Concrete properties

why is concrete great for parking lot and better than asphalt

much better with temperature (ex: florida)

much better for lights because material more light reflection and thus people feel safe and costs less money [street ligthing is extremly expensive]

but note that concrete is more expensive

bleeding

refers to upward movement of water after concrete has been consolidated but before it has set

→ water is the lightest component

BUT sometimes goes wrong → creates tubes of liquid around the particles and act

Bleeding - damage to concrete

when bleeding occur through distinct channels that later act to increase the permeability of hardened concrete → LOCALIZED

→ upper layer of concrete can become rich in cement paste => high W/C ratio

makes very poor connection if you pour another batch of concrete on top

water pockets may form beneath inclusions in the concrete, leaving weak zones and reducing bond

ex: copperdam example from prof → concrete not consolidated completly in the middle

→ causes weakness and bad durability (bc something bad is getting inside) ex: how long chlorides needs time to get to the steel controls how long your bridge lasts

bleeding - when surface is not free

if the water can’t get out, it gets stuck under the aggregates and the reinforcing bars

called a bleed pocket

some people can tell you the top side of concrete based on bleed pockets (microscope)

Bleeding - rate of drying

if bleed water evaporates more quickly than bleeding rates, plastic shrinkage cracking can form → top wants to shrink inside doesnt

creates strain and stress → becasue inside is resisting shrinkage

gets strain if the ends of are free and can shrink and get stress if the ends are fixed and cannot shrink (example of beam)

shrinkage always happen → people now make pretty cracks → control joins

all concrete will have a crack every 10-12 ft in all direction

problematic in hot, dry and/or windy (light breeze in winter) weather (can happen in winter as well → relative humidity is very low → the less humidity the more air can absorb water of concrete) → plastic shrinkage

control joints - happens when concrete is plastic

→ they do a little cut in the slab to reduce cross section and thus control where the crack is (happens after curing)

Slump test

cone made out of sheet stuff (hollow inside)

standardized method

then turn it over on table and measure how much the freed concrete slumps

→ not an absolute measure of workability but tests rough measure of consistency

measure in ranges → measured at construction site

consistency test → the value isn’t as important as the consticency of result

cannot be used for: high slump concrete, low slump concrete, fiber reinforced concrete, certain admixtures (high water reducer)

Laitance

a scum of fine particles or dissolved salts may be carried to the surface in a weak non-durable layer → problematic if at the top of a lift

Dusting

laitance at the final surface

Workability over time

slump decreases over time becasue some jelly effect → before setting starts

→ deformation are elastic

water content vs slump

adding water increases slump and workability BUT TOO MUCH WATER IS BAD CONCRETE → lower quality !!!!

it is technically true but not the rigth way to do it

entrained air

is here for reasons we want to

is good

entrapped air

air trapped that don’t want

it is BAD

tests for air in concrete

tests can only measure the total air of concrete and not the cement paste so not so great + can’t tell you the nature of the air void system

air content is expressed in terms of the volume of concrete though it is only present in the cement paste portion → that is how you specify it in design

3 ways to do it:

• gravimetric test

• volumetric

  • pressure

gravimetric tests

oldest and simplest but not efficient

compares the unit weight of concrete containing air with the calculate unit weigth of air-free concrete

finish flash cards

volumetric method (ASTM C 173)

based upon comparaison of volume of concrete containing air with volume of concrete after air has been removed

→ requires a significant of physical effort to remove air from concrete

Pressure method (ASTM C 231)

most common method used

based upon measurement of change in volume of concrete when subjected to given pressure

Boyle’s law used to calculate the air content

unit weigth

bulk density basically

determined by weighing a known volume of concrete

often performed on the same concrete sample later used for air content determination

presence of air directly affects unit weight

also used a quality check for large variations in air content

setting

refers to the onset of rigidity in fresh concrete

is gradual changes controlled by the hydration of the cement

you cant do anything to the concrete past that point

strandard value (dont mean much) = 500 psi

Hardening

the development of useful and measurable strength → when you can start putting load on it

strandard value (dont mean much) = 4000 psi

rigidity versus time

why is time to set important

• to help regulate mixing and trasit

• to gauge the effectiveness of various set controlling admixtures

• to help plan the scheduling of casting and finishing operation

• as a a quality check to monitor the accelerating/retarding of additives

test to know the setting of concrete

only 1 test currently recognized by ASTM

uses a pseudo-mortar specimen where concrete is tested by penetration → different sizes of needles used to determine penetration resistance of specimen over time (500psi to 4000psi)

PR= F/A

Curing

= after initial set, when adding as much water as possible

essential in producing acceptable concrete performance

affects both mechanical properties and durability characteristics

supposed to give 2 components: prevents loss of moisture through evaporation / supplies additional water for continued hydration

→ excessive evaporation can cause plastic shrinkage cracking

Curing 2

at W/CM ratios of 0.2 of higher, suddicient water is present in the paste to provide complete hydration theoritcally

however water lost due to evaporation or by absorption by the aggregates

once internal relative humidity drops below ~80% the hydration reaction stops as does strength development

curing should be continued as long as feasibly possible,with guideline of ACI 308

slags or pozzolans require even longer curring periods to enure that there is enough water present for teh pozzolanic reaction to take place

concrete production - primary steps

1- batching = weighing out ingredients

2- mixng = central mix, truck mix

3- transporting/placement = mixer truck, pump, chute, tremie …

4- finishing = compaction, floating, troweling, texturing

5- curing = sprinkling, wet covering, ponding…

→ prevent loss of moisture / supply additional moisture

Batching

best done by weight (aggregates and cements)

only water and liquid admixtures can be measured accurately by volume → need HIGH DEGREE OF ACCURACY

ASTM C 94 gives tolerance for ready-mixed concrete

Mixing

thorough mixing = essential → homogeneous concrete

inadequate mixng = lower strengths + greater variations within / btwn batches

overly long mixing times do not improve concrete quality → break aggregates / air pockets

factors for optimum mixing

1- type of mixer

2- condition of mixer → cleanliness

3- speed of rotation

4- size of the charge

5- nature of ingerdients

6- mix design / proportioning

7- ambient temperature

type of mixers

drum mixers - minimm size: 1m3

pan mixers aka bread mixer

continuous mixers minimum size: 1ft3

slide 10-7

ready-mixed concrete

central mixed: (fully mixed) → truck mixer only used to agitate concrete (80% capacity of truck)

transit mixed: (partially / completly mixed) while being transported to the site

truck mixed: completly mixed → water can be added later if aggregates is dry (63% capacity)

placement

very important to prevent segregation

can lead to the formation of rock rockets and honeycombing

concrete should fall vertically → max limits is btwn 3 to 5 feet

fresh concrete properties to produce nice hardened concrete

• easily mixed & transported

• uniform throughout a given batches / btwn batches

• capable of flowing such that fill the tiny corners

• compactable without the need for excessive nrj

• resistant to seggregation

• capable of being finished properly → big issue bc hard labor

Workability

amount of mechanical work required to produce full compaction of the concrete without segregation

Factors affecting workability

segregation and bleeding

segregation

refers to the separation of components of fresh concrete, resulting in nonuniform mix

factors that contribute to the seggregation include:

• large maximum aggregate size & proportion of large particles

• high specific gravity of coarse aggregates realtive to fine

• decreased amount of fines (sand & cement)

• variations in particle shape away from smooth, well-rounded

• mixes that are either too wet or too dry

SCMs can reduce segregation placement + handling = + important

parameters affecting curing

duration of curing

→ if curing stops it will affect rate of strength gain slows down and stop

interrupted curing

temperature

interrupted curing

if interrupted can be restarted with a coresponding increase in strength

ultimate strength will not reach the same level

must maintain water in concrete as a continuous liquid system

large capillariesbegin to epmty due to self-desiccation causing menisci to form

temperature for curing

aka hydration rate

→ affects rate of strength development

at early ages, cement hydrates more rapidly (more increased early strength) → but produce a nonuniform distribution of hydration products → weak zones in the hcp

hot weather → can put ice in the truck then sent to construction

anything under 4°C → take precautions (curing take forever)

methods of curing - water curing

continuous supply of water to concrete surface

ponding - spraying - fogging - sprinkling - saturated coverings

must maintain water in concrete as a continuous liquid sys

slags or pozzolans require even longer curing periods to ensure enough water present for pozzolanic reaction to take place

Sealed curing - methods of curing

much more convenient

= sealing concrete surface to prevent evaporation

→ waterproof paper, plastic sheeting, membrane forming curing compounds

prevent water to get out BUT not supplying any water

should never be used between lifts or bonded overlays or surface painted + not used during the fall on pavements to be exposed to deicing salts

self-desiccation = common problem when sealed curing

Sealed curing (curing compound)

latex + water mixed

→ spray downward angle latex won’t create proper seal

Plastic shrinkage cracking

induced when the rate of evaporation of free water from the concrete surfaces becomes excessive

surface wants to shrink but inside doesn’t

under extreme conditions (high evaporation rate) → can lead to severe cracking of concrete surface

factors are: concrete + air temperature / relative humidity / wind velocity

high evaporation rate rate conditions

hot, dry, windy weather

sometimes happen in quebec winter

hot weather concreting

above 75°F → precautions must be taken

effects : - accelerated slump loss

• increased rate of setting

• difficulty in controlling air content

• increased loss of moisture during curing

• increased risk of plastic shrinkage cracking

• higher concrete temp

• greater thermal variation within concrete

precautions when hot weather concreting

cool concrete materials

cool/protect handling equipment

cool/moisten forms

increase relative humidity of air

avoid hottest period of day for hydration

reduce time between mixing and curing

protect from wind

implement proper curing methods

use set retarding mixtures

use low heat of hydration cement

cold weather conditions

a period for more than 3 days when temp drops below 40°F

cold weather concreting

effects on fresh concrete

• accelerating slump loss

• decreased rate of setting

• increased risk of plast risk shrinkage bc realtive humidity so low and concrete is exothermic (+ light breeze)

• extreme damage if concrete allowed to freeze (1% of water when freezes → 9%)

precautions when cold weather

heat concrete materials

insulate forms

avoid coldest periods of day for hydration

reduce time btwn mixing and curing

proteect from wind (what steals heat)

use shelters/heaters to keep concrete warm (/!\ CO2)

use set accelerating admixture

use high early strength cement

properties of hard concrete

concrete = much lighter than steel

CAREFUL TO W/C RATIO ALWAYS

the faster you load it the more brittle it gets

hard concrete behaviour with creep