There are three types of toppings: bonded, unbonded, and partially bonded. A partially bonded topping is a failure—if portions of the toppings did not bond, it will most likely crack and spall. Partially bonded toppings are typically intended to be bonded but the substrate was not properly prepared to achieve full bond.
Contaminants
Whenever a bonded topping is being installed on an existing slab, contamination is a potential issue. Contaminates can be present within the slab, even when they are not visible at the immediate surface. If the existing slab had finish flooring, even when the adhesive has been stripped the solvent may have penetrated into the existing slab. Other common contaminates include cutting oil in machine shops, metal debris in old foundry slabs, and paints and stains in art studios. Vacuuming may remove some contaminants, but more extensive surface preparation may be required.
Crack Treatment
Any moving cracks in the substrate slab must be stabilized before installing a bonded topping. If moving cracks exist in the base slab, there is a risk they will telegraph through the topping. Cracks can be filled with a polymer structural filler material such as epoxy. Cracks that are exhibiting substantial movement can also be stabilized by cross-stitching where reinforcing bars are epoxied into holes drilled into the slab across the crack in an alternating “stitching” pattern.
Surface Preparation
The key to a successful bonded topping, as with a coating, is proper surface preparation. The slab must be scarified so that the coarse aggregate is exposed and the surface cleaned of dust and debris. A typical surface profile would be a CSP 7 or above as described by the International Concrete Repair Institute’s (ICRI) Concrete Surface Profile (CSP) chips (Selecting and Specifying Concrete Surface Preparation for Sealers, Coatings, Polymer Overlays, and Concrete Repair, ICRI Guideline No. 310.2R-2013). These chips range from a CSP 1 to a CSP 10 and rank the degree of surface roughness to be achieved.
When substantial surface removal is necessary, the surface profile should be created in two stages: initial and secondary. The initial heavy surface preparation can be done by large-scale removal methods such as scarifying, rotomilling, or scabbling. But since these surface removal techniques can create significant microcracking (bruising) in the prepared surface, secondary preparation should be performed such as shot-blasting to remove the weakened surface resulting from the initial surface preparation. For base slabs with a thick mortar layer near the surface, however, the scarification need not expose the base slab coarse aggregate.
Cleaning
Thoroughly cleaning the prepared surface of all loose debris and dust is imperative. The best surface cleaning method is vacuuming using a 2-inch line with a small brush attachment. The dust within the surface pores must be removed to achieve a well-bonded topping. Cleaning the prepared surface by brooming or blowing is unacceptable since the dust will ultimately settle back onto the surface. Avoid sweeping compounds since they may leave an oily residue that can inhibit bond. Vacuum units should be located outdoors. Even units with high filtering systems release dust back into the air when the units are emptied.
Protection of Prepared Surface During Construction
Once the base slab has been prepared to receive a bonded topping, it must be kept clean to avoid collection of contaminants that could affect its bond with the topping. Toppings are often placed months after the base slab. On some projects when toppings are planned, designers will specify a roughened surface finish at the time of placement rather than using mechanical methods to prepare the base slab at a later age. Avoiding mechanical preparation by roughening the base slab during finishing can reduce preparation costs, although there are still usually hidden labor costs to clean the slabs later. Base slabs mechanically prepared just before topping placement are easier to protect from contamination and that action removes any weak surface and laitance resulting from base slab placement and curing.
Saturated Surface Dry Conditioning
Just as aggregate is conditioned to be saturated surface dry (SSD) in a concrete mix, the surface of a prepared base slab should also be conditioned to SSD. While many believe a dry base slab surface will allow the topping mix to be drawn into the surface, conditioning to SSD prevents the base slab from absorbing water from the cement paste and altering the water-cement ratio of the topping mixture. By the same reasoning, the base slab should not be oversaturated such that it gives water to the topping mix increasing its w/c.
Application of Bonding Grout
Getting full contact of the topping with the textured base is critical, whether by brushing in a bonding agent or just brushing in the topping mixture itself. Full contact increases the potential bond area and, consequently, the potential bond strength. I have investigated many debonded toppings where it’s obvious full contact wasn’t achieved. To achieve full contact, specifications will often require application of a bonding grout immediately before the topping concrete is placed. The bonding grout can be cementitious- or epoxy-based.
Placement of the topping before drying of the bonding grout is essential to the performance of the bonded topping. If the bonding grout dries before topping placement, it may form a weak layer that acts as a bond breaker. The rate of drying is dependent on the characteristics of the grout as well as the ambient conditions during placement. Additional water should not be applied to the slurry to prevent drying. Plastic sheeting may protect the slurry from the drying effects of sun and wind, but the better technique is to stay within a few feet of topping placement to insure a constant supply of topping material will be available. If there is a delay in delivery of the topping and the slurry layer dries, it must be removed and the surface preparation repeated before proceeding with topping placement.
Some epoxy bonding agents have extended open times ranging from four to 24 hours. Carefully follow the specification and the manufacturer’s recommendations.
Given the risk of unintentionally installing a bonding grout that acts as a bond breaker, some designers and contractors have eliminated the bonding slurry. The prepared concrete substrate slab can be pre-conditioned by saturating it overnight prior to topping placement. During the saturation period, the slab can be wetted and covered with plastic sheeting. Before topping installation, the plastic sheeting is removed and any standing water is vacuumed to leave the prepared surface in a saturated-surface-dry condition. As the topping concrete is placed, use stiff-bristle brushes to scrub the paste portion of the topping concrete into the SSD substrate ahead of screeding the topping surface.
This article is an excerpt from the upcoming book Guide to the Design and Construction of Toppings for Buildings published by the American Society of Concrete Contractors.
