If the soil or gravel lacks moisture, it can take more effort to compact it well. On the other hand, too much moisture can weaken the base when you compact the gravel. So, it's important to only dampen the gravel before you compact it and prepare it for a concrete slab.
Check that the surfaces you are pouring onto are dry before you begin pouring concrete. Never pour concrete into water-filled cavities or wet surfaces from which the water cannot be displaced and escape.
Whether you pour concrete for a walkway or patio, a strong gravel base is required to prevent the concrete from cracking and shifting. Gravel is especially important in clay soil because it doesn't drain well, which results in water pooling under the concrete slab and slowly eroding the soil as it finally drains.
It is important to keep in mind that dry-pour concrete may not offer the same level of strength and durability as wet-mix concrete. Dry-pour concrete can also be more prone to cracking and shrinkage, which can lead to structural issues and the need for costly repairs over time.
Formwork needs to be hosed down and wet in preparation for the pour. And concrete must be vibrated properly before the pour begins.
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.
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.
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.
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.
If it dries too quickly, the surface of the concrete will be weak, and will be subject to spalling. Spalling happens when a weak surface layer of concrete allows water to infiltrate. The water freezes and breaks up the surface of the concrete.
Gravel for Concrete
Depending on what the driveway will be used for, the base may need to be 4 to 12 inches. Ideally, 6 inches of crushed stone or gravel should be used for most driveways. Proper application requires that the driveway hole be dug out, and the soil has to be prepared.
Loose gravel has 40–50% air in it. You can pour concrete on top, but when that is loaded it will force the gravel into the supporting dirt, so you can expect more settlement. Compacting it first would be wise.
Most driveways require at least 6 inches of stone crushed or gravel. Medium gravel, which is 1.8 cm (3/4 inch) in diameter, is the best material for a concrete base. To cover a 4 inch thick concrete slab, you'll need 3 inches of gravel.
Another "old rule of thumb" for mixing concrete is 1 cement : 2 sand : 3 gravel by volume. Mix the dry ingredients and slowly add water until the concrete is workable. This mixture may need to be modified depending on the aggregate used to provide a concrete of the right workability.
Plan to pour concrete before 10 a.m. or after 8 p.m. in order to avoid the hottest part of the day. If you are concerned about the temperature while working on a summer project, consider pouring concrete at night to give the mixture the best chance to harden and cure before the sun comes up.
Concrete should be left to sit for 2-4 hours after pouring before watering. If it is supposed to rain in that window, the concrete should be covered. Additionally, if you cannot water the concrete as regularly as is recommended, covering concrete helps trap the moisture and slow the evaporation.
As a general rule, stay away from using sand as a subbase. You're better off using gravel; your concrete will last longer and have less of a chance of cracking over time.
Before you can put down a concrete slab, you have to lay down a base. A solid base protects the concrete from settling dirt, water runoff, and other natural hazards. Though all outdoor fixtures are subject to natural wear and tear, concrete will break and deteriorate more quickly without a base to support it.
If you use gravel, you will need to compact in lifts of 8 to 12 inches. An alternative is to use a cementitious product such as flowable fill. Flowable fill is more expensive per cubic yard than structural fill (stone or gravel) but provides several advantages.
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.
To prevent cracking and crumbling, concrete patios need to be built on solid, well-drained soil. If you live in a region with lots of clay in the soil, you will need to dig down further and backfill with layers of compacted sand and gravel.
DO spray new concrete with water. One of the most common methods for curing concrete is to hose it down frequently with water—five to 10 times per day, or as often as you can—for the first seven days. Known as “moist curing,” this allows the moisture in the concrete to evaporate slowly.
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.
Rain can influence the surface appearance of the concrete and its internal strength. It can cause the concrete surface to become soft, which reduces the abrasion resistance and strength of concrete. At the same time, it increases the tendency for dusting and cracking to develop.