Concrete Tip of the Day #03: Why Too Much Water Weakens Your Concrete

Adding water to concrete might seem harmless, even helpful — but it’s one of the most common and costly mistakes in the industry. While water is essential for hydration and workability, using too much can significantly compromise the strength and durability of the finished product.

💧 The Role of Water in Concrete

In a properly proportioned mix, only about 50% of the water is used to chemically react with the cement (hydration). The rest acts as a lubricant to make the concrete workable for placing and finishing.

However, when extra water is added to improve flow or increase slump, it dilutes the mix:

• Reduces density

• Weakens compressive strength

• Increases porosity and risk of cracking

📉 Water-Cement Ratio and Strength

The water-cement ratio is a critical factor in determining concrete strength. The more water added for the same amount of cement, the weaker the resulting cement paste will be.

Even small increases in water can cause noticeable strength loss. For instance:

• A load with low slump will typically yield higher compressive strength.

• Raising the slump by adding water increases the water-cement ratio — and reduces strength.

  
  

According to NZS 3109, controlled water addition on site is permitted — but is limited to 10 litres per cubic metre, and only after confirming with the batching plant that maximum water content won’t be exceeded.

⚠️ Risks of Excess Water

Using too much water in a mix can lead to:

• Excessive shrinkage and cracking, especially in slabs

• Low tensile strength, which impacts structural integrity

• Dusting and crazing, caused by water bleeding to the surface and reworking fines during finishing

🛠 Practical Tips

✔ Stiff concrete is cheaper in labour hours.It may take more effort to place, but it finishes quicker and performs better in the long run.

✔ Avoid adding water to extend workability.Instead, plan for timely placement and ensure enough manpower and equipment are ready before the truck arrives.

✔ Place, don’t pour.Concrete should be placed into position and compacted, not simply poured out and spread thin.

✔ Discharge quickly.Concrete begins to stiffen once mixed. Delays can make it harder to place, leading to the temptation to add water — and compromise strength.

✅ Stay tuned for the next episode of Concrete Tip of the Day!