Question
Asked 20th May, 2015

How can I interpret compressive strength of concrete at 28 days?

It is known that; many of specification stipulates that samples compressive strength of concrete must be at least "fck+delta (MPa)" at 28 days.
But;
1) In-situ construction of cure condition is different from the samples.
2) Hydration improves strength continuously at very long period and It is possible that even if strength is low, it may reach value of 28 days in the long run.
So, how can I interpret compressive strength of concrete at 28 days?

Most recent answer

18th Aug, 2018
Vito Alunno Rossetti
Sapienza University of Rome
Compressive strength of concrete at 28 days doe's not exist.
Make one defined mix of at least water, aggregate and cement, make cylinders and cubes and compact according to your preferred method; make a structure and take cores, chose a method of curing cubes, cylinders and the structure (a temperature and humidity), put the specimens in some testing apparatus, chose a rate of loading and crush at 28 days . If you can, try with multiaxial loading. Don't forget to register the results.
How many different strength values you can get ? and how many if we look for a characteristic strength?
Of course you can adopt also one or more Standard testing methods, so reducing or increasing the number of results, but,
is any result THE compressive strength of that concrete ?

Popular answers (1)

25th May, 2015
Namasivayam Ganesan
National Institute of Technology Calicut
The Ordinary Portland Cement and Portland Pozzolana cements used in making concrete have been found to develop about 80% of compressive strength of concrete at 28 days. Hence this period was thought to be a reasonable period and accepted by many people and incorporated in the code of practice of countries  so that the engineers  can take a decision regarding the quality of concrete in the structure. In case the strength of control specimens like cubes fail to give the compressive strength required for the Grade for which it is designed, it is the responsibility of the QC Engineer to give give instructions to the buider whether to go ahead with the construction or not based on Acceptance Criteria of the Code or conducting necessary NDT tests such as Ultrasonic or Rebound hammer tests . Sometimes QC engineer has to resort to  semi destructive tests such as Core tests. However, the present day concretes are not ordinary concretes , they are High Performance Concretes which consists of both mineral an chemical admixtures in addition to the conventional ingredients. In the case of HPC, more time is required for total hydration to to take place. This total period may be about 56 or 100or 150 days or a year which depends on the type of mineral and chemical admixtures used. Since one cannot wait for a longer period to decide the quality of concrete in various stages of construction, a four week period as adopted by our predecessors can be followed  for both normal and HPC. Also the strength attained beyond 28 days certainly add to the safety of the structure since structures are designed using a specified Grade of concrete based on 28 days strength. However, for fast track construction, one need not wait for 28 days  and accelerated curing  tests   can be made use of for taking appropriate decisions.
3 Recommendations

All Answers (21)

20th May, 2015
Samuel Chuah
Monash University (Australia)
Its simply strength = peak load/area
I recommend ASTM C39
2 Recommendations
20th May, 2015
Paul Barnes
University College London
My understanding is as follows:
Yes the strength of concrete generally continues to improve with time,
sometimes for up to 50 years or more, as the cement phases continue to
hydrate deeper and deeper within the individual grains. But the point of
setting a specific time - 28 days - allows one to make some kind of
like for like comparison between different cements without having
to wait years for the full concrete strength to be measured. The fact
that one concrete might achieve another concrete's 28 day strength after,
say, 2 years is irrelevant because that is no longer a like for like
comparison. Also 28 days is a not an unreasonable length of time to wait,
operationally, so if specification is achieved by 28 days it should
continue or exceed specification thereafter. By standardising every type
to 28 days, the long term values become effecively written in to the
comparison.
1 Recommendation
21st May, 2015
Yavuz Abut
Yalova Üniversitesi
Problems briefly as follows:
1) Fresh concrete samples was taken during construction.
2) Quality of concrete was performed by statistical analysis.
3) For providing 7 days strength of concrete, construction’s molds were removed
4) But at the end fo the 28 days, compressive strength was under the limit.
5) Taking core samples were extracted at the end of 54 days, but the results were under the limit again.
again.. again.. again..
6) At the end of 150 days it was finally achieved compressive strength.
Thus, the researchers realized that the problem of the origin was a superplasticizer.
So, why should 28-day compressive strength  be significantly ?
21st May, 2015
Hisham Qasrawi
Hashemite University
In my opinion, the 28-day compressive strength is only a guide for future progress in the project. In my opinion, the 28-day strength is affected by several factors. These factors can affect the final strength of concrete. For example, curing at higher temperatures will attain the 28-day strength at lower ages but tend to lower the strength at later ages. The type of cement is a critical factor in deciding the 28-day and final strength. The 28-day strength may not have a significant value when pozzolans, slags and (or) silica fume are introduced.
However, to keep things tidy, we have always to refer to something.  A certain strength-age relationship is necessary.
As a further comment, in my opinion,  even the cores do not actually represent the actual strength of concrete in the structure. Many factors affect their results (See literature)
2 Recommendations
21st May, 2015
Naser Kabashi
faculty of civil engineering and architecture,Kosovo
From my opinion the Compressive strength at 28 days is just the verification at the beginning stage in properly maintenance conditions , but is not finally value because the effect of many constituent is oriented in 56 or 90 days Compressive strength for evaluations.
1 Recommendation
22nd May, 2015
Marcos de Oliveira Valin Jr
Instituto Federal de Educação, Ciência e Tecnologia de Mato Grosso (IFMT)
Healing has influence on volume stability of concrete, improves its mechanical performance and their resistance to aggressive agents of the environment.
It is assumed that provides good cure made beyond the desirable characteristics of a concrete piece, strength and durability, a reduction in maintenance costs and also decrease waste from this material.
The job site directly influence the healing process. Very hot environments are to occur accelerating evaporation of the water of the concrete piece, thus requiring a more stringent control of cure.
The greater the care cure the concrete, the better its mechanical performance and its resistance to aggressive environmental agents, but it is known that not all the execution conditions often this procedure is not performed, and the results of testing on the samples do not show the reality of the chassis, and the ineffective cure reduces by 30% the resistance value when compared to cured specimens.
23rd May, 2015
Abdurrahman Güner
Istanbul University
The 28-day (4-week) standard (cured under standard humidity and temperature and having standard dimensions) specimen strength has been adopted for the acceptance control of concrete delivered to the construction site. It is based on the strength gain pace of the Ordinary Portland Cement or CEM I (of classical properties) concrete of appropriate composition, that gains about 70% of its 28-day strength at 7 days, and gains about 85% of its final (at the end of about a year) strength at 28 days. This information is based on the old official specifications. Therefore, the 28-day strength of a concrete mix represents the strength of concrete that can be achieved if properly cured. This duration of 28 days has been adopted and accepted as a reasonable period at the end of which a sound, reliable decision can be made about the strength development of concrete composed of constituents conforming to the relevant specifications. 
However, the strengths of most part of the concrete in the formwork kept under the same hygrothermal conditions as those of the standard specimens is known to exceed those of the standard specimens. This is partly due to the fact that the concrete properly placed and compacted in-situ contains less air voids and less vulnerable to mishandling and improper curing than the small standard specimens, excluding perhaps the concrete slabs.
"Site specimens" kept near the structural elements, or "break-off specimens" connected to the element via an opening, which allows a representative portion of the concrete cast into the formwork to fill a compartment connected to it are used for monitoring the strength development in-situ for determining the proper time for removal of the formwork and scaffolding.
If CEM II, CEM III or CEM IV binders are used, the rate of strength development is  lower; the strength gain ratios beyond 28 days are significantly higher than those of CEM I concretes.
In in-situ cast concrete construction practice, lower Portland klinker content binders are specified, taking into consideration especially the crucial thermal and hydraulic shrinkage problems. This results in lower rates of strength gain rates and a need for the specification of later ages for determination of the characteristic strengths (which are actually going to be needed at ages probably much later than 28 days). However, adoption of later ages for determination of characteristic strength in construction practice will require official agreement statement in the contract.
See
for information.
2 Recommendations
23rd May, 2015
Samuel Chuah
Monash University (Australia)
To answer the 2nd part of your question, the difference in compressive strength can be explained from a few perspectives. 
Chemical composition of cement: if cement contains high alite content,C3S, then u can expect a high early strength as happened in 7 days. The compensated reduction in belite, C2S means your conc rete has a lower strength at late ages. A similar effect can happen with superplasticisers whereby the SP accelerates the formation of alite hydration but delays the C2S hydration.
To overcome these problems in te future, judicious selection of SP and pilot tests are required. Aggregates may also play a role in late strength development. Above all, appropriate curing practice is vital to ensure your concrete continues to gain strength up to and beyond 28 days
2 Recommendations
25th May, 2015
Namasivayam Ganesan
National Institute of Technology Calicut
The Ordinary Portland Cement and Portland Pozzolana cements used in making concrete have been found to develop about 80% of compressive strength of concrete at 28 days. Hence this period was thought to be a reasonable period and accepted by many people and incorporated in the code of practice of countries  so that the engineers  can take a decision regarding the quality of concrete in the structure. In case the strength of control specimens like cubes fail to give the compressive strength required for the Grade for which it is designed, it is the responsibility of the QC Engineer to give give instructions to the buider whether to go ahead with the construction or not based on Acceptance Criteria of the Code or conducting necessary NDT tests such as Ultrasonic or Rebound hammer tests . Sometimes QC engineer has to resort to  semi destructive tests such as Core tests. However, the present day concretes are not ordinary concretes , they are High Performance Concretes which consists of both mineral an chemical admixtures in addition to the conventional ingredients. In the case of HPC, more time is required for total hydration to to take place. This total period may be about 56 or 100or 150 days or a year which depends on the type of mineral and chemical admixtures used. Since one cannot wait for a longer period to decide the quality of concrete in various stages of construction, a four week period as adopted by our predecessors can be followed  for both normal and HPC. Also the strength attained beyond 28 days certainly add to the safety of the structure since structures are designed using a specified Grade of concrete based on 28 days strength. However, for fast track construction, one need not wait for 28 days  and accelerated curing  tests   can be made use of for taking appropriate decisions.
3 Recommendations
26th May, 2015
Jeff Stein
SRA International Inc.
I agree with the answers above.  It is important remember that 28 day strength is based on laboratory mix design and laboratory curing.  If you make concrete cylinders in the field, then you should be able to transport these to the lab and cure them for 28 days in a water tank or moist curing room and meet or exceed the mix design strength.  However if you want to know the field strength of the concrete you have to cure the cylinders in the field under the same conditions as the concrete structure. 
1 Recommendation
26th May, 2015
Tamás Simon
Budapest University of Technology and Economics
There are several interesting and correct answers above. In the mean time my opinion is that, one should always consider the followings:
1. 28th day compressive strength of concrete is only a legal value, based on which calculations, QC and acceptance (payment in many cases) is made.
2. The characteristic compressive strength obtained from laboratory testing        (fck=fm-delta) should be higher than the wrongly named characteristic strength in the sign of the concrete which is specified by the designer. In the sign of concrete the name is wrong, it should be required strength, instead of characteristic.
3. As it is written above, fck obtained from on site poured samples is problematic, because it has no real physical connection with the concrete in the structure. To my understanding it only shows (if pouring, compaction, and curing is proper) if out of that mixture it is possible to make concrete which is from strength point of view according to the specification.
So if the concrete of the questioned structure at the age of 28 days did not reach the specified (required) strength, it means something went wrong, the safety of post hardening (which is the further hardening after 28 days) is lost, meaning that the value of the structure is less than could (or should) have been if the specified strength value was reached.
Please also find:
26th May, 2015
Roland Travnicek
Expert Office Dr Travnicek
Please be aware of the specific weight (depending from air voids) See also the European Standard EN 13791 Appendix A 
1 Recommendation
It is the maximum force which concrete can tolerate divided by area after staying for 28 days in the water.
The procedure is that they put the specimens in some specific device and put two vertical forces on the top and bottom of the concrete. Then the force will increase until the specimen breaks. That force is the considered force for measuring the compressive strength of the concrete. I have the pictures of this procedure in all my papers. Although you might not understand the language of some of them, because they are in Persian, you can follow the pictures and see what is happening. Also there are two more English papers which will be published soon by me. In both of them, you can see more comprehensive answer of your question.
I have attached two pictures of what I have explained for your perusal.
I hope my answer is helpful for you.
1 Recommendation
30th May, 2015
Sayed Mohammadmehdi Jamal
Hormozgan University
The 28 days compressive strength of concrete is the most popular one, because of the time economy. Although the compressive strength with more days than 28 days would be more reliable.  I hope I can have answered your question as your desired. 
30th May, 2015
Derek Mostert
University of Pretoria
To answer from a Concrete Technologists point of view! If an Engineer think that the concrete in the structure will or should be the same as the water cured specimens at a fix temperature are the same, Something in his/her training went horribly wrong! The Characteristic strength is the strength the engineer use to design the structure! Correct? To make sure the structure do its job for what it is design for, and there are safety factors in the design process! Correct?  From the characteristic strength the concrete technologist should design a mix in the lab using the Characteristic strength + (Proportions defective factor for the structure multiply by the standard deviation which is a factor of the quality of the contractor) that can ad up to 13 MPa to the Characteristic strength. If the specimens from site cured the same way as the original lab design mix, fail? Remembering depending on the Proportions defective normally 5% ( meaning 5% of the site cubes should actually below the Characteristic strength) the structure should be either re-designed to be a lower load per square meter or additional support should be put in if possible or take it down which should always be the last option. Taking cores till the structure passed is not the solution, Just to put fat on the fire, core strengths are only an indication on what the cube or cylinder strength could have been! 
Anybody want to differ from me, you can contact me via e-mail at derek.mostert@up.ac.za  I don't think the forum is a place to have private quarrels. And I would really like to get views on this from around the world (Our solution by the way in our country is partial payment! If that is going to keep the structure up I am not convinced??) 
1 Recommendation
30th May, 2015
Tamás Simon
Budapest University of Technology and Economics
I would also like to draw attention to the following article:
The effect of friction on the compressive strength of concrete specimens. In: Balázs L Gy, Lublóy É (szerk.) Innovative Materials and Technologies for Concrete Structures: CCC2011.  pp. 443-446. ISBN: 978-963-313-036-0 
The proceedings may be downloaded from : http://fib.bme.hu/konyvek.html
Tamás
2nd Jun, 2015
Humphrey Danso
Akenten Appiah-Menka University of Skills Training and Entrepreneurial Development
Besides the standard cited, another important thing to do in interpreting 28 day compressive strength of concrete is to consider the strength improvement by age. This means you should have compressive strength for 7, 14 and 21 days and compare the rate of strength improvement with the 28 day. This also helps in the interpretation of the results.  
3rd Jun, 2015
Umer Farooq
Adani Group
Well, it is true that Compressive strength increases with age. but the significance of 28 days is that after 28 days concrete achieves 99% of its strength for which it is mix designed. 
The variation of compressive strength with age varies as:
Age     Strength per cent
1 day       16%
3 days     40%
7 days      65%
14 days    90%
28 days    99%
Note: Curing is a sensitive factor in achieving compressive strength.
Curing plays an important role on strength development and durability of concrete. Properly cured concrete has an adequate amount of moisture for continued hydration and development of strength, volume stability, resistance to freezing and thawing, and abrasion and scaling resistance.
Some researchers have tried to interpret the 28 days compressive strength on the third day using artificial neural networks . Eg. Read this paper on researchgate >>> https://www.researchgate.net/profile/Vahid_Alilou/publication/41845995_Prediction_of_28-day_compressive_strength_of_concrete_on_the_third_day_using_artificial_neural_networks/links/0912f50d36dc87bf39000000.pdf
1 Recommendation
18th Aug, 2018
Vito Alunno Rossetti
Sapienza University of Rome
Compressive strength of concrete at 28 days doe's not exist.
Make one defined mix of at least water, aggregate and cement, make cylinders and cubes and compact according to your preferred method; make a structure and take cores, chose a method of curing cubes, cylinders and the structure (a temperature and humidity), put the specimens in some testing apparatus, chose a rate of loading and crush at 28 days . If you can, try with multiaxial loading. Don't forget to register the results.
How many different strength values you can get ? and how many if we look for a characteristic strength?
Of course you can adopt also one or more Standard testing methods, so reducing or increasing the number of results, but,
is any result THE compressive strength of that concrete ?

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