Portland Cement Concrete Module #10 Prepared by Dr. Randy R. Rapp July 2005.

Portland Cement ConcreteModule #10

Prepared by

Dr. Randy R. RappJuly 2005

2005, Randy R. Rapp 2

Cements and Admixtures

• Cements– Types I through V, p. 250– Special purpose, p. 251

• Admixtures– Accelerators and retarders– Plasticizers– Air entrainment– Water reduction– Corrosion inhibiters– Beware possible adverse interaction; a few % matter

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Temperature Effects

• Lower temperature slows hydration• Higher temperature speeds hydration• Water or aggregate can be artificially

heated or cooled• Freezing during first day or two is

devastating• Shrinkage may exceed tensile strength:

saw joints

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Water: Strength vs. Slump

• Little water for hydration; most for workability• Water-cement ratio, w/c:

– Extremely critical for strength: lower is better– Determined mainly by amount water/sack cement– Δ1 gal/sack cement = Δ500 to 1,000 lb/in2 (psi)

• Slump– Indicates (does not measure) consistency– Match to formwork and reinforcement configuration– Determined mainly by amount of water per yd3

– Δ1 gal/yd3 concrete = Δ1-in slump

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Steel Reinforcement

• Types– Bars– Welded wire fabric (WWF)

• Accessories– Bolsters and chairs– Ties and clips

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Inspection Before Placement

• Plant– Consistency– Mix materials: tests, p. 264– Batching and mixing equipment– Pre-stressed members– Reports, p. 268

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Inspection Before Placement (Cont’d)

• Site– Reinforcement

• Surface condition• Strength• In-place configuration and fastening

– Foundation, Table 10-9– Formwork, Table 10-10

• Commercial, reusable systems• Job-built

– Timely correction is critical

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Inspection During Placement

• Checklist, Table 10-12• Delivery: adequate trucks, tools, labor?• Placement

– Methods– Delivery tickets

• Pumping, pp. 280-81• Construction joints

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Weather Extremes

• Heat > 90oF– Time of day– Crushed ice replaces water lb for lb– Enclosure?

• Cold < 40oF– Heat components– Enclosure

• Not only temperature, but also gradient from time rate of internal temperature change

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Concrete Tests

• Pass, substantially comply, or fail

• Random, representative samples

• Sample from middle 70% of batch

• Waste tested concrete

• Make sure number and type of tests are specified

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Statistics in Quality Control

• Specifications can use statistics to derive target values that should be achieved.

• Tolerances can be based on variations in materials, testing, processes, and sampling.– They consider all variable factors that

influence outcomes.– They can be realistic and enforceable.

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Randomness of Testing

• Obtained by purposeful action– Not haphazard selection– Not merely without intentional bias

• Each part of a batch, each lot, should have same chance of being chosen as any other.

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Randomness of Testing(Cont’d)

• A lot is a prescribed and defined quantity of material from the same process, for the same purpose.– All sampling and testing requirements are in relation

to the lot.– Must establish lot size to determine proper sampling

location and frequency.– Sublots, e.g., for paving batches

• keep sampling and testing spaced out more evenly• reduces chance of extended periods without testing, such as

start and finish periods.

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Slump Test

• Three layers of equal volume, not height

• Measure to center of slumped sample, not highest point

• Changes in slump indicate changes in consistency: find out why

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Air Entrainment Test

• Type A (shown) and Type B meters

• Consolidate to remove voids

• Air pressure collapses entrained air

• Volume change is correlated to bowl volume to find % air

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Yield Tests

• Sample introduced into 0.5-ft3 (≤ 1.5-in aggregate) or 1-ft3 (≤ 3-in aggregate) mold

• Three equal volume layers: rod 25 times in smaller mold, 50 in larger

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Cylinder and Beam Tests

• Cylinders (6-in x 12-in) for compressive strength

• Beams (6-in x 6-in x 20-in) for flexural strength

• Larger molds if aggregate >2-in

• Two samples from middle of batch

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Inspection During Placement (Cont’d)

• Consolidation– Spading or rodding– Vibration

• Finishing– Floating– Trowelling

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Inspection After Placement

• Checklist, Table 10-14

• Curing

• Stripping and reshoring

• Protection

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Records and Reports

• See pp. 300-301