Acceptance and evaluation of concrete strength?

As we all know, in general industrial and civil buildings, concrete members have a great influence on the structure, including the foundation and the main body. Therefore, it is very important to correctly detect and evaluate the strength of concrete. How to test and evaluate? The basis mainly includes: ① Standard for Inspection and Evaluation of Concrete Strength (GBJ107-87); ② Code for Construction and Acceptance of Concrete Structures (GB 50204-92); ③ Standard for Quality Inspection and Evaluation of Construction Engineering (GBJ30 1-88) and Unified Standard for Quality Inspection and Evaluation of Construction and Installation Engineering.

I want to talk about my own understanding and views on concrete strength inspection and evaluation based on some experiences and lessons in work practice, and put them forward for discussion with you.

A, about to participate in the inspection and evaluation of concrete block curing conditions.

I think Comrade Zhang Changqing's exposition in the section "Requirements for Concrete Strength Specimen" is basically correct, but the specimen used for concrete conformity assessment is considered correct only under standard curing conditions. When the specimen cured under the same conditions is used for concrete conformity assessment, the conclusion is "obviously wrong". I think this conclusion is wrong. The reasons are as follows: GBJ 107-87 stipulates that "the standard forming method, standard curing condition and strength test method of concrete specimens used for testing and evaluating the strength of concrete shall conform to the provisions of the current national test method for mechanical properties of ordinary concrete", while the current national test method for mechanical properties of ordinary concrete (GBJ8 1-85) stipulates that "the inspection of cast-in-place concrete projects or Our construction projects in Shenzhen, especially high-rise buildings, are basically cast-in-place concrete structures. It will lead to serious consequences if it is wrong to test the strength of concrete with the test blocks cured in the same condition on site. Obviously, Comrade Zhang Changqing's conclusion is wrong.

Second, about the inspection and evaluation of concrete strength.

It is inappropriate for Comrade Zhang Changqing to simply say that non-statistical methods are only suitable for sporadic production in the inspection and evaluation of concrete.

The inspection and evaluation of concrete strength specified in Code for Construction and Acceptance of Concrete Structure Engineering (GB5024-92) is completely consistent with the Standard for Inspection and Evaluation of Concrete Strength (GBJ 107-87). However, in actual operation, we use commercial concrete. Due to the lack of historical data for calculating the standard deviation S, the "first statistical method" with known variance cannot be adopted. According to the Standard for Quality Inspection and Evaluation of Building Engineering (GBJ30 1-88), the concrete is inspected and evaluated by statistical or non-statistical methods.

To evaluate the strength of concrete, we must first determine the division of concrete acceptance batches. In GBJ 107-87 "Introduction to Several Issues", it is emphasized that the acceptance batches of concrete should not be too large or too small, and "cast-in-place concrete in the construction site should be divided according to the acceptance items of unit projects determined in the" Standards for Quality Inspection and Evaluation of Construction and Installation Engineering ". The Standard for Quality Inspection and Evaluation of Building and Installation Engineering (GBJ300-88) stipulates that a unit project should be composed of several subprojects, and each subproject is composed of several subprojects, and it is very emphasized that "the main subprojects in multi-storey and high-rise residential projects must be divided into subprojects according to floors (sections)" in building and installation engineering. On the one hand, it shows that the concrete with the same label in the foundation and the main body cannot be mixed together for statistics and bidding, on the other hand, it is pointed out that it is the most basic principle to divide concrete batches by layer (segment) in the main body division.

One of the four "guarantee items" of concrete cracking specified in GBJ30 1-88 stipulates that the strength of concrete test blocks should be tested and evaluated by statistical or non-statistical methods. Therefore, under the condition that the concrete strength grade is the same, the concrete age, production technology and mix proportion are basically the same, whether the concrete strength should be tested and evaluated by statistical method or non-statistical method depends on the number n of concrete test blocks; When n≥ 10 group, statistical method can be used, otherwise only non-statistical method can be used. However, no matter what method is used to evaluate concrete, if the concrete project is unqualified, it must be treated. The treatment, inspection and evaluation after treatment must be carried out in accordance with Article 3.0.5 of GBJ300-88.

For a building, the area of each floor is generally not very large. As far as high-rise residential buildings are concerned, the concrete quantity of each floor is generally around 3 ~ 400 m3. According to the regulation that at least one set of test blocks should be reserved for slab concrete every 100m3, every shift and every 100, generally speaking, 3 ~ 4 sets of test blocks should be reserved for each floor, which meets the minimum conditions. In any case, there will be no more than 65,438+00 groups of remaining test blocks for concrete with only 3 ~ 400 m3 per layer, so it is impossible to use statistical methods in the inspection and evaluation of concrete. Therefore, GB300-88 stipulates the hierarchical division of concrete components, which makes the inspection and evaluation of concrete components appear a very harsh condition: when the number of concrete test blocks n is less than 65,438+00 groups,

{mfcu} ≥ 1. 15fcu，k fcu，min≥0.95fcu，k

According to this set of formulas, in order to make concrete (concrete member) qualified, the average mfcu of concrete test block must reach 1. 15 times of the design standard value fcuk. This will be very difficult for concrete with high strength grade of C40 and above. If there is a slight negligence, there will be unqualified concrete batches (concrete members). This is a problem neglected by Comrade Zhang Changqing and even a large number of colleagues in charge of project management and acceptance. I will bring it up for discussion and research today.

How to solve the problem of unqualified strength of concrete test block due to non-statistical inspection and evaluation? I think there are two ways.

First of all, the site supervisor warned the contractor carefully at the preparatory meeting (or the first meeting), telling them that the concept that the main division must divide the project by layers (sections) must be firmly established, and stressed that in each concrete sub-item, there should be as many as 65,438+00 groups of concrete with the same strength grade as possible, so that the concrete sub-items can meet the conditions of inspection and evaluation by statistical methods. I think it is not difficult to solve the problem as long as the contractor's site management personnel realize the significance of retaining the 10 test block.

The second is to remind commercial concrete suppliers that if concrete batches (concrete components) are unqualified, they will suffer losses. Therefore, we should tell them that we must strengthen the quality management of concrete. In our opinion, as long as contractors and commercial concrete suppliers strengthen the management of concrete, generally speaking, for concrete strength grades of C30 and below, even if the number of concrete test blocks is less than 10, concrete members will meet the qualification conditions, because according to the provisions of concrete mix design, the trial mixing strength of concrete fcuo is obtained according to fcuo ≥ fcu.k+1.645s. Some commercial concrete factories in Shenzhen (at least a few concrete manufacturers in our OCT construction unit) still adopt GBJ204-83 Code for Construction and Acceptance of Reinforced Concrete and JGJ55-8 1 Technical Specification for Mix Proportion Design of Ordinary Concrete, that is, the S of C30-C35 is generally 5mpa, and the S of C35 is 6mpa. Thus, for C30, the part where 1.645s is higher than the design strength fcu.k is 1.645x5=8.225Mpa, and when the design strength is 30mpa, the test strength fcu.o of concrete is 38.22MPQ, which is equivalent to 1.277 of fcu. K. Therefore, for C30 concrete, as long as the concrete supplier maintains a certain level of management, practice has proved that it is no problem to make the average strength of concrete test blocks reach 1. 15 FCU. K. The problem lies in the concrete strength of C40 and above. In order to make the concrete block below 10 group, when the concrete is inspected and evaluated by non-statistical method, the batch of concrete can still be qualified, so the S value can only be appropriately increased; Or adjust the formula of concrete test strength to fcuo ≥1.15fcu.k+1.645s to solve the problem, but at this time, s should be selected according to JGJ/T55-96 instead of JGJ55-8 1.

3. As the site supervisor and the construction management personnel of the contractor, we must clearly realize that even if the number of concrete test blocks exceeds 10 groups and the concrete is evaluated by statistical methods, it does not mean that the strength values of each group of test blocks have reached the design standard values, and the concrete batch is bound to be qualified. Examples are as follows:

There are 14 groups of concrete blocks with C20 strength grade, and their values are 29mpa, 27.3mpa, 27.8mpa, 24.7mpa, 23.2mpa, 22.2mpa, 28.6mpa, 24.6mpa, 29.6mpa, 37.8mpa and 23 respectively. 1 MPa。 And because n = 14, we can use statistical methods to check and evaluate concrete, and find λ 1= 1.70 and λ2=0.90. Through calculation, we know that the standard deviation of concrete Sfcn=7.06Mpa and the average mfcu=28.55Mpa are substituted into the formula.

{mfcu} ≥λ 1Sfcu 0.9fcu, k fcu, minimum ≥λ2 fcu, k

The results are as follows:

Because λ2 fcu, k =0.9X20= 18MPa, is less than the minimum set of test blocks of 22.2Mpa, the inequality requirement of the minimum value holds;

Because λ 1 sfckt+0.9 FCU, k =1.7x8.06+0.9x20 = 31.702 MPa, which is greater than the average value Mfch, the inequality of the average value in the formula does not hold. In this way, this batch of concrete is unqualified, that is to say, the division where the concrete is located is unqualified, so the project cannot be accepted. "

Obviously, this is caused by excessive concrete dispersion and bizarre concrete strength change. The strength of concrete test blocks changes too much, and its standard difference is bound to be great, which will inevitably lead to unqualified concrete batches. The only way to change this situation is to strengthen the management of concrete mixing accuracy. In this regard, contractors and site supervisors must not take it lightly.

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