Everyone wants to control the quality of concrete. In fact, the biggest difficulty is that there is no way to start after exhausting all the knowledge in textbooks. It is useless to ask old experts when encountering difficult problems. It is difficult for experts to focus on the crux of the matter without going to the scene and talking in general terms. From a technical point of view, controlling the quality of concrete means controlling every link and process. For example, if the mixing ratio is done well, there will still be problems. That is because you cannot guarantee other links. A master is not full of book knowledge, but has a lot of practical experience. The biggest feeling of many masters is that he understands the properties of various raw materials, not only superficially, but also small details that others do not know. If you only know the common knowledge of the materials commonly used in textbooks, it is no different from others. It is not easy to make comprehensive use of raw materials to prepare concrete that meets the performance requirements. For example, if you know a little more about fly ash than others, it is possible to compound better than others. When problems arise, others will The knowledge base is incomplete, so you don’t know it after much deliberation. You solve the problem by thinking of the knowledge points that others have not mastered.
Four characteristics of cement changes:
The bulk of concrete is cement. Cement is one of the most important factors affecting the quality of concrete. After the 28d age, the strength does not increase, the heat of hydration is high in the early stage, and the cracking is obvious in the early stage. These are some problems that ordinary people cannot understand in the cement used in concrete production. Although there are many factors affecting the quality control of concrete, since the 1920s, from the perspective of the development background of the cement industry, although the appearance of cement has not changed significantly, the mineral composition and strength of cement have changed with the modernization of the cement industry. Great changes have taken place, and these changes have had an important impact on the quality control of concrete today. A. M. Neville pointed out in the book "Properties of concrete" (Fourth Edition) that changes in cement can be summarized in the following four points:
1. The C3S content increased significantly, while the C2S content decreased accordingly
Let’s talk about the UK. In 1960, the C3S content was only about 47%, and in the 1970s it became about 54%, while the C2S content decreased accordingly, and the total calcium silicate content remained at about 70-71%, which is basically not changing. In the mid-1960s in France, the average C3S content of Portland cement was 42%, which increased to 58.4% in 1989, while the C2S content decreased from 28% to 13% at the same time.
2. The strength of early cement mortar increases
Another feature is that the strength of cement mortar varies greatly, mainly because the 7d strength with a fixed water-binder ratio increases significantly, and the 28d strength is also greatly enhanced. However, another very important indicator has not improved, and that is the strength beyond the 28d age. We know that in 1970 a water-cement ratio of 0.50 was required to achieve a concrete cube strength of 32.5MPa. In 1984, a water-cement ratio of 0.57 was required. Keeping the water consumption per cubic meter of concrete at the same 175kg/m3, assuming the work performance remains the same, the cement consumption is reduced from 350kg/m3 to 307kg/m3. In order to be more easily penetrated by carbonized or eroded substances, we gradually adopt higher water-cement ratio and lower cement content to make concrete more permeable. In this way, the durability of general concrete is lower. The rapid gain of early strength also means that sufficient formwork strength is achieved sooner than in concrete using "old" cement, thus stopping early effective curing. As a result, the performance of concrete cannot be improved in the long term.
3. The ratio of cement mortar 28d strength to 7d strength decreases
The strength of cement mortar in the first 28d age increases rapidly, resulting in a decrease in the ratio of 28d strength to 7d strength, which is due to the increase in the ratio of C3S content to C2S content, which is affected by its hydration rate. The ratio of cement mortar 28d strength to 7d strength appears to be lower at low water-binder ratio. This empirical theory leads to more alkali in modern cement. Every cement manufacturer adds alkali secretly, and it is not fixed. It will flexibly add alkali according to the customer's situation and meet the customer's requirements. In 1923, the compressive strength of concrete was linear with the logarithm of age to 25 or 50 years, which was the result of using high C2S content, low fineness cement to formulate concrete. In 1937, the logarithmic relationship between compressive strength and age was about 10 years, which was the result of using low C2S content, high fineness cement to formulate concrete. In recent years, with shorter relationships. This trend can be seen from Figure 3.
4. Continuous improvement of cement fineness
Since the cement industry now adopts new production equipment, which is much more advanced than in the past, especially the continuous updating and progress of new grinding equipment, the specific surface area (fineness) of cement has steadily increased. It should be pointed out that many of us are now gambling with the quality of concrete and our own pocket money. To increase the specific surface area of cement, many people think that it is necessary to reduce production costs, which is also the way to survive. This is a long-term game. Increasing the specific surface area of cement can increase the early hydration rate and strength, thereby increasing the price of cement, but this will inevitably lead to excessive grinding, and the production cost will increase. Therefore, cement manufacturers are more keen to make full use of technological updates of production equipment and find a balance between the two. Now there is a good way, that is to use grinding aids. Adding grinding aids is a common production method to increase the fineness of cement.
Now the cement is very different from the cement 100 years ago. Many existing high-strength cements can use high C3S, low C2S, and high fineness to achieve early high-strength goals. Although the use of "high-strength cement" can reduce the amount of cement with specified strength, speed up formwork removal, and speed up the progress of the project, so that the construction general contractor can obtain huge benefits, but the high water-binder ratio and low cement amount will easily lead to high permeability of concrete. Less durable. Excessive early strength and premature abandonment of the effective maintenance of the formwork also pose safety hazards to the quality of the project.
Others Question:
1. Guarantee rate of cement strength
Mehta mentioned in "Concrete Microstructure, Performance and Materials" that a cost-effective quality control system must be based on statistical methods, and ready-mixed concrete production must be adapted to this. The realization of this goal must be based on the following principles: First, random sampling, to ensure that the samples are random in the process of collecting data, not artificial cheating. The second is to adopt a bell-shaped normal distribution curve, as shown in Figure 5.2-1A below. Statistically, quality control is performed using control charts, which graphically represent the results obtained in a continuous testing program. When the plotted curve approaches or exceeds the upper or lower limit in the graph, it indicates that corresponding measures should be taken.
To adopt statistical methods, there is no shortcut, this is a dead theory, whoever does it, and does it finely, can control the strength of cement. For small-scale cement production enterprises, only controlling the strength of 42.5 grade cement mortar is greater than 42.5MPa, the standard deviation fluctuates greatly, and the maximum strength value is low, which obviously deviates from the principle of mathematical statistics. Therefore, the cement used in concrete production must choose a manufacturer with a high strength guarantee rate to control the risk caused by quality fluctuations.
2. Cement used in high performance concrete
As can be seen from the foregoing, the current cement market is a typical seller's market, and concrete producers use cement passively. It is necessary to propose more reasonable technical performance indicators to cement manufacturers from the perspective of concrete engineering application, so as to solve the problem of concrete performance improvement from the root. Avoid passive use of a single cement product proposed by cement manufacturers, resulting in insufficient durability of concrete and other issues.
The "Technical Guidelines for the Application of High Performance Concrete" jointly issued by the Ministry of Housing and Urban-Rural Development and the Ministry of Industrialization Information stipulates that the specific surface area of Portland cement and ordinary Portland cement should not exceed 350m2/kg; slag Portland cement, pozzolanic silica Salt cement and fly ash Portland cement are represented by sieve residue, and the sieve residue of 80μm square hole sieve is not more than 10% or the sieve residue of 45μm square hole sieve is not more than 30%.
3. The composition of cement is complicated
In recent years, the relevant technologies for coordinating domestic waste and municipal sludge in cement kilns have become increasingly mature. It plays an important role in solving urban syndrome and building a better urban life. However, the source of domestic waste is complicated, and the composition of calcined ash also changes greatly. How to ensure the cement production and use concrete production, but avoid the deterioration of the compatibility of cement and admixtures, is a problem worthy of attention.
The change of cement is gradual, and it is necessary to fully understand its composition change and the development law of performance indicators, combined with engineering characteristics, to give full play to the overall advantages of cement concrete.
