Concrete curing is a process whereby mix-design strengths are achieved to ensure proper structural design requirements are met. Curing is done to ensure proper development of internal matrix of concrete elements through hydration. Concrete curing processes can take up to 56 days for final testing strengths of various mix designs to be achieved, based on typical concrete testing procedures atleast. In some cases, less time is fine to achieve significant strengths for concretes.
Concrete curing is integral to the continued schedule progression of structures with concrete elements supporting one another. Concrete curing, testing and construction methods in general can be reviewed/performed with standards like CSA A23.1
TABLE OF CONTENTS
How Long for Concrete to Cure
High-Early Concrete Mix Designs
Concrete Testing
Concrete Cylinder Samples
Early Breaks & Extra Cylinders
Overview of Concrete Curing
Various Curing Times
Best Curing Conditions for Concrete
Concrete Moisture Testing
Summary
So how long for concrete to cure?
Concrete can achieve significant strength in about 7 days. Final strength with regular mix then should be achieved around the ~28 day mark. Concrete mix designs are provided set strengths, known as MPa, which should be achieved at set time periods, in order for engineers to be satisfied with the results of pours and structural elements. Some specialized high early concrete mixes can achieve up to 35 MPa within 48 hours, which is significant strength in a short time considering up to 7 days is required in adequate curing conditions to even achieve ~70% design strength with regular concrete mixes.
You can typically walk on concrete after 24 to 48 hours, that is the hardening time, but not necessarily indicative of the strength of the concrete (compressive strength achieved through testing).
Even a 56 day period testing duration is offered pretty standard for most commercial / residential concrete projects. If for instance a sample is not achieving strength by a 28 day test, a close eye will be placed on a 56 day sample, or other samples in-between these periods of time.
Samples refers to cylinders of concrete specific to each mix.
Typically an engineer is not going to request that subsequent testing be conducted after the 56 days period for normal concrete mixes, as strength is almost always achieved.
High-early Concrete Mix Designs
High-early or fast-track concrete can cure faster than normal mix-designs.
However these are not typical mix designs, in the sense that they are atypical, with strength (high MPa – megapascal) results designed to be achieved in short durations. For instance, typical high early mix designs would achieve significant (~70%) strength in about 2 days, but with compensation in higher MPa designs, could satisfy a project’s requirements in as short as 7 days. This is much less than the early break 21 day or full cure break 28 day tests so typical of concretes without high-early designation.
This sort of concrete is perfect for pushing schedule faster than anticipated.
What about concrete testing?
Concrete is typically tested in provided curing periods of 7 days, 14 days, 28 days, and even up to 56 days for testing purposes.
(3 days is also used for short term strength gain indication)
The corresponding curing times here are dictated by structural testing requirements for concrete in commercial, residential, and industrial projects. Concrete strength is tied to testing at these time spans typically through any project requirements, will require strength testing, through physical samples or core testing.
For instance ASTM C31 is standard practice for making and testing specimens in field.
Concrete Cylinder Samples
Concrete mix cylinder samples are taken from a truck or batch to test the strength of the concrete. Concrete cylinder molds are used in field testing process for curing small cylinders of concrete for compressive testing.
It is essential to test concrete through compressive strength testing, to indicate the MPa. A concrete cylinder sample is tested/cured for each of the time periods or as dictated by the requirements of the project specifications or drawings. Sometimes more than one sample is tested at a time, it generally depends on the seriousness of the concrete element.
If not enough samples are taken, then additional in-situ concrete coring may be required.
This is one of the reasons that field testing companies need to come to site for concrete pours. Testing of concrete samples through curing of samples in field conditions, placed in a trailer, or curing box, is what facilitates proper concrete testing procedures. The periods of 7/14/21/28/56 days are pretty typical to see, but sometimes earlier breaks and other periods are requested, it depends on the project.
Early Breaks and Extra Cylinders
Other schedule characteristics of a project may dictate that a concrete sample be tested earlier, such as 14 days to achieve a strength rating pertinent for subsequent concrete construction to take place.
Proper foresight with concrete placement, based on schedule conditions may dictate additional samples being tested for at the time of pour. For typical schedules of concrete placement, it may not be completely necessary to get extra samples. However, for rapid scheduling, and subsequent floors that perhaps need to be supported by structures(formwork / shoring) and concrete underneath them, additional samples may be warranted for early testing for MPa strength achievement. If there are implications to schedule progression, a contractor may request additional samples from a concrete testing agency.
Curing of Concrete Overview
Typically concrete curing timelines are dictated by the mix design. Prospective strength(MPa) is dictated by concrete mix designs by specifications and provided by the supply company. Concrete mix design schedules and performance requirements will typically indicate the design expected strength MPa for set times.
Wet curing is another process where moisture is introduced to the concrete top to facilitate stronger curing and ensure proper strengths are achieved. Potential reasons for concrete not to achieve strength include temperature inconsistencies, issues with the mix design, lack of wet cure, or improper heating and hoarding for winter curing. Concrete curing is the period where concrete forms crystallized structures within itself, which is called hydration.
The concrete curing process naturally gives off heat, but even so, if curing conditions are too cold outside, it is possible cement crystals can be inhibited from forming. So that is where processes like heating hoarding are utilized, to make sure that concrete achieves strength, even in winter conditions.
Various Curing Times
Concrete comes in various MPa performance characteristics for any number of elements and structural requirements.
A typical concrete mix design strength is around the 25 to 40 MPa mark. Anything higher than this is leading to a high performance concrete category.
Understanding, that you can get concrete up to even 120 MPa, where other structures might only require ~30 MPa concrete, then proper curing times can vary per mix. Lower water to cement ratio concretes with high MPa (strengths) are what are known as high performance concretes.
The concrete element in itself makes a difference here too. The thicker a concrete element like a slab, the longer it will take to cure.
High early concretes refer to concretes which achieve higher than typical concrete mix strengths and cure quicker, such as into the 50 or 60 range at final curing. The High early mix might achieve much faster considerable strength results than lower MPa concrete, in less time (within a few days), but also cure at final ~14 or ~28 day testing at a higher strength than non-high-performance concretes.
Best Curing Conditions for Concrete
Adequate curing conditions dictate the correct timeline for curing concrete.
For instance, cold temperatures and lack of moisture may result in less than adequate concrete curing. Ideally higher temperature and a hydrated slab will result in ideal concrete curing conditions.
This proper setting and curing of concrete promotes the rapid advancement of schedule for structures that are critical to be built vertically, such as high rises for instance. Some curing methods such as wet curing, membrane curing (burlap / polyethylene) are used to ensure proper hydration for concrete slabs. Other products like concrete curing compounds or sealers can be used to improve the hydration of concrete in the curing process. They also provide the added benefit of increasing hardness or densifying concrete in some cases.
Moisture Testing
After slab cure protection from moisture and precipitation is important. After a concrete slab has achieved proper strength it no longer needs moisture / active hydration trapping to ensure proper finish.
Even after general curing, concrete can still have moisture within in it. There are a couple concrete testing procedures for moisture, including chloride testing and relative humidity of the concrete. The calcium chloride test pulls moisture from the concrete slab for acclimated concrete. Fast track construction with flooring and adhesive products need moisture testing for example. Some concrete moisture tests need concrete preparation work to ensure they are performed properly.
Summary
Concrete continues to harden over time but generally after certain periods of time, strength gains are minimal as to not warrant measurement. Plus, engineers or those parties requesting strength testing may be satisfied as long as design strengths are met after initial testing. Subsequent testing is often documented regardless.
In general concrete curing times are around 28 days for most standard concrete mix designs. Significant strength is expected near the 7 day mark. Curing is a process that continues much past these timelines.
Concrete moisture testing can also be conducted in establishing testing for conditions for application of floors. The testing of concrete samples is essential to establishing proper mix design strengths (MPa) throughout various project elements.
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