G-7 Place and Finish Concrete Flashcards
how could vibration cause a form to fail?
over vibrating near a stressed tie
what are the environmental concerns associated with concrete treatments?
streams and ground water
what tools used to transfer flexible patters
tamper boards
what’s the minimum slump of repair concrete ?
50mm
concrete that has only been cured for 0 days, 3 days, or 7 days will only have which percent strength?
50%, 70%, 90% of it 28 day strength
if two coats of curing compound are used, how is the second one applied?
at 90 degrees or perpendicular to the first coat
Trowel blades
- typically 8” wide - made from thin steel - each pass will increase density of the slab surface
Discolouration in concrete
Prevented by :- clean formwork - maintaining uniform consistency of the concrete mixture - using uniform aggregate types and colour - proper placement of concrete- adequate vibration
screeding or striking off
- levels the survace of concrete to elevation set by screeds-
what is and edger?
used too round the outside corners of the surface of the slab, and create a decretive boarder
how far apart are external vibrators placed?
1200mm apart
wall patches
usually has opening in top to allow placement of patching material
what is a white crystalline growth on the surface of concrete called ?
efflorescense
Curing
process of keeping concrete moist. - concrete not sured at all will only get 50% strength- cured for three days will get 70%- cured for 7 days will get to 90%
Bull float
used for large areas very large wooden or metal float with long handle attached
what is the minimum and maximum slope of a walkway
2% to 8%
what does flaking mean?
very thin flakes of concrete separating from the surface
what’s considered a favourable air temperature for concrete curing ?
15-20 degrees Celsius
what is Efflorescence?
As water from the interior of the concrete migrates to the surface and evaporates, salts are deposited on the surface, usually as a white stain
Fresno
long handled steel trowel used to flatten out the ridges left by the bull float -
Broom finish
NAME?
when are external vibrators used?
metal forms for precast, 1200mm apart
sodium silicate
common hardening agent that is applied to new or existing concrete surfaces to increase hardening
how far into a preceding layer of concrete should a vibrator penetrate?
6” or 150mm
Bond breakers
- chemicals used to break bond between layers of concrete-
what’s the maximum aggregate size that should be used when repairing concrete ?
1/2 depth of patch
how does the aggregate size affect concrete placement?
large aggregates limit pumping
what stages of finishing is an edger used and why?
after bull floating, pushes large aggregate away from surface and form rounds over
what does dusting mean?
fine loose powder caused by the deterioration of a weak surface.
what’s the minimum curing time required to allow the concrete to gain 70% of its 28day strength ?
3 days
edger
used to round corners of the surface of slab . - done after concrete is floated to remove large aggregates away from forms.
what are concrete buckets used for?
moving concrete on tall buildings hoisted by a crane
what’s the max spacing of vibrator insertion points when vibrating a wall?
1/2 the radius of influence, 600mm if unknown
Preventing rapid evaporation
NAME?
Cold weather concrete
formwork left in place to help concrete cure.- once compressive strength reaches 3.5MPa damage caused by freezing is reduced.
list 4 methods of curing concrete
water coating, wet covering, curing compound, mechanical barrier
what’s the difference between dampproofing and waterproofing materials?
dampproofing meant to repel moisture and waterproofing is mean to repel hydrostatic pressure
explain the stripping process for foundation forms.
orderly and safely, remove cleats, 2x4s, walers, ties, sheathing, de-nail, stack lumber
riding power trowels
for large projects- two or three separate trowels are assembled into one - operator rides on top and is able to control movement and rake of blades
list 5 guidelines for placing concrete
check forms & falsework, uniform lifts, place within 120 min of batching
what are 2 ways to keep concrete moist for an extended period of time?
spray-on acrylic curing liquid or plastic sheets
describe how sealers and hardeners also act as curing compounds
coat surface to stop moisture loss
explain the hydration process
chemical reaction of water and cement
what are 2 purposes of chemical sealants?
durable waterproof finish, improves looks, cleanable surface
how long should formwork be left in place during cold weather ?
minimum 3 days or untill it reaches a strength of 3.5mpa or greater
Super plasticizing
Superplasticizers transform stiff, low-slump concrete into flowing, pourable, easily placed concrete. They can improve workability, speed finishing, increase strength, conserve cement and help reduce shrinkage and thermal cracking.
what’s the purpose of hand floating or bull floating ?
push aggregate down and allow bleed water to escape
What does a bull float do?
used too push larger aggregates under the surface and level and smoothen large slabs
what’s the minimum temperature for curing concrete?
10 degrees celcius internal
floating
- after concrete has been placed and screeded level- the surface is floated - this pushes any large aggregate below the surface of concrete slab-
describe how to reduce the segregation of the aggregate from the cement paste
don’t drop through, don’t over vibrate, don’t use wrong rake, don’t slide too far at wrong angle
what causes “plastic shrinkage cracking”?
rapid water evaporation
why are metallic aggregates used in some grouts?
so that metal expansion makes up for concrete shrinkage
Hand floats and kneeboards
tops of walls and small concrete slabs can be done with hand float - if hand floating larger job kneeboards are used
what is a main powdered hardener?
Sodium Silicate, sprinkled over concrete and bull floated in reacting with the lime creating a hard dust-proof surface
Hydration
the chemical reaction that takes place when water reacts with cement. this allows it to harden. must be kept wet so it can get hard.
how can you tell when consolidation is adequate ?
bubbles stop, sheen forms
list 4 problems that can cause formwork to fail
improper stripping, improper rate of placement, improper vibrating, improper bracing
what is curing?
to keep concrete moist as long as possible for maximum strength growth
what are advantages of using pre mixed grouts?
contains chemical bonding agents, just add water
Honeycomb
occurs at the base of concrete columns when concrete is dropped through reenforcing steel and not vibrated adequately- can result is rebar being exposed - no not over vibrate
how soon after the mix water is added to the concrete mixture must the concrete be placed in the forms ?
maximum 120 minutes
how are liquid grouts used ?
forms and poured in
what causes efflorescense
salts amd chemicals in water or aggregate
Presoaking
once surfaces have been swept and vacuumed clean, it should be kept wet for at least two hours to ensure old concrete will not absorb water from patching material
wooden hand floats must be made of?
NAME?
what is the effect on concrete of using fossil fuel burning heaters in an unventilated enclosure?
CO2 weakens concrete and causes dusting
Damp proofing
concrete foundations that are below grade should be coated on the outside with a damp proofing agent
what are 2 required conditions for proper concrete curing?
moisture and favourable temperature
what’s the purpose of a screed
elevated guide for strike off
Accelerators
speed up hydration, which releases more heat warming the concrete.
divider
used to create decretive divisions in walkways an patios- used to create control joints joints-
what does segregation mean?
excessive vibration causing the course course aggregates to settle to the bottom.
concrete voids
spaces caused by trapped air under window bucks and blockouts.must vibrate properly around bucks - drill holes in bottom of bucks to allow air to escape
how far apart, and how far down does the pencil vibrator get placed when vibrating concrete walls?
not more than 600mm apart and 150mm into the previous layer
Chemical Hardeners
NAME?
explain the sack rubbing process
wet the area, rub on sacking, let stand, rub on dry sacking
Retarding admixture
added to concrete for pumping and are particularly useful in hot weather. lengthens the setting time of concrete but does not have any effects on the strength gain of concrete
when is a concrete bucket used to place concrete ?
when building is more than a few stories high and a cranes more feasible to use
What is the soul purpose of vibration?
To remove pockets of air from the concrete
formwork failure
NAME?
Overhead paches
patching material must be stiff and must be braced
what type of tools should be used to pry forms loose when stripping?
wood wedges, wood bars
water proofing
waterproofing membrane applied to foundation walls to resist hydrostatic pressures
Repair concrete Grout
NAME?
Concrete (patching)
NAME?
what safety precautions must be taken with concrete treatments?
read msds
why should additional water not be added to the concrete when placing concrete in hot weather ?
it weakens the concrete
Curing compounds
NAME?
What happens when concrete flash sets?
concrete hardens too quickly resulting in improper finishing and inadequate strength
what does it mean too continually pour concrete within multiple forms or slabs?
Monolithic concrete placement
when are screeds removed from concrete?
after strike off
Floor patches
more rebar could be drilled and place to allow patch to take better
Wall finish
after concrete has been vibrated into place the surface of top of wall is finished level strip or chamfer strip set elevation of top of wall. then hand float is used to finish
Bonding
just prior to patching material, apply bonding material and patch before bonding has a chance to dry
Using power trowel
- make sure concrete is levelled properly 2. should support foot pressure before power trowel 3. make sure bleed water has evaporated 4. move over entire surface evenly 5. allow concrete to harden between passes with trowel
Scaling
small pieces of concrete flake off the surface of a concrete slab. scaling is dude to lowered concrete strength at the surface.
Sealers
chemicals that seal the surface of concrete and prevent the formation of dust on a worn concrete surface
why would non metallic grouts be used?
to avoid rust staining
whats the ideal tempurature range for placing concrete ?
15-20 degrees Celsius
Curing concrete walls
NAME?
what is the difference between a divider and a control joint tool?
a divider is only for decorative purposes where a control joint tool has a deeper cutter for creating control joints
Architectural concrete finish
NAME?
explan when a conveyor is used to transport concrete?
onsite, large jobs, protect from weather.
what does spalling mean?
Spalling is similar to scaling except large chunks instead of just flakes break loose
Curing flatwork
most efficient way to cure concrete slab is to use curing agent. this is sprayed onto slab after finishing it.b
what is sack-rubbing finish mean?
concrete low spots can be filled with the use of a flowable sacking mix mortar and burlap pads. fills voids in the concrete and left flush with surrounding surfaces.
what can cause a concrete slab on grade to sag and crack?
settling or heaving of sub grade
what does crazing mean?
minute cracking of the concrete surface caused by bleed water not fully evaporated disrupting the water/cement ratio
Power trowel
used for finishing concrete slabs- have four spinning concrete trowel blades that spin clockwise
Float blade
- used for final levelling of concrete surface - usually 10” wide - made from thick gauge steel blades are kept flat for initial passsecond pass kept at 90° (crossover floating)
what is a screed/strike off bar and what does it do?
long 2X4 the extends beyond the edge of a slab for to level out the concrete
why should the edges of the area being patch be sloped inward?
to create more surface and bonding area for the patch
what should the slump of a concrete patch be?
no less than 50mm
patching procedure
make test batch and test run a patch, check for discolouration and strength
Heated enclosures
using heated enclosures concrete can continue as usual. fuel burning heaters must have exhaust vented outside because carbon monoxide causes surfaces of concrete to be weak and dusty
explain how-to strip and lift slab tables
using fall protection, lower jacks onto dollies, attach lines to core, roll out 1/4 way, attach first pair of slings, roll out and attach second pair, detach line from core, hoist out
why is a divider tool not used to make control joints?
the groove is not deep enough
when placing concrete for a slab, what precaution should be taken with the reinforcing steel?
keep off ground using chairs or bricks
how are chemical hardeners used?
sprayed or sprinkled onto surface and floated In
what are pigments used for?
colouring concrete or surface
why must rustication be removed very carefully ?
to not damage green concrete
what’re two basic types of concrete pumps?
boom ( crane) & line
formwork and patching
sometimes formwork must be left in place when patching occurs to allow to harden properly
Which of the following chemical elements produced by motorized
equipment on a construction site can poison the cement finisher?
a. Carbon dioxide
b. Carbon monoxide
c. Hydrogen
d. Nitrogen
B
Metallic or aggregate hardeners
NAME?
what is placed just before the patch goes on?
a bonding agent
Chemical sealers
NAME?
What is the capacity of a concrete bucket?
Depends on the lifting capacity of the crane, usually able too care between 0.4 - 1.5m cubed
why are some curing compounds coloured?
to show coverage
Dusting
when surface of concrete is weakened - caused by early age freezing- adding to much water while placing concrete - using unvented heaters - working bleed water back into the surface
Prep for formwork in cold temperatures
NAME?
chemical hardeners
provides a hard-wearing concrete surface and prevent dusting of the concrete
what’s the minimum time that concrete should be cured?
3 days
why should the bleed water be allowed to evaporate ?
to not dilute surface concrete, causing dusting and crazing
why must your wet the area to be patched?
to key the patch
what is cementation?
use of liquid grout In industrial/ commercial projects
Mortar
same as grout but cement content is lower. consistency is much stiffermortar is used to bond bricks together
Describe concrete finishing
top surfaces of cast in place concrete must be finished to a smooth, uniform surface for appearance and durability
Concrete is placed in lifts because why?
so that the overall height of the fresh concrete does not exceed the rate of placement set by the form design
bonding agents
epoxies and adhesives are used to create a good bond between fresh concrete placed over old concrete
Spalling
pieces of concrete chipping off the surface around openings or edges- spalling around rustication strips or window and door bucks is caused by removing forms before concrete s strong enough to resist stripping stresses
how are powdered hardeners applied to concrete ?
telecast on & floated or troweled In
walk behind power trowels
- size determined by diameter of circle it makes- ranges from 24inces to 48 inches in diameter
how is the top of the concrete wall finished
remove cleats and floats
Temperature for curing
keeping concrete warm promotes curing.prime curing temperature is overcast skies at 15-20°
Bonding compounds
used to ensure positive bond between patch and concrete being repaired. woodglue can be used
Stamped concrete
uses manufactured stamps that are set into the partially set surface of concrete.- done after bleed water is evaporated - surface of slab is covered with fine powdered release agent that is designed to allow the stamps to be removed cleanly from the concrete
how long should a patched area be allowed to cure?
3 days minimum
what precautions should be taken when using hot water in the concrete mixture ?
avoid flash set by adding cement last and after scheming mix temperature
describe the procedure used to place concrete around bucks in a wall form
place on one side only to prevent air entrapment/ vibrate and have holes drilled into buck
Preventing dusting
- maintained designated water/cement ratio2. do not add water to finishing process. protect the fresh concrete from direct exposure to the sun, rain and freezing 3. vent heateres!!!!
Mechanical Hardeners
- increase durability of concretes - added at the troweling stage - worked in and bonded to the surface to protect concrete- metal shavings, plastic particles,
what is a groover?
like a hand divider tool but larger with a long handle for making grooves in slabs
explain how to strip and lift gang form panels
using fall protection, partially strip leaving top taper ties in but loose, attach tag lines and crane, tension crane, finish stripping take top taper ties off, clear area
how should concrete be placed on a sloping surface?
start from the bottem work your way up
preparing patched surfaces
edges of area to be patched should be undercut (cut smooth) to provide a key where new patch concrete is being placed.
what are the disadvantages of using liquid grouts?
forms required
Crazing
series of small cracks in surface of concrete. happens due to working the bleed water back into the slab during finishing
what is bleed water?
excess water in the concrete mixture that is not needed for the chemical reaction with the cement. the bleed water will rise too the surface and evaporate.
Pigment
coulering agents may be added during concrete mixing or may be shaken onto surface when slab is being finished
what is honeycombing?
trapped air bubbles from improper vibration that is visible after forms are removed
what is the minimum temperature for curing?
10 degrees celsius
how long should the area being patched stay wet for?
2 hours prior to patching
Vibration
- slabs must be vibrated like walls or columns - edges must be well vibed to reduce bug holes -
how fast should you place the concrete after the water has been added?
120 minutes
how is an exposed aggregate wall surface created?
uniform coat of retardent painted on the surface, don’t slide concrete on surface of treated form, strip after 24hours
how is the concrete mixture cooled in hot weather
spray aggregate with cold water cool the mix with water
what’s the purpose of kneeboards
distribute the finishers weight
sacking mix
sacking is a mixture of portland cement and fine aggregates and admixtures. - used to patch minor defects in concrete like bug holes and honeycomb that can get water in and freeze causing spalling
Hot weather conditions
when temperatures are above 25°C rapid evaporation of water in concrete occurs. water loss will accelerate setting times and create shrinkage cracks.
explain how a broom finish is created?
float, edge, trowel, broom, edge
how is a suspended concrete slab waterproofed?
membrane roofing
what happens when dry shrinkage occurs?
shrinkage happens mostly because of the reduction of capillary water by evaporation and the water in the cement paste. Tensile stress increases causing stress cracks
what is spalling around rustication strips? (keyways)
larger bits or chunks of concrete stuck too the strip and pulled off
Flatwork finishing
NAME?
how can scaling be prevented
quality concrete, placed and finished correctly, low w:c ratio
explain tailgate delivery
trucks chute to forms
Steel trowel
12- 16 inches - longer legnths needed when less pressure is required short is needed hen more pressure is required
what causes spallimg ?
chipping caused by stripping techniques or stripping too soon
when is the placing of concrete completed ?
after floating
Effloresence
white powdery crystal substance that seems to blooms out of cured concrete. dissolved salts in concrete mic - unattractive but generally harmless
