Considerations for Homeowner Concrete Repair Pt 2

Glad you were able to tune in for Part 2 of our concrete series where I will give some tips and tricks on identifying causes of concrete failure and methods for DYI repairs. Before we get started, a brief clarification on the term concrete which is synonymous with Portland Cement Concrete (PCC) and widely accepted as a reciprocal. However, concrete is the mixture of a binder/glue and aggregate, and many types of concrete exist such as asphalt concrete, resin concrete, or the most famous Portland Cement Concrete. Please also note the term cement is used when referring to a neat mixture of binder/glue, think of the plaster of Paris used in sheetrock. With that pet peeve addressed lets dig into identifying some common root causes of PCC failure.

Uncontrolled Cracking - A menacing term with simple meaning, this is any cracking that occurs outside of the intended locations or joints. Typically affects relatively new PCC < 1 year in age and is commonly caused by failing to adequately tool or saw joints in a timely fashion, poor joint spacing, poor PCC panel aspect ratio, poor curing methods, or excess water content during mixing.

Depending on your perspective functionality of PCC, these uncontrolled cracks can range from eye sore to complete removal and replacement (R&R), and are best avoided by following the tips below.  Before using the nuclear option of remove and replace, it is important evaluate the amount of cracking, directionality, and width as unfortunately there are instances where PCC just cracks. I will cover crack remediation methods in Part 3.

Heavily Distressed Panel - Multiple Cracks, Top Crack is Directional Before Splitting into Another Crack, & Lower Crack is Considered Open with Spall Forming between the Lower Pair of Lines

• Amount of Cracking - In an average driveway for a two stall garage, never more than one crack per panel but perhaps a total allowable limit of two cracked panels depending on joint layout. This assumes the cracks are located near a planned joint.
• Directionality - The direction of the crack within the PCC panel can provide indication as to weather other issues are present and causing the failure. If the crack is transverse to a planned joint and fairly straight 3" +/- variance, it is usually acceptable and will not significantly shorten the usable life. Diagonal cracking should be evaluated by length and location within the panel. Best case short diagonal crack across a corner, worst case (R&R) indicating larger issue is full length diagonal across a panel. More than one diagonal crack in a panel is typically grounds for remove and replace.
• Width - Assuming the above criteria is met the final consideration is crack width or if the crack is open or not. An open crack's definition varies throughout the industry with the mean somewhere around an 1/8". Typically if the crack has a discernible shadow I consider it open and will watch it closely for future movement. Closed cracks believe it or not will heal themselves to a degree as calcification further close crack as the PCC continues to cure.

An Example of Evaluating Functionality for PCC cracks

New sidewalk/driveway around your house or garage exhibits cracking meeting the criteria above. Leave in place, possible rout and seal repair

Business front entrance panel has only one crack diagonal to the joints beginning near the mid panel. Crack is open and after observation appears to be growing wider and propagating small elliptical cracks around the main crack. R&R, for several reasons; this is a business front entrance customers notice appearances, PCC has a weak point were damaging chemical deicers can easily enter accelerating corrosion, the crack is continuing to move causing small faulting cracks around the original likely this crack will propagate through the tooled joint into the nearby panels.

The type of cracking in the second scenario is normally the result of movement, potentially caused by either unstable foundation materials or lack of an isolation joint from the nearby structure.

Typically affects relatively new PCC < 1 year in age and is commonly caused by failing to adequately tool or saw joints in a timely fashion, poor joint spacing, poor PCC panel aspect ratio, poor curing methods, or excess water content during mixing.

• Joint Construction is critical for the controlled cracking of PCC, joints can be hand tooled or sawn depending on tools available. Concrete saws are very effective for installing joints however unless your project is an entire driveway they are simply too expensive. Hand tooled joints are formed into the concrete during the plastic stage creating a thinner and hopefully weaker location for the crack to occur in. A trick the pros use is to "stab" the joint once it is formed but not finished opening air voids along the joint within the PCC. Stabbing uses a thin metal rod or stiletto like blade at several equally spaced locations along the joint and can incorporate a small wrist movement to help create an air void. Finish tooling or brushing the joint to conceal an stab marks. 

Crack Propagation from Unpaired Joint

• Joint Spacing and layout is equally as important at joint construction for long term PCC performance. Joint spacing and layout should be completed prior to the pour and account for any reinforcing steel (rebar) if used. Typically a joint should not be planned within 18" of any steel though I have seen this minimum shrink to 12" without negative effect. Joints should be spaced at 12 to 24 times the PCC thickness with a maximum of 15' between joints. It is typically wise to error on the conservative side and place joints closer rather than farther. Do not offset joint lines between neighboring panels, this will cause future carry through crack propagation. When working around appurtenances in the PCC, such as a water shut off, encircle the valve with a piece of 4" PVC the depth of the PCC, additionally if possible attempt to place the joint across the center of the circle

Aspect Ratio Cracking - Avoid Long Narrow Panels

• Aspect ratio of the PCC panels typically affects the longer term performance by preventing fatigue or overload cracking. The ideal shape is a square, however certain conditions sometimes require rectangular panels, which should be a maximum aspect ratio of 1.5, length / width (6' / 4' = 1.5). Never end a panel with a point, try to maintain a minimum width of 18" with 24" being even better.