Experiment No: 14
Determination of Compressive Strength of Cylindrical Concrete Specimens
Introduction
The measure of compressive strength is the most frequently
performed test on hardened concrete. The compressive strength is a fundamental
structural design criterion that guarantees the ability of the structure to
withstand the designed load. Consistently, the compressive strength increases
as the water-cement ratio decreases. With a direct correlation between the
water-cement ratio and concrete quality, compressive strength is frequently
used as an indicator of quality, including longevity and resistance to
deformation caused by weathering. Hence, designers often require a substantial
compressive strength of the concrete to guarantee superior quality, even if
this strength is not necessary for providing structural support. The
compressive strength f′c of normal-weight concrete ranges from 20 to 40
megapascals (3000 and 6000 pounds per square inch).
Factors
Affecting Compressive Strength of Concrete:
·
Water-Cement Ratio
·
Air entrainment
·
Cement type
·
Aggregate type
·
Mixing water
·
Admixtures
·
Curing conditions
·
Compaction and Workmanship
·
Age of Concrete
· Mix Design
Scope
This
test method covers determination of compressive strength of cylindrical
concrete specimens. The results from this test are primarily used to determine
whether the concrete mixture meets the specified strength requirements for
structural applications.
Purpose
To determine the compressive strength of
cylindrical PCC specimens, such as molded cylinders.
ASTM Designation
ASTM C39—Compressive Strength of Cylindrical Concrete Specimens.
Terminology
hydraulic cement —
a
cement that sets and hardens by chemical reaction with water and is capable of
doing so under water.
Portland cement—
a
hydraulic cement produced by pulverizing clinker, consisting essentially of
crystalline hydraulic calcium silicates, and usually containing one or more of
the following: water, calcium sulfate, up to 5 % limestone, and processing
additions.
Workability –
Workability of
concrete is defined in ASTM C-125 as the property determining the effort
required to manipulate a freshly mixed quantity of concrete with minimum loss
of homogeneity. The term manipulate includes the early-age operations of
placing, compacting, and finishing. The effort required to place a concrete
mixture is determined largely by the overall work needed to initiate and
maintain flow, which depends on the rheological property of the lubricant (the
cement paste) and the internal friction between the aggregate particles on the
one hand, and the external friction between the concrete and the surface of the
formwork on the other.
water-cement ratio —
the
ratio of the mass of water, exclusive only of that absorbed by the aggregates,
to the mass of portland cement in concrete, mortar, or grout, stated as a
decimal.
Concrete —
a
composite material that consists essentially of a binding medium within which
are embedded particles or fragments of aggregate; in hydraulic-cement concrete,
the binder is formed from a mixture of hydraulic cement and water.
concrete, fresh —
concrete
that possesses enough of its original workability so that it can be placed and
consolidated by the intended methods.
concrete, hardened —
concrete
that has developed sufficient strength to serve some defined purpose or resist
a stipulated loading without failure.
Consistency — (fresh cementitious mixture)
the relative mobility or ability to flow.
Curing —
action
taken to maintain moisture and temperature conditions in a freshly-placed cementitious
mixture to allow hydraulic cement hydration and (if applicable) pozzolanic
reactions to occur so that the potential properties of the mixture may develop.
Pozzolan —
a
siliceous or siliceous and aluminous material, which in itself possesses little
or no cementitious value but will, in finely divided form and in the presence
of moisture, chemically react with calcium hydroxide at ordinary temperatures
to form compounds possessing cementitious properties.
cementitious material (hydraulic) —
an
inorganic material or a mixture of inorganic materials that sets and develops
strength by chemical reaction with water by formation of hydrates and is
capable of doing so under water.
Fracture:
The
failure of the specimen under compression load.
Significance and Use
Good judgment is necessary when
interpreting the importance of compressive strength measurements obtained by
this test technique, as strength is not an inherent or essential characteristic
of concrete composed of specific elements. The values obtained will be
influenced by several factors, including the dimensions and form of the
specimen, the processes of batching and mixing, the techniques of sampling,
molding, and fabrication, as well as the age, temperature, and moisture
conditions throughout the curing process. The outcomes of this testing method
serve as a foundation for quality control of concrete proportioning, mixing,
and placing operations; assessment of conformity to technical requirements;
evaluation of the effectiveness of admixtures; and other related applications.
The compressive strength of concrete
is a critical measure of the material's ability to withstand axial loads. It is
used for:
·
Quality
Control: Verifying
that concrete delivered to a project meets specified strength requirements.
·
Structural
Design: Providing
engineers with essential data to ensure that a concrete structure can safely
bear loads.
·
Material
Evaluation: Comparing
different concrete mixtures for their relative strengths.
·
Compliance: Meeting regulatory and
construction codes, ensuring that the materials used in construction align with
project specifications.
Apparatus
Testing Machine:
The testing machine shall be of a
type having sufficient capacity and capable of providing the required rates of
loading.
Bearing
Blocks:
The upper and lower bearing blocks.
The dimensions of the
bearing face of the upper bearing block shall not exceed the following values:
|
Nominal
Diameter of Specimen, mm [in.] |
Maximum
Diameter of Round Bearing Face, mm [in.] |
Maximum
Dimensions of Square Bearing Face, mm [in.] |
|
50
[2] |
105
[4] |
105
by 105 [4 by 4] |
|
75 [3] |
130 [5] |
130
by 130 [5 by 5] |
|
100 [4] |
165 [6.5] |
165
by 165 [6.5 by 6.5] |
|
150 [6] |
255
[10] |
255
by 255 [10 by 10] |
|
200 [8] |
280 [11] |
280 by 280 [11 by
11] |
The lower bearing block shall be at
least 25 mm [1.0 in.] thick when new, and at least 22.5 mm [0.9 in.] thick
after resurfacing.
Load
Indication:
The testing machine shall be equipped with either a dial or digital load indicator.
Specimens:
·
Specimens
will not undergo testing if any individual diameter of a cylinder deviates from
any other diameter of the same cylinder by an amount exceeding 2%.
·
Prior
to testing, test specimens must not exceed a deviation of 0.5° from
perpendicularity to the axis, which is equivalent to 1 mm in 100 mm [0.12 in.
in 12 in.].
·
Compression
test specimens that are not level within a range of 0.050 mm must undergo
sawing, grinding, or capping as detailed in Practice ASTM C617/ASTM C617M or,
if allowed, Practice ASTM C1231/ASTM C1231M.
·
The
cross-sectional area of the test specimen must be measured to the closest 0.25
mm [0.01 in.] by averaging two diameters measured at right angles to each other
at approximately mid height of the specimen.
·
Wipe
off any excess moisture from the surface using a towel and determine the mass
of the specimen.
·
When density determination is not
necessary and the length to diameter ratio is less than 1.8 or more than 2.2,
measure the length of the specimen to the nearest 0.05 D.
Procedure
·
Once
moist-cured specimens have been removed from moist storage, compression tests
should be conducted as soon as possible. The test specimens must be maintained
moist using any appropriate method throughout the time between their removal
from moist storage and conducting testing. Their compressive test shall be
tested in moist conditions.
·
Tolerances
for specimen ages are specified below:
|
Test
Age |
Permissible
Tolerance |
|
24 h |
±0.5
h |
|
3 days |
±2
h |
|
7 days |
±6
h |
|
28 days |
±20
h |
|
90 days |
±2
days |
· Placing the Specimen:
Place the lower bearing block on the
testing machine's table or platen, wiping clean the bearing faces of upper and
lower blocks, spacers if used, and specimen. If using unbonded caps, wipe clean
the retainer surfaces and center them on the specimen. Place the specimen on
the lower bearing block and align its axis with the upper bearing block's
center of thrust.
·
Zero
Verification and Block Seating:
Before
testing a specimen, ensure the load indicator is set to zero and adjust it if
necessary. After placing the specimen in the machine, gently tilt the movable
block to ensure the bearing face is parallel to the top of the test specimen
before applying the load.
·
Rate
of Loading:
Apply the
load continuously and without shock. For screw-type machines, use a rate of
loading of 1 mm/min (0.05 in./min). For hydraulically operated machines, apply
the load at a constant rate within the range of 0.15 to 0.35 MPa/s (20 psi/sec
to 50 psi/sec). During the first half of the anticipated loading phase, a
higher rate of loading is permitted. No adjustment in the control of the
testing machine should be made while the specimen is yielding rapidly,
immediately before failure.
· Apply
the load until the specimen fails, and record the maximum load carried by the
specimen during the test. Note the type of failure and the appearance of the
concrete.
Calculation
·
The
compressive strength of a specimen can be determined by dividing its maximum
load carried by the specimen during testing by its average cross-sectional
area.
·
If
the specimen length to diameter ratio is less than 1.8, correct the obtained
result by multiplying the appropriate correction factor as shown in the table.
|
L/D |
Factor |
|
1.75 |
0.98 |
|
1.50 |
0.96 |
|
1.25 |
0.93 |
|
1.0 |
0.87 |
·
Note: The correction factors are
applicable to lightweight concrete weighing between 100 and 120 lb/ft3 and
normal weight concrete, and are applicable to concrete dry or soaked at loading
time. They are also applicable to nominal concrete strengths from 2000 to 6000
psi [15 to 45 MPa]. Values not given in the table shall be calculated by
interpolation.
Report:
1. Identification number
2. Diameter (and length, if outside the
range of 1.8D to 2.2D)
3. Cross-sectional area
4. Maximum load
5. Compressive strength calculated to
the nearest 10 psi [0.1 MPa]
6. Type of fracture, if other than the
usual cone (Fig. 1)
7. Defects in either specimen or caps,
and,
8. Age of specimen
Fig.1: Typical fracture pattern of
concrete cylinders: (1) cones (2) cone and vertical cracks (3) vertical cracks
(4) shear (5) side fracture (6) side fractures with pointed ends (7) cone and
shear
Lab
Questions:
Fundamental
Questions:
1.
What is compressive strength, and why
is it important in concrete?
2. How
does compressive strength relate to the overall performance of concrete in
structures?
3.
What are cylindrical concrete
specimens, and why are they used for compressive strength testing?
4.
What is the standard size of
cylindrical concrete specimens used for compressive strength testing?
5. Why
are specific sizes (e.g., 150 mm x 300 mm or 100 mm x 200 mm) used?
6.
Why is compressive strength measured at
specific intervals such as 7 days and 28 days?
7. What
is the significance of these curing periods?
Procedure-Related
Questions:
8. Can
you describe the procedure for testing the compressive strength of cylindrical
concrete specimens?
9.
How is the load applied to the concrete
cylinder during the test?
10. What
is the rate of loading, and why is it important?
11. What
is the purpose of capping or grinding the ends of the concrete cylinder before
testing?
12. How
does this affect the accuracy of the test?
13. How
do you calculate the compressive strength of a cylindrical concrete specimen?
14. What
is the formula used for calculating compressive strength from the test results?
15. Why
is it important to ensure that the axis of the specimen is aligned with the
axis of the loading machine?
16. What
are the key steps in preparing and curing the concrete specimens before testing
for compressive strength?
Interpretation and
Results:
17. What
is a typical range of compressive strength for normal concrete?
18. How
does high-strength concrete differ in terms of compressive strength?
19. How
do you interpret the test results if the concrete cylinder fails prematurely or
at a lower strength than expected?
20. What
could cause low compressive strength in a concrete specimen?
21. How
do curing conditions affect the compressive strength of concrete?
22. What
are the reasons for observing cracks before reaching the ultimate load during
compressive testing?
23. What
is the significance of the modulus of elasticity in relation to compressive
strength testing?
24. How
does it affect the behavior of concrete under load?
Factors Affecting
Compressive Strength:
25. What
factors influence the compressive strength of concrete specimens?
26. How
do water-cement ratio, aggregate size, and mix design impact strength?
27. How
does the temperature and environment during curing affect the compressive
strength of concrete?
28. What
is the impact of air entrainment on compressive strength?
29. How
does improper compaction of the concrete mix influence the compressive
strength?
30. What
could cause variability in the results when testing different cylinders from
the same batch of concrete?
Equipment and
Safety:
31. What
equipment is used for testing the compressive strength of concrete cylinders?
32. What
is the role of the universal testing machine (UTM) in this process?
33. What
are the safety precautions that should be followed while performing the
compressive strength test?
34. Why
is it important to regularly calibrate the testing machine used for compressive
strength testing?
35. How
do you ensure that the testing machine is applying the load uniformly across
the surface of the specimen?
Standards and
Quality Control:
36. Which
standard or code is followed for determining the compressive strength of
cylindrical concrete specimens?
37. Can
you explain the key requirements of ASTM C39 for this test?
38. What
is the significance of quality control when preparing and testing concrete
cylinders for compressive strength?
39. What
are the limitations of the compressive strength test on cylindrical specimens?
40. How
do you compare the results of compressive strength from lab-cured specimens
versus field-cured specimens?
References
ASCE C39: Standard Test Method for Compressive Strength of Cylindrical Concrete Specimens.