By Eric Barger 
Throughout the construction industry, the common belief is that concrete takes 28 days to cure and reach 100% of its strength.
This evolved from a misunderstanding of what curing actually means. Some regulators characterize concrete as “green” if it has not cured for a certain time frame, not realizing this leads to unnecessary delays and higher project costs. An evaluation of the concepts of measured compressive strength and curing is in order to shed light on the mystery that surrounds the 28-day myth.
Specifying concrete strength is normally done with a minimum compressive strength (psi) at a certain age (days). Specified concrete compressive strength is the minimum compressive strength at which the concrete should fail in standard tests of 28-day-old concrete cylinders. A typical concrete compressive strength specification requires 4,000 to 5,000 psi at 28 days. Some go a step further and mandate that concrete products cannot be installed or used until 28 days after the date of manufacture. This, mistakenly, has given concrete a reputation among some specifiers as being weak or inferior if it has not cured for the full 28 days.
An arbitrary timeframe
The 28-day period is an arbitrary specimen age – though chosen for many good reasons – for testing the compressive strength of concrete. Specification writing authorities chose 28 days as the standard specimen age to establish consistency for testing procedures throughout the industry (1). Such an arbitrary time frame, however, is not relevant to all mix designs. It is estimated that concrete reaches 75% of this 28-day compressive strength in seven days, and its strength will remain stable or even increase over time (2).
A specific ratio can be determined for each specific mix design and curing process to fine-tune this arbitrary age so that the mix design dictates the 28-day compressive strength. Some mix designs reach 5,000 psi of compressive strength in seven days – or even in 24 hours. However, the faster concrete reaches the minimum design compressive strength, the greater the cost of the concrete.
Strength measurement protocol
Measuring the compressive strength of concrete is achieved by taking a sample of concrete at the time of placement. Cylinders, measuring 12 in. high by 6 in. in diameter, or 8 in. by 4 in., are compressed by a break machine that exerts increasing force upon the cylinder until it structurally fractures. When a failure occurs (commonly referred to as a “break”), the compressive strength is measured by dividing the force (lb) measured at the time the cylinder fails by the load-bearing surface area (sq in.) of the concrete sample. At the time of the break, the sample’s age is generally noted for quality assurance purposes. Therefore, to meet the typical concrete compressive strength specification as previously noted, the break results should calculate to at least 5,000 psi at 28 days.
When a specification has performance criteria of 5,000 psi at 28 days, for example, governing authorities will want a test record (two individual cylinder breaks at 28 days) to ensure conformance with the specification. Fortunately, more than two cylinders are typically collected for testing. Through research and empirical data collection, projection of a 28-day strength from a three- or seven-day test break is easy to perform and results in an estimated strength very close to that measured at 28 days. With the accuracy of this early estimation, if a problem arises, it can be reviewed much sooner than 28 days.
Why curing is misunderstood
Curing is the process of controlling the rate and extent of moisture loss from concrete during cement hydration (3). This would refer to a great many processes that are used to cure concrete, including steam curing, moist curing, air curing and more. It is immediately apparent that the phrase “curing” can have different meanings to different people. Strength and age associations behind the word “curing” are overwhelmingly misunderstood.
Digging deeper into the curing process, hydration is the chemical process that allows concrete to go from a plastic state to a hardened state while gaining strength over time. Hydration occurs at a faster rate in the early stages after concrete placement and slows down after a month or so goes by. Many cement pastes will cease hydration before one year, and some may continue to hydrate over the course of several years (4). Because of the variable length of the hydration process, the phrase “green concrete” is a purely subjective characterization.
With so much variation in hydration time, waiting for completion of the hydration process before testing the compressive strength of concrete would be impractical. In determining a more practical age for testing concrete strength, the 28-day myth began. So, while curing does help the hydration process, “28 days” is not an inclusive rule dictating a specific time to produce minimum compressive strengths. Simply stated, as the concrete cures and hydration takes place, the concrete gets stronger – and hydration may continue long after the minimum required compressive strength is reached.
Curing is a process, not a measure of strength
A specified compressive strength may be for any strength at any age of concrete. There is nothing wrong with specifying 5,000-psi compressive strength at one, seven, 11, 14 or 56 days. It all depends on the mix design, circumstances and project requirements. The American Concrete Institute recognizes 28 days or “the test age designated for determination of the specified compressive strength.”5 Therefore, when a specifier calls for 5,000 psi concrete at 28 days, this tells the concrete producer to select a mix design that will attain a minimum of 5,000 psi 28 days after manufacture.
In precast concrete manufacturing, however, very early attainment of specified strengths – reaching the minimum required compressive strength much sooner than 28 days – is the norm. It is extremely rare for a specifier to require a specific cure time. Rather, specifications might state that installation prior to 28 days is acceptable, provided that two cylinder breaks prove the minimum required compressive strength has already been reached. With a select mix design, concrete could easily reach its minimum required compressive strength one day after it is manufactured and therefore be specification-compliant for installation.
The main thing to remember is that curing is a process and not a measure of concrete strength. The 28-day stipulation provides a consistent industry-wide basis for comparing the compressive strength of concrete products. The 28-day time frame is not directly related to whether or not a specific product meets strength requirements for a particular application. As long as the minimum compressive strength is met before the product is put in service, the time frame to reach the minimum compressive strength should not be relevant.
Eric Barger is vice president of C.R. Barger & Sons Inc., Lenoir City, Tenn., a family-owned business spanning five generations. Eric is a civil engineer but admits that he learned all he knows by growing up in a precast plant and listening to his father’s motivational speeches. Contact him at: [email protected].
Notes:
- Cement & Concrete FAQs, Portland Cement Association (PCA); www.cement.org/basics/concretebasics_faqs.asp
- Kosmatka, S.H., Panarese, W.C., Allen, G.E. and Cumming, S. Design and Control of Concrete Mixtures, PCA, Skokie, Ill. 2002.
- “Curing of Concrete,” April 2006; www.concrete.net.au/publications/pdf/Curing06.pdf
- “Overview of the Concrete Hydration Process,” Section 5.1; www.iti.northwestern.edu/cement/monograph/Monograph5_1.html
- Building Code Requirements for Structural Concrete and Commentary (ACI 318-11), ACI, Farmington Hills, Mich.
Impressive clarification of curing! I’m facing an issue that how many percentage of concrete strength is proper for precast member lifting and I think that this shall be related to the curing days after concrete pouring. Could you advise any suggestion if any?
This depends on a lot of different factors. Minimum product stripping strength should be determined on a per product basis. Most products we manufacture are stripped the following day they are poured. This places us in the 1800 psi range.
I would advise talking to a Professional Engineer and performing test lifts if there is any doubt.
Eric Barger
I want to know maximum compressive strength attained by concrete after curing for 28days
Thank you for your comment hamis. I will gladly forward your questions to our Technical Services engineers. What specifically about compressive strength would you like to know? Also, if you could explain a bit more about your project, that will help us provide a more accurate response.
Excellent article! Based on your point that 5000 psi is the minimum strength that the concrete will reach after 7 days, 28 days or longer, are you also saying that if I buy 4000 psi concrete that it could possibly reach 5000 psi over a period of time?
Yes it can. Most plants set up the cement power ratio to optimize the materials used for the region, and on most mixes the 28 day strength is surpassed. On certain projects you may find that a 42,56 or 90 day break is required. ( one project asked for 90 breaks and they had a 4000 psi at 28 days– my results were about 6500 psi on the 90 day breaks) I have used a hold cylinder to break as a 42 or 56 day as the 28 day record was lower than the specified. The engineer has signed off on a passing break at 42 days before. Those that did not, the material was removed and replaced.
Amy, if you purchase 4,000 psi concrete you need to also ask “when is it expected to reach that strength”? It is almost a given that the concrete was designed to meet 4,000 psi in 28 days.
Could it possibly reach 5,000 psi over a longer period of time? Yes, it is possible. The possibility would depend on factors that play a role in the strength of concrete and it’s long term gain. The variables would have to be narrowed down a bit in order to say how likely the concrete is to reach 5,000 psi with a designed 4,000 psi mix design.
I personally believe the chances are better for concrete NOT to reach 5,000 psi if the order was placed for 4,000 psi @ 28 days.
Hi all,
I am searching for the source(ACI or ASTM) percentage of concrete at various ages.
I designed a trial mix concrete for 30 MPa but average result of 28 days compressive strength was 28 MPa.
Does it pass or Not?
This has to be incorporated into standards for practioneers
What would you say would be an acceptable difference in mpa between a pair of 28 day cylinders. Also what would be an acceptable difference in cylinders cast by two different technicians from the same batch.
Thanks Gerard for the comment. Phillip Cutler, P.E., one of our technical engineers, has provided the following answer:
“With the multitude of variables that need consideration there is no easy answer or estimate to provide. Obviously, the more variables you can eliminate the better your prediction for strength variations will be. For example, if you have data that allows you to prove that the mix is plant produced within all applicable equipment tolerances using certified quality materials. It is likely that you could predict and track results that would range as low as 5 mpa (725 psi), possibly lower in some cases, between individual cylinders taken from the same batch and tested at the same age. You may obtain additional information for these type of predictions and statistical analysis by referencing ASTM C39, “Standard Test Method for Compressive Strength of Cylindrical Concrete Specimens;” ACI 318, “Building Code Requirements for Structural Concrete;” ACI 211.1, “Standard Practice for Selecting Proportions for Normal, Heavyweight, and Mass Concrete;” ACI 214, “Guide to Evaluation of Strength Test Results of Concrete;” and the PCA “Design and Control of Concrete Mixtures.”
I would suggest that you contact a local testing firm and inquire about an expected range of cylinder break tests by two separate technicians on the same batch of concrete at the same age. In addition, you might consider posing both questions to a third party firm and ask if they have any supporting data to validate their responses.”
The 28-day period is an arbitrary specimen age – though chosen for many good reasons – for testing the compressive strength of concrete. Specification writing authorities chose 28 days as the standard specimen age to establish consistency for testing procedures throughout the industry (1). Such an arbitrary time frame, however, is not relevant to all mix designs. It is estimated that concrete reaches 75% of this 28-day compressive strength in seven days, and its strength will remain stable or even increase over time (2).
A specific ratio can be determined for each specific mix design and curing process to fine-tune this arbitrary age so that the mix design dictates the 28-day compressive strength. Some mix designs reach 5,000 psi of compressive strength in seven days – or even in 24 hours. However, the faster concrete reaches the minimum design compressive strength, the greater the cost of the concrete.
what should I do if the precast column that already installed,then the 28days cube test failed?
Any corrective action?
Thank you for your comment Eddy. Evan Gurley, one of our technical engineers, has provided the following response:
“If the 28 day tests fall short of the specified compressive strength, you have a few options to consider.
The first option is to take core samples in accordance with IBC Section 1905.6.6 and ACI 318 Section 5.6.5, which was developed for investigating low-strength test results. After the core samples are analyzed, and you follow the steps in ACI 5.6.3.3, if you are within the 500-psi limit but below the required f’c, you are required to take steps to increase the concrete strength. If you do fall below the 500-psi limit, you have to satisfy the capacity of the structure. The steps are clearly outlined in ACI 5.6.5.
Keep in mind following the steps per ACI is important. While tearing out, replacing and repairing the deficient section sounds like the safest route, ACI procedures should still be followed first. Then, if it is determined the concrete is truly insufficient and a life-safety problem exists, you will have fully documented your steps – identifying and addressing the problem.
The next option is to contact the Structural Engineer of Record for further instruction. If additional 56-day cylinders were cast, tested and achieved the required design strength, it is up to the Structural Engineer of Record whether or not they would accept the 56-day tests as an indication that the concrete has reached its design strength.
Other options to consider:
-Taking additional core samples for analysis
-Performing a load test
-Provide alternatives to repair and strengthen the structure
-Reject the piece and structure altogether”
In Nigeria, if cube compressive strength fall short of the specified strength, curing of the structural member is adopted as the next option to clear the client’s doubt. If cured samples fails strength test, load test is adopted and if no convincing result is achieved from the above methodologies, client reserve the right to issue you a Non-conformance / rejection of the structural member(s) In order to avoid the embarrassment concrete failure may cause, you need to analyse all concrete ingredients like fine and coarse agg, water etc. Your construction water should be as par WHO standard ie fit for drinking. Also lay much emphasis on you cement content. I think these will help.
Is there any way to estimate 28 day strength accross the board in order to make calculations more simple, in terms of rigid pavement design?
28 day flexural strength is a parameter which you calculate from compressive strength and i understand that making assumptions on assumptions dramatically increases error, but this is a personal project to see how roads hold up designed in this way.
Thanks in advance.
I am wondering what factors (like temperature or moisture) effect the curing rate?
Thanks for the comment Rolfe. Kayla Hanson, one of our technical services engineers, responded with the following information:
“Thank you for your comment on The 28-Day Myth article on the NPCA website. Numerous factors affect the rate at which concrete cures, including, but not limited to, the following:
– Ambient temperature at the time of mixing
– Ambient temperature at the time of casting
– Ambient temperature during the curing process
– Temperature of mix water
– Water to cementitious materials ratio (w/cm)
– The mix design’s proportions
– Moisture content of the aggregates
– Any chemical or mineral admixtures used in the concrete mix, including products specifically designed to increase or decrease the rate at which concrete cures
– Any compounds applied to the concrete after casting or stripping
– The chosen method(s) used for curing, which could be accelerated curing by use of applied heat or steam; moist curing; use of tarps, plastic sheeting, or burlap to retain moisture; etc.
– Weather throughout the curing process, including wind, sun, rain or snow
– The product dimensions (thicker sections or mass concrete will take longer to cure than thin sections)
– The type of cement used in the concrete mix
NPCA has numerous resources available on this topic that can help explain this further. Please see the following links, below, for some additional information:
NPCA Articles:
To Cure or Not To Cure?
Material Matters: Liquid Curing Compounds
Just the Cure for You
NPCA FAQs:
“What is proper curing?”
Other Resources:
Role of Concrete Curing
Curing Concrete – Normal, Hot and Cold Weather “
Have you ever worked with an additive called Komponent to decrease the shrinkage of concrete so the checker board placement is not needed for shrinkage between pours and have you ever experienced slow curing rates after initial set of concrete there for low breaks at 28 days.
Thanks you for the comment Jim. Evan Gurley, one of our technical services engineers, has provided the following response.
“NPCA is a trade Association so we do not work directly with additives and we do not manufacture precast concrete products. After doing some research on the additive you have called out, it may be beneficial to also contact the NRMCA or even the Portland Cement Association as they may have more experience with their members using this particular additive since this seems to be used for large scale flat/floor pours.
When this expansive mineral additive is combined with portland cement it forms what is described as an expansive hydraulic cement, which unlike portland cement, expands during the early hydration period after setting. ASTM C845 Standard Specification for Expansive Hydraulic Cement outlines the chemical composition and physical requirements for this type of cement.
When it comes to proper curing of expansive cement concrete, the requirements are more stringent than a typical portland cement concrete. Inadequate curing can substantially reduce the expansion level. ACI 223 states that after expansion (if properly consolidated, finished and cured) the tensile, flexural, and compressive strength development of expansive cement concrete is similar in rate and magnitude to that of a standard portland cement concrete. I would strongly recommend looking over the ACI 223 document.
It looks like additives similar to the one you have called out are used in numerous applications. There is even an example of an ASTM C845 cement being used on the construction of a bridge deck and if you click here, you’ll be able to review a report that details the benefits in this application.
Also, from what I have found on the additive and the “checkerboard” placement, ACI Committee 302 recommends that the checkerboard sequence of placement not be used.”
please resolve my doubt ?
for concrete cylinders/cubes 7days crushing should be taken consideration from the date of sampled ? or from the next day ?
eg: if i take sample on 10th than the seven days crushing will be 16th or 17th ? from the date of sample is to be taken in consideration or from the next day ? please guide me with correct answer as early as possible
Thank you for the comment Mohammad. Eric Carleton, P.E., vice president of technical services, provided the following response.
“The intent of the days of testing is each 24 hour period from the date you made the cylinder or cube from the concrete being cast. So in your example, the concrete was cast on the 10th which is when you fabricated your test specimens. So from 10th to 11th (one 24 hour period), 11th to 12th, 12th to 13th, 13th to 14th, 14th to 15th and finally 16th to 17th makes for seven each 24 hour periods. ASTM C39/C39M, Section 7.3 provides allowable tolerances to the time to conduct the strength test; for 7 days the standard states ± 6 hours or 3.6%.
It should be noted if your sampling is to be made from concrete cores cut from the cast concrete all the required date information of the sample is stated within ASTM C42/C42M section 7.10. However, when describing a test for a 14 day old core, it is implied that is when the concrete was cast and not from the date the sample was taken.”
Hi,
How would the curing process/time be affected when the concrete is casted underwater? Would it cure faster because is surrounded by water or slower because it will be at a low temperature? Will the salt in seawater affect this?
Thanks,
Thank you for the comment Alfredo. Seawater actually does not affect the curing process for precast concrete products since it’s completed well before the product is placed underwater. Your questions are directed more toward ready-mix applications. If you do have an interest in marine products, I suggest heading over to our “Find Precast & Supplies” section on the website. There, you can contact any of our precast concrete members who manufacture marine products and they would be happy to help answer your questions.
Creep and shrinkage are extremely important concrete properties which are not measured by crushing concrete samples. High strength concrete (high cement content) will have high shrinkage. Pump mixes often have lower stone content (easy to pump) and will have high creep deflections. Many serviceability failures have occurred due to excessive creep and shrinkage-despite the crushing tests having passed.
No experienced engineer would accept concrete which does not conform with his specifications-this is not up for debate-end of story.
what is the minimum required compressive and flexural strength of concrete in ASTM and ACI??..
i am involved in a major concrete foundation pour in a day or two and like the the engineer to allow me further work on the basis of 3 day results instead of specified 7 day results.This i am getting by use of admixture.What arguments or safeguard i can give to the engineer to allow me to allow after 3 days ? (I like your statement that curing is a devise to control moisture to expedite hydration process and not measure of strength)
Pls, has curing anything to do with the heat engulfed by the concrete mass?
Also, has curing anything to do with cracks?
If yes, I need an overview. Thank u.
Thank you for your comment Patrick. Do you have specific questions you are seeking information for regarding curing? I’d be happy to direct those questions to our technical services engineers.
Yes the concrete made with sandstones did not achieve more strength past 56 days of curing. The strength gain was found to be increasing little and sometimes stable but never decreasing.
Any valuable comments on this?
Break out strength depends on the slump it is poured on( how much water is added). Vibrating the concrete as it is being laid will make it break out stronger.I have no degree but have poured many different concrete company ‘s mix design and have worked concrete for 27 year now daily
I dont know of anyone who confuses 28day required strength with maximum strength attained by concrete. There has never been a misconception of what the 28day requirement is, and it is not arbitrary. The reasons are various, from handling additional load(other than its own weight), back filling, form stripping purposes and so on. On numerous occasions I’ve personally seen contractors use mix designs with substantially higher design strengths, just so the could strip forms sooner. In precast operations, the mix of the concrete is down to the suppliers preference, and time frame. Use the mix that obtains your required results in the time frame that is set before you. Objectively you should be able to note that the requirements are job specific and not arbitrary and randomly chosen to delay work. In fact, most times, work is never stopped regardless of what the break strength. Ive seen numerous engineers that will accept concrete that has reached 80% of its required break strength at 28 days, with full knowledge that concrete continues to gain strength.
I have a question Sir,
Concrete in 28 days has design 28 mpa but after 28 days its result is 26mpa so it is failed, now what to do to make it passed.
Jahnagir,
I forwarded your question to our Technical Services engineers. The following response is from Kayla Hanson:
We would recommend performing more tests to ensure the 26 MPa was accurate. It’s concerning that the concrete in question hasn’t reached 4,000 psi in 28 days. In many cases, precast concrete specimens can display strengths exceeding 4,000 psi in a matter of days (or less). Also, it would be beneficial to verify the mix design is appropriate, batching equipment has been calibrated, and production and curing conditions are appropriate.It may be useful at some point to perform a petrographic analysis on the specimens to determine the cast parameters of the concrete question.
Hi, we build a new foundation of 9 feet x 14 feet x 16 inch 40 MPA for a CNC machine weighting 12 000 pounds (6 tons).
The contractor says we could put the machine on the foundation after 4 days without any problems.
We were planning leveling and starting the CNC after 8 days of curing time
But the CNC manufacturer is refusing to do this before 28 days saying the concrete isn’t cured before that !
Could you please give us your advice ?
Is the concrete still expand or retract after 8 days ?
(meaning the leveling of the CNC on it would be void after a few days)
Thank you for your comment Eric. I forward your question to our Technical Services engineers. The following response is from Kayla Hanson.
It really depends on a lot of factors. It is possible that the concrete could reach its design strength in only a few days. however, it is also possible that the concrete may need a longer period to reach design strength, and although it’s unlikely it would take 28 days, it is not impossible. As for volumetric changes, concrete can expand and contract years after manufacturing depending on local conditions. The intent is to design the concrete and reinforcing to adequately absorb the resulting stresses and resist cracking.
Some questions to ask include:
– Was water added to the fresh concrete on site to make it more workable?
– What curing conditions and ambient conditions is the foundation being exposed to?
– Did you happen to cast compressive strength cylinders made from the same batch of concrete the foundation was poured from?
Typically, most of the dramatic volume change is likely to occur leading up to final set, but if the concrete hasn’t reached an appropriate compressive strength before excessive loads are applied to it, additional volume fluctuations are possible.
Creep is a type of deformation caused by applied loads over a certain period of time, and it’s another very important consideration. According to the Portland Cement Association, “The amount of creep is dependent upon (1) the magnitude of the applied stress, (2) the age and strength of the concrete when stress is applied, and (3) the length of time the concrete is stressed. It is also affected by other factors related to the quality of the concrete and conditions of exposure, such as: (1) type, amount, and maximum size of aggregate; (2) type of cementing materials; (3) amount of cement paste; (4) size and shape of the concrete element; (5) volume to surface ratio of the concrete element; (6) amount of steel reinforcement; (7) prior curing conditions; and (8) the ambient temperature and humidity.”
Hello sir,
Myself Hussain Bharmal from Babji cement products, Gondal, India. I am owning cement products manufacturing company.
I want to know how we can increase the strength of cement products and also want to know more in curing and hydration process.
I am also willing to understand the proportions of raw material to mixed during precast from you sir.
Hope for the positive reply..
Thanks
Hussain Bharmal
Babji Cement Products
Thank you for the comment Hussain. The best way to find the information you are seeking is to search our website using keywords such as “cure,” “SCMs,” “hydration,” or “mix design.” Those keywords will bring up numerous technical documents or related articles and blogs that will aid in your research. If there is a specific technical question that you still need answered, please let me know and I’ll forward it to our technical services engineers.
Is the hydration period accelerated (or shortened) in the 4,000 psi Quikrete product, compared to hydration timeline models found in various industry articles? I’m curious as to the level of hydration that has occurred at 24 hours, since the data sheet suggests to wait 24 hours before post is subjected to any strain.
Thank you for your comment John. I forward your question to our Technical Services engineers. The following response is from Kayla Hanson.
We would recommend inquiring with Quikrete regarding the hydration timeline for their materials, as we do not have access to those resources. However, this may help provide a little insight.
Not only do the raw materials matter, but the curing conditions will also strongly influence the rate at which strength develops. In many cases, products are allowed to cure for about 24 hours before they’re stripped from their forms. This is because the stresses exerted on a product during stripping and handling can cause cracks or other damage if it hasn’t gained the necessary strength to withstand those applied stresses. In cases where products are stripped and handled prior to 24 hours, there is usually some other factor contributing to the strength development that is allowing the early stripping to happen. The mix may be altered with admixtures, a different type or blend of cementitious materials, or the products may be subjected to accelerated curing methods so strength develops faster and products are able to be stripped sooner. Sometimes products will require more than 24 hours before they can be handled safely. The best way to determine how far the product has come in the hydration process is to cast cylinders and test their compressive strength at various stages.
Quikrete may in fact have added materials to accelerate strength gain, but that information would be included with the product you purchased.
Hi,
What are the requirements for curing and attaining specified compressive strength for freezer slabs? We have a client who would like to start the draw down / pull down process before the recommended 28 days. Would appreciate any references to ASTM or ACI regarding this. Additionaly, the floor slab is specified to have flyash.
Thanks
Thank you for your comment R Zachariah. Requesting more information on the Freezer slabs. Were they manufactured using precast or cast-in-place concrete? If its the latter, I’d suggest contacting the National Ready Mixed Concrete Association to get a more accurate response to your questions. Please let me know and I’ll forward this information to our Technical Services engineers for a response.
Dear Sir ,
My house builder did not put any water on the concrete slab.The season is summer and temp is reaching uo to 46-47 degree centegrade.He put water after 27 days.
Please advice the conciquences.
Thanks ®ards
Thank you for the comment Raghu. From the description of your project, it sounds like the concrete slab was constructed with ready-mixed concrete. NPCA represents precast concrete manufacturers and wouldn’t be able to accurately respond to your question. I recommend contacting the National Ready Mixed Concrete Association about your project. However, if the concrete slab is precast concrete. I will gladly forward your questions to our Technical Services engineers for a response.
we have some concrete samples who were test after 60 days some were even 100 days . What is the maximum days it must be tested and any tables of that can be use as basis for this matter.
Thanks and Best Regards,
Dominic
Since concrete continues to increase strength throughout its lifetime, there is no maximum time that concrete samples must be tested. It is important to understand that strengths from compressive tests may not be applied retroactively. Therefore, when you test your samples, the resulting strengths may only be recorded at 60 days or 100 days, depending on when the specimen was tested. The 60-day strength may not be recorded as 28 days.
Is is possible to increase strength of concrete after 7days text and before 28 days. the 7days test failed and am thinking is there any additives or solution that can be added in the curing water to increase the strength before 28days.
Thank you Lyiola for the comment. I forwarded your inquiry to our technical services engineers. The following response is from Mitch Rector.
While there are curing compounds that can be applied to concrete to keep the moisture in, we are not aware of any additives that can be applied to a concrete curing tank that will increase strength gain. Once your mix has been poured and has began curing, little else can be done besides making sure it is in a moist environment.
Concrete usually reaches 75% of the 28-day strength at seven days. If your concrete is not reaching the proper 28-day strength, I would troubleshoot each step of the process to determine why.
Here are example questions you can ask:
Q: Is the mix design correct? A high water-cement ratio will result in a concrete with reduced strength.
Q: Is the concrete curing properly? Concrete cures by the reaction of water and cement. If the sample is in a dry environment or is losing water through evaporation, it will not be able to react fully.
Q: Is your compressive testing machine properly calibrated?
These are not the only problems that can occur, but it a good place to start.
This is an impressive article, Eric which demolishes a widespread myth – on this side of the Pond as well. I have linked to it in an article to be published on LinkedIn later this week concerning the shape of the Time/Temperature curve of a cure and how that, though widely used, is only an inferred measurement, but is the most reliable one. It is however the best available on-site measurement to ensure complete attainment of compressive strength without sample pours and compressive testing.
I have credited you and this article in mine, which will be published fistly on LinkedIn and then in the Trade Magazines and the Journal of the Concrete of The Concrete Society.
Thank you so much for the clarity with which you express yourself and the passion with which you write.
Steve Alker
hello Sir, I am a civil engineering student. Actually my question is irrelevent to this topic. but i want to know that ” Can we find out Strength of concrete within an hour of mixing it?”
” i know that there is no any instrument for this but Can we fabricate a new instrument for this purpose and compare its results with 7days or 28days strength of concrete?”
Thank you for the comment Pratik. When it comes to design and fabrication, we must defer to our NPCA members. If you’d like to find a precaster or associate that may be able to help you with a project like this, visit preast.org/find. If you have any other questions, please let me know.
Maybe the most concisely written article published on design strength and concrete compressive strength test result misinterpretations (myths). Bravo! The industry needs more articles that focus on real issues rather than showing off the latest project. I agree with Steve above, your article is brilliant.
Cheers
Oliver
Hi thrre. Is there any issue if strengths are much higher than the minimum after 28 days, or even 7days?
We are pouring foundatiins for pipe supports and a road culvert in the hot climate of the middle east.
Concrete is specified C50, min 50n/mm after 28 days. For field cured cylinder, we saw figures of 64 after 28 days. On another occasion we saw 50 after only seven days!
Would this represent an issue long term?
Look forward to your feedback. Thank you
Thank you for your question Tariq. The first thing I would be curious about is making sure that your cylinders are representative of the concrete used in the product. You said that you are using field cured cylinders, so I would look into what the standard deviation is on the strength for the specified mix you are using. It is possible that the variability for the mix is so large that it has a higher average strength than what is specified just to compensate for the variability.
To answer your question though on if the higher strength would represent a long-term issue, the only concern that I am aware of is how the higher strength is affecting your elastic modulus. A higher strength typically means a higher elastic modulus which means stiffer concrete. This could make the concrete more susceptible to cracking or surface imperfections if cured incorrectly. If you are using concrete with a relatively high cement content, you may experience volumetric changes which, coupled with the stiffness of the concrete, could result in cracks forming.
Ultimately, I think that as long as you are ensuring your products are being cured properly, a higher strength than what is specified would not be a cause for alarm.
Your all article are excellent and useful specially this one…
I am looking for a model or set of models and equations that will allow me and my company to make a general prediction of the 28-day break strength of concrete, given the weight, volume and density of cementitious materials, aggregates, air-entrainer, water reducer, other various admixtures, w/c ratio, slump, aggregate size, volume of air, etc.
I know I learned about a method for predicting concrete strength in my Civil Engineering Materials course but I feel as though it is too fundamental for what my supervisor is asking of me.
Thanks for the comment Ethan. If you’re looking for more information and research about mix designs, curing and/or any technical-related information, we have many articles and tech papers available on our website. If you can share a bit more about what type of project or product you are researching for, I can relay that back to our technical services engineers for a response. You can also contact us at (800) 366-7731 if you have more questions.
In the article, “The 28 Day Myth”, the term ‘hydration’ is used to describe the loss of water from concrete during the curing process. My understanding of ‘water loss’ is it is referred to as ‘dehydration’ and water gain is ‘hydration’. Does the cement/concrete industry use these two terms in reverse, and, if so, why?
Also, during a discussion with a home builder about our driveway slab and curing time, he said he had heard the cement in the Hoover Dam is still curing. Is that possible given the time that has elapsed since its construction?
Thank you Jerry for the comment. Alex Morales, director of workforce development, provided the following response:
“Q. In the article, “The 28 Day Myth”, the term ‘hydration’ is used to describe the loss of water from concrete during the curing process. My understanding of ‘water loss’ is it is referred to as ‘dehydration’ and water gain is ‘hydration’. Does the cement/concrete industry use these two terms in reverse, and, if so, why?
A. This is a great question. In everyday use, the term hydration is indeed understood as ‘water gain.’ Dictionary.com specifically defines it as “the process of causing something to absorb water,” and uses the following sentence as an example: the human body requires adequate hydration to function properly. In chemistry, however, hydration is defined as the process of combining a substance chemically with water molecules and that is the meaning of the term hydration when discussing cement’s role in concrete.
Cement hydration is a chemical reaction. It is specifically an exothermic reaction, meaning that the chemical process produces heat as it progresses. Contrary to popular belief, water does not evaporate during the cement hydration process. Rather, it is consumed chemically as the exothermic reaction converts the paste into a harden mass. (See Cement Hydration Kinetics article here: https://precast.org/2016/03/cement-hydration-kinetics) As heat is generated by the chemical reaction, water can reach evaporation temperatures. This is why a freshly place concrete mixture is covered at least until initial set (and many times for longer periods) – precast concrete producers want to prevent the evaporation of water because water released into the atmosphere is then unavailable to hydrate cement.
Producing concrete is a plant environment allows for better control of moisture loss during early life of a freshly placed concrete and is one of the many reasons precast concrete producers (and NPCA Certified plants, in particular) can produced concrete of higher quality than concrete that is cast in place.
Q. Also, during a discussion with a home builder about our driveway slab and curing time, he said he had heard the cement in the Hoover Dam is still curing. Is that possible given the time that has elapsed since its construction?
The short answer is yes, it’s possible. It is a common adage in the concrete industry that the hoover dam is stronger today than it was yesterday! The hydration process does continue in perpetuity, as long as there is both water and unhydrated cement present, helping concrete to continually gain strength over time. However, it is likely that additional strength gain of concrete after many years is infinitesimally small and difficult to quantify.”
if we have made sure that concrete strength 28days upto 5000 psi than why need the ratio M10,M15,M20,M25 and that specification.
M10, M15, M20, and M25 refer to different mix design proportions. Various mix designs can achieve a 5,000 psi compressive strength using entirely different proportions of raw materials. It’s important to not only ensure the concrete mix design will provide the necessary compressive strength, but also ensure the raw materials and the proportions in which they are batched into the mix provide the desired fresh and hardened concrete properties. The economy of the mix is another important consideration in proportioning.
Ensuring concrete reaches the minimum required compressive strength is not enough to ensure it will perform as required for the duration of its service life. A concrete mix could achieve a 5,000 psi compressive strength, but may not perform well in service because of the type, quantity or proportions in which the other raw materials are used. For example, a mix could reach the required compressive strength while having a water-to-cement ratio that’s too high. This will make the concrete susceptible to watertightness and durability issues. It’s possible a mix could reach the required compressive strength while using inappropriately sized aggregates, which could increase paste demand and increase the cost of the concrete and potential shrinkage-related issues. The mix proportions affect all fresh and hardened concrete properties, and compressive strength is only one of many important factors to consider. This necessitates defining specific mix proportions.
Great read! Thanks for taking the time to write such an informative article on curing concrete. I’m sure many in the industry will be grateful for your effort and dedication.
A question that has come up is how long does it take for concrete to reach 100% of its compressive strength?
This is a tricky question, because in specific, exact terms one could say concrete never reaches 100% of its compressive strength, as there typically remains some small measure of un-hydrated cementitious materials when in the presence of moisture could continue the chemical process increasing paste bond, and consequently strength. This particularly is a consideration with mass pour concrete. It is reported the internal concrete within the Hoover Dam is still curing.
However, from a practical standpoint the increase in compressive strength curve will slow down and basically become asymptotic virtually reaching a maximum strength for a given design in normal curing and project application. The time to actually reach that point is dependent upon many variables such as the mix itself and what admixtures have been added to accelerate or delay hydration, the curing conditions such as temperature and moisture present. Within the concrete industry research and test data indicates that by 28 days following mixing and casting the concrete in question should have reached its design strength. It is not to say the concrete has stopped all curing and no gain will occur, but as mentioned at this point the strength gain to time ratio is greatly reduced. Depending on the situation such as ability to strip formwork or critical building components, an owner or inspector may determine the product is not in compliance and a suitable remedy is required. There may be a project criteria which will permit strength testing for compliance stretched to 56 days. The Portland Cement Association has an excellent article on the effects of curing and concrete strength gain which can be found here:
https://www.cement.org/learn/concrete-technology/concrete-construction/curing-in-construction
As you read within the NPCA article, the “28 day myth” occurs when an inspector or owner believes the concrete must set for 28 days prior to verification of strength or use, when in fact the design strength required can often be met many days or weeks prior to this 28 day limit.
I just read your article on The 28-Day Myth. It was really interesting! I’m glad you provided some insight into the misunderstanding of what curing actually means. Some regulators characterize concrete as “green” if it has not cured for a certain time frame, not realizing this leads to unnecessary delays and higher project costs. An evaluation of the concepts of measured compressive strength and curing is in order to shed light on the mystery that surrounds the 28-day myth. Can’t wait to see what you come up with next!
Thanks for the great article on concrete! I learned a lot.
Thank you for sharing this informative article of yours. I hope there’s a lot of people can read this article and be guided.