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First and foremost when designing a Fiber Reinforced Concrete (FRC) mix make sure there is sufficient mortar (cement and sand) to coat the extra surface area contributed by the fibers to the mix. When using the standard dosage rates for Microsynthetic Fibers that range from 0.5 to 1.5 lbs/CY (300 to 900 grams/m³) there is typically no reason to consider any modifications to the mix design. It is not until we reach a synthetic fiber dosage level, either Microsynthetic or Macrosynthetic, of 2.0 lbs/CY (1.2 kgs/m3) or more would we be looking at adjusting the coarse aggregate to mortar ratios. Definitely at 3.0 lbs/CY and above when working with the macrosynthetic fibers the ratio of coarse aggregate to mortar will need to be adjusted to compensate for the greater surface area contributed by the macrosynthetic fiber. For Steel Fibers, 35 lbs/CY (20 kgs/m³) is the dividing line.

Length and configuration are additional factors that will affect workability and consolidation of the FRC. These factors should also be taken into consideration when adjusting FRC mixes with elevated dosage levels.

When required to make the change it is highly recommended that the calculated changes be reproduced in laboratory trial mixes. The actual first step is to reduce the coarse aggregate quantity by some value. A good starting point would be a reduction of 15% by volume. Replace the removed volume of coarse aggregate with a like volume of fine aggregate. Adjustments in the cement will be based on any changes in the compressive strength of the concrete. Water requirements may also need to be adjusted.

Visual after all of the ingredients of the adjusted design have been mixed for the appropriate time and one sees coarse aggregate and fibers then we know we still have too much of the coarse aggregate requiring another reduction in the coarse aggregate. Workability and consolidation are important factors in making these ingredient adjustments but performance is the ultimate reason for this exercise. Without a proper matrix to anchor the fibers the maximum performance level will not be achieved.

The use of a water reducer is a definite consideration when higher fiber dosage levels are required. The mid-range water reducer would typically be considered first when the synthetic fiber dosage level runs between 2 and 4 lbs/CY (1.2 and 2.4 kgs/m³). A high range water reducer would be the choice when the synthetic fiber dosage level exceeds 4 lbs/CY (2.4 kgs/m³). As for the steel fibers a mid-range water reducer would be considered a candidate when the dosage range is 25 to 40 lbs/CY (15 to 24 kgs/m³). Whereas any dosage level above 40 lbs/CY (24 kgs/m³) would dictate the use of a high range water reducer.

When dealing with a mix that will be pumped, ACI in 211 acknowledges that a 10% adjustment in the coarse (decrease) and fine (increase) aggregate is permissible. In the case of FRC, the quantity of fiber added as well as the type and the length will be a factor in adjusting the proportions. Pumping of FRC is not an issue if the ingredients are properly balanced.

Two important considerations:

1. The slump is a measurement of consistency from truck to truck and is not a measure of workability.
2. All modifications to a mix design should be verified with a trial run in the laboratory…at a minimum.
3. In the case of higher FRC dosage level mixes a field trial/dress rehearsal utilizing the personnel and equipment that will be part of the construction team is recommended.

Workability of FRC is directly related to providing sufficient mortar to coat the fibers in a given mix. If there is sufficient mortar present to accommodate the fibers the loss of workability, if any, will be minimized. Therefore we highly recommend batching laboratory trial mixes using the ingredients that will comprise the project mix. As recommended above, a partial or full truckload of macrosynthetic fiber or steel fiber reinforced concrete should be delivered to the jobsite to be placed and finished by the contractor’s crew providing an opportunity for all involved to hone their skills.

All projects can always use a dumpster pad and this trial placement will pay dividends when the project starts. We recommend that all of the personnel and finishing equipment be used on the project be available for the trial.

3. There will be less bleed water found at the surface of a fiber reinforced concrete slab than a plain concrete slab with the same water-cement ratio. The reason for this difference can be assigned to the fibers holding the concrete together so that the heavier material (cement and aggregate) does not sink and the water rise. This means there is less plastic settlement. Furthermore, less bleed water results in fewer and smaller bleed water channels, which translates into less permeability. The rate of bleeding will also be slightly slower.

With less migration of bleed water/free water, there is less potential for a wide range in the water-cement ratio from the top to the bottom of the slab. This creates a more homogeneous concrete when it comes to compressive strength and plastic and drying shrinkage.

This also means that the surface of the concrete will have a greater resistance to wear thus extending the service life of the concrete.

4. When placing macrosynthetic fiber and steel fiber reinforced concrete we recommend that a vibrating screed, a roller screed or a laser screed be used to level the surface. These screeds will help embed the fibers in the concrete so they are not exposed. At the higher fiber dosage levels, the use of these screeds also enhances uniform consolidation of the FRC.
5. When a textured surface is required on an exterior slab a stiff bristled broom is needed. Apply light pressure and pull the broom in one direction and only one direction. Pulling in opposite directions will pull up the fibers.
6. The finishers may need to wait for an extra 10-15 minutes before getting on the fiber reinforced concrete to allow for all of the bleed water to surface.

7. Apply a curing compound as soon as possible to eliminate evaporation and loss of water needed to hydrate the cement.
8. Sawing joints is very important. If soft-cut saws are used check the FRC before making the first cut. With synthetic fiber, a small time delay may be required. On the other hand with steel fibers, to reduce the raveling of the joints, a delay of 15 or more minutes may be required. We suggest trying a trial cut in a corner of the slab before proceeding. As to conventional saw cutting the joints should be cut within the first 12 hours after the concrete has been placed and the joint shall be 1/3 the depth of the slab.

9. The ambient temperature and the concrete temperature when delivered and placed are factors in cement hydration process. Cement hydration produces an exothermic reaction. Ambient temperatures below 50 degrees F will reduce or halt the hydration of the cement thus reducing or totally arresting the strength gain of the concrete. Temperatures below freezing will stop the hydration of the cement with no potential of the hydration of the cement resuming when the temperature increases above freezing. Fiber Reinforced Concrete will perform the same as plain concrete in the event the concrete freezes. Thermal blankets will help protect the concrete. An option would be heaters. Heaters must be ventilated properly or the exhaust gases from the heaters could damage the concrete.
10. Ambient temperatures above 80 degrees F and more so 90 degrees F can also create problems with the hydration of the cement. The high heat of hydration can produce drying shrinkage cracks and dramatically reduce the durability of the concrete.  The fact FRC may retain more moisture than plain concrete may help to moderate this potential problem.

– R.C. Zellers, PE/PLS, Director, Engineering Services