Compressive strength of concrete: Out of many test applied to the concrete, this is the utmost important which gives an idea about all the characteristics of concrete. By this single test one judge that whether Concreting has been done properly or not. For cube test two types of specimens either cubes of 15 cm X 15 cm X 15 cm or 10cm X 10 cm x 10 cm depending upon the size of aggregate are used. For most of the works cubical moulds of size 15 cm x 15cm x 15 cm are commonly used.
This concrete is poured in the mould and tempered properly so as not to have any voids. After 24 hours these moulds are removed and test specimens are put in water for curing. The top surface of these specimen should be made even and smooth. This is done by putting cement paste and spreading smoothly on whole area of specimen.
These specimens are tested by compression testing machine after 7 days curing or 28 days curing. Load should be applied gradually at the rate of 140 kg/cm2 per minute till the Specimens fails. Load at the failure divided by area of specimen gives the compressive strength of concrete.
Following are the procedure for Compressive strength test of Concrete Cubes
APPARATUS
Compression testing machine
PREPARATION OF CUBE SPECIMENS
The proportion and material for making these test specimens are from the same concrete used in the field.
SPECIMEN
6 cubes of 15 cm size Mix. M15 or above
MIXING
Mix the concrete either by hand or in a laboratory batch mixer
HAND MIXING
(i)Mix the cement and fine aggregate on a water tight none-absorbent platform until the mixture is thoroughly blended and is of uniform color
(ii)Add the coarse aggregate and mix with cement and fine aggregate until the coarse aggregate is uniformly distributed throughout the batch
(iii)Add water and mix it until the concrete appears to be homogeneous and of the desired consistency
SAMPLING
(i) Clean the mounds and apply oil
(ii) Fill the concrete in the molds in layers approximately 5cm thick
(iii) Compact each layer with not less than 35strokes per layer using a tamping rod (steel bar 16mm diameter and 60cm long, bullet pointed at lower end)
(iv) Level the top surface and smoothen it with a trowel
CURING
The test specimens are stored in moist air for 24hours and after this period the specimens are marked and removed from the molds and kept submerged in clear fresh water until taken out prior to test.
PRECAUTIONS
The water for curing should be tested every 7days and the temperature of water must be at 27+-2oC.
PROCEDURE
(I) Remove the specimen from water after specified curing time and wipe out excess water from the surface.
(II) Take the dimension of the specimen to the nearest 0.2m
(III) Clean the bearing surface of the testing machine
(IV) Place the specimen in the machine in such a manner that the load shall be applied to the opposite sides of the cube cast.
(V) Align the specimen centrally on the base plate of the machine.
(VI) Rotate the movable portion gently by hand so that it touches the top surface of the specimen.
(VII) Apply the load gradually without shock and continuously at the rate of 140kg/cm2/minute till the specimen fails
(VIII) Record the maximum load and note any unusual features in the type of failure.
NOTE
Minimum three specimens should be tested at each selected age. If strength of any specimen varies by more than 15 per cent of average strength, results of such specimen should be rejected. Average of there specimens gives the crushing strength of concrete. The strength requirements of concrete.
CALCULATIONS
Size of the cube =15cm x15cm x15cm
Area of the specimen (calculated from the mean size of the specimen )=225cm2
Characteristic compressive strength(f ck)at 7 days =
Expected maximum load =fck x area x f.s
Range to be selected is …………………..
Similar calculation should be done for 28 day compressive strength
Maximum load applied =……….tones = ………….N
Compressive strength = (Load in N/ Area in mm2)=……………N/mm2
=……………………….N/mm2
REPORT
a) Identification mark
b) Date of test
c) Age of specimen
d) Curing conditions, including date of manufacture of specimen
f) Appearance of fractured faces of concrete and the type of fracture if they are unusual
RESULT
Average compressive strength of the concrete cube = ………….N/ mm2 (at 7 days)
Average compressive strength of the concrete cube =………. N/mm2 (at 28 days)
Percentage strength of concrete at various ages:
The strength of concrete increases with age. Table shows the strength of concrete at different ages in comparison with the strength at 28 days after casting.
Age | Strength per cent |
1 day | 16% |
3 days | 40% |
7 days | 65% |
14 days | 90% |
28 days | 99% |
Compressive strength of different grades of concrete at 7 and 28 days
Grade of Concrete | Minimum compressive strength N/mm2 at 7 days | Specified characteristic compressive strength (N/mm2) at 28 days |
M15 | 10 | 15 |
M20 | 13.5 | 20 |
M25 | 17 | 25 |
M30 | 20 | 30 |
M35 | 23.5 | 35 |
M40 | 27 | 40 |
M45 | 30 | 45 |
Dear Sir,
will you provide me the minimum strength to be gained by the sample of cube for 3 and 7 days.
concrete gains about 50 % strength in 3 days of casting, 70% strength in 7 days of casting and 90% of its strength in 14 days of casting.
Dear Mr.Mishra
I have seen your mail mentioned that the concrete gains about 50 % strength in 3 days of casting, 70% strength in 7 days of casting and 90% of its strength in 14 days of casting. Could you give any reference to check and prove to my management.
Thangaraj
sir,
im dng my m.tech project on concrete.. project name is ” durability of concrete when exposed to base & acid. do u have any projects regarding this.. plz suggest me…
A cube of concrete is crushed to find its compressive strength. If the 150mm cube breaks at a load of 67.5 tonnes, what is its compressive strength?
its compressive strength will be 67.5tonnes divided by area, i.e 67.5/(150×150) tonnes/sq.mm
67.5 tonnes = 67.5×10000 N. so convert it into N/sq.mm
i have 2 cm height x 1.5 cm Length x 1.5 cm wide. Cubic sample. I stand it with height 2 cm. then loading the test and i toke 1.5 tons max Load. how to calculate the Compressive strength because its not same in size. ???? please help me. thankz
compressive strength is calculated by load divided by area perpendicular to that load.
dear sir,
how about the height of sample, I have 3 sample with same mixture, same Area perpendicullar, but different height and maximum load resulted so different. offcourse I think the Compressive Strength are different for 3 samples. can you explain it sir? About height correction.
dear sir,
how about the height of sample, I have 3 sample with same mixture, same Area perpendicullar, but different height and maximum load resulted so different. offcourse I think the Compressive Strength are different for 3 samples. can you explain it sir? About height correction
How can i carringout a project on effect of paint waste water on compressive strength of concrete.
Please i wish to know which elements should core and hammer test should carried being it,foundation,pedestal,roof or base foundation(lean concrete) or columms in accordance with BS standard or Specifications
Thank you
What is the value of compressive strenght of concrete used for constructing roads in INDIA ?
Thank you.
dear sir,
in how many layer need to make cubes for compressive strength test according to BSEN standard, concrete with s4
dear sir,
how many layer need to make cubes for compressive strength test according to BSEN standard, with s4 concrete
What is the value of compressive strength of concrete according to CS I and/or I EN?
sir, what is the minimum and maximum compressive strength of concrete in 7 days of casting?
read answer to first comment on this page..
Concrete slab (12 cm thick) of surface area of about 600 sq meter is to be cast with M20 concrete. Compressive strength is to be tested after 7 & 28 days. How many cube samples should be taken ?
take minimum 6 cubes… 3 cubes for 7 days strength and 3 for 28 days…
Why we are testing concrete cubes after 7,14 or 28 days, why not 2,5,20 days.
we are testing week wise… 1 week, 2 week, 3 week etc. Standard test results were formatted on these days 7 days, 14 days, 28 days etc. We can plot a graph and can find the relative strength on 1 day, 2 day or 20 days also..
Mr. Gopal,
Is there any specific required strenghth shown in/mention in IS or BS code.for example : for 3,7,14,and 28 days in percentage wise.
Hi Nitin,
As per my knowledge, IS Code does not specify 3, 7 and 14 days strength. But it only says 28 days strength.
what is the conversion factor from pounds per sq. mm to newton per sq. mm
sir how much strength we can achieve of concrete cube 15cm.
Hi Gopal Ji,
What is the minimum L/d ratio required for conducting low strain pile integrity test.
Thanks
Nitin K.
Dear SIR,
will you please specify about the standard which says that 50 % is ok for concrete in 3 days compressive strength or any information on compressive strength of poly propylene fibre mixed concrete.in 3 days compressive strength . Regards Sooraj.
for cube of 150mm size and ram dia 165 mm and load is 600kn what is the compressive strength of concrete?
for a civil engineer, C25 means concrete having a compressive strength of 25Mpa but how can i explain this to someone who doesn’t know civil engineering.
what is the conversion factor from 7days compressive strength to 28 days compressive strength of concrete.
what is the conversion factor from 7days compressive strength to 28 days compressive strength of concrete.
please, i need a mathematical relationship between the compressive strength of concrete with respect to the days of curing.i’ll appreciate if i get it early,cos i need it for my project calculation.THANKS
thanx 4 helping me in completing my work
SIR,FOR M-20 GRADE CONCRETE THE STRENGTH IS M-20 @ 28 DAYS, BUT IF @28DAYS THE STRENGTH ACHIVED IS LESS THAN 20 N/MM2 , THEN WOULD THE WHOLE CONCRETING IS REJECTED? , DOES THERE IS ANY CRITERIA FOR THIS PROBLEM
for M20 grade, minimum strength should be 20N/mm2. If its not achieved, then concrete is rejected. Normally for M20, strength you should get is more than 25 N/mm2 for nominal mix concrete if good quality control is achieved.
why 2 samples of concrete cubes? why not 1 sample, why not 3 samples?
While assessing compressive strength of concrete by testing 150mmX150mm cube, shall we take stress at the point of failure noticed (in the form of crack) or allow the stressing till needle of dial gauge moves? Please indicate the relevant code (IS code, if available, would be preferable).
While assessing compressive strength of concrete by testing 150mmX150mm cube, shall we take stress at the point of failure noticed (in the form of crack)?.-Ans:-..The Crack point is the point of failure/breaking point..
What machine is used to measure the same?
sir,
Is adding admixtures to concrete changes the compressive strength ?if so which admixture will yield maximum compressive strength?and please mention their ratio?
please mention acceptance criteria of cube testing after 28 days according to IS codes ?
very usefull
sir,if the top of the cylinder is rough,by how much is the compressive strength reduced?
if top surface of cylinder is rough, then the stress distribution is not uniform over the surface. Then it depends on the amount of roughness.
can u give me minimum ages for handling and transporting for precast concrete. which BS u follow???
means that the strength of curing period??
actually, for precast concrete, members should attain minimum strength for handling and transportation. that minimum strength is calculated based on many factors.
The minimum concrete strength will be based on the calculated concrete stresses at the lifting points, or on calculated stresses caused by the transfer of
prestressing forces or handling. Curing period as I have said earlier is not correct.
actually, for precast concrete, members should attain minimum strength for handling and transportation. that minimum strength is calculated based on many factors.
my question: can u list to me the factors of minimum strength??
go to this site for details about factors.
Click here
Dear Sir,
I would like to know, for compressive strength measurement of cubes in which cement partially replaced with composites, can we use 10 cm mould? Is there a practical advantage of using 15 cm over 10 cm ?
15 cm is the standard mould size and based on that only we get the actual strength of concrete after testing. You can use 10 cm mould, but you should be able to convert the strength values corresponding to 15 cm mould (std mould).
why a cylindrical concrete specimen has about 20 % strength less than the cubic concrete speciment ??
” need the answer ”
when you test a number of standard cube and a number of standard cylinderical specimen of concrete, and draw the normal distribution curve and then calculate the characteristic strength from cube and from cylinder, you will find the following
Fck(cube) = 1.25 Fck (cylinder)
=> Fck of cylinder = 0.8 Fck (cube)
Fck being characteristic strength.
This relation has been obtained from number of tests done on different samples.
dear sir,
one thing iam not understanding i.e why concrete cube size is 150*150*150mm
not 200mm. please reply me asap.
cube size is 150x150x150, because it has been set as standard size, the value of compressive strength of which has been standardized at 28 days.We can also use 200mm cube, but you have to convert the compressive strength of 200mm cube to the equivalent of 150mm cube to know the actual strength of the concrete.
sir my 28 days cube test results for M15 are very from 12n/mm2 to 30 N/mm2 what is the problem in my volumetric mix .
There can be many problems like thorough mixing of concrete, compaction, cement content, water content, size of aggregates etc. These are the main things to consider while mixing concrete. Also, transporting and placing can be a problem, as segregation can take place during transporting and placing. If you take care of your mixing, cement content, aggregate size proportion, water content, compaction etc, you will get good strength concrete.
Prabhat, Can specify the content you took for mixing-i.e cement,aggragate,sand,water, admixture
mr. Gopal
i have confusion. cube gave more strength than cylinder. its ok. but why we use cylinder more or preferrabely in our constructions
you can use cylinder or cube for testing. We generally use cubes in India, may be at your place, you use cylinders for testing. But in India, we define strength as strength of cube after 28 days of casting. So we prefer cubes.
Hi Mr Gopal
How many cube samples should be taken ?
If i m pouring 100m3 of concrete
It depends upon the specifications of the client for whom you work. Generally, you can take 3 cube samples per 20 m3 of concrete.
How many cube samples should be taken ?
read the reply to your earlier comment just above.
thanks
what is the standard value of slump test
values of slump test depends on the type of work.. refer to the link below for standard values of slump tests for different works.
Slump Test & Its Values
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mana compressive strength korang hah?
why the rate of loading is 140 kg/sqr. c.m./min.if anybody knows pls explain me.
Rate of loading is kept uniform, preferably 140kg/cm2/min. If you calculate the same load on 15cm cube per second, it will be around 5.25kN/sec. The load on structure in actual case is not sudden, and impact strength and compressive strength differs, so we take some uniform loading on cubes, such that cube is tested compression but not impact. So, uniform load of 140kg/cm2/min is used which means 5.25kN/sec, i.e. a M20 concrete will fail in 5 to 6 seconds.
Dear sir my calculations says that it will take 1.43 minutes to fail M-20 concrete.
nice post
Thanks, and I need notes about concrete technology books.
thanks. helpful!
I know what will be the cube breaking value for the 28 days, 14 days and 7 days curing of M20 concrete mix..
Very useful
What would be the earliest time to start curing of a slab.
10 hrs.
Tamping rod is 1 inch sq.bar and not bullet pointed at lower end…..pl. conform?
nice post Mr. gopal
cube results will vary and more deviation will found if rate of loading is differ.either pass or fail.
thaks for this.. it helps a lot on our field study…….
If the temperature of water in curing tank of cube is increased, will it effect the strength of cubes…..
Mama over quality person aitiya da?
will effect also if the water temperature decrease and be careful from salty water
hey pradeep ,it will also increase the cube strength
Yes
How much increased the temperature of water in curing tank sir ?
May I know what will be the cube breaking value for the 28 days, 14 days and 7 days curing of M20 concrete mix..
at 7 day you will get probably 80% of your target strength
Russell Barry Independent Materials Testing Brisbane Aust. May I ask which test method is referenced in this guide? Like the site – very useful thks.
We casted Concrete Class 30/20 , and We are getting compressive strength of 28 days lower then 7 days suspected higher Sulphate Content it's other reason?
please what will be the value of a mix concrete in a ratio of 1:2:4 and curing for 28 days.
kindly refer IS-516 for more details………
During testing of cubes, why is the marked face of cube always placed in front?
will the strength of 26 days be different from that of 28 days?
very nice
why we must place some grease into the mould?
so the sand we fill in it does not stick to the mold and can be easily taken out and wighted
cuz we don't want the concrete to stick inside the mold
not grease only shuttering oil and why because by oiling the cubes can easily out from cube mould.
because the concrete should not stick to the mould surface and it can be easily demould
kevvu keka
can u guyz provide me the exact cube crushing strength % increment day by day up to 14 days.thnx.
how does amount of water affect on compressive strength?/.
how to convert 5.2kn/s to140kg/cm2/min.
nice
need help in replacement of fine aggregate with copper slag. has anyone performed the research.
hi guys i'm doing in GBFS in replacement with fine aggregate can u tell me the values after curing fro 3,7,28 days………………
wats the max compression strength achieved by M20 grade concrete @ 7 days & 28 days?
hi , can we use wet aggregate and wet sand to making concreet so the compressivestrength will be effect or not?
yes
thanks
good
so the send we fiill in dstick the mould and can be easily taken out and weightedoes not.
why reinforce concrete is 1:2:4 but not 1:3:6.
This is my final year project! Very funny.
it can useful in psc exam.
Thanks for the info :) dude.
we using m40 to do 6 cubes for my customer, for its 28days cubes test result coming out is average 65.5n/mm , my customer ask me to give reason why cubes strength very abnormal.
what if used are setting cement to make the concrete wont it set before.and stick to the walls.
what about machine mixing.
I like this.This is worth reading.I like my profession.
In concrete compression test, normally 150mm x150mm x150mm concrete cube samples is used for testing. Why isn’t 100mm x100mm x100mm concrete cube samples used in the test instead of 150mmx150mmx150mm concrete cube samples?
This has made my report alot easier..
hai daddy
factors affecting the strength of concrete.
we realy need this limits thanks.
what is the max stress for the road concrete according to BS?
its good for us.
1st good curing & quantity of cement, aggregate, sand, water & admixure then actual process of cube casting, so no chances of cube fail.
percentage of strength attain in the stage of curing……….