G7 Flashcards
place finish and cure concrete
explan when a conveyor is used to transport concrete?
onsite, large jobs, protect from weather.
how soon after the mix water is added to the concrete mixture must the concrete be placed in the forms ?
maximum 120 minutes
explain tailgate delivery
trucks chute to forms
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’re two basic types of concrete pumps?
boom ( crane) & line
how does the aggregate size affect concrete placement?
large aggregates limit pumping
list 5 guidelines for placing concrete
check forms & falsework, uniform lifts, place within 120 min of batching
how should concrete be placed on a sloping surface?
start from the bottem work your way up
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’s the max spacing of vibrator insertion points when vibrating a wall?
1/2 the radius of influence, 600mm if unknown
how far into a preceding layer of concrete should a vibrator penetrate?
6” or 150mm
how can you tell when consolidation is adequate ?
bubbles stop, sheen forms
when are external vibrators used?
metal forms for precast, 1200mm apart
what stages of finishing is an edger used and why?
after bull floating, pushes large aggregate away from surface and form rounds over
what’s the purpose of hand floating or bull floating ?
push aggregate down and allow bleed water to escape
why is a divider tool not used to make control joints?
the groove is not deep enough
when is the placing of concrete completed ?
after floating
why should the bleed water be allowed to evaporate ?
to not dilute surface concrete, causing dusting and crazing
explain how a broom finish is created?
float, edge, trowel, broom, edge
when are screeds removed from concrete?
after strike off
what’s the purpose of a screed
elevated guide for strike off
when placing concrete for a slab, what precaution should be taken with the reinforcing steel?
keep off ground using chairs or bricks
how is the top of the concrete wall finished
remove cleats and floats
what’s the purpose of kneeboards
distribute the finishers weight
what tools used to transfer flexible patters
tamper boards
what are 2 required conditions for proper concrete curing?
moisture and favourable temperature
what’s considered a favourable air temperature for concrete curing ?
15-20 degrees Celsius
explain the hydration process
chemical reaction of water and cement
list 4 methods of curing concrete
water coating, wet covering, curing compound, mechanical barrier
how are chemical hardeners used?
sprayed or sprinkled onto surface and floated In
why are some curing compounds coloured?
to show coverage
if two coats of curing compound are used, how is the second one applied?
at 90 degrees or perpendicular to the first coat
what’s the minimum curing time required to allow the concrete to gain 70% of its 28day strength ?
3 days
what are 2 purposes of chemical sealants?
durable waterproof finish, improves looks, cleanable surface
how are powdered hardeners applied to concrete ?
telecast on & floated or troweled In
what type of tools should be used to pry forms loose when stripping?
wood wedges, wood bars
why must rustication be removed very carefully ?
to not damage green concrete
explain the stripping process for foundation forms.
orderly and safely, remove cleats, 2x4s, walers, ties, sheathing, de-nail, stack lumber
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
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
whats the ideal tempurature range for placing concrete ?
15-20 degrees Celsius
why should additional water not be added to the concrete when placing concrete in hot weather ?
it weakens the concrete
how is the concrete mixture cooled in hot weather
spray aggregate with cold water cool the mix with water
what causes “plastic shrinkage cracking”?
rapid water evaporation
what’s the minimum temperature for curing concrete?
10 degrees celcius internal
what’s the minimum time that concrete should be cured?
3 days
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
what is the effect on concrete of using fossil fuel burning heaters in an unventilated enclosure?
CO2 weakens concrete and causes dusting
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
list 4 problems that can cause formwork to fail
improper stripping, improper rate of placement, improper vibrating, improper bracing
how could vibration cause a form to fail?
over vibrating near a stressed tie
what can cause a concrete slab on grade to sag and crack?
settling or heaving of sub grade
what causes spallimg ?
chipping caused by stripping techniques or stripping too soon
what is a white crystalline growth on the surface of concrete called ?
efflorescense
how can scaling be prevented
quality concrete, placed and finished correctly, low w:c ratio
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
what causes efflorescense
salts amd chemicals in water or aggregate
what’s the maximum aggregate size that should be used when repairing concrete ?
1/2 depth of patch
what are advantages of using pre mixed grouts?
contains chemical bonding agents, just add water
why must your wet the area to be patched?
to key the patch
what’s the minimum slump of repair concrete ?
50mm
how long should a patched area be allowed to cure?
3 days minimum
explain the sack rubbing process
wet the area, rub on sacking, let stand, rub on dry sacking
why are metallic aggregates used in some grouts?
so that metal expansion makes up for concrete shrinkage
why would non metallic grouts be used?
to avoid rust staining
how are liquid grouts used ?
forms and poured in
what are the disadvantages of using liquid grouts?
forms required
what is cementation?
use of liquid grout In industrial/ commercial projects
what safety precautions must be taken with concrete treatments?
read msds
what are the environmental concerns associated with concrete treatments?
streams and ground water
describe how sealers and hardeners also act as curing compounds
coat surface to stop moisture loss
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
what’s the difference between dampproofing and waterproofing materials?
dampproofing meant to repel moisture and waterproofing is mean to repel hydrostatic pressure
how is a suspended concrete slab waterproofed?
membrane roofing
what are pigments used for?
colouring concrete or surface
