ABS 321- CH. 13: Concrete Construction

What are the ingredients of concrete?

-Fine aggregate (sand) -Coarse aggregate (gravel) =together these provide the structural mass of the concrete and constitute the majority of the volume -Portland cement, binds the aggregate -Water, for the chemical hydration of the cement and the hardening of the concrete

advantages of air-entraining cement

-greater resistance to freeze-thaw damage -improved workability when wet but -reduced concrete strength, unless the proportions of other ingredients in the mix are adjusted to compensate

concrete for a exterior planter

-low-strength -good resistance to weather -smooth and pleasing finished appearance

in the us, what does a standard bag of cement contain?

1 cubic foot of material and weighs 94 lbs.

admixtures

Added ingredients that alter or improve concrete properties: Air-entraining Water-reducing Cure accelerating or retarding Workability modifying Shrinkage-reducing Corrosion inhibiting Freeze protecting (for cold weather concreting) Coloring

specified maximum slump

3-5 inches

When identically-sized particles are closely packed, the void space remaining between particles is approximately __________________________ percent of the total volume.

30 to 35

Specially formulated, ultra-high performance concretes have compressive strengths as high as

30,000 psi

Roman concrete date

300 bc

cement production accounts for what percent of worldwide CO2 emissions

5%

high-strength concrete

6,000-20,000 psi supplementary cementitious materials are req. to reach higher strengths Lower water content, required for higher strength, results in a stiff, unworkable mixture when wet. To compensate, water reducing admixtures are used to improve workability. Reduce column sizes in tall buildings. Achieve higher earlier strength, allowing construction to proceed more quickly. Satisfy more stringent structural requirements.

WWR designation example

6x12-W12xW5 W12 wires spaced at 6 in., crossed by W5 wires spaced at 12 in.

aggregates

Coarse and fine aggregate make up 60 to 80 percent of the total concrete volume. Most aggregate comes from natural sand and gravel deposits or is made from crushed rock.

Consolidating Concrete

Compaction eliminates voids and air pockets within the concrete. Hand rodding or tamping Screeding (top) Internal vibration (bottom) External vibration

curing concrete

Concrete hardens through chemical bonding of water and cement. If concrete dries out prematurely, the hydration process stops and maximum strength is not achieved. Hydration, along with increasing strength and durability,

formwork

Construction, usually temporary, to hold freshly poured concrete in the desired shape until the concrete gains sufficient strength to support itself helps to protect newly poured concrete from drying too quickly must be strong enough can account for half or more of total costs of concrete construction designing the formwork itself

slump test

A rough measure of the workability of wet concrete as it arrives on site. Concrete is placed into a conical cylinder; the cylinder is removed, and the loss in height of the concrete mass is measured.

Vermiculite:

Thermally expanded mica

Perlite:

Thermally expanded volcanic glass

ASTM C150 Cement Types

Type 1: general purpose Types II and V: for concrete in contact with soils or water with high sulfate concentrations

Portland cement

a hydraulic cement made from a finely ground mixture of limestone and clay materials are extracted from many sources (limestone, chalk, marble, etc)

air-entraining cement

added ingredients generate bubbles during concrete mixing that create small, distributed voids in the finished concrete air content in the range of 2-8% of the total concrete volume is typical

what happens in the finished concrete?

aggregate particles are surrounded by the hydrated cement and bound into a solid mass

when a concrete beam is prestressed

all the concrete acts in compression the off-center location of the prestressing steel causes a camber in the beam

where is most concrete prepared

at batch plants and delivered to the construction site in transit mix (ready mix) trucks

reinforcing

bars or wires laid into concrete along lines of tension, to provide resistance to these forces

ties prevent the vertical bars from

buckling outwards

Hydraulic cement

cement that chemically combines with water and when hardened, is no longer water soluble knowledge lost with fall of roman empire

vertical bars add strength in

compression and resistance to buckling or bending

reinforcing fabrication

cut to length, bent, and possibly partially assembled by fabricator before transportation to the construction site

Exposed surfaces of newly poured concrete must be protected from

evaporation and drying

Type III Cement Type

for cold weather construction, concrete precast in plants, accelerated construction schedules

when concrete is crushed and recycled, it is capable of absorbing up to _________________ of the CO2 originally generated in the cement manufacture

half

concrete strength

has no useful tensile strength (resistance to bending/stretching forces)

Simple concrete beam reinforcing includes

heavy bottom bars, lighter top bars, and U-shaped shear stirrups

what to do to moderate the temp of concrete in very hot weather

ice or a retarder

why must over-consolidation be avoided

it can lead to segregation of aggregate as larger particles descend and finer components rise to toward the surface

When normal service loads are added, the prestressed concrete is subject to

less tensile force

in a reinforced concrete beam

less than half the concrete is in compression and cracks will appear in the bottom beam under full load

The placement of reinforcing in a concrete beam approximates the

lines of tension, but is simplified to reduce fabrication costs.

Type IV Cement Type

massive structures such as dams, where the heat generated during hydration must be limited to avoid excessive temps.

how is Portland cement processed?

materials are extracted from earth transported crushed, ground, proportioned blended into a very fine powder

pozzolans

materials that react with hydration byproducts in wet concrete to form additional hydraulic cementing compounds require the presence of other cementing ingredient with which to react are not, on their own, hydraulic cements

too low slump

may be difficult to place

artificial aggregates

may come from blast furnace slag, fly ash (right), recycled concrete, thermally treated clay, shale, and other minerals.

too high slump

may have too much water added

reinforcing bars

most common reinforcing material Rebar are hot-rolled steel, deformed with surface ridges so as to bond with concrete.

rebar sizing

numbers correspond to diameter in 8ths of an inch #4 is 1/2" #8 1"

form release compounds

oils, waxes or plastic coatings applied to formwork surfaces to prevent adhesion of the formwork to ease removal

what to do to moderate the temp of concrete in very cold weather

pre-heating or accelerator

welded wire reinforcing (wwr)

prefab welded grids on reinforcing bars or wires common for slabs

how to prevent evaporation and drying of concrete during the curing time

regularly misted, covered with a moisture-retaining material, or treated with a chemical surface sealer (may cause discoloration or swirling)

manufacturing 1000 lbs of Portland cement generates on average, how much CO2?

roughly 900 lbs of carbon dioxide

steel and concrete work well together because

similar rates of thermal expansion/contraction (steel is the only metal that will do this) alkaline chemistry of concrete protects the steel from corrosion the two materials bond well, allowing them to work as a single structural composite

Prestressed members are more _____________________ compared to conventionally reinforced members.

slender and lighter

how to reduceCO2 emissions in concrete

substitution of industrial waste products with cementing properties (fly ash, silica fume, blast furnace slag)

carbon dioxide emissions

the burning of fuel to heat the cement kiln generates vast quantities of CO2 the chemical conversion of limestone (calcium carbonate) within the kiln releases additional large quantities of CO2

What is the other key component of modern concrete?

the embedding of steel reinforcing to increase tensile strength and resilience

reinforcing a simple beam

the greatest tension forces occur at the bottom middle of the beam shear forces occur where tension forces cross compression forces closer to the beam ends

under loading

the prestressed beam becomes flatter but all the concrete still acts in compression and no cracks appear

Where the vertical bars extend beyond the ties..

they are bent slightly inward toward the center of the column

what happens to the blended Portland cement materials?

they are burned or sintered in a high-temperature, rotating kiln, creating clinker the clinker is cooled and ground to a fine powder

concrete strength

varies with design mix and ingredients normal strength: up to 6000 psi compressive strength made with conventional ingredients

reinforcing a column

vertical or column bars: larger-diameter bars placed vertically in the column (common) ties or spirals: wrap around the vertical bars

An optimum balance of characteristics is achieved in concrete mixes with

void volumes somewhat greater than the least possible

maximum aggregate size

with larger aggregates, less cement is required, reducing cost the largest aggregate must fit comfortably btwn reinforcing bars and w/I the overall thickness of the pour

concrete quality

workability when wet: ease of placing, consolidating and finishing structural properties when hardened: strength, stiffness, durability, cost

water

Essential ingredient Combines chemically with the cement as the concrete hardens. Water must be free of contaminants. Water that is potable is acceptable for use in making concrete. Waste water from washing out concrete trucks or runoff at concrete production facilities is also permitted. The quantity of water in the concrete mix must be controlled as closely as any other ingredient: Adding unneeded water weakens the hardened concrete.

reinforcing erection

Fabrication continues on site. Eventually reinforcing is assembled in its final configuration. After inspection by the engineer, concrete may be poured

nonstructural lightweight concrete

For insulating roof toppings and fill material Roughly 60 percent or less of the weight of ordinary concrete

whose reinforced patent system is used today?

Francois Hennebique in 1892

aggregate size distribution

Grading: Distribution of aggregate particle sizes Generally, a lower percentage of void space between particles results in a stronger concrete that requires less cement.

macrofiber reinforcing

Greater concentrations of fibers, that also resist longer-term cracking due to drying and thermal stresses Steel fiber reinforcing also increases the durability of the concrete surface

bars are made with varying strength

Higher strength bars are used to reduce rebar congestion, where reinforcing becomes crowded and concrete placement becomes more difficult. Use of higher strength steel allows for bars smaller in diameter or with greater spacing

90% of US manufactured cements are types

I,II or I/II

Water-Cement (W/C) Ratio

Important determinant of concrete strength. Lowering the proportion of water to cement: Increases concrete strength and durability Decreases workability Increases cost W/C ratio is measured by weight, not volume. When supplementary cementitious materials are added to the mix, the ratio is measured as the water-cementitious materials ratio.

prestressing

Initial force, applied to improve structural capacity. High-strength steel strands are stretched tightly and then restrained by the concrete, putting the concrete into initial compression.

Who patented Portland cement and when?

Joseph Aspdin in 1824

microfiber reinforcing

Low amounts of fibers, to aid concrete in resisting plastic shrinkage cracking that occurs during early curing

Supplementary Cementitious Materials (SCMs)

Materials added to concrete as a partial substitute for portland cement Increased concrete strength and durability Higher early strength Improved workability for wet concrete Reduced concrete drying shrinkage Reduced reliance on portland cement

Hydraulic cements:

Materials with intrinsic hydraulic cementing properties Hydraulic cements do not require the presence of other cementing materials with which to react so as to function as hydraulic cement.

Concrete that has not stiffened can have water added prior to placing, provided that:

Maximum w/c ratio is not exceeded Maximum slump is not exceeded Agitation limits are not exceeded

test cylinders

May be cast from each batch of concrete delivered to the site. Concrete is placed in a cylindrical mold and consolidated to eliminate voids. Cylinders are returned to laboratory, cured, and then strength-tested at appropriate times, to verify the quality of the concrete.

Expanded shale, clay, slate, slag:

Minerals or industrial waste products thermally treated such that they expand and take on a less dense, cellular structure

Cinders and other volcanic rocks:

Naturally occurring light-weight volcanic materials

SCM benefits

Over half of all North American concrete includes SCMs in the mix. Reduces reliance on portland cement, lessening energy consumption and greenhouse gas emissions. Reduce embodied energy of finished concrete (up to 1/3). Divert industrial waste from landfills.

Strength of concrete depends on aggregate:

Porosity Size distribution Moisture absorption Shape and surface texture Strength, elasticity, density, soundness Contamination or detrimental substances

structural lightweight aggregates

Roughly 80 percent or less of the weight of ordinary concrete Reduced structure weight saves costs Lower thermal conductivity increases resistance to building fires

high-strength concrete for very tall buildings

SCMs, for greater strength Water reducers, to increase concrete strength while maintaining workability Admixtures to improve pumpability Retarding admixtures, to allow adequate time for placing

segregation

Separation of large aggregate from the finer portions of the mix, must be avoided. Place concrete as close to final position as possible.

fibrous reinforcing

Short fibers of glass, steel (above), or polypropylene (below), added to the concrete mix will tend to reinforce slabs, not used for major structural elements

posttensioning

Steel strands are tensioned after concrete has been cast and reached adequate strength. Concrete cast on the construction site can be posttensioned.

prestensioning

Steel strands are tensioned before concrete is cast. This requires heavy abutments to restrain the strands and is normally only done with concrete in precasting plants, not on the construction site.

when did steel reinforced concrete first appear?

1850s

hydraulic cement was rediscovered in

18th c

when is concrete strength normally specified

28 days

cement production accounts for what percent of US CO2 emissions

1.5-2%

placing concrete

Avoid delays, during which concrete can stiffen and become difficult to place. Depending on conditions, concrete can placed up to 90 minutes after mixing commences.

Pozzolanic ash

Volcanic ash found at Pozzouli, Itlay; high in silica and alumina content and when mixed with water, hardens like stone even when submerged under water