Hardness Standards for Concrete Prefabricated Houses

The hardness standards for concrete prefabricated houses primarily depend on the concrete strength grade, typically classified by the standard value of cube compressive strength, such as C15, C20, C25, etc. The higher the number, the greater the strength and hardness.

Relationship Between Concrete Strength Grade and Hardness

The concrete strength grade is a core indicator for measuring the hardness of prefabricated houses, with its value representing the standard value of cube compressive strength (unit: MPa). For example, C15 indicates that the 28-day compressive strength of the concrete is not less than 15 MPa, while C30 is not less than 30 MPa. Strength and hardness are positively correlated: the higher the strength, the stronger the material's resistance to deformation and damage, thereby enhancing the overall hardness of the house. For rural self-built houses (including prefabricated structures), commonly used strength grades range from C20 to C30, depending on the house structure type (e.g., frame, brick-concrete), load magnitude (e.g., number of floors, intended use), and geological conditions (e.g., foundation bearing capacity).

Hardness Standard Range for Residential Concrete

In residential construction, concrete hardness standards typically cover different grades from C15 to C50. During design, it is essential to use 70% of the standard value as a benchmark to ensure that the actual load-bearing capacity meets safety requirements. For example:

• Ordinary buildings: 32.5 or 42.5 grade cement (corresponding to a 28-day compressive strength of not less than 32.5 MPa and 42.5 MPa, respectively) can be selected, suitable for low-rise residential buildings or structures with smaller loads.
• High-strength projects: 52.5 grade or higher-hardness cement is required to meet the needs of multi-story or large-span prefabricated houses.

Construction and Testing Requirements

1. Pouring and Vibration: During construction, ensure continuous concrete pouring and thorough vibration to avoid defects such as honeycombing and holes that can affect hardness.
2. Strength Testing: Conduct spot checks on component strength using methods such as the rebound method or core drilling method to ensure it meets the design grade. For example, the rebound method estimates compressive strength by measuring the surface hardness of the concrete, while the core drilling method involves direct sampling and testing.
3. Special Performance Requirements: For prefabricated components, attention should be paid to the cement hardening speed (e.g., rapid-hardening sulphoaluminate cement is suitable for emergency repair projects) or the addition of hardening agents (e.g., wall mortar hardening agents can enhance surface hardness).

Cement Type Selection

The type of cement directly affects the hardening speed and final hardness of the concrete. For example:

• Ordinary Portland cement: Suitable for general prefabricated houses, with a moderate hardening speed and stable strength.
• Rapid-hardening cement: Used for emergency repair projects requiring rapid construction, but attention should be paid to post-hardening shrinkage issues.
• High-strength cement: Such as 52.5 grade cement, is suitable for projects with extremely high hardness requirements, but at a higher cost.

By reasonably selecting the concrete strength grade and cement type, and strictly controlling the construction and testing processes, the hardness of concrete prefabricated houses can be ensured to meet safety and usage requirements.