1. Significant Changes in Cement Properties

After nearly 30 years of rapid development, cement properties have undergone significant changes. (1) Early strength of cement has increased, while later strength growth has slowed; at 60-90 days, cement mortar strength shows little or no growth, sometimes even declining. (2) Cement is becoming finer, hydration speed is increasing, and heat release is concentrated, increasing the temperature difference between the inside and outside of the concrete and leading to an increase in micro-cracks within the concrete structure, increasing the chance of cracking. (3) Excessive admixture amounts cause cement properties to differ significantly from those of traditional silicate cements. (4) Widespread use of grinding aids has decreased the compatibility of cement with admixtures. Therefore, researching cement properties, rationally utilizing cement characteristics, and formulating concrete that meets engineering design requirements is essential.

2. Difficulty in Distinguishing Genuine and Counterfeit Fly Ash

Fly ash is a crucial material in concrete, significantly affecting its properties. However, the market has fly ash of varying qualities, making it difficult to distinguish between genuine and counterfeit products. Some severely substandard fly ash, according to current standards, is difficult to assess for purity; it's even impossible to distinguish genuine from fake. Common examples include "desulfurization ash," "denitrification ash," and "floating black ash," which are often passed off as high-quality products. Standard testing cannot differentiate them, and their use severely impacts concrete quality in practice, leading to problems such as high water demand, poor workability, rapid slump loss, and cracking.

3. The Need for Synchronous Control of Sand Quality

Fluctuations in sand quality currently affect concrete quality, primarily manifesting in changes in the density of close packing, stone content, water absorption, and gradation. Variations in sand density of close packing are mainly due to the diversification of mineral resources, changes in the density of the parent rock, and fluctuations in the content of stone powder in manufactured sand. Changes in sand stone content primarily result from sand segregation during production, transportation, and stacking. During concrete production, loaders tend to scoop up the outer sand containing coarse particles first, and then use the sand with more fine powder in the middle, leading to different contents of small stones over 4.75mm in the concrete production process. Variations in sand water absorption are due to differences in the open porosity of the parent rock, sand powder content, and sand moisture content. Poor sand gradation is mainly caused by segregation during stacking. This requires concrete companies to establish feasible measures to monitor changes in sand quality during production.

4. The Need to Quantify the Goals of Concrete Durability Design

Concrete is a man-made stone. To improve its durability to match that of natural stone, the mortar must completely encapsulate the aggregates to create a unified whole that resists external forces. The interfacial bond strength between the mortar and aggregates should be high enough to prevent cracking and aggregate-mortar separation under external forces. The mortar itself must be uniform, dense, stable, and free from defects to prevent corrosion from aggressive media. To improve the integrity of concrete and achieve complete encapsulation of aggregates by mortar, the optimum amount of cementitious material must be determined. To improve the uniformity of concrete texture and prevent aggregate-mortar separation under external forces causing cracking, a reasonable mix ratio and production process must be determined to improve interfacial bond strength. To prevent the penetration of aggressive media from damaging the concrete, the density of the concrete mortar must be increased to improve its impermeability. In addition, admixtures that enhance concrete durability should be used, such as air-entraining agents, expansion fiber crack-resistant waterproofing agents, and crack-resistant anti-corrosion waterproofing agents.

1. Controlling Slump Loss of Concrete Mixtures

Slump loss is a common problem in concrete construction. The main causes are cement, slag powder, and fly ash. Increased clay content in natural sand and increased powder content in manufactured sand absorb a corresponding proportion of admixtures, increasing slump loss. The main current solution involves adjusting the workability of concrete with water reducers, placing higher demands on the technology of water reducers.

2. The Need to Eliminate On-Site Secondary Water Addition

Adding water on-site is a common problem in current concrete construction. One reason is the blind addition of water by concrete workers to reduce their workload, which severely affects concrete strength. Another is the addition of water due to high viscosity and low slump of the concrete mixture, making it difficult to pump. Therefore, accurately analyzing the causes of secondary water addition during construction is necessary. For the first case, strengthening management can resolve the issue; for the second, technical measures must be implemented. Therefore, it is essential for R&D personnel to develop reasonable solutions.

3. Using Substandard Raw Materials to Produce Qualified Concrete

The quality of concrete raw materials currently varies greatly. Using substandard raw materials in practice to prepare concrete according to individual standards causes various problems. The mix ratio used in concrete construction should be tested based on the actual raw materials. Based on the different properties of the raw materials, the sand ratio, water-cement ratio, and aggregate proportion should be adjusted to produce qualified concrete, rather than using coarse sand with a fineness modulus exceeding 3.0 and high stone content while maintaining the normal sand ratio, resulting in poor workability, poor encapsulation, and easy bleeding.

4. The Importance of Admixtures in Concrete

Admixtures have become an indispensable part of concrete, particularly water reducers, which are widely used in the precast concrete industry. The diverse requirements of different projects and the varied performance requirements of concrete are met through the addition of admixtures. This is especially true now that the problems with concrete raw materials are becoming increasingly severe, leading to higher demands on admixtures. Therefore, accurately identifying the function of admixtures in concrete is necessary.