Concrete Thumb Rules

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Site Cast Concrete:

Contents:

Structure

Spans

One Way Systems

Two Way Systems

Roof

Column

Wall

Foundation

Joints

Additives

Functions

Thermal Mass

Insulated Panels

Sound Transmittance

Aesthetics

Exposed Aggregates

Terrazzo

Translucent

Formwork

Structure:Structure:Structure:Structure: Concrete is very stable in compression but brittle in tension. Due to steel's

coefficient of thermal expansion being nearly alike, concrete is typically reinforced with

steel in order to give it the tensile strength it lacks. Likewise, the concrete may be

reinforced by natural or synthetic fibers. It is inherently fireresistant. Concrete is typically

150 pcf, lightweight concrete may weigh 85 to 115 pcf, and insulating concrete <60 pcf.

Concrete gains its ultimate strength 28 days after placement.

Spans:Spans:Spans:Spans:

One Way Systems:

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04-Aug-14http://www.arch.ttu.edu/courses/2013/spring/3502_perl/Dobry/02/Default.htm

Beam and Slab suitable for spans from 6' to 18'. Concrete beams depth in

increments of 2".

Rule of Thumb Slab: floor depth = span/30 (4" minimum) roof depth =

span/36. Depth = span/16. Width 1/3 to 1/2 of depth (of 2" or 3"

increments) and greater than the width of the supporting column

Slab and Joist suitable for spans from 15' to 36'. A collection closely spaced ribs

supported by parallel beams. One distribution rib required for spans ≥20'.

Rule of Thumb slab depth span/24 (3" to 4 1/2"). Joist width 5" to 9". Joist

spacing based on molds of 20" to 30" widths and 6" to 20" depths (of 2"

increments).

Beam and Slab Slab and Joist

Two Way Systems:

Slab and Beam slab cast integrally with supporting beams and columns on four

sides. Most efficient when square/nearly square bays. Suitable for heavy loads

and spans from 15' to 40'.

Rule of Thumb slab depth = slab perimeter/180 (4" minimum). Ribs based

upon molds of 19" to 30" widths and 8" to 20" depths (of 2" increments).

19" width molds with 5" rib widths create 2' module, whereas 30" width

molds with 6" rib widths create a 3' module.

Flat Slab slab with reinforced column supports. Suitable for heavy loads and 20'

to 40' spans.

Rule of Thumb slab depth = span/36 (6" to 12"). Slab depth around

column 5/4 * slab depth. Column reinforcement width 1/3 span.

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Flat Plate slab reinforced in two or more directions and supported by columns

without beams or girders. Suitable for 12' to 24' spans.

Rule of Thumb slab depth = span/33 (5" to 12".)

Waffle Slab concrete slab with reinforcing ribs in two directions. Suitable for

spans from 24' to 54'. Can be cantilevered in two directions up to 1/3 of the

main span.

Rule of Thumb slab depth = span/24 (3" to 4 1/2"). Rib width 5" to 6".

Reinforcement a minimum of 1" spacing. A minimum of 1 1/2" from exposed

surface. 3/4" bevel or chamfer.

Pre and Post tensioning stressing of reinforcing bars to compensate for an

expected load.

Slab and Beam Flat Slab

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Flat Plate Waffle Slab

Roof slope of 1/4" : 1' (1:50) for drainage.

Columns:

Rounded 10" diameter minimum.

Rectangular 8" width minimum and 96in² minimum.

Reinforcement 1 1/2" cover minimum.

Rule of Thumb 12"column can support 2000 sqft floor/roof area | 16" column

can support 3000 sqft floor/roof area | 20" column can support 4000 sqft

floor/roof area.

Walls:

Minimums: 6" for bearing walls (or 1/25 of unsupported height or length

between stiffening elements) | 4" nonbearing (or 1/36 of unsupported length or

height) | 2" nonbearing interior partitions | 6" unreinforced (height to thickness

ratio < 22) | 8" basement/foundation/party walls

Reinforcement 3/4" cover when not exposed to ground or weather; 1 1/2" when

exposed. Extend 24" beyond corners of fenestrations.

Foundations:

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Strip Footings a continuous footing supporting a shear wall.

Isolated Footing an individual footing

Continuous Footing a footing that supports multiple columns.

Mat a thick slab that serves as a monolithic footing for use with soils with low

bearing capacities. Minimum of 4" thickness.

Floating when the weight of the excavated soil is equal to the construction

weight.

Reinforcement steel reinforcing bars in concrete footings should be placed a

minimum of 3" from the bottom of the footing and a minimum of 6" from the top

of the footing.

Piling a group of friction piles that are driven to suitable or stable soil/bedrock

and capped at a footing in order to transfer building load into the ground. Cap

should be located below the frostline.

Reinforcement should be placed a minimum of 3" above the tops of the

piles and be a minimum of 12" below the top of the footing.

Caisson a large auger with a belled base bored into the ground and filled with

concrete. A minimum of 2' 6" width to inspect the bottom. The bell is a 60° cone

below the main shaft.

Strip Isolated Continuous

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Floating Piling Caisson

Joints:

Expansion joints allow for movement between slab and column or wall

Construction joints end of a pour, may be keyed or doweled; also function as

expansion and control joints

Control joints create a weak point in the concrete to control where cracking

occurs. Should be placed 15' 20' and to create rectangular sections. Typically

1/8" wide and 1/4th the depth of the slab.

Isolation Control

Additives:

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Chemical:

Air Entrained reduces damage from freezethaw cycles (reduces

compressive strength 1% air to 5% compressive strength loss).

Accelerators speeds up hydration of concrete.

Retarders slow hydration of concrete for large pours.

Surfactants aids mixing of water.

Plasticizers increase workability of concrete. Superplasticizers have fewer

detrimental properties than typical plasticizers.

Pigments changes color of concrete.

Corrosion Inhibitors minimizes corrosion of steel in concrete.

Mineral:

Flyash coal byproduct. Partially replaces cement (60% of mass). May be

pozzolanic or hydraulic.

Ground Granulated Blast Furnace Slag steel byproduct. Partially replaces

cement (80% of mass). Hydraulic.

Silica Fume silicon/ferrosilicon byproducts. Particle size 100 times

smaller than flyash resulting in an accelerated pozzolanic reaction.

Increases strength and durability of concrete; typically requires

superplasticizers. Produces a dark gray or black coloring.

High Reactivity Metakaolin similar strength and durability increase to

silica fume. Produces a white coloring.

Functions:

Thermal Mass: Due to the density of concrete, it will delay the transmission of heat

through its thickness. About ten inches of concrete will have a time lag of seven

hours.

Heat Flushing in order for the concrete to cool and absorb heat the next day it

must be flushed of heat (commonly done as a night flush).

Sound Transmittance due to the relative density of concrete, it has low transmittance

of airborne soundwaves.

Thermal Resistance sand/gravel aggregate .08 R/inch | lightweight aggregate .6

R/inch.

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Insulated Concrete Forms R20. In comparison to framed buildings, ICF has reduced

energy loss through conduction and convection, paired with a high thermal mass,

producing a low flux of interior temperatures.

ICF

Aesthetics:

Exposed Aggregates produced by sandblasting, acid etching,

or scrubbing to remove the outer layer of cement paste. May

use recycled glass as a finish material.

Translucent embedded fiber optics allow the penetration of light.

Formwork exposed surface will retain the shape of the

formwork. Wood grain may be impressed on the surface via sandblasted plywood,

sheathing lumber to produce impressions, etc.

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Treatments paints/dyes | sandblasted/polished surface | bush/jackhammered for

courser texture

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Concrete Thumb Rules

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