Concrete Grades and Mix Design Standards in Vietnam

In the construction industry, ensuring concrete quality is paramount for structural integrity and longevity. According to older Vietnamese building codes (TCVN 3105:1993, TCVN 4453:1995), a 150mm x 150mm x 150mm concrete cube serves as the standard specimen for compressive strength testing.

This seemingly small cube holds crucial information about the mechanical properties of the concrete. By testing its compressive strength, engineers can evaluate its strength, durability, and other essential characteristics. The testing process involves placing the cube in a hydraulic press and gradually increasing the pressure until failure. The maximum compressive force the cube withstands before breaking is recorded and used to calculate the compressive strength. This test result forms the basis for assessing concrete quality and making informed decisions about its suitability for a project. Concrete quality control through cube testing is vital for guaranteeing the safety and service life of buildings and infrastructure.

Concrete Strength Development and Concrete Grade

Concrete strength is not a fixed value from the moment it's poured. In reality, it undergoes a "maturation" process over time, much like wine that improves with age. Concrete grade is an indicator of this potential strength, representing the compressive strength of concrete after a specific curing period, typically 28 days.

After concrete placement, a complex chemical reaction known as cement hydration occurs. During this process, cement reacts with water, forming hydration products that bind the aggregates, creating a solid concrete structure. Hydration continues over time, resulting in increased strength and hardness.

Concrete grade, expressed in MPa (N/mm²) or daN/cm² (kg/cm²), is determined by compressive strength testing of standard concrete specimens at 28 days. This value indicates the strength concrete can achieve after the initial hydration phase. However, it's important to note that concrete strength continues to develop beyond 28 days, albeit at a slower rate. Environmental factors such as temperature and humidity significantly influence the rate and extent of strength development.

A comprehensive understanding of concrete grade and the process of concrete strength development is essential for effective structural design, construction, and maintenance, ensuring long-term safety and durability.

CLASSIFICATION OF CONCRETE GRADES IN THE CURRENT VIETNAMESE MARKET

According to Vietnamese standards, concrete is classified into grades ranging from 100, 150, 200, 250, 300, 400, 500 to 600. Each grade corresponds to a specific compressive strength value. For example, concrete grade 200 signifies that a standard concrete specimen reaches a compressive strength of 200 kg/cm² (or 20 MPa) at 28 days.

This classification system is crucial for structural design and construction. Engineers select the appropriate concrete grade based on the load-bearing requirements of each structural element. For instance, high-grade concrete (like 400, 500) is typically used for heavy-duty structures such as bridges, high-rise buildings, and dams, while lower grades (like 100, 150) are suitable for smaller residential projects.

However, concrete grade is just one of many factors to consider when evaluating concrete quality. Other factors like flexural strength, slump, and permeability also play significant roles. Moreover, stringent quality control during concrete production and placement is essential to ensure the structure achieves the designed durability and safety levels.

Concrete Mix Design Table for Grades 150, 200, 250 according to the Ministry of Construction

Water, despite being the simplest component in a concrete mixture, plays a crucial role in determining the material's final strength. The water-cement ratio is the key factor in balancing workability and strength development.

Insufficient water results in a stiff mix that is difficult to place and sets quickly. This hinders complete cement hydration, preventing the concrete from reaching its optimal strength. Conversely, excessive water increases slump, leading to segregation, bleeding, extended setting times, and reduced concrete strength.

While advancements in concrete technology have enabled the development of ultra-high-performance concrete with compressive strengths reaching 1000-2000 kg/cm², the principle of water control remains paramount in ensuring concrete quality and performance.

It's important to note that the size of concrete specimens used for strength testing varies across different standards. For instance, American standards utilize cylindrical specimens with a diameter of 150mm and a height of 300mm for axial compression tests, while older Vietnamese standards employ 150mm cubic specimens. To ensure equivalency between standards, applying conversion factors is necessary.

CONCRETE MIX DESIGN FOR GRADE PC30

DETERMINING CONCRETE GRADE, STRENGTH CLASS, AND COMPRESSIVE STRENGTH

To determine the actual concrete grade, a minimum of one set of specimens, consisting of three identical concrete samples (in terms of sampling location and method, and curing conditions), must be taken on-site. For large structures, these sets should be taken from different locations and in sufficient numbers to be representative of the entire structure. The average compressive strength at the time of failure (determined by compression testing) of all three specimens in the set is used to determine the concrete grade (at 28 days).

If the specimens are not tested at 28 days after concrete setting (usually at 3 or 7 days), the concrete grade is determined indirectly using the corresponding standard concrete strength development chart. The results of compression tests at 3 or 7 days are considered quick checks and are not official. The compression test result at 28 days is considered the actual concrete grade.

According to current Vietnamese construction standards, the symbol for concrete grade has been changed from "M" to "B," representing the concrete's strength class. The strength class is determined by compression testing of cylindrical specimens. This means that instead of using cubic specimens, cylindrical specimens are used and compressed to determine the compressive strength.

Compressive strength is the crushing stress of concrete. This strength is calculated by the force acting on a unit area (kg/cm² or N/mm²). In construction, the focus is primarily on the compressive strength of concrete, while tensile strength is generally less critical.

In many design documents, instead of stating the concrete grade as 100, 200, etc., the concrete grade is recorded according to strength class B (for example, B7.5, B10, B12.5, etc.), which can cause confusion for supervising engineers. Therefore, for easier understanding and memorization, LAS-XD 1043 Laboratory provides a conversion table of concrete grade (M) corresponding to strength class (B) from TCVN 5574:2012 for easy reference, as follows:

Concrete Grade - Strength Class Conversion Table (*)

(*) Extracted from Table A1 of TCVN 5574:2012 - Concrete and Reinforced Concrete Structures. Design Standard.

Cast-in-place concrete structures are considered to meet the design grade requirements (specified in the design) when the average strength of each set of specimens (actual grade) is not less than the design grade, and at the same time, no individual specimen within any set has a test result lower than 85% of the design grade.

REGULATIONS ON CONCRETE SAMPLING

Concrete sampling is a critical step in the quality control process, ensuring the concrete meets design requirements and complies with technical standards. Sampling must adhere to strict regulations to guarantee the representativeness and accuracy of test results.

Purposes of Concrete Sampling

- Quality Control: To assess the mechanical and physical properties of concrete, such as compressive strength, flexural strength, slump, and permeability.

- Compliance with Design Standards: To determine whether the concrete meets the technical requirements of the project.

- Mix Adjustment: In cases where the concrete does not meet the requirements, test results provide a basis for adjusting the concrete mix proportions accordingly.

Concrete Sampling Standards

In Vietnam, concrete sampling is regulated by national standards such as:

- TCVN 3105:2022: Concrete and concrete mixes - Sampling, making, and curing test specimens.

- TCVN 4453:1995: Normal concrete - Acceptance, sampling, and testing.

These standards detail the technical requirements for sampling, including:

- Timing of Sampling: Samples should be taken while the concrete is being poured or mixed.

- Sampling Location: Samples should be taken at locations representative of the concrete batch, avoiding special locations like formwork edges or near reinforcement.

- Number of Samples: The number of samples depends on the volume of concrete, the type of structure, and testing requirements.

- Sampling Method: The sampling method must ensure representativeness and avoid altering the concrete's properties.

- Specimen Size and Shape: Specimens are typically cast in cubic or cylindrical shapes with specified dimensions.

- Specimen Preparation and Curing: Concrete specimens must be prepared and cured according to the correct procedures to ensure accurate test results.

Specific Regulations

- Mass Concrete: If the volume of concrete poured exceeds 1000 m3, one set of samples is taken for every 500 m3. If the volume is less than 1000 m3, one set is taken for every 250 m3.

- Foundation Concrete: One set of samples is taken for every 100 m3, but not less than one set for a foundation block.

- Frame and Foundation Structures: One set of samples is taken for every 20 m3.

- Slab and Pavement Concrete: One set of samples is taken for every 200 m3, but at least one set is taken even if the volume is less than 200 m3.

Important Notes

- Concrete sampling should be carried out by qualified and experienced personnel.

- Concrete specimens must be carefully handled and protected from impacts, moisture loss, or other factors that could affect their quality.

Adhering to concrete sampling regulations is crucial for ensuring accurate test results, which in turn enable proper assessment of concrete quality and guarantee the safety of the structure.

NOTES IN CONCRETE MAINTENANCE

Concrete Curing: Ensuring Strength and Durability

Following concrete placement, the curing phase plays a crucial role in ensuring the effectiveness and longevity of the structure. Proper concrete curing not only helps the concrete achieve optimal strength but also prevents potential damage, extending the lifespan of the construction.

Key Objectives of Concrete Curing:

- Moisture Retention: Concrete needs to maintain adequate moisture during cement hydration to achieve desired strength and durability. Rapid moisture loss can lead to shrinkage, causing surface cracks that affect both aesthetics and load-bearing capacity.

- Minimizing Vibration: In the initial stages after pouring, concrete is vulnerable to mechanical stress. Excessive vibration can disrupt the bond between concrete components, reducing strength and durability.

Concrete Curing Methods:

- Formwork Protection: Avoid strong impacts on the formwork to prevent concrete vibration. Also, inspect the formwork for tightness to prevent leakage of concrete or water.

- Surface Covering: Cover the concrete with a thin plastic sheet or specialized curing materials to limit evaporation and maintain necessary moisture.

- Water Spraying: Regularly spray water on the concrete surface to provide moisture, especially in hot and dry weather. Use a fine mist spray to avoid surface erosion.

- Temperature Control: Ambient temperature also influences the hydration process and concrete quality. In high-temperature conditions, implement measures like shading and cooling to prevent the concrete from drying too quickly.

In addition to these methods, other curing techniques include using curing compounds, steam curing, and electrochemical curing. The appropriate curing method depends on the type of concrete, environmental conditions, and project requirements.

In conclusion, concrete curing is a vital stage in the construction process that cannot be overlooked. Implementing proper curing measures ensures optimal strength development, guaranteeing concrete quality and the longevity of the structure.

APPLICATION EQUIPMENT FOR CEMENT CONCRETE MIXES IN REAL CONSTRUCTION

In the past, concrete construction was a labor-intensive and time-consuming process. Construction workers had to manually mix the materials in the desired proportions to create the concrete mixture, then transport it using manual methods to the construction site. This process not only demanded significant physical effort but also presented safety hazards and made it challenging to ensure accuracy and consistency in the concrete.

However, the advent of modern mechanized equipment has revolutionized concrete placement. These machines automate various stages, from mixing and transporting to pouring concrete, resulting in remarkable improvements in productivity, accuracy, and safety.

Below are some prime examples of this equipment:

Concrete Pavers: Enhancing Efficiency and Quality in Concrete Construction

Concrete pavers are essential machinery in modern construction, playing a key role in projects such as highways, airport runways, bridges, and parking lots. These machines not only increase productivity and construction quality but also ensure accuracy, safety, and aesthetics.  

Operating Principle: Concrete pavers operate by spreading and compacting fresh concrete on a prepared surface. Concrete is transported to the paver by a concrete mixer truck and then evenly distributed across the work area using a conveyor belt and paving mold. The machine is equipped with a vibrating screed to compact the concrete, eliminate air voids, and ensure a dense and strong structure.  

Types of Concrete Pavers:

- Slipform Pavers: These move on rails or tracks and are commonly used for large-scale projects that demand high precision, such as airport runways and highways.

- Wheel-mounted Pavers: These offer greater maneuverability and are suitable for smaller projects with complex terrain.

Advantages of Concrete Pavers:

- Increased Productivity: Concrete pavers can operate at high speeds, significantly reducing construction time compared to manual methods. A modern paver can lay hundreds of cubic meters of concrete per hour.

- Quality Assurance: Concrete pavers allow for precise control over the thickness, slope, and evenness of the concrete layer, ensuring consistent quality throughout the project.

- Enhanced Aesthetics: Surfaces constructed with concrete pavers are typically smooth and visually appealing, contributing to the overall aesthetics of the project.

- Improved Safety: Using concrete pavers reduces manual labor, limiting workers' exposure to dust, chemicals, and potential hazards.

Applications of Concrete Pavers:

- Road Construction: Highways, national roads, urban and rural roads.

- Airport Construction: Runways, taxiways, and aprons.

- Bridge and Port Construction: Bridge decks, wharves, and breakwaters.

- Industrial Construction: Factories, warehouses, and parking lots.

Development Trends: Modern concrete pavers are increasingly integrating advanced technologies such as automatic control systems, GPS positioning, and laser guidance to enhance accuracy, efficiency, and safety during construction.  

Concrete pavers are indispensable in modern construction, playing a vital role in modernizing and improving the quality of infrastructure projects.

Concrete Pump Trucks: The Lifeline of Modern Construction

Concrete pump trucks are indispensable in modern construction, especially for high-rise buildings, tunnels, bridges, and dams. These machines serve as the "lifeline," efficiently and safely transporting fresh concrete from the production or mixing site to the pouring locations.

Structure and Operating Principle:

A concrete pump truck consists of two main components: the truck chassis and the pumping system. The truck chassis carries the pumping system to the construction site. The pumping system includes:

- Hopper: Receives concrete from the concrete mixer truck.

- Hydraulic Pump: Generates pressure to propel the concrete through the pipeline.

- Pipeline: A system of high-pressure resistant steel or rubber pipes that deliver concrete to the pouring point.

- Boom (for boom pumps): An articulated, multi-section boom with adjustable height and reach, enabling concrete placement at various locations.

The operating principle is quite simple: concrete is poured into the hopper, and the hydraulic pump generates pressure to push the concrete through the pipeline to the placement area. For boom pumps, the boom is precisely controlled to deliver concrete to the desired locations.

Types of Concrete Pumps:

- Boom Pumps: Equipped with a placing boom, these pumps can reach high and distant locations, making them suitable for high-rise buildings, tunnels, and bridges.

+ Advantages: Long reach, flexibility, high efficiency.

+ Disadvantages: Higher cost, require skilled operators.

- Stationary Pumps: Lacking a boom, these pumps concrete through a fixed pipeline system and are often used for smaller projects with level terrain.

+ Advantages: Lower cost, easier operation.

+ Disadvantages: Limited reach, less flexible.

Advantages of Using Concrete Pumps:

- Increased Productivity: Pumps concrete at high speed, reducing construction time, particularly for large projects.

- Labor Savings: Minimizes manual labor and reduces the required workforce.

- Concrete Quality Assurance: Continuous concrete transportation prevents segregation, ensuring consistent quality.

- Safety: Reduces occupational hazards, especially when working at heights or in challenging locations.

- Access to Difficult Areas: Delivers concrete to areas inaccessible by manual methods.

Applications of Concrete Pumps:

- High-Rise Building Construction: Transports concrete to upper floors for pouring slabs, columns, and beams.

- Tunnel Construction: Pumps concrete into deep underground locations.

- Bridge Construction: Places concrete for bridge components like piers, beams, and decks.

- Dam Construction: Used in the construction of hydroelectric and irrigation dams.

- Underground Construction: For projects like underground sewer systems and cable lines.

Concrete pump trucks are vital for modernizing and increasing the efficiency of concrete construction. Choosing the right type of pump based on the project's scale and characteristics helps optimize costs and ensure project quality.

Benefits of Using Mechanized Equipment in Concrete Construction

The application of mechanized equipment in concrete construction is not just an inevitable trend in the modern construction industry but also brings tangible benefits, contributing to improved efficiency, quality, and safety in projects. Here are some notable advantages, supported by data and real-world examples:

1. Enhanced Labor Productivity:

- Reduced Construction Time: According to a study by the Institute of Construction Science and Technology, using concrete pavers can increase construction productivity by up to 30% compared to manual methods. For instance, in the Da Nang - Quang Ngai

- Expressway project, utilizing concrete pavers helped cut road surface construction time in half.

- Reduced Manual Labor: Machines replace human labor in strenuous tasks like mixing, transporting, and pouring concrete, significantly reducing physical strain and the risk of accidents for workers.

- Workforce Optimization: Automation allows for a reduction in the number of workers needed, optimizing human resources and lowering labor costs.

2. Improved Concrete Quality:

- Precise Control of Mix Proportions: Modern concrete mixing equipment features automated weighing systems, ensuring accurate mix proportions as per design specifications, thereby enhancing concrete quality and consistency.

- Uniformity: Machine mixing ensures even distribution of components in the concrete mixture, preventing segregation and ensuring uniform quality.

- Increased Load-Bearing Capacity: Concrete placed using machinery typically has higher density and fewer voids, increasing its load-bearing capacity and durability.

3. Enhanced Safety:

- Reduced Accidents: Using machines to replace humans in hazardous tasks minimizes the risk of occupational accidents such as falls from heights, collisions, and exposure to dust and chemicals.

- Improved Working Conditions: Machines reduce labor intensity and improve working conditions for workers, contributing to their health and well-being.

4. Cost Savings:

- Reduced Labor Costs: Workforce optimization helps reduce labor costs, especially with rising labor expenses.

- Reduced Material Waste: Modern equipment allows for precise control of material usage, minimizing waste and saving on material costs.

- Increased Service Life of Structures: High-quality concrete, constructed with proper techniques, increases the lifespan of structures, reducing maintenance and repair costs.

The use of mechanized equipment in concrete construction offers significant benefits in productivity, quality, safety, and cost. It is an essential trend in modern construction, contributing to the sustainable development of the industry.

Conclusion: The Importance of Concrete Grade

Concrete grade is a crucial indicator that plays a key role in the design, construction, and quality assessment of structures. Understanding concrete grade, the factors influencing concrete strength, and the quality control process helps in selecting the right type of concrete to ensure the safety and durability of the project.

In the context of the ever-evolving construction industry, applying advanced technology, modern equipment, and new materials is crucial. However, fundamental knowledge about concrete, including concrete grade, remains essential. The harmonious combination of traditional experience and modern technology will create sustainable, safe, and aesthetically pleasing structures, contributing to a solid future.