Overlays are 6 mm or thicker surface depositions that can be glued, partially bonded, or unbonded to the concrete’s surface. Polymer concrete, Portland cement concrete, epoxies, polyesters, and polymer-modified concrete are all possible overlay materials. Overlay concrete alters the look, texture, and elevation of the original concrete surface, and comes in a variety of colours and finishes. Nonmoving cracks may be bridged by overlays, but active cracks may reappear. Overlays are utilised as a wear course, give protection against water and chloride ions intrusion, provide abrasion resistance, and can be attractive.
Overlays are excellent for sealing regions with a large number of cracks where treating each one individually would be prohibitively costly. Extensible overlays must be utilised to seal active cracks (must have the ability to elongate and shorten). It’s worth noting that the cracks are just surface-sealed with unbonded overlays, not filled from the inside. The most typical type of overlay is a two or three-ply felt membrane with tar between the plies, which is then protected with gravel, concrete, or bricks. Almost all types of overlays work well for latent cracks.Dormant fractures in structural slabs and pavements can be repaired using a bonded overlay or a surface treatment.
Repair in Concrete Structures
Overlays can be troweled, screeded, or sprayed over the concrete surface in one or more layers. In the examination of an existing structure, the additional dead load of overlays must be taken into account. Overlays can be constructed to work in tandem with the existing structure, with extra reinforcement in the form of welded-wire fabric, reinforcing steel, or fibres. The surface to which an overlay is bonded must be clean, sound, and sufficiently roughened in order for it to work correctly. Where there is active cracking or structural movement, bonded overlays are not employed. In such cases, unbonded overlays may be employed.
Before applying the overlay, the surface must be well cleaned and free of laitance, weak material, grease, or oil, and a bond coat of finer overlay material or an epoxy glue must be put immediately before the overlay is applied. Polymer-modified Portland cement mortar or concrete, or silica fume concrete, are the most common overlays. Styrene butadiene or acrylic latexes are commonly used polymers, which are added to Portland cement in amounts ranging from 15 to 20% by weight. Overlays with a minimum thickness of 30mm can be used to hide fine latent cracks. In warm weather, polymer-modified overlays can be mixed, laid, and finished in 15 minutes, although they may require a 24-hour moist curing period.
In freezing conditions, a vapour barrier overlay on slabs-on-ground is not recommended. Because of their high surface-to-volume ratio, thin cementitious overlays are sensitive to plastic-shrinkage cracking under drying circumstances. At least one of four factors can cause cracking in bonded overlays:
1. Surface tearing as a result of late finishing operations.
2. Excessive drying causes plastic to shrink.
3. Temperature differences or drying shrinkage cause differential movement between the deck and the overlay.
Existing fissures were reflected in the overlay. Testing of the in-place concrete is required both before and after surface preparation to establish the concrete’s modulus of rupture and to determine bond strength. Overlay systems can be classified into the following categories:
Unbonded or partially bonded:
These systems are utilised to provide wearing courses and create drainage slopes over waterproofing membranes on bridge decks, plaza decks, parking garages, over office spaces, and over waterproofing membranes. They can also treat exposed patios and driveways with architectural finishes. Asphalt, asphalt concrete, or cement concrete can be used as an overlay. Asphalt and asphalt concrete overlays are porous, provide little waterproofing, and cause deck damage as a result of freezing and thawing. Bonded PCC overlays are simple horizontal concrete layers that are applied to a well prepared existing concrete surface. This substance is used to repair a spalled or disintegrating surface, offer a protective barrier, enhance cover over reinforcing steel, boost slip resistance, level floors, and repair abrasion-damaged concrete surfaces.
Surface treatments:
Portland cement overlays are sometimes reinforced with fibers and other additives. Low-slump concrete overlays and bonded Portland-cement-concrete overlays are used to provide a protective barrier to deicing salts and to increase the load-carrying capacity of the underlying concrete.Surface treatments: Low solids and low-viscosity resin-based systems, using materials such as urethanes, epoxies, polyesters, and acrylics, can be used to seal the concrete surfaces that are not subjected to traffic. These methods are most effective if cracks are treated by injecting with epoxy or by routing and sealing before applying surface treatment. The thickness of this treatment may vary from 1 to 50 mm and aggregates can be sprinkled on the top to improve skid-resistance.
Types of cracks
There are two types of cracks: static and moving. Static cracks, such as craze cracks and plastic shrinkage cracks, are minute imperfections that only impact the surface. They usually don’t need much repair, so you can get away with standard surface prep. Active cracks, also known as moving cracks, are more dangerous. They are frequently structural in character and extend the full depth of the concrete. Inadequate control joint spacing and sequencing, failure to isolate fresh concrete from old concrete, and incorrect subgrade compaction are also causes.If the crack is the diameter of a credit card or wider, it’s most likely a moving crack that needs to be corrected before resurfacing the slab. Also, borderline cracks (not quite credit card width but close) should be treated.
Methods of repair Consult your material provider for suggestions on the best repair method to employ with its product. Also, make sure that each crack is completely cleansed before filling it. The following are some strategies that I’ve found to be effective:Crack, chase, clean, and fill are the steps to take. Using an angle grinder and a V-grooved diamond blade, remove the crack. Using pressurised air or a shop vac, remove all debris. Fill the crack with a semi-rigid substance, particularly one based on polyurea.
Replace the control joints with new ones. After you’ve filled the cracks, I strongly advise you to saw-cut a new control joint close to the crack to help relieve the slab’s expansion and contraction strains.Injection of epoxy resin. This approach is unlikely to cause cracks to recur, but it needs skill and can be time-consuming and costly. Install ports in the crack at intervals equal to the concrete thickness; use a low-pressure pump to inject epoxy through the ports to fill the crack. Stitching with a crack. On both sides of the crack, drill holes. Using wires or U-shaped metal strips, span the damaged region and grout or epoxy in place. (Some installers use a saw to cut a wide kerf across the fracture and then epoxy rebar into the region to join the two sides.)
Concrete Crack Repair
During the life of a concrete construction, cracking is a regular occurrence. This crack must be treated with care and attention, and it must be repaired at the appropriate time. Concrete crack repair is fraught with time and financial constraints. There are various methods and strategies for repairing concrete cracks.Choosing the right crack repair procedures can save you a lot of time, money, and energy while also providing long-term results. To choose the most appropriate and cost–effective way of repair, it is necessary to understand the type and nature of the cracks that have occurred in the building. It’s also crucial to figure out what’s causing the cracks so that they can be fixed and the cracks don’t return.
If your driveway needs a whole new look, a decorative concrete overlay will create a new finish
To use epoxy injection to repair a crack, the crack is first cleaned by vacuuming.
Wide cracks in concrete are best patched and sealed with a concrete patching compound.
Also Check
Strength and Failure of Bituminous Pavement Materials