by For example, the measurement of concrete compressive strength of an actual concrete project: the rebound value data of the test piece is 36 ~ 38Mpa, and the failure data of the concrete pressure piece is 43 ~ 45MPa; The concrete strength of another project: 43 ~ 45MPa according to the rebound value of the specimen and 63 ~ 68mpa according to the compressive failure data of the concrete.
The surface layer of concrete component after setting and hardening has greater deformation and buffer space under the action of external force.
For example, there is no definite corresponding relationship between non-destructive rebound test and 28d specimen compressive strength, and between core drilling test and rebound test and 28d specimen compressive strength.
The data of rebound value can only play the role of inference or estimation of concrete strength.
Concrete is a widely used bulk building material.
The good and poor workability of the mixture directly affects the compressive strength of the concrete after setting and hardening, that is, if the workability is good, the concrete will be dense and the higher the strength.
However, the rebound value obtained from the rebound test is not necessarily high and may be low.
It is required that the measurement result of rebound inspection should be one to two strength levels higher than the design strength, otherwise core drilling inspection is required.
From the traditional on-site mixing to today’s modern commercial concrete (ready mixed concrete), especially the scientific development of concrete materials in recent 20 years, mineral active admixtures and high-efficiency multi-functional composite admixtures are widely used to produce high-strength, high-performance and high fluidity concrete, which provides good workability for concrete engineering construction, and has the characteristics of crack resistance and seepage prevention, improving later strength and pumpability.
Its theoretical basis is that there is no coarse aggregate, the mixture is nearly homogeneous and symmetrical, has better construction performance, there is no weakest transition zone in the concrete, and the strength of the component surface, inside and each layer is very uniform.
Using the traditional classical rebound method to detect and estimate the strength of modern commercial concrete will only mislead the evaluation of on-site concrete quality.
This acceptance method is contrary to national standards.
The recognized conclusion is to reduce the particle size of coarse aggregate and improve the bonding performance and utilization efficiency of cementitious materials.
In order to be in line with the world cement quality level, the specific surface area of cement is more than 350m2 / kg and the fineness is reduced to less than 2%.
Therefore, it is very difficult to get a set of symmetrical test data by springback in a test area on the component surface or in a building.
As for the rebound test, the main reason why the rebound value of concrete components is low is that the quality of cement in China has changed qualitatively.
It is unreliable to evaluate and accept the engineering structure based on this data.
The strength reaches 800MPa ~ 200MPa.
In the project, the average particle size of expressways above C40 is 5 ~ 20mm; The average particle size of pipe pile above C80 is 5 ~ 12mm; The average particle size of foreign coarse aggregate is mostly 5 ~ 12mm, and that of China is mostly 5 ~ 31.5mm.
It also brings wrangling and shirking of responsibility to the construction party, the construction party and the concrete supplier.
In other words, the rebound value indicates the surface hardness of the material, and strength are two different concepts.
However, for concrete, which uses the simplest manufacturing process and such a complex heterogeneous material, there is no unique relationship between rebound value and compressive strength; Not only do different strength grades of concrete have the same hardness compressive strength relationship, but even concrete with the same strength grade does not have the same composition and microstructure; Even for a given concrete, there will be different rebound values due to the different hardness between aggregate and matrix and the change of aggregate in mineralogy.
Why use this method to test? Obviously unreasonable..
However, there is still a lack of systematic and scientific specifications for the quality inspection and strength evaluation of concrete engineering components after setting and hardening.
The more uniform the aggregate in the mortar matrix, the more balanced and stable the strength will be.
At present, the only mandatory national standard is gb50107, which stipulates that random sampling at the pouring site shall be carried out for concrete inspection and evaluation based on the same condition curing and standard condition curing.
Comparing the two sets of data fully shows that there is a relationship between material surface and material strength, but it is not the only correlation.
The method of evaluating concrete strength fairly and scientifically is to implement the existing standard gbj50107.
This practice limits the comprehensive utilization of industrial waste residue and mineral admixtures to a great extent; It is also not conducive to the promotion, application and implementation of national environmental protection policies; It is not in line with the objective development law of concrete material science.
The fundamental reason why the concrete after setting and hardening is multiphase heterogeneous material is that there is the weakest transition zone around the coarse aggregate, and its failure strength characteristics – mechanical properties show nonlinear changes.
Although the core sample test can truly reflect the internal strength of components, some construction sites add water at will during the construction process, resulting in the increase of water cement ratio and the decrease of strength, resulting in the considerable dispersion of core sample results, which can not be statistically evaluated.
However, some local construction competent quality supervision departments always take rebound inspection and core drilling inspection as the basis for inspection and acceptance of safety and quality inspection every quarter of the year, and misunderstand that it is a cost-effective commercial behavior to mix common active mineral admixtures such as fly ash and slag powder into concrete; It is thought that adding water reducing agent is simply to facilitate concrete transportation and prolong setting time.
In order to solve the uneven strength of concrete structural materials, experts and scholars all over the world continue to conduct in-depth research.
At present, as a new type of cement-based structural material, reactive powder concrete (RPC) without coarse aggregate has become a reality in France and has been applied in engineering.
However, it is conceptually wrong to use the classical rebound test method and introduce the “reduction coefficient” to calculate the strength of concrete based on the thickness of carbonated layer.
Random sampling shall be conducted at the pouring site, and the strength of concrete specimen shall be taken as the evaluation basis of acceptance project.
