NBN Online for the week of April 25, 2005

Basement Snorkeling

I just read an article in USA Weekend by Lou Manfredini entitled “Avoid Basement Floods This Spring.” The gist was not particularly earth-shaking: use a sump pump. I understand that Lou targets his column more to little old ladies in rocking chairs than massively brainy builders like my target audience. But I think a better explanation is in order.

Before discussing fixes to any problem, one should understand the causes. If you construct a basement in an area of high groundwater and do not install a functional behind-wall drainage system, said groundwater will press against your basement walls, and upwards on your basement floor.

For every foot of depth, groundwater exerts 62.4 pounds of pressure per square foot of surface it presses on (called hydrostatic pressure). So the deeper the groundwater, the more hydrostatic pressure your walls and floor must resist. This is the same pressure your ears feel at the bottom of a swimming pool.

Most basements are constructed of concrete, which can be waterproof if built right. Consider the thousands of concrete water tanks in service, if you doubt this is true. It’s the “constructed right” part that typically causes the rub. To ensure waterproof concrete, you must:

• Vibrate (a.k.a. consolidate) the wet concrete the right amount. Too much consolidation can cause separation of aggregate and paste; too little can result in rock pockets and voids. Banging on the forms does not constitute proper consolidation — use a stinger.
   
• Use a good mix. I recommend small aggregate (pea gravel or 5/8” max), and six sacks of cement per cubic yard. This should result in a 3,500 psi mix, minimum.  Don’t use too much water! (See my previous column, “Concrete Too Wet Equals Weak Concrete”) If there is a lot of rebar clogging up the forms and you need a thinner mix, use a water-reducing admixture, aka plasticizer.
   
• Use air entrainment of 4%-6%. This helps “flowability” during placement and protects against freeze-thaw. It also makes concrete more waterproof.
   
• Use rebar in adequate quantities and in the right places. Basement walls are retaining walls; braced retaining walls to be more specific. They are held laterally at their base by the floor slab and at their top by a framed floor. As soil pushes on the wall and it tends to bow inward, tension will develop on the inside face of the wall. This is very different than a cantilever retaining wall, which has tension on the dirt face (see my past column, “The Broken Concrete Guy Leaning Badly”). The point is, you want vertical rebar toward the tension face; in the case of a basement wall, toward the inside face. You could also center vertical rebar in the wall, but doing so will necessitate a thicker wall, say 10-inches vs. eight.
  
  Horizontal rebar should be placed toward the wall center and at no greater spacing than 24-inches. I know the new International Building Code (IBC) doesn’t require this much horizontal rebar, but I disagree. Horizontal bar helps hold things together in the event of earth movement or settlement, and it also helps distribute loads around should a particular load-resisting element fail. Interestingly, the 1997 Uniform Building Code (UBC) requires horizontal rebar at 18-inches or less.
  
  The actual amount of vertical rebar needed and the thickness of wall depends on wall height, groundwater height (if any), gravity loads from above, soil type and other factors. Retaining walls are a lot more complicated than most people realize. My rule of thumb is any retaining wall over 4-feet tall should be engineered.
  
  
• You can also use a water-proofing admixture. This type of product is added to concrete during batching. It is extra insurance against leaks, but not absolutely necessary if you get the above items right; and it won’t help if you wind up with big cracks or voids. Consult your redi-mix supplier for recommended brands.

But what if your walls and floor already exist and they leak? You have several options:

1. Remove the water from the outside. This is the most fail-safe method of stopping leaks. However, it is usually the most expensive. Generally, a ditch is dug immediately outside the wall(s) to the depth of the bottom of the footing, and a drain tile system is installed. You can use perforated pipe and washed gravel or a proprietary against-the-wall system (MiraDrain is one brand). Water is collected at the bottom of the footing and routed either downhill to daylight, or to a sump or other drainage structure, and then pumped out. This doesn’t fix the cracks, it just removes water before it gets to them. If you’re going to the trouble of installing a drainage system, I’d also recommend waterproofing the outside of the walls via a spray-on or roll-on membrane. Cheap insurance. (I might note, even the dimmest bulbs understand that a behind-wall drain/waterproofing system is a heck of a lot cheaper and easier to install up-front during construction than after the fact.)
   
   
2. Collect leakage on the inside of the wall and route it to an interior sump. There are certain proprietary methods of collecting water in small troughs or gouges cut into the floor and routing it to an in-floor sump pump. Basement Systems is one.
   
   
3. Use a proprietary concrete sealer/plugging agent. Products exist that can be applied to the inside of a leaking wall or floor that will absolutely stop water from coming through. Kryton is one brand. Be forewarned, however, that these products are only as good as their application, so a factory-certified applier is important. Also, if your walls or floor are not structurally sound — i.e. they’re cracked, settling or otherwise moving — this system is not likely to work over the long term.
   

But what if your floors or walls are seriously cracked, settling or moving? Leaks may not be your biggest problem. Hire a well-experienced engineer to investigate.

To summarize, basement walls and floors are a bugger to fix. The best solution is build them right in the first place.

Tim Garrison of ConstructionCalc.com, is a professional engineer, author and software producer for the building industry. Send e-mail to buildersengineer@constructioncalc.com. Tim reads every one.

This column cannot be reprinted without permission from the author.