provided by the shear
reinforcement is equal to the capacity of an individual stirrup
multiplied by the number of stirrups over the length d, which is
approximately equal to “d/s”. The shear carried by the stirrups is
given as;

11. Recent Approaches to Shear
Design of NSRC.
 i. Compression Field Theory
 ii. Truss approaches with concrete
contributions
 iii. Shear friction theory
 iv. Strut and Tie Model.
 v. Some recent empirical equations

12. Compression Field Approaches.
 In this approach the tensile stresses along
the cracked concrete is also taken into
consideration, which was neglected in the
earlier approaches.
 CFT uses four conditions for the analysis of a
section:
1. Equilibrium of the section is considered under
external shear force, respective components of
the concrete diagonal compression force,
vertical stirrups and longitudinal steel.
2. Strain compatibility of the cracked concrete.
3. Stress strain relationship of reinforcement
4. Stress stains relationship of cracked concrete
in compression

13. Modified Compression Filed Theory MCFT
 Vechhio and Collins (1986) further
developed the CFT into Modified
Compression Field Theory (MCFT)
that accounts for the influence of
tensile stresses on the post cracking
shear behavior of concrete.
 The highest value of longitudinal strain is approximated
to the strain in the tension chord and is given by

14. Simplified Modified Compression Field Theory
 Bentz et al. (2006), proposed a simplified MCFT for quick
and convenient calculation of the shear strength of RC
beams. This method according to authors provided excellent
predication of shear strength of RC concrete beams with
only 13% coefficient of variation. Based on the assumption
of the theory the following expressions were developed.

18. 2. Truss Approaches.
 In the traditional approaches assumed that compression
struts are formed parallel to cracks and no stresses are
transferred across the cracks, hence the concrete
contribution due to transfer of stresses across the cracks is
usually neglected, but this often leads to conservative results.
 The more recent approaches also take into account the
following two contributions;
 Tensile stresses that exists transverse to the crack.
 Shear stress that is transferred along the inclined crack by aggregates
interlocking.
 The Vc suggested by Ramirez and Breen ( 1991) is given as;
 Variable Truss angle was later modified
in the range of 25-65 degrees.

19. 3. Shear Friction Approach
 The roughness may be visualized
as series of saw toothed frictionless
fine saw toothed ramp having a
slope of tan. The separation is
sufficient to yield reinforcement
across the crack. This nominal
shear resistance of concrete is
given as Vn = μ Asfy.
 The shear friction was adopted by
ACI-318 code in 1973 and the value
of μ has been reduced as against
suggested by Birkeland and
Birkeland (1966) . The Canadian
Code has recently introduced
modified friction formula.

20. Strut and Tie Model (STM)
 Structures are sometimes classified
as either B- (Beam or Bernoulli)
Regions or D- (Disturbed or
Discontinuity) Regions, for selection
of appropriate design procedure.
 B-Regions are parts of a structure
in which Bernoulli's hypothesis of
straight-line strain profiles applies.

21. High Strength and High Performance Concrete.
(HPC)

22. High Strength Concrete
 At times the compressive strength of 40 MPa was considered
as high strength.
 With improved mixed design, ultra high range water reducers
(Superplasticisers), and mineral admixtures, concretes with
compressive strength above 100 MPa are easily obtained in
the field.
 ACI-318 committee revealed that in the 1960’s, 52 MPa
(7500 psi) concrete was considered high-Strength concrete
and in the 1970’s, 62 MPa (9000psi) concrete was
considered as HSC.
 The committee also recognized that the definition of the high-
strength concrete varies on a geographical basis
 In regions where 62 Mpa (9000 psi) concrete is already being
produced commercially, high-strength concrete might be in
the range of 83 to 103 MPa (12,000 to 15,000psi).

23. Behavior of HSC-Stress Strain curves.
A point to ponder
The main variations in these
curves are illustrated as follows
 The stress strain curve is
getting more linear with the
increase of compressive
strength of concrete.
 A relatively higher strain is
observed for corresponding
values of stress, which is more
pronounced at the maximum
stress.
 The descending part of the
curve after the peak is steeper
in case of HSC.
Complete Stress Strain Curves for
Normal and High Strength
Concrete
Ref: .Wang, PT; Shah, S.P, and Naaman, A.E., “
Stress Strain Curves of Normal and Light Weight
Concrete in Compression” ACI journal, proceedings
V.75 No. 10 1978, pp 603-611.

24. Cracking Behavior of HSC
 The high strength concrete exhibits ductile behavior as
compared to normal strength concrete
 The difference in rigidity between cement paste and
aggregates leads to concentration of stresses at the
contact zones of the two ingredient of HSC and at certain
overall stress level, a distributed micro crack pattern
forms at the contact points.
 When the overall stress level further increases, a
substantial part of the increased energy is used in
developing a clearer crack pattern. The stress strain curve
at this stage tends to deviate from linear elastic line.
 With further increase of stresses, the micro crack pattern
will provide an efficient re-distribution of the stress and a
tough and brittle failure is obtained.

25. Some latest work on HSC
 Duthin and Carino ( 1996) has pointed out that most of the
current shear design techniques either do not acknowledge
the loss in the aggregate interlock mechanism in high
strength concrete or simply do not account for the influence
of adding shear reinforcement to other shear transfer
mechanisms.
 Johnson and Ramirez (1989), reported that for a constant low
shear reinforcement, the overall reserve shear strength after
diagonal cracking diminishes with increase in the
compressive strength of concrete.

26. Provisions of International Building and Bridges
Codes for Shear Design of RC beams

27. Summary of Research results on HSRC
beams

28. Member Type
All RC-beams Pre-stressed beams. Inferences
With or without
AV
Both RC
No Av
RC
With Av
Both PC
No Av
PC
With Av
- CSA and LRFD
has given best
results particularly
Code No 1359 878 718 160 481 321 160
ACI Mean 1.44 1.51 1.54 1.35 1.32 1.38 1.21
CoV 0.371 0.404 0.418 0.277 0.248 0.247 0.221
LRFD Mean 1.38 1.37 1.39 1.27 1.40 1.44 1.32
CoV 0.262 0.262 0.266 0.224 0.261 0.290 0.154
CSA Mean 1.31 1.25 1.27 1.19 1.41 1.46 1.31
CoV 0.275 0.274 0.282 0.218 0.261 0.287 0.147
JSCE Mean 1.51 1.36 1.35 1.38 1.80 1.85 1.70
CoV 0.321 0.28 0.293 0.216 0.292 0.297 0.272
EC2 Mean 1.85 1.75 1.75 1.70 2.06 2.13 1.91
CoV 0.409 0.328 0.328 0.373 0.470 0.43 0.687
DIN Mean 2.05 2.10 2.10 1.25 2.25 2.59 1.58
CoV 0.395 0.327 0.327 0.267 0.413 0.345 0.357

29. National Cooperative Highway Research
Program (NCHRP), USA
 NCHRP conducted a research project “Simplified Shear Design
of structural Concrete Members”.
 The simplified provisions must be directly usable without
iteration for shear design and evaluation of the shear
capacity of the members.
 Must be useful in conducting field evaluation to estimate
the failure loads for the shear cracking by the site
Engineers.
 Must be easy to explain by the Engineers to others.
 Allow reliable and hand based design method.
 Provide safe and accurate estimates for the RC members
in the selected test database.
 The shear reinforcement as a result must be reasonable

30. Recommendations of NCHRP
 The web shear cracking Vcw was simplified as
 Flexure-shear cracking strength:
 For value of

31. Summary of Literature review on HSRC beams
 1. The relationship between aggregate interlocking share
and compressive strength of concrete in shear strength of
HSRC beams. A reduction factor may be explored to
correlate the aggregates interlocking share of HSC and
compressive strength of concrete.
 2. The level of minimum web reinforcement for HSRC
beams, to avoid sudden and brittle failure of beams may be
identified.
 3. Parametric study for the shear strength of HSRC beams,
incorporating important parameters affecting the shear
strength of HSRC beams is required, which can supported
with experimental work to develop more rational equations
for the shear strength of HSRC beams.

32. Experimental Program
( Beam Region)

33. Details of Experimental works :Phase-1
 70 beams in two sets of 35 beams each
 Beams dimensions: 9in x 12 in
 a/d ratio: 3, 3.5, 4, 4.5 , 5, 5, 6
 Longitudinal steel ratio: 0.33%, 0.75%
,1.00%, 1.5% and 2.0 %.
 Shear reinforcement used in 35 beams.
Av = 0.16% ,fc′ =52 MPa

34. Span
Proving Ring
Hydraulic Loading
Typical Loading Arrangement for testing of beams

35. Concrete Mix Design and Re-bars details

36. Basic Shear Failure modes and Observations
 1. Diagonal Tension failure:
 Diagonal crack starts from the last flexural crack.
 Turns more and more inclined under the shear loading.
 It encounters resistance as it moves up into the tension zone,
 Becomes flatter and stops at some point 1 as shown in the Fig.
This is sometimes called primary shear crack as well.
 With further increase of load, the tension crack extends at low
slope mostly across the tension region of the beam and causes
failure of the beams at point 2;
 The diagonal shear failure may take place in beams with large
shear span usually.

37. Basic Shear Failure modes and Observations
 2. Shear compression failure:
 In small shear span, web shear cracks may initiate at 45
degrees,
 Crosses neutral axis even before proper flexural shear
cracks appear in the beams.
 Thus the shear cracks are crowded in smaller depth and
extended towards the supports or reactions.
 The compression failure of concrete occurs with further
increases of loads.
 This is more common failure mode in short beams where,

38. 3. Splitting or true shear failure:
 For deep beams, where the shear span is less than the
depth of the beam, the shear load is taken by the arch
action between the load and reaction .
 Ordinary diagonal tension concept is no more valid here.
 The final failure may take place due to compression
failure at the reactions or as splitting failure of concrete.
 The concept of
 Truss and Tie Model ( STM) is more applicable to such
kind of shear failure
Basic Shear Failure modes and Observations

39. Basic equations to identify the beam
failure mode
 According to (Calogero Cucchiara, 2004);
 Hence Mu/Mfl has been worked out for all the five sets of
beams to identify the region of beam failure.
 The minimum value from graphs shown on next slide shows
that for critical values ensuring beams failure is about 2.5 or
more.

41. Comparison of theoretical flexural capacity of beams and actually observed values of
failure loads for beams without stirrups.

42. Shear transferred to beams without web
reinforcement at peak load.

43. Beam shear Failure

44. Flexural shear failure

48. Failure loads of beams with web reinforcement

51. Failure modes

53. Observations
 Effect of longitudinal steel on the
shear strength of HSC-RC beams.
 For constant values of a/d, the shear
strength of beams has been increased
with the increase in longitudinal steel
mainly due to dowel action.
 The width of shear crack has been
reduced with the increase of longitudinal
steel.
 The dowel action is more prominent in the
beams with web reinforcement as there is
continuous increase in the shear strength
of beams with the increase of longitudinal
steel for same value of shear span to
depth ratio a/d.
 This increase can be attributed to better
packing of the longitudinal steel with the
help of stirrups.

54. General Observations
 The actual failure loads, when compared with the theoretical flexure strength
of beams, reveal the following facts;
 The actual failure loads have been increased with the increase in
longitudinal steel for given span and a/d ratio, which is well illustrated fact.
 For beams having ρ<1%, the failure and theoretical flexural values are
reasonably close and it is believed that the failure has been caused mainly
due flexure.
 For beams with ρ>1%, the failure loads less than the theoretical flexure
strength of beams and the failure is caused typically by the shear cracks.
The shear cracks normally appear before the failure of the beams and are
measured at the instant when secondary cracks have just emerged. This
has also been observed from the cracking pattern of beams discussed in the
subsequent sections. Hence for shear failure the rest of the discussion has
been based on the results of beams with ρ>1%. The selection of this
threshold value of ρ is surely a subjective matter; however this is supported
with the actual observation of cracks and failure of beams.
 .

55.  The actual failure strength of beams has been substantially increased
with stirrups, as shown in table.
 This increase can be attributed to better packing of longitudinal steel by
the stirrups. An average increase in flexural capacity of beams for ρ <
1% is more as compared to beams with ρ>1%.
 The shear strength of beams, where failure is mainly due to shear, the
total loads at the first cracks, Pcr*, have been observed, which
corresponds to the shear strength of beams section

56. Comparison of Shear capacity of beams with and without shear reinforcement
for same values of longitudinal steel and a/d ratio (for ρ< 1%).
Shear stress(MPa) Stirrups contribution
Steel ratio
(ρ%)
a / d
B e a m w i t h o u t s t i r r u p s B e a m
w i t h
s t i r r u p s
A C I
E q A c t u a l
C o l . 1 2
3 4 5 6 = 4 - 3
0 . 3 3 3 . 0
0 . 9 5 0 . 9 9 0 . 4 2 2 0 . 0 4
0 . 3 3 3 . 5
0 . 8 8 0 . 8 8
0.42 0
0.33 4.0 0.7 0.8 0.42 0.1
0.33 4.5 0.56 0.73 0.42 0.17
0.33 5.0 0.48 0.57 0.42 0.09
0.33 5.5 0.41 0.46 0.42 0.05
0.33 6.0 0.25 0.41 0.42 0.16
0.73 3.0 0.95 1.7 0.42 0.75
0.73 3.5 0.88 1.38 0.42 0.5
0.73 4.0 0.8 1.22 0.42 0.42
0.73 4.5 0.73 1.06 0.42 0.33
0.73 5.0 0.65 0.98 0.42 0.33
0.73 5.5 0.57 0.82 0.42 0.25
0.73 6.0 0.41 0.75 0.42 0.34
1 3.0 0.99 1.7 0.422 0.71
1 3.5 0.96 1.54 0.42 0.58
1 4.0 0.88 1.45 0.42 0.57
1 4.5 0.81 1.39 0.42 0.58
1 5.0 0.73 1.23 0.42 0.5
1 5.5 0.66 1.07 0.42 0.41
1 6.0
0.58 0.91
0.42
0.33

59. Behavior of Beams without web
reinforcement:
 The shear failure of the beams without web reinforcement is
generally sudden and brittle typical for high strength concrete.
 In beams with ρ <1%, the failure is mainly due to flexural
cracks concentrated along the mid span region. In such beams
relatively more vertical cracks appear near the mid-span region,
which ultimately extends along the full depth of the beam web
and cause the failure of the beams.
 Typical flexural failure of beams has been shown in next slides.
In most of the cases the failure is caused by the primary cracks,
which originate in the beginning and extends further. The failure
crack angles range from 50degree to 65degree

60.  ρ=0.73%, Span =7.5 ( 225cm) feet
a/d = 5.0
ρ=0.73%, Span =7.5 ( 225cm) feet a/d = 5.0
Typical Flexural failure of the beams with longitudinal steel < 1%

61. Experimental Observations of beams having
longitudinal steel less than 1%
ρ=0.33%, Span = 5 feet (150cm) a/d = 3.0
ρ=0.73%, Span = 5 feet(150cm) a/d =
3.0

62.  However in cases, with longitudinal reinforcement is
more than 1%, a single crack appears at the critical
section for shear near support, which further extends
along the length of beam. With further in crease of loads,
second branch of the crack originate from the first
branch which is sometimes called secondary crack. This
crack extends further and causes the failure of the
beams at the point of application of loads.
 The typical shear failure of the beams having ρ>1% has
been observed. The crack causing the failure of beams
is shallower and more pronounced. This has also been
exhibited on next slide, where the failure loads are less
than theoretical flexural capacity of the section. Hence
the beams are assumed to have failed in shear

63. Photograph No.3: Typical shear failure of slender beam without web reinforcement at ρ > 1%
( ρ = 1.5 % a/d = 5.5 , Span = 8’ – 4”[ 203 cm} )

64. ρ=1.5%, Span =8.5 (254cm) feet a/d = 5.5
The shear failure of beams without web reinforcement.
(The brick has been placed under the beam to avoid the splitting of beam for photograph. The failure is more brittle and sudden).

65. Behavior of beams with web reinforcement.
 The failure load for all beams has been increased with stirrups
almost in all cases, regardless the nature of their failure. This is
mainly due to the fact with stirrups are helping to pack the steel
for better and unified resistance. The proper placement of
longitudinal steel ensures more integrated resistance from the
longitudinal steel.
 For beams with ρ <1%, the failure loads are more than the
theoretical flexural capacity of the beams, hence flexural failure
is more prominent here. The cracks angles are relatively
shallower, as the cracks are intercepted by the stirrups. The
crack angles are ranging from 40o to 55o as compared to 50o
to 65o for beams without stirrups. This can be attributed to the
role of stirrups in resisting the shear.
 Typical shear failure has been observed in the beams with ρ
>1% in case of beams with stirrups.

66. Contribution of stirrups in shear strength of
HSRC beams
 The strength of all beams has been substantially increased with stirrups for
all cases of beams. The beams with ρ <1%, which failed due to flexure
previously seems to have been failed due flexure after adding stirrups, yet
the flexural strength of these beams has been increased substantially with
the stirrups.
 The increase is due to better packing of the longitudinal steel by stirrups.
However the crack angles show that the beams have failed without
developing the shear cracks.
 For beams with ρ>1%, the failure is mainly due to shear, but the shear
strength has been increased substantially by stirrups and the increase in the
shear strength of concrete can be attributed both to the shear resisting role
of stirrups and packing of the main steel by stirrups. The shear force at
service load is estimated to be failure load divided by 1.80, to account for
load and partial material safety factor.

67. Effect of longitudinal steel on the shear strength
of HSRC beams.
 As shown in the table, for constant values of a/d, the shear strength of
beams has been increased with the increase in longitudinal steel due to
dowel action.
 The dowel action is more prominent in the beams with web reinforcement as
there is continuous increase in the shear strength of beams with the
increase of longitudinal steel for same value of shear span to depth ratio a/d.
 This increase can be attributed to better packing of the longitudinal steel with
the help of stirrups. Hence the dowel action has been increased when
stirrups has been used. This is an additional advantage. The dowel action
for beams without stirrups is not more consistent.
 Thus it seems not advisable to include the dowel action in the beams without
stirrups in resisting the shear. Only strength contribution of concrete is more
prominent in such beams. This is one of the basic reasons for using
simplified formula of ACI for shear capacity of beams.

68. Effect of longitudinal steel on the shear strength of HSC beams without stirrups
a/d=4.0
a/d=3.0
a/d=3.5
a/d=5.5
a/d=5
a/d=4.5
0
20
40
60
80
100
120
140
160
1% 1.50% 2%
Longitudinal steel ( %)
Shear
strength
of
beam
(
KN)
a/d=3
a/d=3.5
a/d=4
a/d=4.5
a/d=5
a/d=5.5
a/d=6

69. Effect of longitudinal steel on the shear strength of HSC beams with stirrups
0
20
40
60
80
100
120
140
160
a/d=3 a/d=3.5 a/d=4 a/d=4.5 a/d=5 a/d=5.5 a/d=6
Longitudinal steel ( %)
Shear
strength
of
beam
(
KN)
1%
1.50%
2%

70. Effect of shear span to depth ratio a/d to the
shear capacity of beams.
 The shear strength of the beams decrease with
the increase of shear span to depth ratio a/d for
all cases of beams but again the decrease is
more pronounced in case of beams without
stirrups.
 This reveals that the location of the load has
direct effect on the shear strength of beams.

71. Effect of shear span to depth ratio on the shear strength of HSC beams without stirrups
0
20
40
60
80
100
120
3 3.5 4 4.5 5 5.5 6
Shear Span to depth ratio a/d
Shear
strength
of
beam
(
KN)
1%
1.50%
2%

72. Effect of longitudinal shear span to depth ratio on the shear strength of HSC beams
with stirrups
0
20
40
60
80
100
120
140
3 3.5 4 4.5 5 5.5 6
Shear Span to depth ratio a/d
Shear
strength
of
beam
(
KN)
1%
1.50%
2%

73.  The following broader conclusions are drawn from the
above discussions;
 The shear failure is more prominent for HSRC beams
having ρ >1%.
 The stirrups have increased the flexural strengths of
beams in general.
 The flexural and shear strengths of all beams have been
increased with the increase of longitudinal steel.
 The shear failure of HSRC beams is more brittle in case of
beams without stirrups and hence minimum transverse
steel must be provided to check this sudden failure
particularly in the areas prone to earthquakes.
 The shear strength of HSRC beams has been increased
with the increase of compressive strength. However this
increase is not by the amount of increase for NSC-RC
beams.

74.  The shear strength has been decreased with the increase
in the shear span to depth ratio of beams.
 The role of stirrups in resisting the shear of HSRC beams
increases with the increase of compressive strength of
concrete.
 The shear strength of HSRC beams with stirrups was not
equal to sum of the shear strength of HSRC beams without
stirrups and stirrups contributions as assumed by most of
the codes. The obvious stirrup contribution is relatively less
than the theoretical values.
 The deflection of beams has been reduced with stirrups for
a particular section, compressive strength, longitudinal
steel, shear span to depth ratio and loads. Thu the stirrups
has also increased the rigidity of beams.

75. Comparison of actual and Code results

77. General Comments on the provisions of the Codes for
Shear Strength of HSC
1. The provisions of building codes are general non-conservative
for HSC-RC slender
beams having a/d > 4. This is also true for NSC-RC beams, but in
case of HSC-RC
beams, the degree of risk is higher due to small Vtest/Vcode
values on one side and
brittle behavior of HSC beams on the other side.
2. The provisions of Eurocode EC2 are non conservative almost
for all values of the longitudinal steel; hence extra care is
required in designing the HSC-RC beams with EC2 building
code.
3. The provisions of Canadian Code CSA are over conservative
for HSC-RC beams,
where as in case of NSC-RC beams, it is a good predictor of
shear strength.
4. The Provisions of LRFD based on MCFT, appears best
predictor of shear strength of
HSC-RC beams for both types of beams with and without
stirrups.

78. In all codes, the ratio of Vtest/Vcode for HS-RC
beams with stirrups is less than the values for
the beams without stirrups, which can be
mainly due to the fact that shear strength of HS-
RC beams with stirrups is not obtained by
merely adding the individual role of the stirrups
to the shear strength of concrete, as assumed
by most of the codes. It is rather a complex
phenomenon requiring more intelligent
analysis.

79. Conclusion and Recommendations
 The shear failure of HSC-RC beams without web
reinforcement is generally more abrupt and sudden as
there is very little reserve stress available after initiating of
shear cracks and failure of the beams.
 The typical shear failure in beams is more prominent and
phenomenal in beams with longitudinal steel ratio ρ ≥ 1%,
whereas in beams with ρ < 1%, typical flexural failure has
been observed.
 The shear strength of HSC-RC beams has been
increased for both sets of beams with and without web
reinforcement; however the increase is more evident in
beams with web reinforcement. Even the stirrups have
increased the flexural strength of the beams, failing in
flexure. This is attributed to better packing of the
longitudinal steel bars by the stirrups, which leads to
integrated action of the steel bars.

80.  The minimum web reinforcement proposed by ACI 318-
05 code, used in this experimental research has
improved the shear strength of HSC-RC beams and
have avoided the sudden and abrupt failure of the
beams. This has been observed as a major contribution
of the stirrups in HSC-RC beams, unlike NSC-RC
beams. Hence the use of minimum web reinforcement in
HSC-RC can play a vital role, as the reserve strength
and distribution of stirrups along the full length of beams
is an essential requirement, the designers need to follow.
 The shear strength of HSC-RC beams for both types of
beams has been increased, with the increase in
longitudinal steel, normally called dowel effect. This fact
is well illustrated by most of the codes. The increase is
however more in case of stirrups, apparently due to
better packing of steel bars.

81.  The shear strength decreases with increase of shear span to
depth ratio, which is also called size effect for both types of
beams. However this decrease is more pronounced in beams
without web reinforcement.
 The stirrups effectiveness has been observed to increase with
the increase in the compressive strength of concrete in general;
however there seemed no regular and consistent trend in this
increase. The typical approach of most of the codes to view the
stirrups contribution independent of the concrete strength
seems more rational.
 The shear strength of beams with web reinforcement is
assumed as the sum of individual contributions of concrete and
stirrups by most of the building codes. However the
experimental results have not validated this basic assumption.
The shear failure of HSC-RC beams with web reinforcement
normally takes place at much earlier stage before the yielding
of stirrups. Hence additional care is required in spacing of
stirrups for HSC-RC beams.

82.  The provisions of the building codes are generally un-
conservative for the shear design of HSC-RC beams without
web reinforcement at lower level of longitudinal steel and
require special care in application
 The ACI 318-05 provisions for shear design are reasonably
safe for both types of beams with and without web
reinforcement. However for beams without web reinformcent,
the provisions are relatively less conservative, which may be
mainly due to poor predication of the role of stirrups in resisting
the shear.
 The provisions of AASHTO, LRFD based on Modified
Compassion Filed Theory (MCFT) fits reasonably well into the
observed values of shear strength of HSC-RC beams and are
good predictors for shear strength of beams both with and
without web reinforcement. However to make it easier as field
design solution, the simplified Compression Field theory
reported in ACI Structural Journal----- can be a reasonably
good alternative.
 The provisions of Eurocode EC2, adopted in 1992, are poor
predictor for the shear strength for the HSC-RC beams without
web reinforcement.

83. Research work published/Accepted for publication in refereed
Journals from PhD work
 A.Shah,. S.Ahmad., “An experimental investigation into shear capacity
of High Strength Concrete beams” Asian Journal of Civil
Engineering (Building and Housing) Iran, Vol. 8(5) (2007)-pages 545-
558.
 Shah,Attaullah., Ahmad, Saeed., “Effect of Longitudinal Steel, Shear
Span to Depth Ratio on the Shear Strength of High Strength Concrete
Beams” Mehran University Journal of Engineering & Technology Vol
27(2)- April, 2008 Jamshoro UET-Pakistan. HEC approved journal.
 Ahmad, S., Shah.A., Zaman, S., “ Shear Design of four pile caps using
STM” Accepted for publication in Journal of Chinese Institute of
Engineers ( JCIE), in vol.32(2009).
 Ahmad, Saeed., Shah, Attaullah. “Evaluation of Shear Strength of
High Strength Concrete Corbels using Strut and Tie Model (STM)” Final
editing done for publication in Arabian Journal of Science and
Engineering ( AJSE)- Impact Journal.

88. An extract from my PhD thesis dissertations
………..
 PhD studies at UET Taxila had been an enterprising experience of my life
which transformed me from a predominantly Servicing officer into an
academician with more thirst for learning, knowledge and interaction with
scholarly people.
 My PhD supervisor Prof Dr. Saeed Ahmad actively involved me in the
research work of post graduate students, their examination and viva voce
exams right from the beginning and provided me an opportunity to learn
more about the latest trends and developments in the Civil Engineering,
besides my core area of research. In these endeavors I had been able to
work on many projects with him which mainly included, High Range Water
Reducers, (Superplasticizers), Self Compacting Concrete, Very Early
Strength (VES) Concrete, High Strength Concrete (HSC), Retrofitting and
Rehabilitation of the damaged structures etc.
 These efforts on the part of my supervisor enabled me to bridge the
knowledge gap and tackle the PhD studies more seriously and rigorously. I
must appreciate his patience and straightforwardness as I have always
found him a sincere and upright person. He had been very kind throughout
the research work and provided me, his guidance at all stages of my studies.

89. Essence of Survival
Thanking For your patience
“Every morning in Africa, a Gazelle( Small deer) wakes up,
it knows it must run faster than the fastest Lion or it will be
killed.
Every morning a Lion wakes up, it knows it must outrun the
slowest Gazelle or it will starve to death.
It does not matter whether you are a Lion or a Gazelle – when
the sun comes up; You’d better be running.”