Preparation of Substrates  for Concrete Repair & Restoration

Preparation of supports is undoubtedly the major phase of any Concrete Repair project. No doubt, most problems encountered are directly related to inadequate substrate preparation. It is of course important to determine methods and equipment required for a sound preparation of concrete surfaces.

The profile of a substrate is an important element of the preparation of substrates. The profile is the degree of roughness of the surface, or the mean distance between high and low points. The profile shall be proportionate to the thickness of the patching mortar. For instance, for the installation of an epoxy coating or a thinset mortar coat (less than 3mm/ 1/8”), the profile shall be low (approximately 0,8 mm/ 1/32”), while for a major 50mm (2”) repair job, the profile shall be 6mm (1/4”) minimum.

A certain number of mechanical substrate preparation methods exist, each having its advantages and disadvantages. A brief description of the main methods follows:

Sandblasting

This Method consists of projecting silica sand, with or without water, on the surface to prepare. The degree of abrasion depends on the size of sand particles, quantity used, projection speed and nozzle displacement speed across the surface.

Profile:

Varies depending on size, quantity, speed and duration of application.

Productivity:

Up to 75m2/hr (807 sq. ft./hr).

Advantages:

Quick method

Effective on walls, floors and ceilings

Disadvantages:

Produces lots of dust or water

Tedious cleaning

May polish concrete

High Pressure Water Blasting

This method consists of projecting water at very high velocity on the surface. The degree of abrasion depends on the pressure developed by the equipment, the type of nozzle, as well as on the nozzle displacement speed across the surface.

Profile:

Varies depending on the pressure and nozzle displacement speed.

Productivity:

50 to 80 m2/hr for 6mm (538 to 861 sq. ft./hr for ¼”)

Advantages:

No dust produced

Very low noise level

No vibration

Disintegrates damaged concrete and leaves sound concrete intact.

Effective on walls, floors and ceilings.

Doesn’t fracture aggregates

Doesn’t require a saw cut at the perimeter, thus eliminating smooth perimeter areas liable to hamper lateral adhesion.

Disadvantages:

Uses a lot of water.

Cumbersome disposal of effluents.

Shot blasting

This method, commonly referred to as “Blastrac”, named after a manufacturer, consists of projecting small steel abrasive pellets on the concrete surface and recovering the debris produced with a vacuum cleaner.

Profile obtained varies depending on the size, shape and quantity of pellets used and on the speed and power of the equipment used.

Profile:

0.1 to 3 mm (3/64” to 1/8”)

Productivity:

Up to 100 m2/hr (1076 sq. ft./hr)

Advantages:

Fast (up to 100 m2/hr/1076 sq. ft./hr)

Clean (very little dust produced)

Economical for large areas

Disadvantages:

Profile not deep enough for thick repairs

High cost for small areas

Scarification

This method consists of preparing the concrete surface with rotating cutters. Different types of cutters are available depending on the profile or surface requirement. Depth of the profile is adjustable to 12mm (1/2). For deeper profiles, it is recommended to use several passes.

Profile:

3 to 6 mm (1/8” to ¼”)

Productivity:

Between 25 and 50 m2/hr. for 6mm (269 and 538 sq.ft./hr for ¼”)

Advantages:

Excellent mechanical bond for mortar application. Versatile (can perform several types of surface preparation depending on the types of cutters used)

Disadvantages:

Produces a lot of dust

Noisy

May fracture aggregates

Bush-Hammering

This method consists of hammering the concrete surface with tungsten points. This operation breaks the Concrete Repair surface and allows obtaining a profile adequate for the installation of medium to high build repair mortars. A preparation similar to bush-hammering can be obtained using an air hammer.

Profile:

1 to 6 mm (3/4” to ¼”)

Productivity:

5 to 10 m2/hr for 3mm (53 to 107 sq. ft./hr for ¼”)

Advantages:

Excellent mechanical bond for mortar

Adequate for small areas

Disadvantages:

Low productivity

Noisy

Can fracture aggregates

Other Methods
Several other types of substrate preparation equipments are available. These shall be used based on the profile anticipated and on the feasibility of using the equipment on the job site.

Acid etching is another substrate preparation method. This method consists of applying a hydrochloric based acid (muriatic acid) or phosphoric acid on the concrete surface. These acids etch the concrete surface and destroy the surface laitance. This method has several disadvantages and shall only be used when deemed essential.

Indeed, acids shall be used at accurate concentrations and duration times. Furthermore, this method requires a thorough cleaning to remove all traces of residual acid. May also cause environmental problems.

On the other hand, acid etched concrete surfaces are weaker than mechanically prepared surfaces. Thus prepared surfaces offer a weaker bond to mortar coatings. For these reasons, emphasis shall be placed on mechanical surface preparation.

Perimeter preparation

Another important element to consider when repairing concrete consists in making a saw cut at the perimeter of the area to repair. The saw cut creates a clean separation between the existing area and that to be repaired and determines the minimum mortar thickness. Furthermore, this method eliminated weak points due to thin applications of mortar. However, the smooth surface created by the saw cut shall be roughened to ensure maximum lateral bonding.

Substrate Conditioning

Ideally, concrete surfaces that are to receive a repair mortar shall be water saturated and surface dry. Dry concrete has the tendency to absorb the mortar’s water and can possibly dry-up the mortar before hydration process is complete.

It is thus important that the concrete be saturated with water 24 hours prior to the application. However, the surface shall not contain excess water, which would hamper adhesion of mortar. It is thus recommended to dry up the surface before placing the mortar.

This method applies to all types of mortar r cementitious-based concrete installations, except when an epoxy-bonding agent is used.

Treatment of Rebar

Treatment of rebar is an important phase of the restoration of concrete structures. One can note that delaminated or completely loose concrete occurs mainly above rebar’s. Corrosion increases the volume of the steel bars and thus creates stresses in the concrete matrix and causes elimination between concrete and rebar’s. This problem occurs when the concrete is exposed to large quantities of deicing salts or when the concrete cover (distance between rebar’s and surface of concrete member) is too small.

If such is the case, treatment of reinforcing bars is recommended. The first stage involves the removal of delaminated concrete to the level of sound concrete beneath the rebar. Clean rebar by sand blasting to completely remove rust and corrosion. Rebars must then be treated using a cementitious or epoxy treatment process.

Epoxy

This system offers excellent resistance to deicing salts. Repair mortar adheres very strongly when the epoxy binder is still sticky. However, special care shall be given to concrete placing to prevent the formation of fish eyes in the coating. Furthermore, if working time is exceeded, the epoxy coating crystallizes and adheres weakly to mortar.

Cementitious Corrosion

This system consists of a water based polymer, cement, aggregates and corrosion inhibitor. The main advantage of this system is its compatibility with concrete and steel. In fact, in its plastic phase, it adheres very strongly to steel and, after curing, its roughness ensures a mechanical bond to concrete that is more effective than epoxies. This bonding characteristic prevents the slipping phenomenon that occurs between mortars or concretes in contact with epoxy treated rebars.

Anchorage

An anchor is a device used to ensure a solid bond between two structures. There are two types of anchor binders: Epoxy and Cementitious.

Epoxy

This method consists of drilling a hole approximately 3mm (1/8”) larger than the bar to anchor.

The walls of the holes must be cleaned, the bar inserted in the hole and the space around the bar filled with the epoxy binder. For horizontal anchors, liquid epoxy binder is recommended, while for all other vertical and ceiling applications, gel epoxy binder is the best choice.

Cementitious

The placing method required is similar to that of epoxy binders. However, diameter of the drilled hole shall be at least 8mm (5/16”) larger than the steel bar.

Cementitious anchorages are more economical than their epoxy counterparts, but have lower mechanical strengths.

Bonding Agents

It is always recommended to use a bonding agent before application of a repair mortar. This operation increases the adhesion of mortar to existing concrete. There are several types of bonding primers detailed below. Bonding agaents are common in concrete repair projects.

Slurry Bond Coat

Fluid mix of cement, fine aggregates, water or a polymer used as a binder between existing concrete and new concrete.

When a repair mortar is used, simply mix part of the powder with water or latex and brush apply the mix on the bonding surface. When the grout is still wet, apply the repair mortar.

Epoxy Adhesive

Two-component epoxy adhesive used to bond a concrete or a mortar to existing concrete. Mix Part A and Part B. Prepare surface to repair by applying the epoxy adhesive on the existing substrate, when it is still sticky to touch, then apply the repair mortar.

Advantages:

· Long working time

· High strength

· Superior adhesion

· Can be applied on slightly damp surfaces

· 100% solids, solvent free

Disadvantages:

· Acts as a vapor barrier

· Costs more

· Recommended for interior applications only

Bond Coat

Two-component primer intended to improve adhesion and cohesion of the repair consists of using galvanized wiremesh firmly anchored to the substrate. This technique is especially used for ceiling and wall repairs. Wiremesh shall be kept at a distance from the substrate by means of spacers and shall not be supported directly on the substrate.

Repair Methods and Materials

The choice of repair methods and materials shall be based on the following criteria:

· Evaluation of the structure

· Future use of the element to repair

· Environmental conditions in which repair work is to be performed

· Type of repair

· Feasibility on the job site

· Costs

· Products

· Labor available

· Etc.

Interior Horizontal Applications

Self-leveling

This method consists of applying a very fluid mortar on an uneven concrete surface to create a flat surface. The Main advantages of this method are: quick implementation, flatness of surface obtained and short period of time required before installation of resilient flooring materials. However, some products require specialized labor and very accurate mixing.

It is possible to level from 200 to 500 m2 (2152 to 5382 sq.ft.) per day, depending on floor configurations. It is to be noted that the in-place thickness of products vary between 3 and 12mm (1/8” and ½”) per coat.

Leveling Topping

When floor repairs are in excess of 25mm (1”), self-leveling agents are not recommended. It is preferable to use quick-setting repair mortars. The main advantages of using a hydraulic binder would be:

Relatively low cost mix using sand and/or aggregates

No shrinkage crack

Fast setting (traffic allowed after 4 hours)

Fast drying (less than2.5% moisture content after 24 hours)

Installation of ceramic tiles after only 4 hours

Installation of waterproofing membranes after 24 hours

Doesn’t need curing (in normal conditions)

High performance repair mortars are recommended for applications in industrial environments. These repairs are often exposed to extensive heavy traffic.

Exterior Horizontal Applications

For the repair of an exterior concrete slab, it is very important to select repair mortars highly resistant to freeze/thaw cycles, deicing salts, air pollutants and acid rains. In fact, a large number of exterior concrete slabs have been damaged by concurrent phenomena.

For successful repairs, special care shall be given to the preparation of substrates for such applications and to the curing phase.

Exterior Toppings in excess of 40 mm (1 5/8”) thick

For this type of repair, one can consider using a conventional concrete having a good mechanical strength (4350 psi) minimum and adequate air entrained to ensure and improved long term resistance and to reduce permeability of concrete and increase its resistance to freeze /thaw cycles and deicing salts. A liquid polymer can be added to concrete. A slurry bond coat incorporated will improve adhesion of topping to substrate.

Illustration to be uploaded later

To speed up the subsequent work activities such as the installation of a waterproofing membrane, it is recommended to use a repair mortar such as Quick CemTop 202 with 205 stone aggregates. The waiting period before installation of the membrane will be reduced from 28 days to only 24 hours.

Exterior Repairs between 6 (1 ¼”) and 40 mm (1 5/8”) Thick

For this type of repair, it is recommended to use a ready-to-use pre-blended mortar . The main difference between mortars lies in their cementitious base. Some will be based on Portland cement, while others are based on a special hydraulic binder. Advantages of the special hydraulic binder are as follows:

No curing required (in normal conditions)
No shrinkage crack
Fast drying (less than 2.5% moisture content after 24 hours)
High early strength

Exterior Repairs less than 6 mm (1/4”) Thick

For this type of repair, it is recommended to use a mortar having particle size smaller than that used for thicker repairs. Portend Based mortar are a good choice in that respect. However, this product requires curing.

Interior and Exterior Vertical Applications

Two main methods can be used for the repair of walls, ceilings and columns. The first consist in the spray application of a thixotropic fiber mortar on the surface to repair. The second consists in building a formwork around the element to repair and to pour a fluid repair mortar

Repair Method for Vertical Surfaces

Outline the surface to repair with a saw cut

Mechanically prepare the surface

Clean substrate

Install #12 wiremesh (if required)

Saturated concrete with water for a period of 24 hours minimum

Install the formwork and pour fluid fiber mortar

Or

spray apply and finish the thixotropic fiber mortar on the surface

water cure for 3 days or use a water- based curing agent (2 coats)

Waterproofing

Cementitious Waterproofing Mortar/Coating

A waterproofing cementitious mortar is intended to stop water migration through the concrete substrate. It waterproofs under negative or positive pressures. Water exerts a positive pressure when mortar is applied inside a reservoir for instance and negative when the mortar is applied on the interior face of foundation walls.

Flexible Cementitious Fiber Mortar/Coating

The Flexible Cementitious fiber mortar is intended to protect concrete against effects of freeze/thaw cycles and deicing salts. The mortar is applied in thin coats and has a flexibility that considerably reduces cracking hazards. This type of mortar is mainly used to protect and resurface concrete and masonry elements

Water Agents
Water repellent is a liquid that penetrates the substrate and reacts to form a hydrophobic compound. The compound protects and the substrate against the penetration of water and pollutants and doesn’t act as a vapor barrier. It is recommended for concrete structures exposed to high concentrations of deicing salts or for porous concrete or masonry walls exposed to acid rains or air pollutants.

Curing

Curing is critical to any repair job where Portland cement mortar is used. In fact, Portland cement should be kept damp from 7 to 14 days to allow complete hydration. When curing is inadequate, mortars and concretes have the tendency to crack and leave a weak and powdery surface. Certain hydraulic binder-based repair mortars require no curing such as PLANICEM 50, QUICKCEMTOP 102, 202.

Water curing

Water curing is considered the better method

Another effective curing method consist in covering the concrete or mortar with a sheet of polyethylene, however, this method is not recommended for exterior applications considering the possible greenhouse effect of the polyethylene/sun combination liable to significantly raise temperature of the concrete. Furthermore, when polyethylene is used alone, no water is added to the system and this could cause hydration problems when the repair mortar is applied on a porous substrate.

Curing Agent

This method consists in applying a liquid curing agent on the mortar or concrete surface. The agent forms a thin film that considerably slows down the evaporation of water present in the substrate. In the case of polymer-based concretes or mortars, water-based curing agents are recommended. However, as for the polyethylene sheet method, no water is added to the concrete and the residual film is not compatible with coatings and with most paints and adhesives. To improve effectiveness of these products, application of two coats is often recommended.

Phases of a Successful Repair Job

1. Determine methods and materials required.

2. Control quality of labor

3. Prepare substrate:

- Perimeter saw cut

- Make surface profile using a mechanical mean adapted to repair

- Concrete should be saturated with water but surface dry

4. Treat Rebars (if required)

5. Apply a slurry bond coat

6. Apply the repair mortar

7. Perform appropriate curing (for Portland cement-based mortars)

When followed correctly, these methods are the best available means of restoring concrete surface through Concrete Repair.