Experiment No: 08
Determination of Compressive Strength of Hydraulic Cement Mortars
Introduction
The principal characteristic of hardened cement is its compressive strength. Hence, it is expected that the cement undergoes thorough strength testing in the laboratory prior to its application in significant industrial projects. Strength tests are not conducted on pure cement paste due to the challenges posed by severe shrinkage and consequent breaking of the cement solid. The strength of cement is determined indirectly by the particular amounts of cementitious sand mortar. To determine the compressive strength of mortar, 50-mm (2 in.) cubes are formed and then compressed following the guidelines of ASTM C109. The mortar is made by combining cement, water, and cementitious sand (ASTM C778). ASTM C150 provides specific minimum compressive strength thresholds for various cement kinds at different stages of development. A direct proportionality exists between the compressive strength of mortar cubes and the compressive strength of concrete cylinders. However, the compressive strength of the concrete cannot be predicted reliably from mortar cube strength, as the concrete strength is also affected by the aggregate qualities, the concrete mixing, and the construction strategies.
Scope
This test method involves
determination of the compressive strength of hydraulic cement mortars,
utilizing 50 mm [2 in.] cube specimens.
Purpose
The purpose of this test is to determine the compressive strength
of cement mortars, which is essential for ensuring that the cement used in
construction meets the required strength criteria.
ASTM
Designation
ASTM C109— Standard Test Method for
Compressive Strength of Hydraulic Cement Mortars (Using 50 mm [2 in.] Cube
Specimens).
Terminology
Hydration:
The chemical reaction between
hydraulic cement and water forming new compounds most of which have
strength-producing properties.
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.
Normal consistency:
A degree of plasticity of
a hydraulic cement paste that is appropriate for testing as measured by a
stipulated method.
Compressive Strength:
The compressive
strength of cement mortar is a measure of the mortar's ability to resist
compressive forces or loads. It is defined as the maximum amount of compressive
stress that a mortar specimen can withstand before failure or cracking. This
property is typically expressed in units of force per unit area, such as pounds
per square inch (psi) or megapascals (MPa).
Mortar:
A mixture of cement,
sand, and water used in construction to bond building blocks such as stones,
bricks, and concrete masonry units.
Significance and Use
The
compressive strength test is a fundamental quality control measure for
hydraulic cement. The test results are used to verify that the cement complies
with strength requirements specified in various construction codes and
standards. It also provides valuable information for mix design, ensuring that
the mortar will perform adequately in structural applications. The test is
significant in the early stages of construction as it influences the
durability, safety, and stability of the structures.
This test method provides a means of determining the
compressive strength of hydraulic cement and other mortars and findings may be
used to determine conformity with standards. Further, this test method is
referenced by numerous other standards and test methods. Caution must be used
in utilizing the findings of this test technique to forecast the strength of
concretes.
Apparatus
Reference Masses:
The
physical testing methods for hydraulic cement and related materials must
conform to Class 6 reference masses in Specification ASTM E617. Values from
Table 1 of Specification ASTM E617 for Class 6 metric reference masses:
TABLE 1 Tolerances for Reference Masses
|
Mass, g |
Acceptance Tolerance, ±mg
|
Maintenance Tolerance, ±mg |
|
10000 |
1000 |
2000 |
|
5000 |
500 |
1000 |
|
3000 |
300 |
600 |
|
2000 |
200 |
400 |
|
1000 |
100 |
200 |
|
500 |
50 |
100 |
|
300 |
30 |
60 |
|
200 |
20 |
40 |
|
100 |
10 |
20 |
|
50 |
7 |
14 |
|
30 |
5 |
10 |
|
20 |
3 |
6 |
|
10 |
2 |
4 |
|
5 |
2 |
4 |
|
3 |
2 |
4 |
|
2 |
2 |
4 |
|
1 |
2 |
4 |
Scales and
Balances:
The
maintenance tolerance for a mass determination device should not exceed 0.05%
of the test load throughout the test load range. The mass determination devices
with a capacity that exceeds 1000 g must have a readability of no more than 0.1
g. The index scale of a mass determination device with a capacity of 1000 g or
less must be readable up to 0.01 g.
Glass
Graduates:
200 mL or 250 mL capacity, and conforming to the
Specification ASTM C1005.
Specimen Molds:
Specimen Molds, for the 50 mm [2
in.] cube specimens shall be tight fitting. The molds should contain not more
than three cube compartments and shall be removable into not more than two
portions. The pieces of the molds when joined should be positively kept
together. The molds must be made of strong metal not damaged by the cement
mortar. For new molds the Rockwell hardness number of the metal should be not
less than 55 HRB. The sidewalls of the molds should be sufficiently stiff to
avoid spreading or warping. The inside faces of the molds shall be level
surfaces.
Flat
Trowel:
Having a sharpened straight-edged steel blade
100 mm to 150 mm in length.
Testing Machine: Manual / Automatic
Materials
Graded Standard
Sand:
The sand used for making test specimens shall be natural
silica sand conforming to the requirements for graded standard sand in
Specification C778.
TABLE 1 Standard Sand
Requirements
Temperature and
Humidity
·
The temperature of the
air in the vicinity of the mixing slab, molds, and base plates shall be
maintained at 23.0 ± 4.0 °C.
·
The temperature of the
mixing water used to prepare cement paste and mortar specimens shall be 23.0 ±
2.0 °C.
·
The relative humidity of
the laboratory shall be not less than 50 %. The moist closet or moist room
shall conform to the requirements of Specification C511.
·
The temperature of the
mixing water, moist closet or moist room, and water in the storage tank shall
be set at 23 °C ± 2 °C
Preparation of
Specimen Molds
·
After cleaning molds and
non-absorptive base plates, apply a thin coating of release agent to the
interior faces and apply oils and greases using an impregnated cloth. Wipe the
surfaces with a cloth to remove excess release agent and achieve a thin, even
coating. Spray the release agent directly onto the mold faces and base plate
using an aerosol lubricant, ensuring complete coverage. Wipe the surface with a
cloth to remove excess aerosol lubricant. The residue coating should be enough
to allow a distinct finger print to remain following light finger pressure.
·
Seal the surfaces where the halves
of the mold join by applying a coating of light cup grease such as petrolatum.
The amount should be sufficient to extrude slightly when the two halves are
tightened together. Remove any excess grease with a cloth.
·
To seal molds to their base plates,
use a watertight sealant like microcrystalline wax or a mixture of paraffin wax
and rosin [Ratio 3:5]. Liquefy the wax by heating it to a temperature between
110°C and 120°C. Apply the liquefied sealant at the contact lines between the
mold and its base plate to create a watertight seal.
·
Optionally, a watertight sealant of
petroleum jelly is permitted for clamped molds. Apply a small amount of
petroleum jelly to the entire surface of the face of the mold that will be
contacting the base plate. Clamp the mold to the base plate and wipe any excess
sealant from the interior of the mold and base plate.
Specimen
Preparation Procedure
Composition of
Mortars
The essential components for the standard
mortar consist of cement, graded standard sand, and water. The specific
quantities of materials to be combined simultaneously in the mortar batch for
producing six, nine, and twelve test specimens have to conform to the
guidelines provided in Table 2. Follow the prescribed water content guidelines
for all cements made from Portland, Portland limestone, air-entraining
Portland, or air-entraining Portland limestone.
TABLE 2 Standard Test
Mortar Proportions
|
Materials |
Number
of Specimens |
||
|
6 |
9 |
12 |
|
|
Cement, g |
500 |
740 |
1060 |
|
Sand, g |
1375 |
2035 |
2915 |
|
Water, mL |
|||
|
Portland or
Portland-limestone cements |
242 |
359 |
514 |
|
Air-entraining
Portland or air-entraining Portland-limestone cements |
230 |
340 |
488 |
|
Other cements (to
flow of 110 ± 5) |
… |
… |
… |
Preparation of
Mortar:
Mechanically mix:
Place the dry paddle and the dry
bowl in the mixing position in the mixer. Then introduce the materials for a
batch into the bowl and mix in the following manner:
·
Place all the mixing water in the
bowl.
·
Add the cement to the water; then
start the mixer and mix at the slow speed (140 ± 5 r/min) for 30 s.
·
Add the entire quantity of sand
slowly over a 30-s period, while mixing at slow speed.
·
Stop the mixer, change to medium
speed (285 ± 10 r/min), and mix for 30 s.
·
Stop the mixer and let the mortar
stand for 90 s. During the first 15 s of this interval, quickly scrape down
into the batch any mortar that may have collected on the side of the bowl; then
for the remainder of this interval, close the mixer enclosure or cover the bowl
with the lid.
·
Finish by mixing for 60 s at medium
speed (285 ± 10 r/min).
·
In any case requiring a remixing
interval, any mortar adhering to the side of the bowl shall be quickly scraped
down into the batch with the scraper prior to remixing.
Manual
Mix:
Take standard sand, cement in a
non-porous enamel tray and mix them with a trowel for one minute, then add
prescribed quantity of water and mix the three ingredients thoroughly until the
mixture is of uniform colour. The time of mixing should not be less than 3
minutes nor more than 4 minutes. Immediately after mixing, the mortar is filled
into a cube mould.
Molding Test
Specimens:
Perform the consolidation of the
mortar within the moulds either by hand tamping or by a suitable alternative
technique. Some alternative approaches include, but are not restricted to, the
use of a vibrating table or mechanical apparatus.
Hand Tamping
·
Start the process of molding the
specimens within a maximum duration of 2 minutes and 30 seconds after the first
mixing of the mortar mixture.
·
Apply a 25 mm [1 in.] layer of
mortar that is roughly half the depth of the mold into each of the cube
sections.
·
Tamp the mortar in each cube
compartment 32 times in about 10 s in four rounds, each round to be at right
angles to the other and consisting of eight adjacent strokes across the surface
of the specimen, as illustrated in Fig. 1.
·
The tamping pressure must be just
adequate to guarantee homogeneous filling of the molds. The four rounds of
tamping (32 strokes) of the mortar should be completed in one cube before
continuing to the next.
·
When the tamping of the first layer
in all of the cube compartments is completed, fill the compartments with the
remaining mortar and then tamp as indicated for the first layer.
·
During tamping of the second layer,
bring in the mortar driven out onto the tops of the molds after each round of
tamping by means of the gloved fingers and the tamper at completion of each
round and before beginning the next round of tamping.
·
On completion of the tamping, the
tops of all cubes should extend slightly over the tops of the molds. Bring in
the mortar that has been driven out onto the tops of the molds with a trowel
and smooth off the cubes by dragging the flat side of the trowel (with the
leading edge slightly raised) once over the top of each cube at right angles to
the length of the mold.
·
Then, for the goal of leveling the
mortar and making the mortar that protrudes above the top of the mold of more
uniform thickness, drag the flat side of the trowel (with the leading edge
slightly elevated) lightly once along the length of the mold.
·
Cut off the mortar to a plane
surface level with the top of the mold by dragging the straight edge of the
trowel (held almost perpendicular to the mold) with a sawing action across the
length of the mold.
Fig. 1 Order of Tamping in Molding
of Test Specimens
Storage of Test
Specimens
·
Immediately upon completion of
molding, store the test specimens in the damp closet or moist room.
·
Keep all test specimens, immediately
after molding, in the molds on the base plates in the damp closet or moist room
from 20 h to 72 h with their upper surfaces exposed to the moist air but
protected from dripping water.
·
If the specimens are withdrawn from
the molds before 24 h, keep them on the shelves of the wet closet or moist room
until they are 24-h old, and then submerge the specimens, except those for the
24-h test, in saturated lime water in storage tanks built of noncorroding
materials.
·
Keep the storage water clean by
changing as needed
Determination
of Compressive Strength
·
Test the specimens immediately after
their removal from the wet closet in the case of 24-h specimens, and from
storage water in the case of all other specimens. All test specimens at a
specific test age should be broken within the permissible tolerance provided as
follows:
|
Test
Age |
Permissible
Tolerance
|
|
24
h |
±
½ h |
|
3
day |
±
1 h |
|
7
day |
±
3 h |
|
28
day |
±
12 h |
·
If more than one specimen at a time
is withdrawn from the moist closet for the 24-h tests, maintain these specimens
covered with a wet towel until time of testing. If more than one specimen at a
time is taken from the storage water for testing, maintain these specimens in
water at a temperature of 23 °C ± 2 °C and at sufficient depth to completely
submerge each specimen until time of testing.
·
Wipe each specimen to a surface-dry
condition, and remove any loose sand grains or incrustations from the faces
that will be in contact with the bearing blocks of the testing machine. Check
these faces by applying a straightedge. If there is substantial curvature,
grind the face or faces to plane surfaces or discard the specimen.
·
Apply
the load on specimen faces that were in contact with the real plane surfaces of
the mold. Carefully position the specimen in the testing machine below the
middle of the upper bearing block. Prior to the testing of each cube, it should
be ascertained that the spherically seated block is free to tilt. Use no
cushioning or sleeping materials. Bring the spherically seated block into
uniform contact with the surface of the specimen. Apply the load rate at a
relative rate of movement between the upper and lower platens equivalent to a
loading on the specimen with the range of 900 N/s to 1800 N/s [200 lbf ⁄s to
400 lbf/s]. Obtain this predetermined rate of movement of the platen during the
first half of the anticipated maximum load and make no change in the rate of
movement of the platen in the later half of the loading especially while the
cube is yielding before failure.
Calculation
Record the total maximum load indicated by the
testing machine, and calculate the compressive strength as follows:
fm
= P/A
where,
fm
= compressive strength in MPa [psi]
P = total
maximum load in N [lbf], and
A = area of
loaded surface mm2 [in.2].
Either 2-in. or [50-mm] cube
specimens may be utilized for the determination of compressive strength,
whether inch-pound or SI units are employed. However, consistent units for load
and area must be used to determine strength in the units specified. If the
cross-sectional area of a specimen changes more than 1.5 % from the nominal,
utilize the actual area for the calculation of the compressive strength. The
compressive strength of all appropriate test specimens made from the same
sample and tested at the same period should be averaged and reported to the
nearest 10 psi [0.1 MPa].
Report
The compressive strength of Ordinary
Portland cement for 3, 7 and 28 days.
Faulty Specimens
and Retests
·
In
determining the compressive strength, do not consider specimens that are
manifestly faulty.
·
The
maximum permissible range between specimens from the same mortar batch, at the
same test age is 8.7 % of the average when three cubes represent a test age and
7.6 % when two cubes represent a test age.
·
If
the range of three specimens exceeds the maximum, remove the result which
varies most from the average and verify the range of the remaining two
specimens. Make a retest of the sample if less than two specimens remain after
disgarding defective specimens or disgarding tests that fail to conform with
the maximum permitted range of two specimens.
LAB Assignment Questions:
Basic Understanding
1. What
is the compressive strength of hydraulic cement mortar?
2. Why
is it important to determine the compressive strength of cement mortar?
3. How
does the compressive strength of cement mortar relate to the overall strength
of concrete?
Procedure and Equipment
4. What
is the standard mix ratio of cement, sand, and water used in the compressive
strength test of cement mortar?
5. Can
you explain the procedure for preparing the cement mortar for compressive
strength testing?
6. What
is the role of the cube or prism mold in this test?
7. How
is the mortar placed and compacted in the mold?
8. What
is the purpose of curing the mortar before testing its compressive strength?
9. How
is the compressive strength of cement mortar tested using a compression testing
machine?
10. What
precautions must be taken while removing the mortar specimen from the mold?
Calculation and Interpretation
11. How
do you calculate the compressive strength of the cement mortar from the test
data?
12. What
factors can influence the compressive strength of cement mortar?
13. What
is the significance of the surface area of the specimen in calculating
compressive strength?
14. How
does the curing time affect the compressive strength of cement mortar?
15. What
would you conclude if the mortar shows abnormally high or low compressive
strength?
Standards and Specifications
16. Which
standards (e.g., ASTM, IS) are followed for the compressive strength test of
cement mortar?
17. What
are the standard curing durations for compressive strength testing (e.g., 7
days, 28 days)?
18. What
is the typical compressive strength of Ordinary Portland Cement (OPC) mortar?
19. What
are the allowable tolerances for compressive strength in construction
applications?
Application and Practical Considerations
20. How
does the compressive strength of cement mortar influence the quality of
concrete structures?
21. What
are the potential causes of failure during a compressive strength test?
22. How
can the compressive strength of cement mortar be improved?
23. How
does the water-cement ratio affect the compressive strength of mortar?
24. What
are the practical applications of determining the compressive strength of
hydraulic cement mortars?
25. What
precautions should be taken to ensure accurate compressive strength results?
References:
ASTM C109: Standard
Test Method for Compressive Strength of Hydraulic Cement Mortars (Using 50 mm
[2 in.] Cube Specimens).