Finishing Tips with Microfibers: Managing Apparent Slump Changes & Surface Quality

Oct 21, 2025 | Microfiber Concrete, Polypropylene Fibers, Concrete Curing

Synthetic microfibers for concrete have become a go-to solution for effective plastic shrinkage crack control and enhanced surface durability. Yet, for many finishers, the first experience with fiber reinforced concrete microfibers brings up a common question: what about the slump? Understanding how these fibers affect fresh concrete is the key to achieving a flawless, high-quality finish every time.

• Understanding Apparent Slump Loss

  When you add polypropylene concrete fibers to a mix, you may notice a lower slump measurement. This is known as "apparent slump loss". The fibers create a supportive internal network that holds the concrete together during the test, resulting in a lower reading. It’s a change in measurement, not a true loss of workability. The concrete often remains easy to pump, place, and consolidate. 

  The most important rule is to not add water to "fix" this apparent slump. Doing so will increase the water-cement ratio and compromise the concrete's strength. If a tighter finishing window requires more flow, consider using a compatible water-reducing admixture in the mix design.

• Practical Steps for a Perfect Finish

  Achieving a great surface with microfibers is straightforward when you follow a few best practices.

  1.  Mix Design and Mixing: 

  Start with a base mix that has a minimum slump of 80 mm before adding fibers. For best results, introduce the fibers during batching at the ready-mix plant. A typical dosage of PSI FIBERSTRAND 150 is 0.4 to 0.6 kg/m³. Mix for 3 to 5 minutes at maximum speed to ensure uniform dispersion and prevent clumps.

  2.  Placing and Finishing:

  • Patience is Key: The fiber network can reduce visible bleed water. Wait until all bleed water has evaporated before starting any finishing operations to prevent delamination.
  • Tools and Technique: Use a magnesium bull float to level the surface, as its light weight helps prevent dragging. Follow with a standard steel trowel sequence, starting with the trowel flat and increasing the angle on subsequent passes as the concrete hardens.
  • Avoid Overworking: Normal vibration is sufficient for consolidation. Avoid over-vibrating or overworking the surface, as this can bring excess paste to the top and create finishing issues.
  • Power Trowels: Power trowels work well with microfiber concrete. Begin with float pans once the concrete can support the machine, and then switch to finishing blades as it sets.
  • Curing: Proper curing is essential. Use a quality curing compound to retain moisture, which allows the concrete to achieve its full strength and locks in your high-quality finish.

• Quick Troubleshooting Guide

  • Tearing or Dragging: If the surface tears during floating, you’ve likely started too early. Wait for the concrete to set further.
  • Trowel Drag: If your trowel drags, the surface may be too dry or your trowel angle is too steep. Lower the angle to be flatter against the surface.
  • Exposed Fibers: A "fuzzy" surface can result from overworking the concrete. A final, hard steel trowel pass can help embed stray fibers.

By Nadine Schürmann