By Daniel Green
They’re heavy things, concrete slabs. Heavy and brittle. And they generally don’t like fighting gravity. But when yours is crooked, has sunken, is cracked or needs removing, remediation works via lifting and relevelling brings all sorts of challenges.
Concrete forms the basis for many durable aspects of modern construction. Floors, walls, driveways, footpaths and even ceilings. Determining the best method for lifting them depends on what type of slab you’re moving in the first place, and why. Read on.
Some Common Scenarios
Often less than 100mm thick, these lightweights are designed for foot traffic or a few bicycles only. Most times poor ground preparation, poor drainage, tree roots or extreme weather events are the biggest factors in sinking or cracked footpaths. These conditions compromise the structural integrity of the earth underneath the slab, leaving behind cavernous hollows. The rest of the damage is done by gravity and traffic.
The modular design of footpaths means that should they become uneven then remedial repair works are relatively easy in that individual panels can be easily repaired or replaced.
Concrete walls are not new. Modular concrete walls that are fastened in place are. More commonly known at tilt-up construction, pre-fabricated concrete walls are manufactured off-site to precision standards. This leaves very little fitting during installation. This also means that unless there is damage to the wall panel itself, removal and replacement is possible. In the case of wall slabs that have been poured in situ or fastened with chemset rod then there is very little hope other than demolition and rebuilding.
Often the cause of misaligned wall slabs is due to more complicated issues further upstream, namely those associated with the floor slab underneath. Concrete wall misalignment can be from building or vehicular impact but this is obviously rare. One thing to remember is that the going thickness these days of tilt-up wall slabs is 175mm, making them extremely heavy.
Usually at least 250mm thick with an abundance of rebar, floor concrete slabs are obviously designed with strength in mind. As with footpaths the causes are usually poor ground preparation, poor drainage, tree roots or extreme weather events. In some cases the resultant damage is downstream of concrete cancer symptoms such as corroded structural steel.
Floor slabs are usually poured first, meaning every other structural item is laid on top, making it very difficult to constrain damage. It also dampens the ability to make remediation plans. It is not impossible however and some alternative resetting methods can be employed.
Suspended ceilings are obviously extremely well situated. Their sheer mass makes it a necessity. In almost all cases the need to lift them comes down to two primary reasons: refurbishment with design changes to the existing structure or damage. Even then these are mostly partial-lifts, although full ceiling slab lifts are not impossible. In either case the offending area will be cut away and removed or relocated.
The Realignment Process
Working with heavy, brittle, often damaged materials brings its own unique challenges. Add a multitude of causes into the mix and the correction process isn’t so clear. There are however, some common procedures to adhere to.
As we’ve mentioned, the realignment of vertical surfaces is entirely dependent on if they’re modular or not.
Constructed In Situ
If the wall slab has been poured in situ like Rediwall and other brands, then it will require extraction without damaging or interfering with the surrounding construction. This means demolition saws, ring saws or depending on the depth – wall saws. This is of course followed by traditional heavy demolition using manual means or hydraulic breakers.
Once the demolition is complete, a more full damage assessment can be completed. From this further repair works may need to occur. If the wall is to replaced then the methodology will depend on the end use.
If on other hand, the wall is of a pre-cast nature, it will likely have been fixed with bracketed that can simply be unbolted. Often, the sheer bulk of the structure will require machinery to lift it. This will usually be in the form of a crane or large excavator, depending on the weight and size. One other advantage of pre-fabricated concrete panels is that they most likely have lifting points cast into them during manufacturing.
Once the structure has been rigged and is free, it can be removed and;
a] replaced, or
b] other aspects of the building can now be assessed for repair
Just keep in mind that during both processes, the structure is now ‘open’ and no longer has any security value.
Unlike wall slabs, floor slabs have several repair methods available. This depends entirely on the type of damage, the integrity of the surrounding construction and the building in general, the end use, the propensity for further damage to occur, budget and time.
The most common scenario is where the slab has sunken or cracked due to a loss of the underlying earth. Weather events, poor construction techniques, poor earthworks, unusual or irregular water table activity, underground streams, corroded steel causing foundational collapse, tree root growth and scores of other reasons can be the cause. In any of these scenarios the earthen foundations have moved significantly or disappeared altogether, leaving vast voids and the concrete simply cannot cope. It cracks, sinks or both. In many cases full demolition is required. Usually this can occur without damaging other aspects of the house but it’s not always necessary.
As the name suggests, this is where the concrete is jacked up to its original poured height, often in one or several large pieces. There’s four primary methods of underpinning:
- Mass concrete. Using hydraulic jacks to raise the slab back up, concrete is pumped in the void, capped off under pressure and allowed to cure. As the mass pour is far thicker than the original floor slab [hence the term, mass], the chances of sinking again are drastically reduced.
- Steel pile. This method sees steel piles driven into the earth around the sunken slab. Brackets link the two, preventing further sinking. This won’t relevel the slab but it will prevent further sinking.
- Cement grout. Like steel piling this has its limitations, but only in terms of excessively large or heavy slabs. Holes are drilled in the concrete at set intervals and liquid grout is pumped in under pressure. Once the void is filled the pumping continues until the slab rises to its original location.
- Polyurethane injection is very similar to cement grout, except the material is a more complex chemical compound with better working and longevity properties.
Crane or Excavator
Should slab jacking not be suitable, cranes are one option to raise existing slabs. Rigging points are anchored into the concrete to allow an even and controlled lift. As we know,, some pre-cast panels have lifting points cast into them during manufacturing. Once the slab has been lifted and is in situ, it must be held in place using fasteners, often those engineered specifically for that application. Only then can the repair take place. Once a mass pour, grout or urethane application has been applied the supporting understructure can be removed for finishing works.
Once the slab has been reset and works have been completed, some measures should be undertaken to prevent damage from reoccurring.
- Waterproofing and membranes. Uncontrolled water flow is often the cause of sunken slabs. A waterproofing professional will guide you on how and what to do to shape the flow of water so that you retain foundational integrity.
- Drainage, or lack thereof, is another cause of sinking slabs. If your property is downstream of a large water flow [even an underground one], or is subject to natural springs or water tables, sorting these drainage issues will help protect your need for such extensive remedial works again.
- Tree roots. If these were the cause of your slab issues then clearly they must be taken care of or they will return. Do not underestimate the resilience of mother nature and her ability to regrow. Underground root barriers may need to be constructed or, sad as it is to say, the tree may need to be removed altogether.
Repairing sunken and cracked slabs remain one of the most complex remediation processes there are. Many variables go into deciding what methodology to use and opinions may vary. Finding an experienced professional is the best outcome as they will guide you through these tricky repairs every step of the way.
For more information on stabilising damaged slabs, contact Tom at Perfect Remediation and Refurbishment on 0407 491 888.
We Get It Done.
For more information on our services, take a look at our webpage or request a free quote.contact us today