Concrete in civil engineering columns requires careful curing after formwork removal.

According to Dong Lam Cement Joint Stock Company, the phenomenon of concrete ceiling cracking leading to water seepage, causing aesthetic damage and reducing the functionality of the floor after construction, is relatively common nowadays.

Floor cracks can stem from various causes such as weak foundations, formwork shifting before concrete hardens, improper concrete pouring, and concrete expansion and contraction due to changes in natural conditions like humidity and temperature.

In construction projects, concrete work plays a crucial role in structural components. Reinforced concrete is considered the backbone of a structure, therefore ensuring the quality of concrete in construction is a prerequisite for the durability of the structure.

Mr. Ton That Tung Thanh, Head of the Product Quality Testing and Inspection Department (Dong Lam Cement Joint Stock Company), shared that concrete mix design is the process of calculating to find the appropriate ratio of sand, aggregate, cement, and water additives (if any) for the mixture, in accordance with the standards of concrete materials, in order to achieve quality standards and suit the actual usage conditions.

Concrete mix design depends on the concrete grade, the size of sand and aggregate particles, the type of cement, and the quality and composition of additives (if any). In practice, when building individual houses, some builders work based on experience and intuition, mixing concrete and adjusting the sand and cement until it's just right and has the right consistency. Very few construction teams carefully measure the quantities when preparing the concrete mix for construction.

This can result in poor-quality concrete, leading to waste, uneven quality, and increased risk of cracking and leakage. To ensure concrete quality, based on calculations and experiments, the Ministry of Construction has issued a table of material consumption norms in construction, including detailed material mix proportions for common types of civil concrete. Cement manufacturers have specified these norms to match product quality and printed instructions on mortar and concrete mix ratios on cement bags.

Ensuring the quality of concrete in construction is a prerequisite for the sustainability of the project.

Accordingly, regarding the selection of cement materials, for conventional concrete, the commonly used cement types for residential construction projects today are PCB40 and PCB30, which comply with TCVN 6260:2020 standards. The general principle is that low-grade cement should not be used to produce high-grade concrete and vice versa. Following this principle, users can choose the appropriate type of cement for each construction item.

Yellow sand, a type of river sand with evenly distributed grains ranging in diameter from 0.5 to 3.0 mm, and low in impurities, dust, and silt, should be used. Screening the sand before use is a good solution before mixing it into concrete or mortar.

The aggregate must be hard, clean, free from debris and impurities such as dust, mud, and clay; avoid aggregates containing many elongated or flattened particles, and use clean water for mixing concrete. The water should be free from oil slicks, grease, iron, organic impurities, and corrosive salts.

If using additives, ensure you use only those from reputable brands with clear origins. Always consult with a manufacturer's representative or carefully read the instructions before use. Never exceed the manufacturer's recommended dosage.

According to the predetermined mix design, aggregates are weighed/measured for mixing into the concrete mixture. The key at this stage is to reduce the amount of mixing water in the concrete mixture to the appropriate level, and to mix thoroughly and uniformly in each batch and throughout the entire pouring process.

According to Mr. Ton That Tung Thanh, the water ratio in the concrete mix greatly affects and plays a decisive role in the quality of the concrete. A low water ratio will help the concrete have higher durability, improve waterproofing, and limit cracking, especially large cracks.

Installation and dismantling of scaffolding for concrete pouring in residential construction projects.

Typically, the water-to-cement ratio in concrete mixes does not exceed 0.5. There are many ways to mix concrete efficiently, including manual mixing or using specialized mixers. Nowadays, most construction projects use concrete mixers to save time, improve concrete quality, and reduce labor costs.

Furthermore, to obtain a quality concrete batch that meets the design strength, the preparation for pouring, compaction, and surface finishing of the concrete, as well as the maintenance and formwork removal, are extremely important and greatly affect the quality of the concrete.

Proper curing is essential to maintain the moisture content of the concrete, protecting it from rapid water loss and adverse effects during the initial hardening process. Inadequate curing can lead to surface cracking, white discoloration, and failure of the concrete to reach its designed strength.

When removing formwork for concrete pouring in low-rise columns and walls, construction workers typically remove the formwork the following morning. However, this is incorrect and risks the concrete peeling off or collapsing because it may not have hardened yet. Before removing the formwork, it is important to check the exposed areas to see if the concrete has hardened and plan the removal accordingly.

For concrete slabs and beams, the concrete strength (grade) usually needs to reach about 80% of the design grade before formwork can be removed. Currently, for civil concrete mixes without additives, formwork can be removed after 21 days. However, this also depends on the type and amount of cement, the quality of raw materials, etc.

Text and photos: Bach Nguyen