02:22:46 AM 1/26, 2021

How to Confirm the Strength of Concrete ?

2020-08-183Thomas Huang

Sand, gravel and cement are mixed together, and concrete is artificially made stone. This artificial stone is divided into different strength levels.
 So the question is, how to check the strength of concrete? First make a test block.
 A mold is needed to make the test block:
 Q1: Why is the concrete compressive test block square, but the impermeable test block is round?
 A: The compression test block is to test the strength of concrete, and the impermeability test block is to test the waterproof performance of concrete. The impermeability test is to inject water from the middle to pressurize. The shape of the penetration is round, so the shape of the test block is also cylindrical.
 Before making the test block, brush the inner surface of the test mold with a layer of clean engine oil as a release agent.
 Q2: Can edible oil be used as a release agent?
 A: The main function of the release agent is to prevent the concrete from sticking to the mold. Edible oil is also good.
 Then, follow these 6 steps to carefully vibrate and compact.
 Q3: There is a thick layer of scum on the upper layer of the made test block, and the scum has no strength at all. Why?
A: At present, most concrete is pumped concrete, and the sand rate (the proportion of sand) is very large. The top layer of scum is caused by the sinking of the stones after vibrating. The reason for this is excessive vibration.
 Solution: After the test block is vibrated, select a few stones from the mixed concrete (do not add dry stones) and add them.
 Q4: The test block on the construction site has not been subjected to parallel inspection. The company has come to inspect and let us do the parallel inspection. How many days should the test block be cured? How to notify?
 A: Parallel inspection is a kind of supervision method. The supervision method is divided into side station, inspection and parallel inspection.
 Parallel inspection here means that the construction unit makes test blocks, and the supervisor also makes test blocks, each of which is used to test the pressure to see if there is any fraud when the construction unit makes the test blocks. The method of the test block test is completely consistent with that of the construction unit. However, this method is rarely used, because the supervision will be tiring to do so.
 Q5: This is a question of preventing human cheating. How can concrete test blocks be prevented from being dropped during the production process in an intelligent way at low cost?
 A: The easiest way is to bury the chip under the supervision of the supervisor when making the test block.
 Each set of test blocks is 3 pieces. Standard curing for 28 days.
 Q6: Can the test piece be soaked in water? Can it be buried in the sand?
 A: The standard curing test block requires constant temperature (20℃±3℃) and constant humidity (90% humidity). If the humidity is too high in water, the temperature may not be 20℃±3℃. The standard curing test block is generally in the standard It is carried out in the raising room or the standard raising box.
 Q7: Will the concrete test block be cured in water to reduce its strength?
 A: Not really, because the process of cement hardening is a chemical reaction with water.
Take the test on the 28th day until the test block is crushed. At this time, the force recorded by the testing machine divided by the area (225 square millimeters) is the strength of this test block.
One set of test blocks is 3 blocks. What is the strength of this set of test blocks? It needs to be calculated like this.
 Q8: When testing the strength of concrete test blocks, are there requirements for the temperature and humidity in the laboratory?
 A: No, the temperature and humidity requirements are put forward for the maintenance of the test block.
 Q9: Is the compression test block stronger as the time is longer?
 A: Yes, but the growth of concrete strength is fast in the early stage and then slower and slower.
 Q10: What should I do if the test block is overdue during holidays and Spring Festival?
 A: Some experimental studies show that after 28 days, the strength increases very slowly, but it is still increasing. If the purpose of the test is not to conduct scientific research, but to test the strength of concrete, the effect of a few days beyond the expiration date is not too great. However, this intensity is not the 28-day standard curing intensity after all, so it cannot be evaluated.
 Q11: Why should the 100×100×100 test block and the 150×150×150 test block be subjected to the compression test, the results can be equivalent?
 A: Correct your mistake. When sampling, you can make a cube test block of 100×100×100, or a test block of 150×150×150. The compressive strength obtained from the test should be multiplied by a conversion factor of 0.95. , The data obtained is the compressive strength of this group of test blocks.
 Every time the concrete is poured, it does not mean that only one set of test blocks is finished. Many sets are needed.
 Q12: Is the concrete test block of the concrete cast-in-place pile retained as standard or the same conditions? Which standard?
 A: There must be one set of test blocks for every 50m3 of pouring. For piles smaller than 50m3, there must be one set of test blocks for each pile.
Q13: Do I need to retain test blocks for masonry structural columns? How many sets should be retained if necessary? Thank you!
A: Each inspection lot of masonry structural columns shall not be less than 1 set of test blocks, and there shall be no less than 3 sets of test blocks in total.
 What I just talked about is the standard maintenance test block. In addition, the same condition maintenance test block must be made.
 The day when the test block is cured to 600°C under the same conditions.
 What is the significance of keeping several sets of test blocks in the same condition?
 Q14: Why do we need to leave the demoulding test block?
 A: The demoulding test block is mainly used to judge whether the concrete strength meets the strength requirement of formwork demolishing, and it also needs to be maintained under the same conditions. The collapse of the cooling tower construction platform of Jiangxi Fengcheng Power Plant was caused by the low strength of concrete when the mold was removed. If the test block test of the mold removal is done carefully, such a safety accident can be completely avoided.
 Q15: What is the significance of leaving a set of 7-day compressive strength test blocks?
 A: Sometimes we consciously do more test blocks, mainly to prepare for unexpected situations. For example, if the pressure test of the demolding test block does not reach the strength, the template needs to be demolished a few days later, and after a few days, you can take the spare test block to test the pressure.
 Q16: Will the concrete test block of the same condition expire? Which description is regulated?
 A: Test the pressure at 600℃, it is overdue if it exceeds 600℃.
 Q17: How to make the critical concrete block?
 A: Critical strength is the minimum strength that concrete poured in winter must reach before freezing. The critical strength test block is also a curing test block under the same conditions to determine whether the concrete can be removed for heat preservation.
Q18: The difference between the delivery inspection test block and the standard maintenance test block
A: The delivery inspection test block is not equal to the opening appraisal test block.
 Concrete is a special commodity. When the concrete mixing plant is delivered, it is impossible to judge whether the concrete can reach its strength. After delivery, if the site maintenance conditions are poor, the concrete strength will also be affected. Once there is a strength problem, both parties are easy to wrestle.
 When the concrete is delivered, a set of test blocks are made, cured in accordance with the standards and then pressure tested to prove whether the concrete delivered by the mixing station is qualified.
 Q19: There is a failure of the concrete curing test block and the standard curing test block under the same conditions, how to evaluate the eligibility?
 A: This question is very well asked. This is the question we will talk about next.
 Concrete strength evaluation.
 There are two methods for evaluation, more than 10 groups use statistical methods, and less than 10 groups use non-statistical methods.
 Q20: Some test blocks are too strong, are they fake test blocks?
 A: It's because some construction units have falsified. The strength of the test block is ridiculously high, and it is indeed suspected of fraud. There are local standards in various places. For example, if we believe that the strength of the test block is 2 levels higher, it is judged as a fake test block.
 Q21: There are batches of failures in the compressive resistance of C45 concrete test blocks?
 A: The strength of individual test blocks is lower than the standard value of strength. It is not too low. The assessment may still pass, but if many test blocks fail, the final assessment of such concrete must be unqualified.
 Q22: Should the strength of C35 underwater concrete test block be greater than 35MPa or greater than 40MPa?
 A: In order to ensure the quality of underwater pouring, the mix ratio of underwater concrete should be one grade higher than conventional concrete for trial mix. The acceptance is still based on the design strength, which is still 35MPa.
 Q23: According to the specification, the sample size of an inspection lot should be three consecutive sets of test pieces. If there is only one set of concrete test blocks, how to evaluate it?
 A: If there is only one set of test blocks, it must be greater than the design strength.
For the evaluation of concrete strength, firstly make standard curing test blocks, but also keep the same condition test blocks.
In the case that the test block under the same conditions is unqualified, the rebound test can be performed. If the rebound test is still unqualified, core drilling is required. If it still fails, then you need to:
 Q24: The rebound strength of concrete is insufficient, what should I do if the strength of the test block is qualified?
 A: The data obtained by the rebound method to test the strength of concrete is conservative. There are two possibilities for this result: 1. The strength is qualified, 2. The test block is falsified.
 Q25: What should I do if there is a set of digging piles without test blocks but the upper structure has been constructed for 2 floors?
 A: In this case, there is no rebound or core drill to sample. It is really difficult. If it is continuously poured concrete, it seems that there is no basis for an approximation. If it is necessary to know whether the pile is qualified, a load test may be the best solution.


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Email:thomas@camelway.com;whatsapp:+86 19937387378

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