Building Foundations Foundation Walls
Footings A base Monolithic construction Independent construction
Footings - A base that provides a larger bearing surface against soil for load-bearing parts of structure. Generally made of concrete poured into place In Monolithic construction the footing and foundation are poured as a single unit. Independent construction - We will focus on separate installation of footings.
Spread footings Undisturbed soil Foundation-Wall Footings Spread footings – generally size is specified on plan. Depth & width determined by loads it must bear, bearing capacity of soil, & local codes. Must always rest on undisturbed soil. Reduces chance for uneven settling of foundation.
Footing Design Prevent settling or cracks
Minimum of 12” below grade. Below frost line Thickness = thickness of foundation wall Width = 2x thickness of foundation wall Footing Design Proper footings prevent settling or cracks in foundation wall. Should be placed minimum of 12” below grade. In cold climates must be below frost line. Often footings are placed deeper than code to form basements. Width of footing depends on soil bearing capacity and is specified by local codes. General guide line for footings on standard soil: Thickness should be equal to thickness of foundation wall; Width should be twice the thickness of foundation wall.
Rebar embedded 2 lengths of ½” diameter (#4) rebar Positioned minimum of 3” above bottom Footing Reinforcement Strength greatly improved when rebar is embedded in it. Usually 2 lengths of ½” diameter (#4) rebar. Must be positioned minimum of 3” above bottom of footing.
Footing Forms Form Keyway
steel, lumber, or combination of lumber & plywood. Keyway Footing Forms - created by pouring concrete into dirt trenches or into a form. Form is any framework designed to contain wet concrete. Can be made of steel, lumber, or combination of lumber & plywood. Commonly 2X lumber with bracing to prevent spreading when filled with concrete. Often assembled with duplex head nails to make disassembly easy. Keyway should be formed in top of footing. Locks foundation wall to footing. Helps prevent leaks between wall & footing. Created by 2x4 pressed into concrete after pour and removed after curing.
Other Types of Footings
Pier and post Stepped Footings Other Types of Footings Pier and post – Block of concrete usually separate from main foundation. Often used to support decks. Stepped Footings – Often used on a lot that slopes. Instead of being at same height around entire foundation these footings “step” down the sloped site.
Footing Drains 4” diameter pipes at base of footing
Carries water away from house. Sloped at least 1/8” per foot Landscaping Fabric Footing Drains 4” diameter pipes placed alongside base of footing on top of gravel bed. Usually connected to storm sewers. Many small holes along bottom edge of pipe. When water rises into pipes it is carried away from the house. Should be sloped toward drain at least 1/8” per foot. After placed covered with filter fabric (landscaping fabric). Allows water through but prevents soil from getting in and clogging pipes. Then covered with more gravel and backfilled up to grade with dirt.
Poured-Concrete Foundation Walls
Durable & water resistant Most building sites Support any type of house 8 –10” thick Min. compressive strength 2500 psi Most 8’ high Poured Concrete Foundation Walls Durable & water resistant Can be installed on most building sites Can support any type of house Range from 8 –10” thick Min. compressive strength 2500 psi Most 8’ high – provides clearance of 7’8” from top of finished floor to bottom of first floor joists.
Full Height Walls Formwork Reusable forms Wales Full Height Walls
Formwork constructed for each wall face Reusable forms most cost effective when used regularly. Must be accurately constructed & properly braced to withstand forces of pouring & vibrating operation & pressure from fluid concrete Wales – horizontal bracing members are usually sufficient with reusable forms
Standard Wall Forms Wood or metal Built on site Prefabricated forms
Snap-ties 3 –7 days Standard Wall Forms Made from wood or metal depending on desired durability Many made from plywood & lumber Exterior-grade plywood can be used. Special form-grade plywood is available Forms may be built on site and taken apart after concrete hardens Generally more cost effective and efficient to use and reuse prefabricated forms. 2 sides of forms fastened together with clips or other ties. Thin metal rods (snap-ties) are commonly used. Extend through foundation. Metal brackets on ends of rods prevent forms from spreading. After forms are stripped off protruding ends of rods are snapped off. Forms should be left in place for 3 –7 days. Slows curing process and results in stronger walls.
Insulating Wall Forms ICFs (Insulating Concrete Forms) Permanent
Basic components planks, sheets, or hollow blocks Standard wall Grid wall Insulating Wall Forms Fairly new formwork, made of rigid foam insulation (expanded or extruded polystyrene) called ICF's (Insulating Concrete Forms) Are not stripped off after concrete cures. Are Left in place permanently. Increases insulating power of foundation walls. Can also be used for above-grade walls. Light weight makes installation easy. Must still be braced with care. Basic components can be planks, sheets, or hollow blocks. Held together with steel connectors. Standard wall – Concrete forms solid wall identical to wall poured between traditional concrete forms. Grid wall – Concrete forms waffle like grid and varies in thickness at different places. Uses less concrete than other ICF foundations. Should be reinforced with rebar.
Placement Poured continuously without interruption Cold joint
Water content Remove Air pockets Concrete vibrator (stinger) Placement Concrete should be poured continuously without interruption. Helps prevent a cold joint. Cold joint – occurs where fresh concrete is poured on top of or next to concrete that has already begun to cure. Cold joints are more likely to leak and are weaker than surrounding walls. Water content is very important. Concrete should always be as stiff as it is practical. Concrete should be worked to remove air pockets and help it flow. May be jabbed with shovel or pipe. May also use a concrete vibrator (stinger)
Crawl-space Walls Advantage – reduced cost Soil cover Ventilated
Insulated Piers support Crawl Space Walls Advantage – reduced cost over full basement. Little to no excavation required except for footings. Soil beneath house must be covered with a material to block moisture. Bare dirt should be covered with 6-mil plastic sheeting to prevent fungi from growing on bottom of floor joists. Plastic may also be covered with pea gravel to protect it from damage. Usually must be ventilated. Check local codes Floor framing above crawl space should be insulated to reduce heat loss Poured-concrete or concrete-block piers often used to support floor girders. Should be no closer than 12” to the ground.
Reinforcing Concrete Walls
Rebar centered in wall Lintel Reinforcing Concrete Walls Rebar should be centered in wall Where openings occur in foundations a steel or reinforced concrete lintel should be installed over the opening. Lintel is a horizontal member that supports weight of the wall above. Directs load around the opening.
Sill Plate Anchors Anchor bolts Metal strap anchors Sill sealer
Anchor bolts - ½” Diameter L-shaped bolts (most common) – Embedded in concrete after top of foundation walls have been floated smooth. Spaced not more than 8’ apart & no more than 12” from ends of any plate section. Set 8” deep or more in poured concrete walls. Large flat washer used at head end of bolt. Metal strap anchors – strap embedded in concrete. Legs of strap fit around plate Sill sealer placed under sill plate to smooth any uneven spots. Also helps prevent damage to wood due to moisture from concrete.
Foundation Wall Details
Special features: Brick-veneer siding Utility Sleeves Foundation Wall Details Foundation wall must often support special features: Brick-veneer siding Supporting ledge or offset about 5” wide. Results in space about 1” between masonry & sheathing to make brick laying easier. Utility Sleeves Often necessary for pipes (main drain to sewer or septic system) to pass through foundation Where a pass-through is required, a tight-fitting foam block is placed within formwork and secured with nails. Concrete is poured around them. After forms are stripped blocks are removed. Excess space is later filled with hydraulic cement and waterproofed. These features must be accounted for in the design of the foundation
Foundation Vents And Windows
Metal vents Rust-resistant steel frame windows Wood framing (pressure treated) Foundation Vents and Windows In crawl-spaces metal vents may be installed within forms before concrete is poured Rust-resistant steel frames for small grade-level windows may be placed in forms for full walls Larger openings may require wood framing (pressure treated). Sometimes left in place after forms are stripped. Small anchor bolts should be inserted into pre-drilled holes in frame before concrete is poured
Beam Pockets Girder flush with top of sill plate
Beam Pockets - A wall notch or pocket needed for basement beams or girders. Allows top of girder to be flush with top of sill plate.
Stripping and Maintaining Forms
Wood wedges (no metal pry bars) Stiff bristle (not wire) brush Form-release agent – Bio-slick Stripping and Maintaining Forms Forms should not be removed until concrete is strong enough to support loads of early construction. Metal pry bars should not be used to strip wood forms. May damage faces of panels. Use wood wedges to pry away from concrete. Clean forms with a stiff bristle (not wire) brush to clean off concrete residue. Should be recoated with a form-release agent before reusing.
Moisture Protection Bituminous (tar) coating
Top of footings to finished grade level Moisture Protection Walls should be coated with a bituminous (tar) coating to help with waterproofing exterior of foundation walls Should be coated from top of footings to finished grade level
Backfilling Filling in excavation area
Too soon, push in foundation walls First floor framing Temporary bracing Backfilling - Process of filling in excavation area around foundation with soil Brings area up to rough grade around house. If done too soon, the weight of the earth pushing on walls may push in foundation walls. In ideal conditions, first floor framing should be in place before backfilling. Helps to brace walls as earth is pushed against them. May also use temporary bracing on inside of walls.
Concrete Block Walls Popular for foundation walls No formwork
Blocks inexpensive Work stopped and started Concrete Block Walls Popular for foundation walls. Do not require formwork Blocks are fairly inexpensive Work can be stopped and started as needed
Concrete Block Basics Concrete Masonry Unit (CMU) Head Joint Bed Joint
Common bond Pilasters Protect from freezing & moisture Concrete Block Basics Concrete Masonry Unit (CMU) – any hollow masonry unit (concrete block) Most common made with Portland Cement, a fine aggregate & water Come in many shapes & sizes – Most common 8”, 10”, & 12” wide (nominal dimensions) 7 5/8”x15 5/8” (actual) Head Joint – vertical mortar joint Bed Joint – horizontal mortar joint In cold climates, block walls usually constructed of eleven courses (layers) above footings with solid 4” cap block. Block courses laid in common bond. No head joints line up from course to course. Joints tooled smooth to seal against water seepage. Mortar spread on all contact surface of block called full bedding. Pilasters – projections that look like columns. Used to strengthen wall under a beam or girder. Pilasters placed on interior side of wall & constructed to bottom of beam or girder they support. Freshly laid block walls should be protected in temps below 32ºF. Freezing of mortar before it sets results in low adhesion, low strength, and joint failure. Block should be protected from moisture before and during installation
Cutting Block Brick hammer and chisel Portable masonry saw
Blocks usually available in half lengths but sometimes you may have to cut to size. Brick hammer and chisel – score block on both sides to make clean break. Portable masonry saw for fast, neat cutting
Mortar Mortar bond strength depends on: Type & quantity
Workability, or plasticity Surface texture bedding areas Rate at which masonry units absorb moisture from mortar Water retention Skill of person laying block Mortar - Good mortar is essential for a strong, solid wall. Mortar bond strength depends on: Type & quantity of mortar Workability, or plasticity of mortar Surface texture of mortar bedding areas Rate at which masonry units absorb moisture from mortar Water retention of mortar Skill of person laying block
Mortar Mixtures Portland cement, hydrated lime, sand, & water.
Prepackaged mortar mix Type N Type M Type S Type O Mortar Mixtures A mixture of Portland cement, hydrated lime, sand, & water. High proportions of cement increase strength Lime reduces compressive strength but increases flexibility & makes it “stickier” Sand reduces shrinkage during curing Several different types of prepackaged mortar mix. Type N – average strength for most general above grade masonry work. Moderate compressive strength. Type M – high compressive strength, very durable. Good for heavily loaded or below-grade foundation walls Type S – high tensile strength & high compressive strength. Good for earthquake & high wind areas. Type O – low compressive strength. Used primarily for interior walls
Mixing and Placing Mortar
Evaporation Hydration 2 ½ hour when air temp is 80ºF or higher 3 ½ hours when air temp is below 80ºF. Mixing and Placing Mortar Stiffens on mortar board because of evaporation or hydration. Evaporation is because of lost moisture. May add water to restore workability Mortar stiffened by hydration should be thrown away. Mortar should be used within 2 ½ hour when air temp is 80ºF or higher - & within 3 ½ hours when air temp is below 80ºF.
Laying Block Foundation Walls
Skilled masons Corners built first Story pole (course pole) Laying Block Foundation Walls - A job for skilled masons Corners are built first, usually four or five courses high Each course should be carefully checked for alignment. A story pole (course pole) is a board with markings 8” apart. Can be used to check top of masonry for alignment on each course. Fill in blocks for wall between corners making sure they are carefully aligned.
Intersections Control joint
Intersections - Control joints should be placed at junctions of bearing as well as nonbearing walls. Control joint – controls movement caused by stress in the walls. Permit slight movement without cracking masonry
Tooling the Joints Weather tight joints & neat block walls
“Thumbprint hard” Tooling compacts Head joints, then bed joints Tooling the Joints Weather tight joints and neat block walls depend on proper tooling. Done after mortar has become “thumbprint hard” (thumb makes no indentation) Tooling compacts mortar & forces it tightly against masonry on each side of joint & makes uniform, neat appearance Head joints should be done first, then bed joints
Completing the Walls Hollow block walls capped Strengthened with rebar
Hollow block walls must be capped with course of solid blocks. Sometimes might be strengthened with rebar – Each channel containing rebar is then filled with concrete
Moisture Protection Damp proofed or waterproofed Parging
Block walls must be damp proofed or waterproofed Sometimes done by parging. Spreading mortar or cement plaster over block and forming a cove (rounded area) where wall joins footing.
Radon Colorless & odorless radioactive gas Extremely toxic Lung cancer
Soluble in water 9X heavier than air Radon - A colorless & odorless radioactive gas that travels through soil May be extremely toxic to humans if it builds up inside a house. Long-term exposure linked to increased risk of lung cancer May enter through floor & wall cracks, expansion joints, gaps around pipes, or pores in concrete. Is soluble in water – can enter through water seepage and water vapor Radon is 9X heavier than air – tends to accumulate in basements. But air circulation helps distribute radon throughout a house
Gas-permeable layer Soil-gas retarder Sealants Vent pipe Cap course Radon Resistant Construction Gas-permeable layer – 4” thick layer of drainage gravel under floor slab. Allows radon to move freely under house Soil-gas retarder – Polyethylene sheeting 6-mil thick placed on top of gas-permeable layer. Prevents radon from moving through slab. Sealants – All opening and joints in foundation floor sealed to reduce radon entry. High performance caulk & plastic covers over sump pits. Vent pipe – 3” – 4” dia. PVC pipe connected to gas-permeable layer. Leads to roof and acts as exhaust to safely vent radon outside house. Cap course – Solid masonry course prevents radon from moving through hollow cores of block.
Review What is the minimum depth required for an exterior-wall footing? Describe the type and position of reinforcement that is commonly added to strengthen a footing. What is a keyway and how is it formed? For nearly all poured-concrete foundation walls, formwork must be constructed for each wall face. What holds the formwork together? What creates a cold joint? Why are the joints in a concrete block wall tooled? When using mortar, why is it helpful to know the differences between hydration and evaporation?
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