Concrete Slabs – The Basics When you are building with concrete slabs, a general rule of thumb is the thickness is around 10cm or 4 inches. This should be the minimum thickness of your concrete slab. The harshness of the sun and expansion from the heat will make thinner pieces crack and break.
How thin can you pour concrete over concrete? Your new layer needs to be at least 2 inches thick. If it's any thinner than that, it'll easily crack under pressure and won't adequately bond to your surface.
The thinnest that is usually used when pouring concrete, in general, is about 2 to 2 ½ inches. That also applies to this pouring new concrete over old concrete. Any thinner than 2 inches would be too thin.
A crack in a slab of 1/8 inch or less is typically a normal shrinkage crack and not a cause for concern. If the crack is larger or growing larger (an “active” crack), or one side of the crack is higher than the other, then you may need to have the work reviewed by a structural engineer.
Break Up the Concrete
For slabs less than 3 inches thick, use a sledgehammer. For more than 3 inches thick, use a demolition hammer.
For the most part, if the cracks are less than 1.5mm in width and the length is less than those specified in Australian Standards (generally ranging from 5mm to 15mm) then it can be assumed to be minor cracking that is acceptable.
If the weight of the concrete deforms the dirt base in any way, the entire concrete structure will be flawed. All of that is not to say that it is impossible to pour concrete over dirt. You can absolutely do it successfully, but you need to follow a specific process.
The general rule of thumb is that concrete takes about 28 days to dry for every inch of slab thickness. Within 24 to 48 hours, the concrete will be ready for foot traffic.
Concrete that is not moist-cured at all dries too rapidly, and reaches less than half its potential design strength. It will also have a greater number of shrinkage cracks.
Rest assured that even with the best floor design and proper construction, concrete cracks are very common and, in some situations, inevitable.
As the slab loses moisture while curing it gets a bit smaller. As the concrete shrinks, the slab could crack in order to relieve tension. Shrinkage cracks are common and can occur as early as a few hours after the slab has been poured and finished.
Standard Concrete Slab Thickness In Australia
In Australia, concrete slabs must be fixed in adherence with the Australian Standards AS 2870 and AS 3600. The most common slab thickness in Australia is between 85mm and 100 mm. The thickness of concrete slabs is usually stated in specification documents.
40-mm minimum concrete cover to the reinforcement is required (up to 75 mm may be specified if soil has aggres- sive ground water). In some areas greater depth and heavier reinforcement is required. Bottom-steel must be placed on bar chairs or trench mesh spacers.
Hi, a 50mm topping outdoors will crack fairly quickly.
Moist curing is a common method of concrete curing. It involves wetting the concrete slab often with water (5-7 times per day) for the first 7 days. This method ensures your concrete slab will be extremely strong and durable, because it allows the moisture to evaporate slowly, preventing cracks and shrinks.
Many construction specialists recommend watering it down five to ten times per day, for the first week, starting 2-4 hours after it has been poured. The first two to four hours lets the concrete “finish”, which refers to the setting process.
If water evaporates too quickly, it will weaken the finished product with stresses and cracking. To put it simply, the goal is to keep the concrete saturated during the first 28 days. The first 7 days after installation you should spray the slab with water 5-10 times per day, or as often as possible.
Because concrete is a very porous material, it will absorb any moisture that it contacts. This can cause pooling. Without crushed stone, pooling water will settle under it and erode your slab. Adding a layer of crushed stone will add proper drainage, as well as create a barrier between your slab and the ground.
The simple answer is to prevent the concrete from cracking and breaking.
Technically you can, but you really shouldn't pour concrete over grass. To maintain its tensile strength, concrete needs to be laid on a firm, even, and dry foundation. Grass and soil will naturally channel moisture into the concrete from below, which will likely result in it cracking over time.
Generally, cracks with a width of less than 0.3 mm are acceptable and do not affect the structure. However, they may develop and grow to become structural cracks. Therefore, one should monitor small cracks on houses or structures. Crack width greater than 0.3 mm can create problems for the durability of the structure.
In the majority of cases, a 100mm thickness is recommended.
'Plastic Shrinkage' From Water Loss
Before concrete dries, it's referred to as “plastic,” and this is a particularly temperamental stage. Both hairline cracks and shrinkage cracks occur because of rapid moisture loss, also known as “plastic shrinkage,” before it dries.