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CE 331 Laboratory Experiment #14

Concrete Mixing and Strength Testing

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Concrete Mixing & Strength Testing

Objectives: 1)  Investigating the relationship between the

w/c ratio and compressive strength 2)  Investigate effect of air entrainment on

strength. 3)  Investigating the statistical nature of

concrete 4)  Visualize the difference b/t mixtures of

various qualities (appearance & behavior)

Mixing

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General Concepts

•  Slump (more water, more workability, more slump)

•  w/c ratio (the greater the w/c the lower the strength)

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Procedures •  Separate into 4 groups •  Each group will weigh and mix

3 batches by hand

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Procedures •  The slump of each mix will be

measured as per ASTM C-143 •  Each group will cast 3 samples in

the 3" by 6" cylinders provided as per ASTM C-192

Field Testing- Slump Test Procedure

•  To obtain a representative sample, take samples from 2 or more regular intervals throughout the discharge of the mixer or truck. DO NOT take samples at the beginning or the end of the discharge.

•  Dampen inside of cone and place on a smooth, moist, non-absorbent, level surface large enough to accommodate both the slumped concrete and the slump cone. Stand on foot pieces throughout the test procedure to hold the cone firmly in place

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Slump Test Procedure •  Fill cone 1/3 full by volume and rod 25 times with a 5/8-

inch-diameter x 24-inch-long hemispherical tip steel tamping rod (This is a specification requirement which will produce non-standard results unless followed exactly). Distribute rodding evenly over the entire cross section of the sample.

•  Fill cone 2/3 full by volume. Rod this layer 25 times with rod penetrating into, but not through, the first layer. Distribute evenly like before.

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Slump Test Procedure •  Fill cone to overflowing. Rod this layer 25 times with rod

penetrating into, but not through, second layer. Distribute evenly.

•  Remove the excess concrete from the top of the cone, using tamping rod as a screed. Clean overflow from the base of the cone.

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Slump Test Procedure •  Immediately lift cone vertically with slow, even motion.

Do not jar the concrete or tilt the cone during this process. Place cone next to, but not touching the slumped concrete. (Perform in 5-10 seconds with no lateral or torsional motion.)

•  Lay a straight edge across the top of the slump cone. Measure the amount of slump in inches from the bottom of the straight edge to the top of the slumped concrete at the point over the original center of the base. The slump operation shall be completed in a max. elapsed time of 2 ½ minutes. Discard concrete, Do not use in any other test.

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Procedures •  Samples will be removed after 24hrs and placed in the

curing room for the next week

Curing •  Concrete must be properly cured for optimum property development

–  3 key factors: time, temperature, and moisture –  With proper curing

•  Strength increases, less permeable, more abrasion resistant, and more durable

–  3 curing methods •  Add Water (curing bath of lime water, humidity chamber,

saturated mats, fogging) •  Water Neutral (Plastic and impervious membranes) •  Loses Water (accelerate curing by adding heat)

–  If stored in dry location, confining stress caused by capillary pressure in small pores of cement can artificially raise the breaking strength by 10% or more

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Materials Used Mix # w/c Portland

Cement Fine Aggregate

Coarse Aggregate (C.A) no.4 to 3/8

C.A. no. 3/8 to 1/2

C.A. no. ½ to ¾

Water

1 0.5 6.61 lb 3.00 kg

13.32 lb 6.04 kg

10.14 lb 4.6 kg

5.07 lb 2.3 kg

5.07 lb 2.3 kg

1500 ml

2 0.55 6.61 lb 3.00 kg

13.32 lb 6.04 kg

10.14 lb 4.6 kg

5.07 lb 2.3 kg

5.07 lb 2.3 kg

1637ml

3 0.6 6.61 lb 3.00 kg

13.32 lb 6.04 kg

10.14 lb 4.6 kg

5.07 lb 2.3 kg

5.07 lb 2.3 kg

1762ml

•  2 groups will mix plain concrete •  2 groups will have the same mix proportions plus air

entrainment

What to Hand In at the End of Class

•  Each group needs to hand in a table of their collected data, and a master sheet of all labs will be emailed to you

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Group # Mix Empty Full Slump # wt. (g) wt. (g) (in) 1 2 3

Group # Mix Empty Full Slump # wt. (g) wt. (g) (in)

1AE 2AE 3AE

The Amount of Air Added

•  Example

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