Pouring a Basement Slab properly can nearly eliminate water penetration from below

Pouring a Basement Slab properly can nearly eliminate water penetration from below.

Constructing a good basement slab is very easy when a few simple rules are followed. Concrete used in slabs is often the most abused material used in the home’s construction.  Our intent is to save a homeowner time and money when you have your contractor follow these simple rules.

Make sure your contractor followsthe General Structural Notes on the cover page of your structural engineer’s drawings.  Why spend the time and money creating the plans if your contractors will simply be ignoring them.  The plans in some cases clearly outline site preparation, compaction, and structural fill to be used.  It also spells out the ACI standards which should be followed, for example, ACI 301 & 318, which are the latest edition for concrete for buildings and code requirements.  Even small details like how the exposed edges of the concrete shall be finished, i.e., Chamfered.  Important details like what type of cement, sand gravel, or crushed stone will be used.  Note the PSI strength listed on them for the footings, interior and exterior slabs along with the amount of slump outlined.  Lastly, make sure they adhere to the steel reinforcement, stirrups, splices, welding if you have any, and grout strength outlined in the plans.

Granular fill should be always be used below the base of the basement.

Granular fill in simple terms is gravel, crushed stone, and some man-made products, but regardless of which material is used it has to be well compacted and level. An uneven surface will restrict slab shrinkage movement.

Damp-proof membranes serve a very important function in the construction of a basement slab.

Their primary purpose is to stop the passage of water vapor from rising into the slab.  Without it moisture would soak the foundation and damage base plates of walls, floor coverings, etc.  It is also mandatory under all living areas but only recommend for use in the garage area.

Some type of gravel material is required under the damp-proof membrane to prevent groundwater from pushing its way through the slab by capillary action.

The damp-proof membrane should be made of a water-proof polythene material.

To reduce the chance of being punctured, it should be placed on a layer of sand, but some contractors like using building paper on top of it as an added layer of protection.

All tears, holes, and laps need to be taped or sealed to prevent moisture from escaping.

Concrete itself

By minimizing the amount of water used in the mixing,the risk of it cracking shortly after it is placed will be reduced.

A stronger structural mix prepared at a batch plant is always preferred over a pump mix prepared on site.  The option to use a concrete additive or super-plasticizer is a great method for significantly reducing the amount of water used in the mixing of the concrete.  It will also greatly reduce the amount of water rising to the surface and pooling on top of the freshly poured concrete

Watch for shallow (plastic) cracking with this type of mix.

Avoid using added water on site to increase slump for the ease of the concrete being placed and pumped because it will reduce the concrete’s PSI and increase concrete shrinkage.

Cracking in Plastic Concrete

The cracks normally appear on the day the concrete is being poured.

The concrete has to be covered to reduce evaporation, the water bled to the surface when screed boards are used.  This type of cracking can easily be avoided by waiting for the concrete to stiffen a little and avoid excessive screeding.  Once the concrete is allowed to stiffen a little, then finish it and allow it to properly cure.  This is when the concrete is most at risk.

For best results use a water mist or garden hose with a fine sprayer.

One of the most effective materials to cover the freshly poured concrete is plastic sheeting (Visqueen).  Plastic not only repels water from above, but also from below.  This means that water vapor rising up through the slab will not be able to evaporate from the surface, but will instead be trapped there by the impermeable membrane.  That’s what makes curing concrete with plastic so effective: the plastic keeps the concrete damp and drying out too quickly during the summer.

When it is placed above the concrete it is better than curing blankets and any other method in my opinion because it does not leave a discolored surface.

Remember to avoid the plastic touching the wet concrete because it causes condensed water to form and pool wherever the plastic touches the slab.  When water is allowed to pool it causes the wet concrete to cure differently than the dry spots.  Later when the slab is uncovered, the result is a very blotchy, mottled slab with odd-shaped light and dark shapes throughout.

The only other thing to watch out for is while the concrete is being screed and leveled out.  If the concrete does not appear to be bleeding to the surface you will be at risk of plastic cracking occurring.

Early-Age Cracking

Temperature changes in the freshly poured concrete may mean that the slab will crack overnight.

To reduce the risk considerably of early-age cracking all joints should be cut or placed into position after the concrete stiffens a little during its placement.

This involves the use of crack inducers, control joints installed, or early-age cut joints.

Many cracks will be seen within the first two days.  (They look exactly like a drying shrinkage crack.)

Cutting control joints have been the traditional method for installing control joints.  Cutting control joints to reduce the cracking should be done as soon as the concrete begin to harden, usually within twenty-four hours.

The process should be carried out even sooner, between twelve to eighteen hours, if the slab is unprotected from high temperature changes or drying winds.

The earlier the control joints are cut or installed the better, because failure to cut them in a timely manner will significantly increase the probability of uncontrolled cracking in the slab.

Concrete is usually placed using one of two methods

Currently during the desired method in the residential market is for the concrete to be pumped into the basement.  The advantages are pumping trucks can shoot mix up to hundreds of feet if needed.  Other methods require the trucks to back up directly to the poured area.  Pumping is the fastest and easiest method available today. The disadvantage of pumping is its potential for shrinkage cracks.

A conventional or structural concrete mix uses air-entraining admixtures for critical areas because the risk of shrinkage is significantly reduced.

The less water used onsite the better

Increasing the amount of water added to the concrete mix will reduce its potential strength and compromise its stability over time.

Added water increases the slump during placement which will weaken the concrete over all if a higher slump was designed.

It will also increase the shrinkage movement that it will undergo as it cures.

Adding water to the mix without the approval of the ready-mix company places the liability squarely upon the shoulders of the person adding the water.

It particularly transfers any financial consequences for this action to that person as well.

In clear language: Don’t allow anyone to add water to the concrete mix unless the ready-mix supplier approves it.

The driver is usually the person who adds the extra water, but only on the orders of the batcher.

How should concrete be cured?

It is very important that concrete is not allowed to dry out quickly after it is poured.

Apply the curing as soon as the concrete can withstand the process. The most effective method for curing concrete is by water spraying, ponding, or using plastic as mentioned earlier.

Points to remember:

Poor curing of floor slabs can reduce concrete strength by 50%, resulting in greater risk of random shrinkage-induced cracking, despite cutting control joints.

Effective curing will improve the durability and the abrasion resistance of the concrete.

The curing period should be at least three days. Longer is better, and can be easily done with good planning.

These key points are very important if a good quality slab is to be produced.