2/3 times the 28 days strength
The compressive strength of 7 days shall be about 70% of the 28-days compressive strength. If your compressive strength at 28-days is 25 MPa, then the 7-days compressive strength expected to be (0.70X25MPa)=17.5 MPa Regards, Sami
15
More than 50 %
fibre
f ′c: concrete compressive strength at 28 days (compression is negative) fcu: concrete crushing strength (compression is negative)
The compressive strength of M20 concrete at 28 days typically falls in the range of 20 to 25 megapascals (MPa), which is equivalent to 2900 to 3600 pounds per square inch (psi). M20 is a mix designation for concrete, where "M" stands for "mix" and the number 20 represents the characteristic compressive strength of the concrete mix in MPa. Please note that the actual compressive strength of M20 concrete can vary depending on several factors, including the quality of materials used, the water-cement ratio, the curing conditions, and the testing procedures. To ensure the desired strength, it's essential to follow the recommended mix proportions and proper construction practices during the preparation, placing, and curing of the concrete. Additionally, testing of concrete samples is typically conducted in a laboratory environment to determine the 28-day compressive strength accurately.
Compressive strength of concrete is defined as the concrete cube of 150 mm x 150 mm x 150 mm with specified proportion with 28 days curing.Target mean strength - In order that not more than the specified proportion of test results are likely to fall below the characteristic strength, the concrete mix has to be designed for a somewhat higher target average compressive strength (fck).__fck = fck + t * swhere ___fck is target average compressive strength at 28 days,fck is a characteristic compressive strength at 28 days ands standard deviationt a statistic, depending upon the accepted proportion of low results and the number of tests.
It is attributed to two reasons: 1. Strength of concrete grows faster in initial days and this process of gaining strength slow down with time. This can be said as stabilization in value of gained strength of concrete. After approximately 20 or some more days, strength value get stabilize and does not change too much with time. Therefore, compressive strength is taken after approximatley 3 weeks time. 2. These tests have to performed practically and date record is needed to be maintained. Therefore, if test duration is a multiple of 7 (7 weekdays) then it is more easy to remind. It is the reason we have 28=7x4 days to note down compressive strength of concrete. for ex: if we start test on monday then we know that after 4 weeks on same time on monday we can finish the compressive strength test.
The compressive strength of M25 concrete at 7 days will be between 6-7.5 N/mm2 .
35N/mm2 actually, the '35' respresents the characteristic design strength of the concrete. this strength is pivotal in design as its the 'lowest fair estimate' of strength.in reality tere is a margin for error (between 5-10MPa)and a 5% defectives built in to a normal probability curve.in my latest research i have found that in some cases the characteristic design strength (in your case 35MPa) can almost be underestimated by 20%.the importance is on-site testing, but as previously stated, if you design to 35MPa... you are definitely on the safe side of concretes inevitable variability
concrete strenght gain at first 7 days much increase so we consider 7days strenght
The modulus of elasticity of concrete is denoted by Ec .Ec = 5000 sqrt fck as per IS standardswhere, fck- The compressive strength of concrete at 28 days in N/mm2 .