Figures 3, 4: Milling, scabbling, bush hammering, and scarifying may cause bruising and delamination of the prepared slab's surface.
Figures 3, 4: Milling, scabbling, bush hammering, and scarifying may cause bruising and delamination of the prepared slab's surface.

Question: How can we thoroughly clean the slab after surface removal?

Answer: It is extremely important to completely remove all loose dust and debris from the slab surface, as well as from the permeable pores. Brooming and air blowing create airborne particulate that only settles back onto the prepared slab surface.

Vacuum removal is the best method. However, hose attachments that widen at the end decrease the effectiveness of the vacuum. Using a 3-inch-diameter hose without attachment typically removes loose material from permeable pores best.

Question: We agree it's important to control airborne dust. Can we broom if we use a sweeping compound?

Answer: Sweeping compounds contain oily substances that control dust from becoming airborne. However, these substances can contaminate a prepared slab surface and should not be used where finish flooring is to be installed.

Question: Sometimes slabs are prepared using milling or scabbling. Do you recommend these methods?

Answer: There are various methods available for concrete surface removal. They are described in detail in Guideline No. 03732, available from the International Concrete Repair Institute (ICRI).

More aggressive techniques such as milling, scabbling, bush hammering, and scarifying are capable of removing more concrete but also risk microfracturing (bruising) the prepared slab surface. Examples of bruising failures are shown in Figure 3and Figure 4.

When significant surface removal is required, you should follow these primary surface preparation techniques with secondary techniques such as shotblasting (with shot <S280), sandblasting, or hydroblasting. For these techniques, the amount of surface removal is also related to equipment speed. After secondary preparation, thoroughly clean the slab before installing flooring.

In summary, proper surface preparation is imperative to the performance of finish flooring, underlayments, and bonded toppings. Separation or delamination beneath the bond interface can occur if you do not pay attention to this critical step.

The integrity of a slab surface can be tested using the tensile bond pull-off method outlined in ASTM D 4541. You can perform this test before or after surface preparation, and after the flooring system has been installed to verify whether you have achieved the necessary bond.

Scott Tarr is a principal engineer in the Structural Evaluation section of CTLGroup. E-mailstarr@ctlgroup.com.