Concrete Tip of the Day #2: How Concrete Is Tested – From Slump to Strength

Understanding how concrete is tested is critical to ensuring quality, compliance, and long-term performance. From site sampling to laboratory testing, each step must be performed correctly to produce reliable results and meet project specifications.

🧪 Why Test Concrete?

Concrete testing provides vital technical assurance to builders, engineers, and contractors. Testing ensures that the mix delivered to site meets the standards for strength, consistency, and durability.

Dedicated laboratories across the country routinely carry out thousands of tests each year on hundreds of different mix designs. These labs help monitor quality control and support the building and construction industries with expert advice.

🏗️ On-Site Concrete Testing

On request, trained technicians can visit job sites to:

• Measure slump (consistency of fresh concrete)

• Cast cylinders for compressive and tensile strength

• Cast beams for flexural strength

Specimens are collected the next day, cured under standard conditions, and tested in the lab using calibrated equipment.

🧪 The Slump Test – Measuring Workability

The slump test, as described in NZS 3112 Part 1, is one of the most common field tests for fresh concrete. It helps determine whether the mix meets project requirements for consistency and workability.

✔️ Best Practice for Slump Testing:

1. Prepare a non-absorbent, level surface and moisten the slump cone.

2. Fill the cone in 3 layers, each compacted with 25 strokes using a standard 16mm steel rod.

3. Strike off the top and remove the cone slowly over ~2 seconds without jarring.

4. Measure the slump from the top of the cone to the highest point of the slumped concrete.

Poor slump testing technique can cause false rejection or acceptance of concrete loads. Follow procedures carefully.

🧱 Casting Compression Test Cylinders (NZS 3112 Part 2)

To assess compressive strength, standard test cylinders (100mm x 200mm) are prepared and tested.

✔️ Key Steps:

• Mould Preparation: Coat internal surfaces lightly with mineral oil (remove excess).

• Filling & Compaction: Fill the mould in three equal layers, compacting each thoroughly with a tamping rod (25 strokes minimum).

• Vibration: Alternatively, layers can be compacted using vibration, but avoid over-vibrating.

  
  

Once filled:

• Strike off the top and trowel smooth.

• Cover within 1 hour to protect the cap.

• Store in a vibration-free area between 10–25°C.

• Deliver to the lab within 80 hours of casting.

Failure to follow these procedures can result in inaccurate, lower-than-actual strength values.

🧬 Research & Innovation in Concrete

Advanced laboratories are also used for concrete research and development. They explore global advancements in materials and admixtures to develop tailored solutions for local applications.

Ongoing collaboration with admixture technology providers ensures that the most effective and modern formulations are made available, keeping the industry at the forefront of innovation.

Conclusion

Concrete testing is much more than just box-ticking. Done properly, it ensures quality, safety, and durability. Whether you're placing concrete today or analysing results 28 days later, accurate testing and good technique are key to long-term success.

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