Concrete mix and cement content – bags per cubic metre For a concrete mix targeting a 28-day crushing strength of about 280 kg/cm^2 (≈ 28 MPa), the approximate minimum number of 50 kg cement bags required per cubic metre is:

Introduction / Context:
Preliminary mix selection often uses experience-based ranges for cement content. A 28-day strength around 280 kg/cm^2 (≈ 28 MPa) typically corresponds to a nominal design in the M25–M30 range, requiring moderate-to-high cement content.

Given Data / Assumptions:

• Target strength ≈ 28 MPa (280 kg/cm^2).
• Ordinary aggregates, normal workability.
• 50 kg per cement bag; 1 m^3 concrete volume.

Concept / Approach:
For medium–high strength mixes (≈ M25–M30), typical cement content is about 320–380 kg/m^3 depending on exposure and workability. Converting cement mass to 50 kg bags gives an initial estimate of bag count.

Step-by-Step Solution:

Verification / Alternative check:
Trial mixes and water–cement ratio control refine this value, but preliminary sizing of 7 bags/m^3 is widely used for this strength level.

Why Other Options Are Wrong:
5.0–6.5 bags (250–325 kg) are usually low for ≈ 28 MPa unless special materials are used. 7.5–8.0 bags imply 375–400 kg/m^3, more typical for higher strengths or severe exposure.

Common Pitfalls:
Ignoring maximum water–cement ratio limits; assuming bag counts without adjusting for moisture and absorption; neglecting admixtures that can shift the required cement content.

Final Answer:
7.0