Building Foundations

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26 Νοε 2013 (πριν από 3 χρόνια και 8 μήνες)

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Building Foundations

Foundation Walls

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. We
will focus on separate installation of footings.

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


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.


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


Shape 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.


Footing Forms continued


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


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
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.


Check Your Knowledge


What is the purpose of a footing?


What type of reinforcement is commonly
added to strengthen a footing? How is
it placed?


When are stepped footings required?


What is a footing drain, and why is it
important?

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 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


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.

Standard Wall Forms
continued


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


Fairly new formwork, made of rigid foam
insulation (expanded or extruded polystyrene)
called ICFs (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.

Insulating Wall Forms
continued


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


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.

Placement continued


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 over full
basement. Little to no excavation required
except for footings.


Soil beneath house must be covered with a
material to block moisture.


Usually must be ventilated. Check local codes


Floor framing above crawl space should be
insulated to reduce heat loss

Crawl
-
space Walls continued


Poured
-
concrete or concrete
-
block piers often
used to support floor girders. Should be no
closer than 12” to the ground.


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.

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
-

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.

Sill Plate Anchors


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


Foundation wall must often support
special features:


Brick
-
veneer siding


Utility Sleeves


These features must be accounted for
in the design of the foundation

Masonry Ledges


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.


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


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


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


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


Process of filling in excavation area around
foundation with soil


Brings area up to rough grade around house.


If done to 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.

Check Your Knowledge


What is a wale?


What is one advantage of an ICF?


Why must concrete always be poured in
as stiff a mix as is practical?


Name 3 important things you must do in
crawl
-
space foundation construction.

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)


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


Concrete Block Basic
continued


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.


Concrete Block Basics
continued


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 32
ºF.
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


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


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


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.


Mortar Mixtures continued


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


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 80
ºF or higher
-

& within 3 ½ hours
when air temp is below 80ºF.

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 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 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 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


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


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

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.


Check Your Knowledge


List 3 advantages of concrete block
foundation walls, as compared to solid
concrete walls.


Name the type of packaged mortar that is
most suitable for regions where earthquakes
occur.


What is a story pole and how is it used when
laying concrete block?


What is parging and what is its purpose?