Encyclopedia

of Home Improvements

*Concrete*

Concrete is an indispensable building material that fills a variety of structural and decorative needs. Working with concrete may seem intimidating, but it is actually fairly easy, although the work must be done properly to ensure strong durable and attractive results. By understanding what concrete is made of and how it attains strength you can undertake most projects with confidence. Concrete is a mixture of sand, course aggregate, Portland cement and water. The sand used in concrete should be round in shape and in various sizes. The course aggregate is gravel or crushed stone. Portland cement is made of clay, lime, and other ingredients that had been heated in a kiln and grounded into a fine powder.

Mortar

There is essential to all brick and block construction. Mortar is a paste made of water cement, lime, and sand. Lime slows the setting speed making it easier to work. Mortar by itself is not as strong as concrete but has strong adhesive properties. In combination with stone, brick, or block it creates extra strong walls. In addition, mortar serves as an attractive spacer between materials and helps hide their imperfections.

Using concrete

For small jobs such as setting a mailbox post, use a bag of pre mix concrete that simply mixes with water. Each bag is about 80 pounds and contains cement sand and aggregate mix together. Each bag contains enough mixture to make about half a cubic foot of concrete.

For larger jobs as it makes sense to rent a cement mixer. You can either mix the pre mix bags in the mixer or you can by cement, sand, and aggregate separately and make your own concrete. Making small batches allows you to finish one section at a time.

Mixing your own concrete

Shovel the gravel, sand, and cement into your mixing barrel. Mix the material together while dry. Add small amounts of water to the dry mix and stir it thoroughly. This way it is less likely finer ingredients will settle to the bottom. Eventually the mixture will take on the consistency mud. When it turns a uniform shiny gray the mixture is close to the correct consistency. Grab a handful of cement and pack it together like you would snowball, if the ball stays together you probably have the right mix. A mixture that is too wet will weaken concrete.

Demolition

Old concrete forms or walls must often be removed from construction sites in order to remodel. The best tool for this job is a jackhammer and a real strong back. With the jackhammer break off small pieces of concrete. If possible undermine the edge of the slab so the pieces being struck cannot be supported from below.

Preparing your form

All concrete work involves some excavation and grading. Dig carefully when you excavate. Concrete must be placed on undisturbed soil. The base of the excavation should be flat and level, especially on a sloping site. Patios and footings are shallow enough to be a form with simple boards held in place by stakes. Taller structures require more sophisticated form system. Building codes insist on steel reinforcement in certain concrete structures steel - either round bars or welded wire mesh.

Pour concrete immediately after mixing it. Avoid spreading it around to keep the heavier aggregate from sinking to the bottom. When the concrete is in place to square edge shovel to consolidate it. Agitate the concrete enough to set it in place. After the concrete settles, level the top by dragging a screed across the top of the forms. Use a 2 x 4 with a sawing motion to push excess concrete along in front and fill in the low spots behind. Screening does not smooth the service it only levels it. Smooth this surface with a tool called a float. Move the float back and forth over the concrete, pushing the aggregate down and smoothing the surface. Clean the perimeter edges by running a trowel between the forms and the fresh concrete. Footings and simple walls do not require careful finishing.

Drying the concrete

To ensure that the concrete attains full strength keep water from evaporating by covering with plastic. Concrete will never reach full strength if it dries too quickly.

Cutting Concrete

For small cuts I use a grinding wheel with a diamond blade. The small blade lets me do angle cuts, round corners, and remove mortar from joints. For larger cuts in concrete and stone I use a skill saw with a diamond blade. It's easier to cut a straight line with this set up.

Concrete and masonry fasteners:

Lead shields and anchors - Drill a hole in the concrete just large enough for the lead shield. Fill the hole with PL adhesive for concrete and insert the shield. Drill a hole in the material you are attaching, insert the anchor through the material and into the shield. Tighten the anchor with a ratchet.

Ramset shots and pins - 22caliber nail gun that shoots a steel pin through wood and into the concrete

Wedge and sleeve anchors - They work by means of a threaded rod with a cone shaped tip. The tip end is inserted into the hole, and a nut or screw head is tight end on the other end. With wedge anchors, the cone shaped tip pulls against a short sleeve with barbs that wedge in place near the bottom of the hole. They are extremely strong and impossible to remove once installed.

Cut nails- old fashioned but still reliable. They simply just get hammered into the concrete.

Concrete Screws - A blue coated screw so hard that it can be screwed into concrete. Drill a pilot hole first slightly smaller than the screw. The screw cuts its own thread in the concrete. Concrete screws are stronger than all lead and plastic anchors used in residential construction, and are easier and faster to install.

Drilling holes in concrete

A hammer drill and a good carbide tip bit are the way to go here. A hammer drill rotates and hammers the drill bit at the same time. For larger holes drill a small hole first, and depending on the size of the hole repeat this method as many times as necessary. Carbide tip masonry bits are expensive but remember they can be sharpened for a fraction of the price of a new one!!

Repairing Hairline Cracks

You can repair hairline cracks in concrete with a grout made of Portland cement and water.
Scrape out the crack as much as you can and clean out all dust and debris. Dampen the area for a couple hours. Mix the Portland cement and water to a peanut butter consistency and push it into the crack with a putty knife. Smooth out the surface and cover with plastic for a few days.

Repairing large cracks and holes

For larger cracks and holes I recommend enlarging the area to be repaired to at least 1 inch deep. Clean out the hole and etch the old concrete with diluted muriatic acid. Rinse thoroughly and apply a liquid bonding agent (it will help to bond the old concrete to the new concrete). Fill the repair area with new concrete and tamper it down. When it begins to set you can finish it off to match the rest of the old concrete. A metal trowel will give you a smooth finish while a wooden trowel will give you a rougher look.
Cover the patched area with plastic for a couple days to let the concrete cure properly.

Large repairs to driveways and walkways

Follow the same plan for cracks and holes but use a gravel mix for extra strength. Use a mixture of one part Portland cement, two parts sand and three parts gravel.
For active leaks in basement walls or floors use quick setting hydraulic cement. Prepare the crack or hole the same way as described above. Mix the hydraulic cement 1 part water to 3 parts cement. Mix only a small amount because it cures very quickly. Apply the cement to the repair and smooth out even with the surrounding surface.
For active leaks mix the hydraulic cement as before. Wait 2-3 minutes or until the mixture becomes warm to the touch. Force the hydraulic cement into the opening and hold it there for 5 minutes to ensure a complete set and sufficient bond to stop the flow of water. The patched area may be smoothed and leveled in about 5 minutes.
Hydraulic is also ideal for sealing floor and wall joints and settlement cracks in masonry walls.

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