Building a Sure Foundation
A Sure Foundation
There are many types of home foundations on the market today. Since your entire investment is supported by this beginning stage, it is extremely important to make a wise choice. First you have to decide if you’re going to have a full basement or just a crawl space. For a full basement your excavation will need to go much deeper or your house may sit too far out of the ground to accomplish the required ceiling height on this level. It is also very important to know the required frost depth for your area to protect your foundation from the effects of ground frost. Your design professional can assist in this area along with a visit to the local code official. Since there are many different options to choose from, lets take a look at a few of the more popular styles.
1] Pre-engineered Foundations: Many brands of manufactured foundations exist on the market. A careful investigation into each product proves to be beneficial. Some factories may allow a walkthrough of the facility to witness the manufacturing process. This could help to answer any questions that you may have and to better educate you on the differences in the available products. You may also want to visit a job site to witness a foundation erection in progress before making a purchase. References are also helpful in making a decision to purchase. Satisfied customers will help speak to the quality of the product. Remember, just because a product is backed up by a registered design professional does not mean that the product is without deficiencies. Some pre-engineered products perform well in certain soil conditions, and poorly in others. It is important to know what your dealing with and how this product will perform in your area. You can also ask the local authority their opinion on a particular product, though they may not want to offer a product assessment, you can ask if there have been any complaints registered with them for a specific product. It is also wise to have the engineer sign off on the product erection and not just the product manufacturing, and remember to always retain a copy for your personal records. Look at the product specifications and make sure that the installer is doing what the specifications dictate. Many products have been installed different then the products specifications, causing trouble down the road.
2] Poured foundations: On site poured foundation systems have been around for many years. It is important to hire an experienced contractor with the proper equipment for this process. A concrete footing (Footer) is first formed over the excavated area. It is extremely important to make sure the soil is not a fill area or soil unsuitable to support the foundation. Backfilling is required to be properly tamped to an acceptable compression to avoid any unwanted settlement that may cause failure of the footing. It is also unwise to form and pour over frozen ground. Many contractors may overlook frozen soil in an attempt to keep the job moving. This can cause uneven settlement later causing cracks to develop in both the foundation and footings. Some footing designs require ½” or more steel reinforcement to be placed horizontally throughout the footing prior to pouring. It is extremely important for the builder to construct from the approved drawings and not deviate from them without approval. Footing strength may vary depending on the design specifications, but make sure that the concrete strength is equal to or greater than the designed strength. You can obtain a batch delivery slip from the concrete driver to verify what was delivered. Properly installed forms should always be used when pouring concrete. This can assure that your footings remain square, level, and can eliminate the unwanted waste of costly concrete. Most locations require a concrete pump to place the concrete into the forms. Make sure that there is a firm understanding of this cost before proceeding, since a concrete pump can be very expensive.
After the footing has been poured, your contractor may cut in a “keyway” along the top of the footing while it is still wet, to assure a water tight bond with the foundation wall. Vertical uprights may also be placed into the wet concrete to be later used as a starting point for the reinforcement in the walls. Generally, one side of the foundation wall is formed first, and the steel reinforcement is then placed, before the opposite side is formed. Bracing of the wall forms is crucial to prevent a failure during the pour. Many foundation walls have fallen during the pour because of improperly braced forms. Wet concrete tends to have a mind of its own, and will find any weak point in the wall forms, so take the extra time to brace them properly. When pouring the walls it is very important for the concrete to be placed with a vibrator to assure a consistent pour. If a vibrator is not used honeycombs may develop in the walls, causing weak areas of the foundation wall. You also do not want to pour your concrete too wet in an attempt to have it flow through the forms easier. By diluting the concrete, known as “Slump”, to a manageable consistency and not making it too wet, it will prevent the strength of the concrete from becoming too weak.
After the walls have been poured the contractor can wet sink the anchor bolts or straps in the top of the foundation wall. The anchoring system can be drilled at a later date, but it is best to be able to wet sink them before the concrete hardens. Anchor methods come in many different forms, some straps, some bolts, and some installed through drilling at a later date. Typically a bolt is used that has a small hook on the bed to better seat itself into the concrete. Whatever method your designer decides is best for the house design, make sure that it is installed according to the manufactures specifications. Placement should also be as designed in the plans, but generally the bolts or straps are located one foot from each corner, and every 4 to 6 feet apart. High wind areas may require additional strapping, so again follow what your design professional has specified. Caution should be used during the removal of the forms to prevent damaging the walls. Remember that concrete requires a curing process and will need sufficient time to properly dry. The weather conditions may also play a role in this curing process, and protection from freezing is essential for a proper foundation. In the winter months the curing process may take longer than expected, so it is important to not become too hasty.
3] Block Wall Construction: Masonry block walls can also be used for your foundation. It is important to consider the foundation depth when erecting a block wall foundation. Ground pressure can cause serious structural damage to the foundation when the block width has been under sized. Horizontal reinforcement, known as “Dura-wall”, may also be necessary to assure a solid foundation. Sometimes pilasters are erected on the inside of the foundation wall to give additional lateral support at the mid-span sections of the wall. These “Interior piers” are typically erected into the foundation wall as each course of blocks are laid, becoming part of the wall. The block hollows are generally filled solid and re-bar pieces are placed vertical from the footing to the top of the foundation wall. In most cases though, if the blocks are properly sized to retain the imposed ground pressure, no piers will be necessary.
The footing installation will be the same as the footings for a poured wall foundation. It is important for the block wall to be placed properly on the footing and not have portions hanging off, but resting square to the footing. Typically a mix of 1 part masonry cement to 3 parts masonry sand is used to place the blocks. Some mixes may have lime or Portland cement added for additional strength. The blocks are started with a “Bed joint” of cement over the footing, and the placement of the first row of blocks. The corners are erected first and a line block is used to string a line between the corners to maintain a straight wall. A story pole or level is used to make sure the corners remain plumb, and a square will check the proper alignment of the corners. A transit or laser level is necessary to shoot the elevation from corner to corner making sure that the foundation remains level. I have seen masons who paid little attention to the level of the foundation until the last few courses of blocks, making for very inconsistent bed joints. This is an improper way to erect a foundation wall and a mason that builds like that should take up knitting instead. Each course will receive a new layer of cement mortar mix over the block webs, and the process is continued until the foundation has been erected. Horizontal reinforcement is placed between the courses as specified by the design professional, usually every second or third course. The top course will be a 4” solid, 8” solid block (FHA), or poured solid with concrete, and then the anchor system will be set as indicated on the approved drawings. The block joints are usually pressed with a jointer on the dressed or inside of the wall. This process requires pressing the joints, sponging the wall, and pressing the joints again. The outside portion of the wall is generally scraped flush with the blocks to prevent dried pieces of mortar from effecting the exterior waterproofing, but this may not be an issue if the mason is installing a parging over the exterior before the waterproofing. Block wall foundations have been used for many years and have proven to be very affective when properly installed, but they are sometimes time consuming and a lot of work to erect.
4] Pier Construction: Another method of support would be a series of piers constructed to be the foundation system for your home. This method is sometimes used in very wet locations or areas prone to the possibility of flooding. The type and location of a pier system is designed by the design professional and the exact specifications should be closely followed. Also, understand that without a basement or crawl space, square footage is lost for storage, recreations rooms, workshops, and other use areas. Soil conditions may warrant this type of support system, for example, if you live in an area prone to flooding, like by the sea. Make sure the design is suitable for the area. Land that was once used as a dump or a brick yard may have a very inconsistent soil condition that leads the designer to install piers instead of a typical foundation system. You must rely on the professionals in these cases, and make sure that your professional backs up their design with a stamp and signature.
5] Alternative methods of support: There are many other options available on the market today, including pressure treated wood foundations, wood pylons, stone, etc. Make sure that if you are leaning in an alternative direction, first do your home work. Some options may not work for your specific location needs, and remember that visiting an actual construction site could help clear up any confusion on the product of your choice. If your hoping to maintain an historical flare to your home and opt for a stone foundation, follow all modern building methods for a water tight system. Many of the older stone foundations have lasted hundreds of years but they allow water and dampness to permeate into the home causing unwanted troubles.
Now that we have decided on the foundation, let’s pour our interior basement floor. Typically a 4” concrete slab will be poured, making sure that a 6ml plastic vapor barrier, and 6x6 wire mesh or fiber mesh is used for strength. The slab should be brought to a trowel finish, making the surface easier to broom sweep, or to lay a sub floor covering like carpet or tile. The concrete should be a consistent mix, without dry clumps or too much stone. Your mason should also limit the amount of water used since this can weaken the strength of the concrete. If part of your basement will be used as a garage the floor thickness will need to be increased to accommodate the additional loading from your vehicles. Some designers specify using a expansion material between the foundation wall and the new slab. I for one have seen floors poured both ways and have found little difference if the floor is poured properly, but regardless, stay with the specifications called for in the approved plans. The floor is usually poured around the houses support columns if the floor system has been erected before the floor is poured, but regardless, proper anchoring of these columns is crucial especially in the garage area, but we will discuss that in the upcoming chapters. Oh, and by the way, don’t forget to place that new shiny penny in the concrete before it dries.
from "How to Build a House from "A" to "Z" by R.Serino purchase at lulu.com