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Analysis of the Causes of "True" Unqualified and "False" Unqualified Concrete Strength

Thomas Huang posted this article in June 30, 2020

Analysis of the cause of "true" failure of concrete strength
1. Poor quality of raw materials
(1) Poor quality of cement
①Cement actual strength
 The actual strength of cement cannot be very stable, and the production enterprises with good quality control have little fluctuation, but the production enterprises with poor quality control have large fluctuations in their 28d actual strength. Occasionally, there will be a difference of more than 10MPa, which directly affects the strength stability of concrete. It may even cause insufficient strength.
②Cement stability is unqualified
 It is caused by excessive content of free CaO and MgO in cement clinker or too much plaster. CaO and MgO are both "overfired" after high-temperature calcination, hydration is very slow, and in severe cases, it can crack the cement stone. When the content of gypsum is too much, after the cement has hardened, it will also react with the solid cement calcium aluminate to generate high-sulfur hydrated calcium sulfoaluminate, with a volume increase of about 1.5 times, causing cracking of the cement stone. Although the concrete surface prepared by some unsatisfactory cement has no obvious cracks, the strength is extremely low.
(2) Poor aggregate quality
① When the strength of the stone is lower than the strength of the concrete, the strength of the concrete is insufficient.
 ②The total crushing value of artificial sand is large. The total crushing value of the artificial sand has a significant effect on the strength of the concrete. Two kinds of artificial sand with a total crushing value of 30% and 15% are used to prepare the concrete. The former is about 3 to 4 MPa lower than the latter.
 ③The large amount of mud in the aggregate, the large content of the mud block, the high content of harmful substances, the high content of shells in the sea sand, and the large content of needle flake particles in the coarse aggregate, etc., reduce the strength of the concrete.
 ④The volume stability of the stone is poor. Some crushed stones made of porous flint, shale, limestone with expanded clay, etc., under the action of alternating wet and dry or freeze-thaw cycles, often exhibit poor volume stability, which leads to a decrease in concrete strength.
(3) The quality of mixing water is unqualified
The high content of organic impurities in the mixed water, or the use of sewage and industrial wastewater to mix concrete, will cause the concrete strength to decrease.
(4) The quality of admixture is unqualified or the composition ratio is improper
At present, the market for concrete admixtures is highly competitive, and the result of the price war has caused the phenomenon that the raw materials are sub-charged from time to time. One of the reasons is that users are excessively pursuing low prices.
Improper composition ratio of admixture: In antifreeze, the amount of antifreeze composition is insufficient; in pumping agent or retarder water reducing agent, the amount of retarder is too large or hydroxycarboxylate retarder does not vary with the use temperature Increase and decrease by lifting; too much early strength agent dosage, improper proportioning, etc., will cause the concrete strength to decrease.
2. Improper concrete mix ratio and production control
(1) The water consumption is increased, the moisture content of aggregate is not measured in time, and the production mix ratio is adjusted according to the measured moisture content.
 (2) The test of the mix ratio is not verified, and the mix ratio is freely applied.
 (3) If the admixture is excessively mixed, the concrete strength will be permanently insufficient. Similarly, a small amount will reduce the strength of concrete, especially additives with water-reducing function, such as pumping agent, water-reducing agent, etc., due to the reduction of the amount, often increase the amount of water, resulting in insufficient concrete strength.
 (4) The metering device is out of calibration and the production mix ratio is seriously out of control, resulting in insufficient concrete strength.
 (5) The mixing time is too short or too long, resulting in uneven concrete.
 Analysis of the causes of "false" failure of concrete strength
1. Reasons for the demand side
 (1) Individual demand parties have impure motives, come up with false reports of non-conformity or do not produce reports at all, but use unqualified intensity as a reason for debt, or propose to deduct part of the payment.
 (2) At the time of pouring, water is randomly added to the concrete, the vibration is not real, the formwork is seriously leaking, etc.
 (3) Improper curing of the poured concrete, such as early water shortage and drying, freezing.
 (4) Poor management of test blocks.
2. Testing equipment and methods
(1) Use the rebound method to detect the strength of structural solid concrete
 The rebound method has certain limitations and the accuracy is not high. At present, the instrument commonly used in China to detect the strength of the concrete of structures or members is a rebound meter. According to the requirements of the regulations, the rebound meter should be subjected to a rate test on the steel anvil before and after engineering inspection. The rate average value of 80±2 is in line with the requirements .
A high or low rate setting will directly affect the rebound hardness value of the structural solid concrete. If two rebound instruments with a time rate average value of 82 and 78 are used to detect the rebound of the concrete structure, if the difference is 3 times The estimated value of the spring value differs from 5 to 7 MPa. If the deviation of the steel anvil used in the rebound hammer is added, the error is greater when the two are superimposed.
Therefore, the accuracy of the rebound hammer has a significant impact on the estimated strength, and there is a large error. Only the accuracy of the rebound hammer can cause the difference between so-called "qualified" and "unqualified".
(2) Drilling core method to detect concrete strength
The core-like strength does not fully represent the concrete strength in the structural entity. Because the core sample will cause interference and injury during core drilling. The accumulation of this damage will reduce the actual strength of the core, especially for high-strength concrete. In addition, the drilled core samples should be ground flat or grouted before the test. The operation error will cause the core sample compressive strength to be 20% to 30% lower than the actual structural strength; and the operation is complicated, the cost is high, and the structure and damage Core sample.
(3) The influence of the testing machine on compressive strength
Due to the differences in various characteristics of the testing machine, it has a great influence on the results of the concrete strength test. The test pieces of the same quality are tested on different test machines, and the strength difference can sometimes reach more than 20%; even with the same test machine, the test results fluctuate considerably. The difference is not only reflected in the strength, but also the deformation and failure mode of the test piece. different.
 3. Sampling inspection
The homogeneity of concrete is very poor, the system is complex, with the uncertainty of microstructure and the uncertainty of performance. For a certain structural concrete, in order to make the sampling representative, the number of strength samples is generally not less than 10 groups. Too small a quantity is difficult to represent the overall strength of concrete, and the probability of misjudgment is quite large.
But in actual operation, it is impossible to take a large number of samples, which will inevitably affect the representativeness of the inspection results, and it is inevitable to cause negligence errors, especially when only one set of concrete test pieces is formed in the same inspection batch. The occurrence of false "non-conformity".
4. Incorrect thinking
According to regulations, the acceptance of concrete strength is assessed by "batch" rather than by "group". However, there are still many construction units or project supervisors who determine whether they are qualified according to the "group" of the test pieces. Therefore, it is not allowed that the 28d strength of the test piece is less than 100% of the strength standard value, and even requires that it must reach more than 115%; There are also cases where the 7d strength must reach 100%. This is a misconception, does not meet the requirements of the assessment standards, and does not comply with the regulations.
Regardless of the standard or the procedure, the strength pass probability considered is 95%, not 100%. It is entirely possible that the representative value of the strength of a small number of specimen groups is less than the standard value, and it is normal. As long as it is greater than the minimum acceptance limit, the acceptance lot can still be assessed as qualified.

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