University of Nairobi
Department of Civil and Construction Engineering
Name: Owino Elijah Ochieng’
Reg. no: F16/1374/2011
Course Title: Civil Engineering Materials
Course Code: FCE 246
Concrete laboratory report: Testing of dry (hardened) concrete.
Strength of concrete could be defined as the ultimate load that causes failure or its resistance to rapture.
Its units are force units divided by area (N/mm2). Strength of concrete is not very directly affected by
workability, but by the method of manufacture, water/cement ratio, effective water in the mix, gel/space
ratio, aggregate/cement ratio, properties of coarse aggregate, curing and age of the concrete have direct
effect on the strength. Most concrete mixes attain over 70% of their strength after 7 days and almost
maximum strength after 28 days.
To determine the compressive strength of concrete.
To determine the tensile strength of the same concrete.
There are various types of concrete strength, the depending on how the load is applied and the type of
loading. These are:
Compressive strength – compressive strength of a concrete specimen treated in a standard manner which
includes full compaction and wet curing for a specified period give results representing the potential
quality of the concrete. The test is performed using either a concrete cube (usually 150mm by 150mm) or
cylinder (150mm diameter by 300mm height). Compressive strength obtained from cylinder is 0.8 times
that obtained from the cube. There are three types of loading in compression test:
Uniaxial loading – represents the most conservative system and yields the lowest values in
compression. There are three types of failure in uniaxial test; tension (splitting) failure, shear
(sliding) failure, combined (tension and shear) failure.
In compression test, tangential forces are being developed between the end surfaces of the concrete
specimen and the adjacent steel platens of the testing machine. These forces will cause lateral expansion
in concrete. The steel platen will restrain the lateral expansion of the concrete in the parts of the specimen
near its ends; the degree of restraint exercised depends on the friction actually developed.
Tensile strength – although concrete is not designed to resist direct tension, the knowledge of tensile
strength is of value in estimating the load under which cracking will develop. There are two types of test
for strength in tension;
Direct tensile test – is the application of a pure tension force free from eccentricity.
Splitting tension test – is where a concrete cylinder is placed with its axis horizontal between the
platens of a testing machine and the load is increased until failure by indirect tension in the form
of splitting along the diameter takes place.
Flexural strength – is where a plain concrete beam is subjected to flexure using symmetrical two-point
loading until failure occurs.
The cubes and cylinders prepared after workability tests were got after curing for 14 days. The cubes were
divided into four parts on each of the smooth surfaces. The cube were then put in the compression
machine and loaded until it crushes, and the crushing forces recorded. This step was repeated for two
more cubes. One of the cylindrical concrete was then placed vertically on the compression machine and
the crushing load forces recorded. The mode (mechanism) of crushing was also noted. The remaining
cylinder was then placed on the machine horizontally, and the force required to split the cylinder along
the centroid measured.
Cube 1: Force P1 = 430KN
Cube 2: Force P2 = 440KN
Cube 3: Force P3 = 420KN
Average Force P = 430KN = 4.3*105N
All the cubes crushed normally.
Cylinder 1: Force P = 260KN, mode of crushing was across the cylinder, that is, shearing.
Cylinder 2: Force P = 145KN
Cross section area A of the cubes = 150*150 = 22500mm2
Compressive strength 1 = (average force P) / (cross section area A)
= 4.30*105/22500 N/mm2
= 19.11 N/mm2
Compressive strength 2(for the cylinder)
Cross sectional area A of the cylinder = PI*D2/4 = 3.142*150/4mm2
Compressive strength 2 = force P/cross sectional area
= 2.6*105/17671.56 N/mm2
= 14.71 N/mm2
Tensile strength = 2P/ (PI*D*L)
= 2*145000/ (3.142*150*300) N/mm2
= 2.05 N/mm2
The compressive strength of the cube was found to be 19.11N/m2 while that of the cylinder was
14.71N/m2. The ratio of compressive strength of cylinder to that of the cube is 14.71/19.11,
approximately 0.8 as was expected, but this varies from one grade of concrete to the other. The tensile
strength was found to be 2.05N/m2, which is approximately 10% of compressive strength of the cube.
Since the actual strength of the concrete was 20N/m2 given the concrete mix proportion of 1:1.5:3 (grade
M20), the variation could have been due to the friction between the platens of the compression machine
and the test specimen. The loss in strength could have also been due to improper mix proportioning;
effective water/cement ratio, cement/aggregate ratio, gel space and moisture content during testing.
The tensile strength of concrete is very much lower than the compressive strength. This shows that
concrete is more brittle and almost non-ductile and thus should be subjected to tensile loading, unless
reinforced with steel, thus the knowledge of the behavior under tension is vital.
This experiment can be considered successful since the aims were achieved. The compressive strength
and tensile strength of the concrete were obtained and the relationship between the two were ascertained.
It was established that the tensile strength is 10% of the compressive strength thus concrete is weak in
tension and strong under compression due to its brittleness.
1. S. K. Duggal, Building Materials, Third revised edition, 2008, New Age International Publishers.
2. A. M. Neville and J. J. Brooks, Concrete Technology, third edition, 2011, Pearson Education