Concrete Tip of the Day #4: Placing and Finishing Concrete – Getting It Right From the Start

Placing and finishing concrete isn’t just about getting it into the formwork and smoothing the surface. It’s a carefully timed process that directly affects strength, durability, and long-term performance. When done correctly — especially with proper vibration and finishing techniques — it can significantly improve concrete quality.

🔊 Why Vibration Matters

Fresh concrete needs just the right level of workability to move into position around reinforcement and into formwork corners. But when space is tight or reinforcement is dense, mechanical consolidation becomes essential — and vibration is the best solution.

✅ Benefits of Vibrating Concrete:

• Allows placement of low-slump, high-strength mixes

• Reduces air voids and improves density

• Improves uniformity and homogeneity

• Enhances bond with reinforcement

• Minimises plastic settlement and shrinkage

• Helps layers merge into a monolithic structure

  
  

🧱 Using Low-Slump Concrete

Vibration allows for stiffer concrete to be placed efficiently. This enables:

• Lower water-cement ratios

• Higher strength and lower shrinkage

• Better early and long-term performance

  
  

Vibration Duration: Usually 5 to 15 seconds per insertion is sufficient. Indicators of adequate vibration include:

• Uniform movement at the surface

• Appearance of cement paste at form edges

• Steady vibrator tone

• Vibrator “feel” in the operator’s hands

  
  

⚠️ Avoiding Over-Vibration

Wetter mixes are more vulnerable to over-vibration, which can:

• Cause mortar-rich surface layers

• Lead to horizontal stratification

• Reduce durability and surface quality

  
  

Important:

• Don’t use vibrators to move concrete horizontally — it causes segregation.

• Vibrate each layer systematically, inserting the vibrator vertically at 300–500mm intervals, and penetrate 50mm into the previous layer.

• Ensure immersion time is long enough to remove entrapped air — usually around 10 seconds.

  
  

🛠 Finishing Concrete the Right Way

Finishing must wait until bleeding has stopped.Starting too soon means:

• Excess bleed water gets trapped at the surface

• Weak, dusty, and easily damaged top layers form

• Risk of surface delamination later

Weather conditions play a major role:

• High winds, heat, or low humidity can cause moisture to evaporate too quickly

• Even when water isn’t visible, internal bleeding may still be active

  
  

🧴 Curing Compounds and Surface Finishes

If wet curing isn’t practical, use a high-quality curing compound. Consider:

• Breakdown behaviour — some products require grinding to remove

• Compatibility — with coatings, sealers, or hardeners

  
  

Note: Precast release agents and silicate-based hardeners don’t meet curing standards.

🎨 Common Surface Finishes:

• Smooth/polished: Hard-wearing, easy to clean

• Broomed: Better grip but less wear-resistant→ Broom in direction of water runoff to aid drainage→ Increased surface area = increased risk of cracking

  
  

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