4 - Operations for Concrete

Placing Issues

Drop Height: < 3 ft Horizontal movement: Limit to prevent segregation

Curing affects:

Durability, Strength, Resistance to freezing & thawing, Volumetric stability, Resistance to de-icing chemicals

Slump

Ease of placing, consolidating, & finishing Highest slump with no segregation or excessive bleeding * coarse aggregate migrates to bottom & water migrates to top

Concrete properties

Ec = 2000 - 6000 ksi Poisson's ratio, v = 0.11 - 0.21 ACI building code: american concrete institute

Lightweight concrete

* Floating concrete (ASCE concrete canoe) * Costs more by need less because of reduced weight

Cold Weather Concerning

* Freezing before concrete has achieved 500 psi will result in ultimate strengths 50% lower than reference * extended set times * slow strength gain

High - Strength Concrete

* Greater than 6000 psi to roughly 20,000 psi * Supplementary cementious materials are required to reach higher yield strengths * Lower water content, required for higher strength, results in a stiff, unworkable mixture when wet. To compensate, water reducing admixtures or high-range water reducing admixtures (super plasticizers) are used to improve workability Specially formulated, ultra-high performance concretes have compressive strengths as high as 30,000 psi

High-strength Concrete

* at least 6,000 psi strength with normal weight aggregates * very low w/c with superplasticizers up to 20,000 psi

Method Selection Considerations

* avail. of curing materials * size and shape of structure * production facilities (in-place or precast) * economics

Determine proportions of concrete mix ingredients that will:

* be economical * be practical * use available materials * satisfy requirements & specifications * acceptable workability of fresh mix * quality (durability, strength, appearance) of hardened concrete * Economy Depends on project size

Self-Healing Concrete (Smart Concrete)

* bendable concrete * self-healing concrete with the use of bacteria

How to combat HOT weather

* cooling concrete materials * wetting aggregate stockpiles * cooled water * replace portion of water with ice * wetting forms, steel, subgrade, and equipment * Avoid long term transportation times & prolonged mixing * use of retarding admixtures * use of higher levels of fly ash

High Performance Concrete

* high strength sacrifices other properties * by using special aggregate gradation, admixtures, and techniques we can improve several properties at once (workability, strength, toughness, volume stability, and exposure resistance)

Self-Consolidating Concrete (SCC)

* highly flowable, nonsegregating concrete * can spread into place, fill the formwork, and encapsulate the reinforcement, without any mechanical consolidation

Approach 2 - Seal the Surface

* impervious paper of plastic sheets * membrane forming compounds * Leave forms in place

Using water to combat hot weather effects:

* increased water-content ratio * decreased strength * decreased durability * nonuniform surface appearance * increased drying shrinkage

Fiber-reinforced Concrete

* instead of rebar (for corrosion) -becoming more common * possibly flexural strength increased by up to 30% * Steel, nylon, glass, etc. * Control cracking due to plastic and drying skrinkage * Reduces workability * Limited to a max. of 2% by volume of the mix due to workability problem

Approach 3 - Heat

* insulate * steam * good for early strength gain & in freezing winter * heating coils, electrically heated forms or pads * usually in precast plants only

Heavyweight concrete

* massive walls for nuclear, medical, and atomic shielding * very heavy weight aggregates (barite, magnetite, hermatite, lead, steel)

Order of Operations for Concrete (must be in certain order)

* mix design (proportioning) * Trial mixes & testing * Batching ** start the clock * mixing * transporting * Placing * Vibrating (consolidating) ** initial set here * finishing ** Final set here * curing * maintenance

Compressive Strength (f'c) test

* most common test by far * 2:1 cylinders are cast in 3 layers rodded 25 times each layer and cured at 95% humidity. * or specimens are cored from finished structure * 7 day = 60% of 28 day and 28 day = 80% ultimate strength * typical compressive strength is 3000-6000 psi

Approach 1 - Maintaining Presence of Water

* must water periodically * also provides cooling Methods: * ponding: smaller jobs flat-work (floors & pavement) and laboratory * Spraying or fogging: expensive and a lot of water * Wet coverings: burlap, cotton, rugs, etc.

How to combat COLD weather

* portable heaters * enclosed area * insulating forms * using type 3 cement * Adding 100-200lbs portland cement * chemical accelerators

Why use higher strength concrete

* reduce column dimensions in tall buildings * Achieve higher earlier strength, allowing construction to proceed more quickly * Satisfy more stringent structural requirements

Factors that affect water demand

* smaller aggregates increase water demand * angular shaped aggregates increase water demand * higher slumps require more water * water reducing admixtures reduce the water required * increased entrained air decreases the water demand * higher cementious contents require more water * higher ambient temperatures increase required water

Finishing Concrete

* smoothing and imprinting the surface of the concrete with the desired texture * must be completed before final set * many types of colors and textures available * stamped concrete uses rubber stamps to create the look of tile, stone, etc.

Polymer Concrete

* very quick set ( 1 hr) or super high strength (>20,000 psi) * not widely adopted yet due to high cost, recent progress has led to reduction in cost, meaning use of polymer concrete is becoming more widespread * polymer-PC concrete * latex is mixed with portland cement

Normal Strength Concrete

- Up to 6000 psi compressive strength - Made with conventional ingredients

Moisture Corrections

Adjust the weight of water and aggregates to account for the existing moisture content of the aggregate * Wet aggregate weighs more than dry aggregate (we used dried condition) * We assumed SSD and must adjust free mix water if not SSD

Increase slump with:

Admixtures, rounded aggregate

Curing & curing period

Curing: immediately after final set to avoid surface damage Curing period: * Min. 7 days * 70% of f'c (3 days for early strength) * other job requirements

Strength Requirements

Design engineer specifies a strength of concrete used for design calc. (f'c) Concrete strength is variable If material engineer provides a material with an avg strength equal to the strength specified by the designer, then half the concrete will be weaker than specified strength Material engineer designs concrete so only a small proportion of concrete will have a strength less than the strength assumed by design engineer ** see Slides 7 & 8 in lecture 4

Consolidation (compaction)

The elimination of voids and air pockets within the concrete pour * hand rodding or tampering * screeding * internal vibration * external vibration Consolidation is esp. critical w/ stiff concrete mixes or when concrete is placed around densely packed reinforcing arrays Over-consolidation must be avoided, as it can lead to segregation of aggregate as larger particles descend and finer components rise to toward the surface

Split Tension Test

To measure tensile strength * about 10% of f'c

Non- destructive Tests : Ultrasonic Pulse Velocity

Transmitter, receiver, and clock * piexoelectric crystals Velocity = distance / time * faster = more dense like a RR track * cracks and weak spots are slower Usually only used for finding cracks and discontinuities

Slump

Workability is measured by slump test * fill a cone in 3 layers, 25 rods each layer * lift cone off and measure distance it slumps from original height

Ready mixed

in a central plant and delivered in an agitator truck (2-6 rpm)

Truck mixed

mixed completely in a mixer truck (4-16 rpm) MAX 90 MIN FROM START OF MIXING TO DISCHARGE, EVEN WITH RETARDERS

Absolute volume method

most accurate, using specific gravity of each ingredient

Weight method

relatively simple design method, using an assumed or known weight of concrete

Water Content

For a given slump it depends on max size and shape of aggregates Consider SSD condition

Stress-strain relationship

Increasing water-cement ratio decreases both strength (f'c) and stiffness (E) Stronger concrete is more brittle Almost linear at small strains

Curing Concrete

Maintain moisture and temperature in the concrete to promote continued hydration and strength gain Hydration will resume if curing is stopped & resumed

Non- destructive Tests : Rebound Schmidt hammer

Measures energy absorbed by concrete * hardness of surface - correlated to strength Not very accurate * avg. 10-12 readings in one area

Batching

Measuring correct proportions of components and placing in the mixer By weight is more accurate bc air voids don't matter

Water Cement Ratio

Most Important determinant of concrete strength. Lowering the proportion of water to cement: * Increases concrete strength & durability * Decreased workability * Increases cost W/C ratio is measured by weight, not volume Extra water is needed for workability but cause voids * Decrease strength, durability, and bond between concrete & rebar * Increase permeability When supplementary cementitious materials are added to the mix, the ratio is measured at the Water cementatious materials ratio

Shrink Mixed

Partially mixed in plant and delivered in a mixer truck (4-16 rpm)

Nature of Particles (shape, texture, porosity) - (Coarse aggregate requirements)

Round shape and smooth texture improve workability (or less water & cement) check max aggregate size (use smallest)

Water-cement ratio check for maximum allowed

Severe exposure conditions require lower w/c ratios Use lowest w/c ratio of all applicable conditions * Exposure conditions * Sulfate conditions Increase water-cement ratio --> compressive strength decreases

Strength test Cylinders

Test cylinders may be cast from each batch of concrete delivered to construction site. * test cylinders - ASTM C30 * Most common cylinder size is 6 in diameter * 12 in high * Good for aggregates up to 2" nominal max. size * Aggregate larger than 2" diameter must be 3* higher and height should be twice the diameter Concrete is placed in a cylindrical mold and consolidated to eliminate voids

Sampling & Testing

Test on site - slump, air content Prepare samples for later testing - cylinders, beams

Arbitrary Volume Method

1:2:3 = PC : fine aggregates : coarse aggregate

Curing Approaches

1. maintain presence of water in the concrete 2. seal the surface so mix water can't escape 3. heat & additional moisture

Trial Mixes

Check Proportions with trial batches * air content, slump, 28 day compressive strength: 3 cylinders - 6" dia. * 12" H Adjust for optimum workability & economy

Mixing

Until Uniform appearance usually batch mixers (once at a time), but sometimes continuous (conveyors automatically feed components into mixer) Usually start with 10% of the water in the mixer, then solids with 80% of the water, and then remaining water

Gradation & Max. Size (Coarse aggregate requirements)

Use large and most dense gradation for better economy * large aggregate improves the workability (or less water & cement)

Concrete Strength

Varies with design of concrete mix