We also demonstrate the differences in strength between traditional concrete and a newer, innovative concrete technology—Ultra-High Performance Concrete UHPC. This is the most common and well-accepted measurement of concrete strength to assess the performance of a given concrete mixture.
It measures the ability of concrete to withstand loads that will decrease the size of the concrete. Compressive strength is tested by breaking cylindrical concrete specimens in a special machine designed to measure this type of strength.
It is measured in pounds per square inch psi. Compressive strength is important as it is the main criteria used to determine whether a given concrete mixture will meet the needs of a specific job.
Pounds per square inch psi measures the compressive strength of concrete. A higher psi means a given concrete mixture is stronger, so it is usually more expensive. But these stronger concretes are also more durable, meaning they last longer. The ideal concrete psi for a given project depends on various factors, but the bare minimum for any project usually starts around 2, to 3, psi. Each concrete structure has a normally acceptable psi range. Concrete footings and slabs on grade typically require a concrete of 3, to 4, psi.
Suspended slabs, beams, and girders as often found in bridges require 3, to 5, psi. Traditional concrete walls and columns tend to range from 3, to 5, psi, while 4, to 5, psi is needed for pavement. Compressive strength is usually tested at seven days and then again at 28 days to determine the psi. The seven-day test is done to determine early strength gains, and in some cases, it may even be performed as early as three days. But the concrete psi is based on the results from the day test as noted in the American Concrete Institute ACI standards.
Tensile strength is the ability of concrete to resist breaking or cracking under tension. It affects the size of cracks in concrete structures and the extent to which they occur. Cracks occur when tensile forces exceed the tensile strength of the concrete. Traditional concrete has a significantly lower tensile strength as compared to compressive strength. This means that concrete structures undergoing tensile stress must be reinforced with materials that have high tensile strength, such as steel.
It is difficult to directly test the tensile strength of concrete, so indirect methods are used. The most common indirect methods are flexural strength and the split tensile strength.
The split tensile strength of concrete is determined using a split tensile test on concrete cylinders. Flexural strength is used as another indirect measure of tensile strength. It is defined as a measure of an unreinforced concrete slab or beam to resist failure in bending. In other words, it is the ability of the concrete to resist bending. Flexural strength is usually anywhere from 10 to 15 percent of the compressive strength, depending on the specific concrete mixture.
Results are expressed in a Modulus of Rupture MR in psi. Flexural tests are very sensitive to concrete preparation, handling, and curing. The test should be conducted when the specimen is wet. For these reasons, results from compressive strength tests are more typically used when describing the strength of concrete, as these numbers are more reliable.
Other factors contributing to the strength of concrete include:. This refers to the ratio of water to cement in the concrete mixture. A lower water-to-cement ratio makes for a stronger concrete, but it also makes the concrete more difficult to work with. 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? Thanks for the comment Rolfe. Numerous factors affect the rate at which concrete cures, including, but not limited to, the following:.
NPCA has numerous resources available on this topic that can help explain this further. Please see the following links, below, for some additional information:. 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. 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. 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 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 document. It looks like additives similar to the one you have called out are used in numerous applications. Thank you for the comment Mohammad.
Eric Carleton, P. 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. 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.
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? Thank you for the comment Alfredo. Your questions are directed more toward ready-mix applications. 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. 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? 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 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. 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.
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. 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. In many cases, precast concrete specimens can display strengths exceeding 4, 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. The contractor says we could put the machine on the foundation after 4 days without any problems.
Could you please give us your advice? Is the concrete still expand or retract after 8 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. 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?
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. 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.. Thank you for the comment Hussain. Is the hydration period accelerated or shortened in the 4, psi Quikrete product, compared to hydration timeline models found in various industry articles? Thank you for your comment John.
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 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. What are the requirements for curing and attaining specified compressive strength for freezer slabs? Additionaly, the floor slab is specified to have flyash. Thank you for your comment R Zachariah. Requesting more information on the Freezer slabs. Were they manufactured using precast or cast-in-place concrete?
Dear Sir , My house builder did not put any water on the concrete slab. The season is summer and temp is reaching uo to degree centegrade. He put water after 27 days.
Please advice the conciquences. Thank you for the comment Raghu. From the description of your project, it sounds like the concrete slab was constructed with ready-mixed concrete. However, if the concrete slab is precast concrete. First, though a couple of definitions may be useful: The processes of 'setting' and 'hardening' are often confused:.
Setting is the stiffening of the concrete after it has been placed. A concrete can be 'set' in that it is no longer fluid, but it may still be very weak; you may not be able to walk on it, for example.
Setting is due to the formation of ettringite and early-stage calcium silicate hydrate. The terms 'initial set' and 'final set' are commonly used; these are arbitrary definitions of early and later set. There are laboratory procedures for determining these using weighted needles penetrating into cement paste.
Hardening is the process of strength growth and may continue for weeks or months after the concrete has been mixed and placed. Hardening is due largely to the formation of calcium silicate hydrate as the cement continues to hydrate.
The rate at which concrete sets is independent of the rate at which it hardens. Rapid-hardening cement may have similar setting times to ordinary Portland cement. Traditionally, this is done by preparing concrete cubes or prisms, then curing them for specified times.
Common curing times are 2, 7, 28 and 90 days. The curing temperature is typically 20 degrees Centigrade.
The SI unit for concrete strength measurement is the Mega Pascal, although 'Newtons per square millimetre' is still widely used as the numbers are more convenient. Thus 'Fifty Newton concrete,' means concrete which has achieved 50 Newtons per square millimetre, or 50 Mega Pascals.
While measurements based on concrete cubes are widely used in the construction industry, the European standard for cement manufacture, EN , specifies a test procedure based on mortar prisms, not concrete cubes. Whether 'real concrete' made from that cement will achieve There are many relevant factors; some of the more important follow: Concrete porosity: voids in concrete can be filled with air or with water.
Air voids are an obvious and easily-visible example of pores in concrete. Broadly speaking, the more porous the concrete, the weaker it will be. Probably the most important source of porosity in concrete is the ratio of water to cement in the mix, known as the 'water to cement' ratio.
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