concrte Core-testing for compressive strength.pdf

Test smart – Build right
Testing for Concrete Strength
• Quality control and acceptance testing
 Standardized curing conditions
 Indicator of potential strength
 Code compliance
• In-place strength
 Investigate low test results for standard-cured
specimens (cylinders or cubes)
 Determine if constructions loads can be applied
 Evaluate existing structures
1

In-Place Strength
• Evaluation of in-place strength
 Testing cores
 Rebound hammer
 Pullout test
 Pull-off test
 Case studies
 Maturity method
2

Obtaining and Testing Cores
ASTM and ACI Approaches
• Planning
• Apparatus
• Core dimensions
• Moisture conditioning
• End preparation and L/D correction
• Precision
• In-place characteristic strength (ACI 214.4R)
3

Why Take Cores?
• Investigate low test results from standard-
cured specimens
• Develop correlation with other in-place or
nondestructive (NDT) tests
• Confirm interpretation of NDT methods
• Obtain samples for petrographic analysis
• Estimate the in-place “specified strength” for
structural evaluations
4

How Many Cores?
• Will depend on the objective
• For investigating low standard-cured test
results, ACI 318 (design standard) requires at
least three for “area in question”
• For estimating the equivalent in-place specified
strength, a larger number is required,
depending on:
 Variability of in-place strength
 Desired confidence level
5

Where to Take Cores?
• Depends on objective
• For evaluating low standard-cured test results,
take cores from concrete represented by the
low test results
• Forensic investigations:
 Depends on uniformity of concrete in the structure
(ASTM C823/C823M)
• Avoid taking cores from top of placement
(inferior properties)
6

ASTM C823/C823M
7

ASTM C823/C823M—Sampling
• Concrete in structure is similar
 Sampling locations spread randomly or
systematically over the entire structure
 Treat data as belonging to same population
• Concrete in two or more portions likely
to have different properties
 Sample from each portion
 Use statistical methods to establish if
there are differences in properties
8

ASTM C42/C42M
NOTE 1—Appendix X1 provides recommendations for
obtaining and testing sawed beams for flexural performance.
9

ACI 214.4R
10

Some Factors Affecting
Core Strengths
• Core size
• Location of core
• Length-diameter ratio
• End preparation
• Embedded steel
12

Review of ASTM C42/C42M
• Apparatus
• Minimum core diameter
• End preparation
• L/D correction factor
• Presence of reinforcement
• Precision
13

Apparatus for Drilling Cores
• Water-cooled, diamond-impregnated drill bit
• Stable support for drilling machine
• Low feed pressure, high speed
Traditional
Coring
Machine
www.penhall.com
14

Lightweight CORECASE
15

CORECASE Features
Flexible rubber coupling between
drill machine and coring bit;
transfers only torque
Thin walled (2 mm) diamond
bit; less material is cut
Barrel is advanced in axial
direction with no bending.
16

Benefits
• Long coring bit life (~800 cores)
• Coring with little force
(fingertip pressure only)
• Straight cores
• Little space required
• Simple to core in any direction
17

Minimum Core Diameter
• Dmin = 94 mm [3.7 in.] or 2 times nominal
maximum size of aggregate, whichever is
larger Dmin
18

Length-Diameter Ratio (L/D)
• If specified strength based on
cylinder
 Preferred L/D: 1.9 to 2.1
 L/D cannot be less than 1
 For L/D < 1.75, strength correction
required
• If specified strength based on cube
 L/D = 1.0
L
D
19

Moisture Conditioning
• In the past, cores were tested after a period
of air drying or after being submerged for at
least 40 h
• In high w/c concrete, storage under water
for 40 h resulted in saturation
• With modern concrete and lower w/c, storage
under water leads to moisture gradient
20

Research Findings
21

Moisture Gradients
Immediately After Wet Drilling
• Moistened concrete
tends to swell
• Swelling is restrained
by dry interior
• Results in internal
stresses; outer region
in compression
• Measured strength is
reduced
Compression
Tension
22

Joint
Industry
Study
www.cement.org
CT003
23

Effect of Core Conditioning
on Strength
CT003
24

Moisture Conditioning
ASTM C42/C42M
• Wipe off drilling water, surface dry
• Place in watertight containers
• Wait at least 5 days between wetting due to
drilling or sawing and testing
• Other procedure permitted when required by
the “specifier of tests”
25

End Preparation
• Capping with sulfur mortar ASTM C617/C617M)
 Ends of cores have to be relatively flat and close to
perpendicular to core axis
26

End Preparation
• Grinding
 Ends of cores must meet ASTM
C39/C39M requirements for molded
cylinders
 Plane within 0.05 mm [0.002 in.]
 Perpendicular to within 0.5 degrees
• Unbonded caps (ASTM
C1231/C1231M)
 Approved in 2011 for cores
27

Unbonded Caps
Retainer
Rubber Pads
Retainer
Source: PCA

Bonded vs. Unbonded Caps
Sulfur mortar caps Unbonded caps
Source: PCA
29

Metal retainer
Rubber pad
Unbonded Cap
30

Unbonded Cap
• Pad conforms to
end surface
• Retainer prevents
pad from lateral
flow
31

“Flow” of Pad During Test
Retaining ring diameter: 1.02 to 1.07 core diameter
32

• Ends need to be ≈ perpendicular (<0.5°)
• No depressions > 5 mm
• Pad hardness depends on core strength range
33

Testing for Compressive
Strength
• Before capping and testing, measure mass
of core to obtain estimate of density
 In 2011 made mandatory
• Test in accordance with ASTM C39/39M
• If L/D < 1.75, multiply the measured
compressive strength by a strength
correction factor
34

L/D Correction Factor
• Convert measured strength to equivalent
strength for L/D = 2
0.86
0.88
0.90
0.92
0.94
0.96
0.98
1.00
0.8 1 1.2 1.4 1.6 1.8
Strength
Correction
Factor
Length/Diameter
35

Why Do We Need a
Correction Factor?
• The apparent compressive strength of
a cylindrical specimen increases as L/D
decreases
• This is due to the effect of friction
between the ends of the specimens and
the loading plates of the testing
machine
36

Effect of End Friction –
Triaxial Compression
Frictional
Stresses
Zones of
triaxial
compress
ion
37

As L/D Decreases
Strength Increases
38

L/D Correction Factor
• Convert measured strength to equivalent
strength for L/D = 2
0.86
0.88
0.90
0.92
0.94
0.96
0.98
1.00
0.8 1 1.2 1.4 1.6 1.8
Strength
Correction
Factor
Length/Diameter
39

How much embedded steel
is permissible in cores?
40

Core with Steel Bar
Restrains lateral
expansion
Causes stress
concentration
41

Effects of Steel Bar
• Depends on:
 Core diameter
 Bar diameter
 L/D
 Bar location
 Strength level
of concrete
L
D
h
r Bar
42

Limited Research
• Strength reduction due to steel varied
from 0 % to >10 %
• No reliable correction factor has been
developed by ASTM
43

Example
150 x 300 mm Molded Cylinders
Gaynor, R.D., “Effect of Horizontal Reinforcing Steel on the Strength of Molded Cylinders,” Problems and Practices in
Journal of the American Concrete Institute, Proceedings, Vol. 62, No. 7, July 1965, pp. 837-840
44

How much embedded steel
is permissible in cores?
ASTM C42/C42M-10
45

Cores with Steel
• Preferred approach
 Trim core to remove steel
 Maintain L/D ≥ 1.0
• In 2011, text was revised; permitted testing
cores with steel if it can't be avoided:
46
dmin L/D

Some Factors Affecting
Core Strengths
• Core size
• Location of core
• Length-diameter ratio
• End preparation
• Embedded steel
47

Core Strength Acceptance
Criteria
• In the absence of other legal requirements,
specifier of tests should provide the
acceptance criteria
• ACI 318 criteria is for acceptance of in-place
concrete when standard-cured cylinders fail
to meet requirements (ACI 318-14;
26.12.4.1(d))
48

Precision
Single-Operator
Laboratory A Laboratory B Laboratory C Laboratory D
Test
Results
49

Precision
Between-Laboratory
Laboratory A Laboratory B Laboratory C Laboratory D
Test
Results
50

Precision
51

In-Place “Specified Strength”
• In new design, engineer uses the
specified strength, f’c
• In a strength evaluation, need value of
f’c to use in member capacity equations
52

ACI 214.4R
In-place strength Core strength
Correction for L/D
Correction for D Correction for
moisture content
Correction for “damage”
due to coring
Table 9.1 Provides the values of the F-factors
Convert core strength to in-place
concrete strength.
53

• If 4 by 8 in.
core is tested in
standard
condition:
54
1 1 1 1.06
c core
f f
   

In-Place Strength
• Engineers often assume that core strength is
85 % of actual in-place strength
• Or, in-place strength = core strength/0.85
• This is not a rational approach
55
0.85
core
c
f
f 

Equivalent Specified Strength
• K depends on:
 Number of core tests
 Variability of core strengths
 Confidence level
56
'
,
c eq c
f Kf

'
,
c eq c
f Kf

Equivalent
specified strength
Average in-place
strength
Statistical factor

Summary
• Cores can be taken for different reasons
• When taking cores for evaluating in-place
strength, standard procedures must be
followed to obtain comparable results
 Moisture conditioning is very important
 End preparation in strict accordance with
ASTM C42/C42M
• In the absence of governing provisions, the
licensed design professional is responsible
for defining acceptance criteria

Summary
• For strength evaluation of existing
construction, careful planning to select
location and number of cores
 Use NDT to locate “good” and “bad” concrete
 Number of cores depend on variability and
tolerable uncertainty of population mean
• ACI 214.4R provides rigorous method to
obtain equivalent specified strength

concrte Core-testing for compressive strength.pdf

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