Features of aluminum formwork construction
Why do aluminum formwork results in lower strength tests than wooden formwork when using the same batch of concrete in the same project and at the same age, especially during winter construction? This is mainly due to the inherent properties of aluminum alloy, such as high thermal conductivity, low water absorption, and good airtightness. To address this issue, we need to face the current situation, analyze the reasons in depth, and formulate targeted preventative measures to continuously improve the quality of concrete structure engineering.
Compared to wooden formwork, aluminum formwork has significant advantages such as greater overall rigidity, higher structural stability, and a higher number of reuses, and it aligns perfectly with the concept of high-quality green development, thus becoming the mainstream formwork system in construction engineering.
At the same time, we must confront the problem: the rebound strength of aluminum formwork concrete on current construction sites generally does not meet the standards. Analysis of relevant cases in recent years reveals the following main reasons for this problem: Aluminum formwork has high thermal conductivity, resulting in rapid heat loss during concrete hydration and slow early surface strength development, especially during winter construction.
Aluminum formwork has low water absorption, resulting in a high water-cement ratio on the concrete surface. Vibration can easily lead to the formation of a low-strength laitance layer on the surface.
Aluminum formwork has good sealing properties, making it difficult for internal air bubbles to escape during concrete pouring, leading to the formation of closed air bubbles on the surface.
Addressing these problems and analyzing their causes aims to implement precise measures and preventative measures to reduce the incidence of concrete surface defects and substandard rebound strength during aluminum formwork construction. The following are relevant recommendations: Due to the high thermal conductivity of aluminum formwork, which slows down the early hydration process, the demolding time must be accurately determined based on the ambient temperature to prevent premature demolding and subsequent damage to surface strength. During the curing stage, the concrete should be fully covered and watered, or a hydration curing agent should be applied to enhance surface strength. When the ambient temperature is low, heat preservation and moisture retention curing measures should be implemented to optimize surface hydration conditions and promote strength development.
To address the issue of a laitance layer on the surface caused by the low water absorption of aluminum formwork, the water-cement ratio and admixtures need to be optimized during the concrete mix design phase. This reduces the precipitation of free water, improves surface density, and accelerates early strength gain.
Given that the excellent sealing properties of aluminum formwork can lead to surface air bubbles, the multi-grade concrete mix design needs optimization. By using refined aggregate gradation, the volume stability of the concrete is enhanced, porosity during hardening is reduced, and the generation of air bubbles within the concrete itself is minimized. Simultaneously, a vibration process combining high-frequency vibrators and surface low-frequency vibrators is employed to ensure sufficient compaction of both the inner and outer layers of the concrete, effectively expelling air bubbles and alleviating surface looseness.
Furthermore, in some projects, if the aluminum formwork surface is not thoroughly cleaned, a smooth film cannot form, resulting in an uneven concrete surface, insufficient density, and consequently, inadequate rebound strength. Therefore, the aluminum formwork must be thoroughly cleaned before applying release agent. For reusable aluminum formwork, the surface passivation film should be checked regularly and repaired promptly.