Questions & Answers about New Concrete

We are installing a new concrete sidewalk that will abut our home's concrete foundation. The contractor says he wants to pin or "knit" the new pour to the foundation using metal rebar. Is that a good practice, and is it necessary?

When pouring a new walkway or small patio that abuts a house or other structure's foundation, always consult local building codes for guidelines on pinning or knitting the new pour to the existing foundation. In most cases, the new slab should be left separate from the foundation to allow slab movement. If the sidewalk is knitted to the foundation, cracking can occur in those areas.

The only reason to consider knitting the two together would be to prevent the new slab from settling back toward the house, creating a drainage problem. I have heard of situations where rebar is installed in the new slab and allowed to slide in and out of greased holes drilled into the foundation. This allows the slab to move without causing cracks, while keeping it from settling toward the house. However, if the sidewalk is placed over a properly prepared and compacted subbase, there should be no settling problems and no reason to tie the slab and foundation together (see Subgrades and Subbases for Concrete Slabs).

We have a stamped concrete sidewalk that is settling in an area near one of our downspouts, probably due to erosion of the subbase underneath the slab. We plan to redirect the downspout, but how do we relevel the slab without causing damage?

When it comes to lifting and or releveling decorative concrete slabs or concrete foundation slabs that have sunken or subsided due to washout or deterioration of the subbase, you have two options. The first is mudjacking, or slabjacking. This process has been used for decades, and involves pumping a flowable (high-slump) cement-based liquid fill under the slab through holes to lift the concrete back into place (see How Slabjacking Is Done). Once the concrete slabs are level and back in place, the holes are capped off, and the fill material cures to form a solid concrete subbase.

A newer trend is to lift settled concrete using a polymer-modified material instead of a cement-based fill. The polymer material flows easier and solidifies faster, reducing the labor and time required to lift the slab. It also is more resistant to future washout and environmental deterioration.

Both of these methods are proven, and reputable contractors often provide multi-year warranties on the materials and workmanship. Get estimates from several professionals who do this type of work to determine the best option for your particular situation.

We had our driveway replaced with concrete recently. The day of the pour, the air temp was in the mid- 40s and dropped down to 26 degrees that night. The next day, the air temp was about 52 degrees and dropped down to 27 degrees again at night. The concrete is very blotchy, and we can see a pattern of hairline cracks all over the surface. The contractor says this is normal and will go away. It's been almost a month, and the concrete still looks the same. The contractor placed blankets on the concrete after placement, but didn't have enough to cover all the concrete. Only about 70% of the driveway was covered one night, then they removed the blankets and the forms the next morning. What is your opinion? Is this procedure normal and will the concrete hold up long term?

Here a few facts that should help you out. The American Concrete Institute report “Cold Weather Concreting” (ACI 306R-88) defines cold weather as follows. This report has been the accepted standard for cold weather concrete for years.

Cold weather is defined as a period when, for more than 3 consecutive days, the following conditions exist: The average daily air temperature is less than 40 F and the air temperature is not greater than 50 F for more than one-half of any 24-hour period. The average daily air temperature is the average of the highest and the lowest temperatures occurring during the period from midnight to midnight.

Most of this report focuses on large commercial and heavy highway concrete pours. However, cold weather concrete practices should be used on all exterior pours when conditions warrant. The key factors that are a must are proper concrete temperature at placement, proper curing and protection, and proper concrete temperature cooling once placed.

From your description, the contractor knew or was aware of the cold weather; otherwise blankets would not have been used at all. Cold-weather placement of concrete is an all or nothing event. If cold weather blankets were used, then all cold weather practices should have been used, starting with covering the entire slab for a period of time to allow the concrete to slowly come down to 50 F. This usually takes 2 to 5 days.

In regard to the blotchiness, this is from the curing differential caused by moisture trapped under the blankets, and this is normal for winter pours. The blotchiness will minimize over time and will get to a point where it will only be noticeable when the concrete is wet.

The cracking you describe is called shrinkage cracking. This is caused by rapid moisture loss during the initial curing cycle - usually the first few days. While aesthetically unpleasing, it usually is not structurally compromising. In some cases, it can lead to premature surface failure, called scaling and or spalling, but this occurs over time.

In conclusion, I see some real issues with the way your concrete was placed, but I’m not sure any of them warrant ripping out and replacing the slab. The only way I would support removal and replacement would be if the cracking was severe, and if surface testing indicated low strength.

Both of these methods are proven, and reputable contractors often provide multiyear warrantees on the materials and workmanship. Get estimates from several professionals who do this type of work to determine the best option for your particular situation.