One of the most common questions I receive during my inspection orientation concerns concrete cracks and concrete failure. Why is the home’s concrete cracking and is this a shoddy job?The fact is, when installed properly; concrete is one the most durable and long lasting products in your home. But, there are well-established rules and guidelines with respect to installation that can affect its durability, strength, and resistance to cracking.
Let’s look at a few reasons concrete cracks:
•1) Excess water in the mix.
It’s no secret that concrete is heavy. When it’s being poured from the truck the concrete is stiff and hard to maneuver. In an effort to make it lighter and easier to move some contractors often add water to the mix. But the excess water also greatly reduces the strength of the concrete.
As the concrete cures or hardens, it shrinks. This shrinkage is one of the main causes of the cracking and the wetter or soupier the mix the greater the shrinkage. It’s not uncommon to shrink as much as ½ inch in 100 feet. So, a low water to cement ratio during installation is the number one issue effecting concrete quality and any excess water reduces this ratio. The result - increased cracking.
What you can do about it? Well by the time the home is being inspected not a lot. But during the construction period one way to help combat cracking is choosing a reputable contractor who will make certain the concrete has the proper water mix as its being poured. Is it more expensive to pour stiff concrete? In terms of labor cost…yes. But it pays off in the long run.
•2) Rapid Drying of the concrete
Once the concrete is poured and in place, add water! Wait a minute…I thought we just said don’t add water? Well now we want water…that’s right! The ideal technique to cure concrete is to keep the concrete wet as it goes through the curing process.
Concrete that dries too fast will crack, so you want to slow the hydration or chemical reaction process the concrete goes though. As it cures concrete is transformed from a liquid or plastic state to a solid state. This process continues to occur for days and sometime weeks after you pour. Adding water to slow down this reaction time will help lessen the propensity of the concrete to crack. How do we add the water? Several ways:
- Water cure-the concrete is dampened or mist sprayed after the pour to help prevent mix water evaporation.
- Water retaining methods-use coverings such as sand, canvas, burlap, or straw. These products are kept continuously wet for the entire curing period.
- Waterproof paper or plastic film seal- applied as soon as the concrete is hard enough to resist surface damage. Use caution here; plastic films may cause discoloration of the concrete and you don’t want that if you intend to do not stain or use it as a finished floor surface.
- Chemical Membranes- this kind of application should be made as soon as the concrete is finished. Note: curing compounds can affect adherence of resilient flooring, so you may want to contract your flooring contractor and/or chemical membrane manufacturer for further guidance.
The length of time to protect concrete against moisture loss depends on the type of cement used, mix proportions, required strength, size and shape of the concrete mass, weather, and future exposure conditions. For most structural use, the curing period for cast-in-place concrete is usually 3 days to 2 weeks.
•3) Improper strength concrete poured on the job
Concrete’s strength is measured in pounds per square inch (PSI). Once fully cured, the higher the number the harder the concrete and, the higher the PSI the longer the cure time. There are minimum standards for concrete strength. The Federal Housing Administration (FHA) minimum standard for residential construction is a PSI reading of 2,500. Too much water in the mix can lower that number and weaken its ability to resist stress.
•4) Lacks of control joints.
Control joints help concrete crack where you want it to. The joints or seams should be of the depth of the slab and no more than 2-3 times (in feet) of the thickness of the concrete (in inches). So a 4″concrete slab should have joints 8-12′ apart. Longer sections tend to crack more often.
•5) Weak subsurface or pad.
The sub grade must be prepared according to your particular soil conditions. Some flatwork can have concrete poured right on native grade. In other areas 6″of base fill may be required along with steel rebar installed for added strength.
Any areas beneath the slab that are not to be filled with concrete; i.e. plumbing and other mechanical trenches should be brought back to grade in compacted lifts. That means a 24″ deep trench would be backfilled 6″ at a time, each “lift” being mechanically vibrated or packed so it is left compact.
If the excavated areas are not compacted when backfilled, the loose soil will, in time, settle and leave the concrete over that area with no support. That zone then becomes a prime place for concrete to settle. Since the soil next to the excavated area is native soil or tightly compacted, the un-compacted trench can literally become a thoroughfare for water.
•6) Temperature
Never pour concrete in sub-freezing weather conditions. Like the human body, concrete must be warmed or covered in the winter.
Both fresh and newly-hardened concrete lose moisture and heat rapidly in cold-weather conditions. You must protect cold weather concrete against early freezing to assure strength development and job progress.
If you do attempt to try your hand at cold weather pours, use insulation blankets or heated enclosures to maintain concrete temperatures above 50 degrees Fahrenheit for at least three to seven days. Then maintain the concrete temperature above 40 degrees Fahrenheit for at least four more days. Cold air retards the evaporation rate and setting time of concrete.
If you inspect your home and find a cracked driveway, walk or patio and wonder if it’s the builder, the sub-contractor or the supplier, remember that you’ll have to ask a number of other questions before getting a good concrete answer.
The Old Man can be cruel for sure, but all of this is not done in vain. Winter is a time when certain species are in full glory. Some of these plants have been waiting for their time in the spot light and have been producing buds and the promise of a colorful explosion for weeks on end. Whether it is your favorite Camellia in your front yard inviting your neighbor’s eye for a visual treat, the vast sweeping bed of Nandina that lead you to your back yard, or the majestic Hollies that you have as specimen plantings hugging your property line, these plants have been patiently waiting their turn in nature to show you why they are in our plant palette.
the architecture of the trees and reflect on the work that was done to get them to where they are now.
profession, certain standards were adopted. In order to create this standard Mississippi chose the American Society of Home Inspectors’ (ASHI) Standards of Practice as a guideline. The state also adopted ASHI’s Code of Ethics to guide inspector conduct. 
