Concrete Mix Design: The Right Ratio of Cement, Sand, Aggregate and Water

What Is Concrete and How Does It Work?

Concrete is a precisely proportioned mixture of cement, fine aggregate (sand), coarse aggregate (stone chips or gravel), and water — which hardens over time to develop stone-like compressive strength.

Each ingredient plays a specific role:

The ratio in which these four ingredients are combined determines everything — strength, durability, workability, and cost.

---

What Do Concrete Grades Mean? M10, M20, M25 Explained

The "M" stands for Mix. The number that follows is the minimum compressive strength of the concrete after 28 days of curing, measured in N/mm² (or MPa).

M20 = minimum 20 N/mm² compressive strength at 28 days

Commonly Used Concrete Grades in Bangladesh:

Per BNBC (Bangladesh National Building Code):

• Residential RCC structures: minimum M20
• Columns and foundations: M25 recommended
• Buildings of 6 storeys or more: M25–M30 mandatory

---

Correct Mix Ratios by Grade

Nominal Mix Ratios (by volume):

For M30 and above, a laboratory-designed mix is mandatory — nominal ratios are not sufficient for higher grades.

---

How to Calculate Cement, Sand and Aggregate from a Ratio

This is the question every site engineer and contractor needs to answer before a pour. Here is the step-by-step method.

The Core Formula

When dry materials are mixed with water, the volume of the wet concrete produced is less than the total dry volume — due to the filling of voids. To account for this, dry material volume is taken as 1.54 times the required wet concrete volume:

Dry volume of materials = Wet volume × 1.54

---

Worked Example 1: M20 Concrete (1 : 1.5 : 3) — for 1 m³

Step 1 — Calculate dry volume:

Dry volume = 1 m³ × 1.54 = 1.54 m³

Step 2 — Find total parts in the ratio:

1 + 1.5 + 3 = 5.5 parts

Step 3 — Cement quantity:

Volume = (1 ÷ 5.5) × 1.54 = 0.28 m³

Weight = 0.28 × 1,440 kg/m³ = 403 kg ≈ 8 bags (at 50 kg per bag)

Step 4 — Sand quantity:

(1.5 ÷ 5.5) × 1.54 = 0.42 m³

Step 5 — Coarse aggregate quantity:

(3 ÷ 5.5) × 1.54 = 0.84 m³

Step 6 — Water quantity (W/C ratio = 0.50):

Water = 0.50 × 403 = 201 litres

M20 Summary — Materials for 1 m³:

---

Worked Example 2: M25 Concrete (1 : 1 : 2) — for 1 m³

Step 1 — Dry volume: 1 × 1.54 = 1.54 m³

Step 2 — Total parts: 1 + 1 + 2 = 4 parts

Step 3 — Cement:

(1 ÷ 4) × 1.54 = 0.385 m³

0.385 × 1,440 = 554 kg ≈ 11 bags

Step 4 — Sand: (1 ÷ 4) × 1.54 = 0.385 m³

Step 5 — Aggregate: (2 ÷ 4) × 1.54 = 0.77 m³

Step 6 — Water (W/C = 0.45): 0.45 × 554 = 249 litres

M25 Summary — Materials for 1 m³:

---

Universal Formula for Any Grade:

Cement (bags)  = (C ÷ (C+S+A)) × 1.54 × 1440 ÷ 50
Sand (m³)      = (S ÷ (C+S+A)) × 1.54
Aggregate (m³) = (A ÷ (C+S+A)) × 1.54
Water (litres) = W/C ratio × Cement weight in kg

Where C = cement parts, S = sand parts, A = aggregate parts in the mix ratio.

---

The Water-Cement Ratio: The Most Critical Variable

More construction failures trace back to incorrect water-cement (W/C) ratio than any other single factor.

W/C Ratio = Weight of water ÷ Weight of cement

What happens when W/C ratio decreases:

• Compressive strength increases
• Durability and impermeability improve
• Workability reduces (harder to place and compact)

What happens when W/C ratio increases:

• Mix becomes more workable and easier to place
• Strength drops significantly — every 0.05 increase in W/C ratio reduces strength by approximately 10–15%
• Risk of segregation, bleeding, and cracking increases

Maximum W/C Ratios per Exposure Condition (BNBC):

In Bangladesh's humid tropical climate, a W/C ratio of 0.50 or below is strongly recommended for all structural concrete.

---

Material Quality — What to Verify on Site

Sand (Fine Aggregate)

• Silt and clay content must be below 5%
• Simple field test: Fill a glass bottle with sand and water, shake well, and leave for one hour. If the silt layer that settles above the sand exceeds 5mm in depth, reject that sand for structural use.
• Avoid sea sand — chloride content causes steel reinforcement to corrode.

Coarse Aggregate (Stone Chips)

• Use 20mm aggregate for slabs, beams, and columns
• Use 40mm aggregate for mass concrete and large foundations
• Do not use brick chips for RCC structural work — brick chips have significantly lower crushing strength and much higher water absorption than stone, which severely compromises structural concrete performance.

Water

• Must be potable quality (fit for drinking)
• Do not use saline, brackish, or chemically contaminated water
• Per BNBC: chloride content below 500 mg/L, sulphate content below 400 mg/L

---

Curing: The Step That Cannot Be Skipped

Curing is the process of maintaining moisture in freshly placed concrete so that cement hydration can continue and full strength can develop. Without proper curing, surface moisture evaporates before hydration is complete — and strength can be reduced by up to 40%.

Curing methods include keeping surfaces covered with wet hessian or gunny bags, regular water spraying (3–4 times per day), and applying curing compounds on large pours.

---

Common Mistakes Seen on Bangladesh Construction Sites

Adding excess water to make mixing easier — This is the single most common mistake. A wetter mix is easier to pour but dramatically weaker. Never add water beyond the specified W/C ratio.

Using brick chips instead of stone chips for RCC — Still observed on many sites. Brick chips are not appropriate for structural concrete under any circumstances.

Using silt-contaminated sand — Cheap sand with high silt content reduces bond strength and overall concrete quality. Always test before use.

Skipping curing — Concrete does not gain strength simply by drying out. It requires sustained moisture for hydration. Uncured concrete never reaches its design strength.

Estimating ratios by eye — Guessing proportions on site is not an acceptable substitute for measured batching. Even small deviations accumulate across a pour.

---

Frequently Asked Questions

M20 or M25 — which should I use for a residential building?

For 2–4 storey residential construction, M20 is structurally adequate for slabs and beams. Use M25 for columns and all foundation elements. For buildings of 5 storeys or more, use M25 throughout and consult a structural engineer.

How long does concrete take to reach full strength?

Concrete reaches approximately 65–70% of its 28-day strength by day 7, and 99% by day 28. Formwork (shuttering) should not be removed before 7 days for slabs and beams.

Is it safe to pour concrete during rain?

No. Rainwater increases the effective W/C ratio, weakens the surface layer, and washes out cement from freshly placed concrete. Postpone pours until rain has passed.

Is Ready Mix Concrete (RMC) better than site-mixed?

For quality consistency and accuracy of mix proportions, RMC from a batching plant is significantly more reliable than site-mixed concrete. It is strongly recommended for large pours and critical structural elements.

What is the difference between nominal mix and design mix?

A nominal mix uses fixed ratios (e.g., 1:1.5:3 for M20) suitable for smaller projects. A design mix is developed through laboratory testing of actual site materials to optimise strength and economy. Design mix is mandatory for M30 and above.

---

Conclusion

The quality of your concrete is the quality of your structure. A correctly designed and properly executed mix — with the right grade, accurate proportions, quality materials, thorough compaction, and consistent curing — will produce a structure that performs safely for 50 years or more.

At DeltaCore Alliance, we oversee concrete quality at every stage of a project — from mix design and material testing through to pour supervision and cube strength verification. We do not leave structural quality to chance.