QuizB(flashcard)

Handle/ Transport of Concrete

to avoid separation of course aggregate from mortar and of water from other ingredients. Segregation at point of discharge, avoided by down pipe at end of chute. Concrete should drop vertically into the center of receiving bucket, hopper, form

Ways Concrete is handle and Transported

Chutes
Push buggies
Buckets handled by cranes

Pneumatically forcing through a hose (shotcrete)(Gunnite)

Preparation

surfaces should be properly prepared, forms should be erected, reinforcement should be installed

Requirements Common to All Concrete-1

Not be placed frozen subgrade(snow, ice, debris must be removed from forms, excavations and from slab beds
When placed on rock, loose material removed, cut face vertical and horizontal rather than sloping

Requirements Common to All Concrete-2

-Chlorides not be used in concrete which dissimlar metals are embedded, reinforcing steel and heating ducts

-reinforcing steel should be clean and free of loose rust and mill scale at time concrete is placed

-Hardened mortar should be removed from steel

Slabs on Grade-1

Subgrade should be trimmed to spec elevation, uniformly compacted
except when vapor retarder, waterproof membrane, or separator has been installed below slab, subgrade should be moistened to prevent too rapid extraction of water from concrete

Slab on grade-2

At time of placing concrete no mud, soft spots, or free water standing where concrete is to be placed
PCA recommends vapor retarder on grade where impervious floor finish will be installed, where passage of water vapor is undesirable

Slab on Grade-3

Though Vapor retarder placed in contact with concrete, causes excess moisture in the concrete to bleed to surface rather than downward, increasing capillaries in the concrete, weakening the surface, more cracking, higher permeability of the slab

Slab on Grade-4

PCA recommends vapor retarder not be placed directly beneath slab, instead covered by 3in thick layer of granular, self draining(sand)

-when not vapor retarder used, building paper can be used, installed between base course or fill and the slab, not in contact with slab

Curling

warping of the slab

Formed Concrete

tight, adequately braced, constructed of material that impart the desired texture to the concrete
remove dirt, nails, sawdust, wood chips
treated with releasing agent (oil or lacquer)
architecual concrete- agent non-staining lacquer, emulsified stearate
wood forms moistened before concrete placed or they will absorb water from concrete and swell

Placing

placed to point where concrete will stay as close as possible
not in large quantities and allowed to run or worked over large distance
mortar flows ahead of coarser material results sloping surface
placed horizontal layers uniform thickness, each layer compacted before the next is placed
layer must be placed before other layer hardens

Placing Systems

Dumping from truck
Buggy
Dumping from truck to bucket, which crane lifts
Dump from truck through hose
Drop no more than 4 ft to prevent segregation
Not strike against forms, drop chute should be used to prevent spattering of mortar

Order of Placing

on walls, start at ends, work toward center
slabs, start around perimeter
Flat surface, start at far end so that each batch will be dumped against previously placed concrete, not away from it, should not be poured into piles and worked together

Order of Placing

if stone pockets occur, remove larger particles and distributed to areas where more mortar present to surround them
Walls, first batch placed at either end, then progress towards center

Placing on Hardened Concrete-1

should be level, rough, clean, moist, some aggregate particles should be exposed
Laitance (soft layer of mortar) removed from top surface
appreciable thickness of laitance indicates that segregation and bleeding occurred or too much water was used

Prevention of Laitance

using stiffer mix or finer material , particularly in the upper part of the lift

Placing of Hardened Concrete-3

When new placed on old concrete, old must be roughened and cleaned thoroughly, sometimes whole top surface can be removed.

Placing of Hardened Concrete-4

placed on hardened concrete or brick, layer of mortar should be placed down to provide a cushion against new concrete to be placed. prevents stone pockets, endures tight joint. made of same concrete to be placed and slump of 6in. thickness of 1/2 to 1in. In two-course floor construction, coat of portland cement and water paste with consistency of thick paint should be brushed onto hard surface

Shotcrete

pneumatically placed, primarily for swimming pools and in/above ground free-form structures and repair.
not require form work
through hose onto earth, rock, existing concrete, another layer of shotcrete
Reinforced with bars, mesh, steel fiber

Shotcrete Finish

Natural finish left by placement gun
screeded finish by sharp-edge cutting screed
flash coat by spraying on a final coat of shotcretejmade with fine aggergate
float and trowel finsh, though not on thin slabs

Consolidating

Concrete should be compacted for

eliminate stone pockets, large air bubbles
consolidate each layer with one placed earlier
completely embed reinforcing and fixtures
bring just enough fine material to the faces and top surfaces to produce desired finish

Spading and Puddling

Medium to High Slump Concrete(3 to 8in) should be compacted and worked into place by spading or puddling.

Spades ans Sticks used that are long /thin enough to pass through steel and forms, should be used

Vibrating-1

Low to Medium (1 to 6in) mechanical vibrators within concrete (internal vibrator) top of concrete or on the forms(external vibrator). Thin slabs not heavily reinforced may be visbrated by beam or truss screeds. Thick slabs structural reinforcement and thicker concrete members, such footings, walls, columns beams, girders vibrated with internal vibrators

Vibrating-2

friction between CA is temporarily eliminated, concrete behaves as liquid
concrete settles in the forms under action of gravity
trapped air bubbles rise more easily
once vibration is stopped friction is reestablished

Vibrating-3

less water per unit volume of portland cement, concrete will be of better quality
less portland cement unit volume used, concrete will be more more economical
vibrators should not be used to move concrete or push it
Sufficient vibration is seen by mortar line along forms and coarse aggregate sinking below top surface

Finishing

Finished in severalways, depending on its location, concrete material used, esthetic effect desired

Finishing Standard weight Concrete slabs

Steps: screeding, leveling, edging, jointing, floating, troweling, broom finishing or surface texturing

Bleeding

dry materials heavier than water so they settle to bottom, causing water to move towards surface= bleeding
happens more often in non-air entrained
scaling, dusting, crazing if worked on while bleeding is present
most basic rule
Placing, screeding, leveling must be done before it has the chance of happening
Should not be placed faster than it can be spread, struck off, leveled

Screeding

screeding is where surface of newly placed concrete is struck off by moving straight edge back and forth like a saw motion. If mechanical vibrating equipment is used in screeding, leveling may not be needed

Small amount of concrete kept in front of straightedge to fill low spots

Greatest affect on surface tolerance

screeding the surface to a predetermined grade

Leveling-1

bringing of a concrete surface to true grade with enough mortar to produce the desired finish
after a slab has been screeded, use of Darby to level raised spots and fill depressions left after screeding

Bull Float

Long handled, either wood or metal, used sometimes instead of Darbies to smooth and level. Though hard to smooth edges so Darbies are needed then

Darbying or Bull Floating

Leveling is sometimes called Darbying or Bull Floating but if used both must be used or Architect must dictate which one is to be used and which one is inappropriate. PCA uses broader term "leveling"

Leveling-2

purpose to eliminate the ridges and voids left by screeding.
slightly embed the coarse aggregate, preparing the surface for subsequent finishing operations of edging, jointing, floating, troweling
no operation can be preformed until concrete will sustain foot pressure with only about 1/4in indentation

Edging

bleed water and water seen have left surface and concrete starts stiffen
it rounds off the formed edge of a slab to prevent chipping or damage
edger ran back and forth
coarse aggregate covered
not to deep of impression

Edging 2

interior slab on grade doesn't need most of time
most often preformed on sidewalks, driveways, and steps
not used is slab is to be finished with resilient flooring requiring a smooth level subfloor
edge on construction joints ground lightly with silicon carbide stone

Jointing

Except when joints will be later sawed, immediately following or during edging, premolded inserts are placed in concrete slabs to control cracking as results of shrinkage

Floating

after edging and hand jointing, slab should be floated
many variables affect process, concrete temperature, air temperature, relative humidity, wind
Knowledge comes with experience, general though floating may be done when water sheen gone, concrete will hold weight finishing

Floating-2

purpose

embed large aggregate just beneath surface
Remove slight imperfections, humps, voids, produce a level or plane surface
Consolidate mortar at surface in prep for other finishing ops
Open surface to permit excess moisture to escape

Floating-3

marks left by edgers and jointers removed by float
floats made of aluminum or magnesium, especially on air-entrained concrete

This type of float greatly reduces amount of work to be done, because slides easier over concrete, good floating action

Troweling

done on slabs left to exposer, or receive thin finish( resilient flooring, carpet, title or paint
when required, surface should be troweled immediately after floating
customary for a cement mason using hand tools to float and then steel trowel before moving to knee boards

Troweling-2

If necessary, tooled joints and edges should be rerun before and after troweling to maintain true lines, proper depths and uniformity and remove kinks

Troweling-3

purpose to produce smooth hard surface
first troweling by power or hand, trowel must be kept flat, if not and tilted it may cause washboard or chatter
First troweling, a new trowel not recommended
Time lapse of time between troweling let concrete hardened

Troweling-4

as surface stiffens, troweling should be made by smaller trowel tipped at progressively higher angle so that sufficient pressure can be applied
seconde troweling recommend because it gives closer surface tolerances and better surface to receive flooring.

Broom Finishing

slightly roughened to produce a non-slip surface by brushing or brooming them.
brushed surface made by drawing a broom over surface after steel troweling
work who can draw broom right over edge joint but joint not being marred, indicates proper steel trowel

Finishing Air-Entrained concrete slabs

microscopic air bubbles hold the ingredients, including water in suspension
requires less mixing water, still has good workability with same slump, little or no bleeding occures
no need to wait for evaperation of free water, floating and troweling can be started soon as it can hold weight of person

Finishing Lightweight Structural Concrete

tendency for the coarse aggregate to raise to the surface instead of falling, while the mortar raise
to prevent 1. mix properly proportioned, not over/under sanded to meet weight stand. 2. finishing not started early, not overworked/ overvibrated

Finishing Lightweight Structural Concrete-2

well proportioned mix can be placed, struck off, leveled, floated with less effort than normal weight concrete

Excessive leveling, floating are principal causes of finishing problems because heavier mortar is driven down to the bottom, and CA to the top

Special Slab Finishes

suface may be scored or tooled with jointer in decorative and geometric patterns

Exposed Aggregate

chosen for an area where special textural effect is desired
selection of aggregate is so important that test panels should be made before job started
should have colorful gravel, uniform in gradation, sizes ranging from 1/2 to 3/4 in

Exposed Aggregate-2

Flat, sliver-shaped particles or aggregate less than 1/2 should be avoided
5 and one-half to six-sack concrete with max 3in slump should be used
after slab is screed and leveled, aggregate spread by hand
initial embeding done with darby or 2x4 by patting

Exposed Aggregate-3

surface hand floated with float or darby, make sure all aggregate in fully covered, no holes, or openings in surface
shortly after reliable retarder may be sprayed or brushed
exposing time of aggregate very importpant, soon as grout covering aggregate can be remove by brush or hosing

Exposed Aggregate-4

interior areas where smooth surface is desired, exposing aggregate is done by grinding, may be followed by polishing, which produce a similar product of terrazo
Decortive or control joints, cut 4 to 12 hours after slab is placed, also 1/5 depth of slab

Exposed Aggregate-5

small radius edger should be used before and after embedding
another method installing redwood before placing concrete
another method spreading a 1in thick top coarse

Dry Shake Colored Surface

used for interior areas, terraces, decorative walks, driveways
ingredients mineral oxide pigment, white portland cement, specially graded silica sand or fine aggergate
select, proportioning, mixing at site not recommended

Dry Shake Colored Surface-2

floated by hand or power
if by hand, use magnesium or aluminum float, to bring up moisture for combining with dry shake, also removes ridges, dpression, dry shake by hand
if wrong amount applied, peeling may occure

Dry Shake Colored Surface-3

first application about two-thrids total needed, which is usually specified in pounds per square foot of surface
after float and then trowel, there should be a lapse of time which depend on temp and humidity

Dry Shake Colored Surface-4

Exterior surface two trowelings are enough,
Interior should get a third, should be done by hand to produce hard, dense, wearing surface
must be cured
interior may be coated twice with concrete floor wax( same mineral oxide pigment)

Swirl Design

nonskid surface texture produced on a slab using magnesium or aluminum float or steel finishing trowel
called swirl float finish when float is used
called swirl trowel finish when trowel is used
work semicircular or fan like motion by cement mason
moist curing finial operation

Keystone Finish

produces a travertine like texture
used terrace, garden walk, driveway, deck around swimming pool
Not used where it is exposed to freeze/thaw
after screeded, leveled, edged, then broomed with stiff broom to ensure that finish(mortar) bonds

Keystone Finish-2

mortar coat made by mixing one sack white portland cement, 2 cu ft of sand, 1/2 lb of color pigment,
Yellow is usually used
Enough water added to make soupy mix like thick paint

Keystone Finish-3

mortar placed in pails and thrown vigorously on slab with stiff brush called dash brush
this produces uneven surface with ridges and depressions
Ridges should be 1/4 to 1/2 high

Finishing Formed Surfaces

depends on exposure and esthetic effect designer is trying to achieve

Rough Form Finish

rough form finish left where formwork locations will not be seen in the building
improve only by filling tie holes and defective areas, chipping off projections larger than 1/4 in

Smooth Form Finish

used two times

when form liners are used to impart a pattern or texture
when concrete will receive paint, waterproofing, dampproofing, veneer plaster, another thin coating or covering material
tie holes patch or left alone, screw ties with cones used when ties not to be filled

Smooth-Rubbed Finish

applied when smooth form or form liner used
applied to moistened concrete within 1 day after forms removed and rubbing it with abrasive such as Carorundum brick
Cement grout not used

Grout-Cleaned Finish

used on concrete that has been placed against a smooth form or form liner
apply thick-paint like grout of 1 part portland cement and 1 1/2 fine sand, mixed with equal part acrylic or styrene butadiene bonding material and water
stand (gray) and white portland cement used to achive color
completed by rubbing with burlap

Scaling

breaking away of the hardened concrete surface of slab to depth 1/6 to 3/16
occurs at early age of slab
cycles of freezing/thawing, salts on non-entrained air concrete it happens
preforming finsih operation while bleed or water on surface

Scaling-2

not allowing bleeding or excess water to evaporate, cause segregation of surface, brings thin layer of neat portland cement
water should be allowed to evaperate from surface or drag water hose across the top before finsihing

Crazing

numerous fine hair cracks in the surface of newly hardened slab due to shrinkage
looks like crushed egg shell
result of rapid surface drying, caused by high temp, hot sun, drying winds
lessened by curing with water
over vibrating screed, darby, bull float by working excess mortar to top

Dusting

powdery material on surface
excess of harmful fines(clay or silt) in concrete mix with sand and cement at surface
this problem emphasizes the use of clean, well-graded coarse and fine aggregate

Dusting-2

carbon dioxide comes in contact with surface of plastic concrete, it impairs hydration
Condensation usually occurs on surface during spring and fall
concrete should be heated if this anticipated by hot water, or blower type heaters
neat portland cement/mixtures/ fine sand never used as dry shake

Dusting-3

well mixed dry mixture= 1 part portland cement and 1 part well-graded concrete sand
winter heaters may lower relative humidity around concrete such as inhibiting hydration

Curing

fresh concrete has more than enough water for hydration, but at job sites this is often lost by evaporation
Hydration preceeds at much slower rate as temp drops until it reaches 14 degree which it stops all together. at 40degrees it is to slow to proceed

Curing Methods

leaving forms in place
sprinkling, ponding,
use moisture-retention covers or a seal that goes on as liquid then makes hard thin layer

Leaving Forms in place

great way of keepinig water in, in hot dry weather, wood forms dry out and take water in from mixture. These should be kept moist by sprinkling

Sprinkling

must be continuos, not allowing dry to happen between applications

Ponding

used on flat-surfaces
small dam of earth or other water-retaining material placed around perimeter of a surface and enclosed area is flooded

Curing Covers

moisture-retaining covers such as wet sand, burlap, polyetgylene film
kept constantly moist enough to provide flim

Watertight Covers

used on floors and other horizontal areas, non-staining, strong enough to with stand wind and other factors. Seams should be overlapped several inches.

Sealing Compound

sealing colors in red, colorless, white pigmented coatings
some can be applied in one coat but two will give better results
made immediately after concrete in finished, concrete kept moistened
in hot weather, wet concrete for 12hrs before application
covered by 1in of sand or earth or other means, though not sooner than 24hrs after sealing

Curing Temp

Refer to American Concrete Institute (ACI) ACI 305R and ACI 306

Cold Weather Construction

often need to heat concrete ingredients
in Cold weather concrete should have temp of between 50 to 70 f
no cases should it be heated to above 70degrees, if so built in thermal stress will occur
Should be kept between 50 to 70

Cold Weather Construction

70 degrees 3 days, 50 degrees for 5 days when type 1 is used
70 degrees 2 days, 50 degrees 3days when type III is used
no dependence on salt or other chemicals

Mild Weather Construction

weather is betw

Hot Weather Construction

Definition

Length of Curing

should last atleast 7 days no matter what kind of concrete
in mild weather it can be reduced 2 days when type III
temp not to draw below 50 degree
rapid cooling may induce thermal cracking, especially during first 12 to 24hrs

Length of Curing -2

Type I- 5days 70 degrees

7days 50-70degrees

Type II-3 days 50 degrees

All other Types- 7days 70 degrees

14days 50-70 degrees

QuizB(flashcard)

Construction Science 253 with Segner at Texas A&M University

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