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Dr Natalie Lloyd, Senior Lecturer

Andrew Barraclough, PhD Research Candidate

Early Age Tensile & Compressive

Concrete Strength – Impact on anchor pull-out prediction

October 2013

Tensile Anchoring Lateral Tensile Anchoring

Concrete

Load resistance model of an anchor

system in a precast panel

Complimentary Reinforcement Supplementary Reinforcement

• The mechanical interlock of the steel anchors

predisposes a concrete element to a tensile failure

mechanism

• Lifting before 3 days is necessary for prefabricators

• Concrete is more sensitive to tensile failure than

shear failure

• Pure shear failure is hard to create without the

influence of tension

Foot anchor concrete cone Safe Working Load at various concrete strengths

SWL limited by steel

Variables

Anchor embedment

Panel thickness

Distance to the 4 edges

Anchor spacing

Concrete strength

Test data is a collation of results from 4 sources

Used compression tests and cast-in insert pull-out test

as the benchmark comparison

Evaluation of concrete strength tests

- in-direct tensile (splitting)

- uniaxial direct tensile

- compression (cored and moulded)

Application benchmark

Test benchmark

Early age testing compared to the ACI capacity model

77 1.63 – 3.79 2.49 0.4

n, number of Tests Test / Predicted

Range

Average

Test/Predicted

Standard

Deviation

Test/Predicted

AS3600 – Characteristic Uniaxial tensile strength from compressive

𝑓𝑐𝑡′ = 0.36 𝑓𝑐

′

𝑓𝑐𝑡 = 0.9 𝑓𝑐𝑡.𝑠𝑝

Direct tensile strength from compressive strength (Darwin 1996)

AS3600 – Uniaxial tensile strength from splitting test

fct = gt × wc × 𝑓𝑐′

𝑔𝑡 = 0.0069

𝑤𝑐 = Unit Weight of Concrete (kg/m3)

𝑓𝑐𝑡.𝑠𝑝 = 𝑀𝑒𝑎𝑠𝑢𝑟𝑒𝑑 𝑠𝑝𝑙𝑖𝑡𝑡𝑖𝑛𝑔 𝑡𝑒𝑛𝑠𝑖𝑙𝑒 𝑠𝑡𝑟𝑒𝑛𝑔𝑡𝑕 𝑜𝑓

𝑐𝑜𝑛𝑐𝑟𝑒𝑡𝑒, 𝑎𝑠 𝑝𝑒𝑟 𝐴𝑆1012.10 𝑆𝑝𝑙𝑖𝑡𝑡𝑖𝑛𝑔 𝑇𝑒𝑠𝑡

Compression

• Moulded cylinder, ø100mm

• Cored cylinders, ø69mm and ø110mm

Tension

• In-direct splitting, moulded ø150mm

• Uniaxial direct, moulded ø100mm notched

Tension in application

• Cast-in foot anchor, 50mm hef

All tested at 1, 2, 3, 7, 21 and 28 days, using 20MPa and

40MPa concrete design strength

Cast-in

Direct

Splitting

Cored

Concentric alignment is easy to achieve

Non confined specimens. Shrinkage cracks

influence crack propagation

Connection to tensile specimens has been made

easier with the development of epoxy paste

Setup method and time reduces the risk of

adding non-test related cracks (i.e. cores)

• Standard 100mm dia. x 200mm plastic capped cylinders

• Naturally cured (10-25 degrees C)

• Prepared in accordance with AS1012.8

• De-moulded at time of test

• Notched cylinders size: 60mm dia. & 30mm length, 5mm taper

• Compression and tension cylinders stored together

• Compression and tension cylinders poured from the same concrete batch

Un-notched 40MPa tensile result Un-notched 32MPa tensile result

a) Cracks formed at the onset of extension & bridging

b) Throated cylinders typical fracture

a) b)

Typical fracture surface

10

15

20

25

30

35

40

45

50

1 3 7 28

Mean

Co

mp

ressiv

e S

tren

gth

, f c

m (

MP

a)

Time (Days)

Compressive Strength Gain Comparison - Cored v Moulded

Moulded Cylinders

Cored Series1 Panel 1

Cored Series 1 Panel 2

Cored Series 2 Panel 1

Cored Series 2 Panel 2

Cored Series 3 Panel 1

Cored Series 3 Panel 2

20MPa f’c

40MPa f’c Age fcm for f'

c

40MPa

0.36.

√fcm fct.sp fct

1 9.4 1.10 1.9 1.71

2 28 1.90 2.6 2.34

3 33 2.07 3.4 3.06

7 43 2.36 3.2 2.88

21 50 2.55 3.5 3.15

28 50 2.55 3 2.7

Age fcm for f'

c

20MPa

0.36.

√fcm fct.sp fct

1 0.6 0.23 0.1 0.09

2 2.3 0.45 0.5 0.45

3 3.6 0.57 0.6 0.54

7 6.4 0.76 0.9 0.81

21 14.2 1.13 1.5 1.35

28 16 1.20 1.6 1.44

Calculated Uniaxial tensile strength, as per 𝒇𝒄𝒕 = 𝟎. 𝟗 𝒇𝒄𝒕.𝒔𝒑

Age fcm for f'c 40MPa

A

0.36.√fcm

B

gt.√(wc . fcm)

C

fct

1 15.83 1.43 1.35 1.13

2 27.83 1.90 1.78 1.20

3 31.17 2.01 1.89 1.37

7 42.90 2.36 2.21 1.33

21 46.13 2.45 2.30 1.37

Age fcm for f'c 20MPa

A

0.36.√fcm

B

gt . √(wc . fcm)

C

fct

1 3.13 0.64 0.60 0.57

2 7.00 0.95 0.89 0.63

3 10.00 1.14 1.07 0.78

7 14.93 1.39 1.31 1.37

21 21.63 1.67 1.57 1.33

Calculated fct values compared

against tested fct values

A - Characteristic Uniaxial tensile

strength (AS3600) 𝑓𝑐𝑡′ = 0.36 𝑓𝑐

′

B - Direct tensile strength to

compressive strength (Darwin 1996)

fct = gt × wc × 𝑓𝑐′

𝑁𝑜𝑡𝑒: 𝑓𝑐𝑚 𝑢𝑠𝑒𝑑 𝑖𝑛 𝑙𝑖𝑒𝑢 𝑜𝑓 𝑓𝑐′

C – Tested direct tensile strength

0

1

2

3

0 5 10 15 20 25

Ten

sile S

tren

gth

, M

Pa

Concrete Age, Days

Indirect vs Direct Tensile Strength

40MPa Direct Tension 40MPa Split Test

0

10

20

30

40

50

60

0 5 10 15 20 25

Ten

sile

Pu

ll-o

ut

Lo

ad

(k

N)

Days

40𝑀𝑃𝑎 𝑃𝑟𝑒𝑑𝑖𝑐𝑡𝑒𝑑

40𝑀𝑃𝑎 𝑇𝑒𝑠𝑡 𝐴𝑐𝑡𝑢𝑎𝑙

20𝑀𝑃𝑎 𝑇𝑒𝑠𝑡 𝐴𝑐𝑡𝑢𝑎𝑙 20𝑀𝑃𝑎 𝑃𝑟𝑒𝑑𝑖𝑐𝑡𝑒𝑑

Comparison of calculated versus the mean actual pull-out ultimate

load for a hef of 50mm in a 20MPa and 40MPa f’c

Appreciation of the influences that affect concrete strength

Use reliable and repeatable test specimens

Mechanical interlock and adhesion of the course aggregate plays a part in the

early development of tensile capacity

CCD method is conservative enough to be robust

Further extensive direct tensile tests are required to establish a model for

direct tensile strength gain

Correlation to fracture mechanics parameters would assist further

understanding, mainly Fracture Energy and Modulus of Elasticity

Further research on tensile strength gain will be:

- Effects of different concrete mixes in tension

- Effects of temperature controlled curing

- The fracture size effect

- Non-linear FEA modeling

The majority of the tests were conducted in the

ITW Construction systems concrete laboratory at

Chirnside Park, Victoria.

Thanks to ITW Construction systems for their

continued support