High-performance concrete is concrete with excellent workability, mechanical properties and durability that is made to meet the specific requirements of construction projects using high-quality conventional raw materials and an optimised mix design, through green production methods and strict construction measures. As an important green building material, the promotion and application of high-performance concrete in infrastructure construction is of great significance for improving project quality, reducing the comprehensive cost of the entire life cycle of the project, developing a circular economy, achieving energy conservation and emission reduction, and promoting the structural adjustment of the concrete industry.
Technical problems with ready-mixed high-performance concrete
1. Loss of slump over time
In practice, due to insufficient consideration of the loss of slump, it often causes difficulties in pumping or compacting during on-site pouring, thus affecting construction efficiency and concrete quality. Compared with on-site mixing concrete, the time difference between mixing and pouring is greatly extended for ready-mixed concrete. The supply radius of ready-mixed concrete is generally within 10-20 km.
The time difference between mixing ready-mixed concrete and transporting it to the site for pouring can be more than 1 h, including transportation and waiting time. Therefore, the loss of slump of ready-mixed concrete over time is generally large, especially when preparing concrete with higher strength or high fluidity.
When ready-mixed concrete is pumped, the lower the slump, the higher the pump pressure required. Therefore, slump is commonly used to determine the pumpability of concrete. When the slump is too small, the friction in the pipes is high. Increasing the pump pressure can make pumping possible, but at high pressure, the wearing of the pumping parts increases, the concrete is prone to dehydration, and eventually the pipes will become blocked. When the slump is too large, although the pump pressure is reduced, the concrete mix has poor cohesiveness and segregation is obvious, which can also cause blockages in the pipes. Generally speaking, when the concrete slump is controlled within the range of 160-180mm, a thin layer of low-viscosity slurry will form inside the delivery pipe wall, which is conducive to reducing the pumping resistance of the concrete mixture. For high-performance concrete, it is not comprehensive to judge its pumpability only by slump, and the cohesiveness index must also be considered comprehensively.
For ready-mixed high-performance concrete, the loss of slump of the concrete mixture is relatively fast due to the physical and chemical effects during the process of setting and hardening. Therefore, it is necessary to study the mechanism of slump loss and countermeasures to control slump loss.
2. Pumping performance evaluation indicators have not yet been determined
Pumping performance is an important aspect of the workability of ready-mixed concrete. In addition to meeting the mechanical and durability properties required by structural functions, ready-mixed concrete prepared at a mixing plant should also meet the requirements of a concrete mixture with low loss of slump over time during transportation and good pumpability during construction. Both theoretical analysis and practice have shown that during concrete pumping, the concrete mixture forms a whole ‘plug’ movement in the delivery pipeline. There is a thin layer of slurry with low viscosity between the pipe wall and the edge of the ‘plug’, and there is also a very thin water film on the pipe wall, which lubricates the concrete during transportation in the pipeline. Therefore, in order to make the concrete mixture have good pumpability, the following requirements should be met: (1) the concrete mixture can form a thin slurry layer that acts as a lubricant when transported in the pipeline, reducing the resistance of the concrete during transport; (2) the concrete mixture should have sufficient water retention and cohesiveness to ensure that the concrete does not bleed or segregate during pumping, and can be continuously and stably transported in the pipeline.
Under the action of friction in the pipeline and pumping pressure, the factors affecting the pumping performance of concrete are complex, and are closely related to factors such as cement content, water-cement ratio, sand rate, surface properties, particle size and gradation of coarse aggregates, as well as the quality of admixtures, type and dosage of admixtures, etc. Therefore, the pumping performance of ready-mixed high-performance concrete is a comprehensive performance index, and there is currently no precise evaluation index or test method.
3. Prominent early cracking of ready-mixed high-performance concrete
Ready-mixed concrete has a high cement content, a high sand ratio and small coarse aggregate particles, which results in large plastic settlement and makes the concrete prone to cracking in the early stages after pouring. Ready-mixed concrete also has a large slump (generally above 200 mm), and for many high-rise buildings, the slump of the concrete is sometimes more than 220 mm. It has been found in construction that concrete with a large slump and high fluidity is more prone to cracking in the early stages than concrete with a small slump and low fluidity.
Under certain water-to-cement ratios, a large slump requires more water per unit volume, and more water per unit volume leads to an increase in the amount of cement used. Cement is the main component of the cementitious material in concrete, and it undergoes chemical shrinkage (i.e., autogenous shrinkage of concrete) during the process of concrete setting and hardening. For high-performance concrete, when the water-cement ratio is less than 0.35, the relative humidity in the mixture quickly drops below 80%, and the volume reduction of the concrete due to self-shrinkage is about 8%, which is an important cause of cracking in high-performance concrete. In addition, due to the high unit water consumption, in the later stage of concrete hardening, as the internal water evaporates, the concrete shrinks and cracks.
In addition, ready-mixed concrete has a high sand content (generally above 40%) and coarse aggregate particles are smaller (generally 5–25 mm) to meet the requirements of transportation and pumping.