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CGN3501 Civil Engineering Material Lab Preparation and Evaluation of a Concrete Mixture

Laboratory Exercise #2

Preparation and Evaluation of a Concrete Mixture

Introduction: When making Portland cement concrete, there are two sets of criteria that must be considered: i) Short-term requirements while the concrete is still in the plastic state (fresh concrete), generally lumped together under the term workability. ii) Long-term requirements of the hardened concrete such as strength, durability and volume stability. The most important parameters for both sets of requirements are the total water content of the mix and the water/cement (W/C) ratio. The workability of the concrete is primarily a function of the total water content, increasing as the water content increases. If, as is commonly (though incorrectly) done, the water content is increased to improve workability without a proportional increase in the cement content, the W/C ratio will also increase, resulting in a decrease in both strength and durability. The ultimate strength of properly cured concrete is almost entirely a function of the W/C ratio since it is the W/C ratio that determines the porosity of the concrete. Purpose: This laboratory exercise will be broken into two separate components. The first part involves concrete mixing and subsequent evaluation of the fresh concrete properties. The second part will examine hardened concrete properties. The purpose of this laboratory exercise is to introduce the student to proper concrete mixing procedures, the evaluation of fresh concrete properties and the determination of hardened concrete properties. An emphasis will be placed on the W/C ratio and its effect on concrete strength.

1 University of Florida ­ Department of Civil and Coastal Engineering

CGN3501 Civil Engineering Material Lab Preparation and Evaluation of a Concrete Mixture

Part A : Fresh Concrete Properties Concrete Mixing 1) Lay out twelve (12) 4" x 8" cylinders molds 2) Weigh out the following quantities of materials for the mix: · · · · Cement Water Fine Aggregate Coarse Aggregate : : : : 54 21 62 73 lb lb (W/C 0.40) lb lb

3) Rinse the mixer with water. Drain excessive water from the mixer after rinsing. Place all of the coarse and fine aggregates in the mixer and run for 30 seconds. Stop the mixer and add the cement. Run the mixer for another 30 seconds. Stop the mixer and add the water. Turn the mixer on and allow it to run for 3 minutes, followed by a 3 minute rest period. Turn the mixer back on for a final 2 minute mixing period. 4) After mixing is complete, shut the mixer off. Measure the slump, unit weight and air content of the fresh concrete as per the instructions provided hereafter. Record the results. Cast 9 - 4" x 8" cylindrical specimens by placing the concrete in 2 equal layers. Consolidate each layer either by rodding them 25 times or using the vibrating table. (After casting the second layer, the top surface of each specimen should be finished by striking off with the rod or trowel in an attempt to produce the flattest surface possible). Note: the fresh concrete used in the slump and unit weight tests should be returned to the mixer after the cylindrical specimens have been cast. The concrete used in the air content test must be discarded. 5) Add 1.5 to 2.5 lb of water (W/C 0.7), as instructed by the lab instructor, to the concrete remaining in the mixer and mix for another 3 minutes. After mixing is complete, shut the mixer off. Measure the slump and unit weight of the fresh concrete and cast 3 - 4" x 8" cylindrical specimens as described in Step 4. 6) Mark all cylinder molds so that the hardened concrete specimens can be identified and numbered after removal from the molds. Cover the molds with a polyethylene sheet to prevent water evaporation. 7) After at least 20 hours, remove the hardened concrete cylinders from the molds (make sure the T.A. is present during this operation and follow all instructions carefully). Mark the concrete cylinders so that your group number and the respective W/C ratio can later be identified. Place the cylinders in the ponding bin containing lime water. Testing of these cylinders will be performed 28 days after casting. 8) For the lab report, calculate the actual W/C of the two batches of concrete produced, based on the actual weights of the water used and adjusted by the actual moisture contents of the aggregates used.

2 University of Florida ­ Department of Civil and Coastal Engineering

CGN3501 Civil Engineering Material Lab Preparation and Evaluation of a Concrete Mixture

Slump Test for Concrete (ASTM C 143) Record all data on Data Sheet, page 8 1) Dampen the slump cone and place it on a flat, moist, nonabsorbent surface. A special base plate will be available in this lab. 2) Fill the slump cone in three layers, each approximately 1/3 of the volume of the cone. 3) Rod each layer with 25 strokes of the tamping rod uniformly distributed over the cross section of the layer. Rod the bottom layer throughout its full depth. Incline the tamping rod slightly to reach the perimeter of the bottom layer. Rod the second and the top layer throughout their depth so that the strokes just penetrate into the underlying layer. 4) Strike off the surface of the concrete using the tamping rod. Remove any excess concrete from the side of the cone. 5) Lift the slump cone slowly in a vertical direction. Measure the slump by determining the vertical difference between the height of the cone and the height of the displaced concrete, measured with respect to the center of its top surface. 6) If the concrete topples sideways when the cone is removed, it indicates that the materials have not been uniformly distributed in the cone and the test should be redone. Unit Weight Test for Concrete (ASTM C 138) Record all data on Data Sheet, page 8 1) Make sure the measure is dry and clean. Weigh the empty measure and record the result. 2) Place the fresh concrete in the measure in 3 layers of approximately equal volume. Rod each layer with 25 strokes of the tamping rod evenly distributed over the cross section of the measure. Rod the bottom layer throughout its full depth. For the top two layers, the strokes should penetrate about 1 inch into the underlying layer. After each layer is rodded, tap the sides of the measure 10 to 15 times with the rubber mallet to close any voids left by the tamping rod and to release any large air bubbles that may have been trapped. 3) Strike off the top surface of the concrete and finish it as smooth and level as possible with a flat plate or the tamping rod. 4) Clean all excess concrete from the exterior weigh the container filled with concrete. 5) Determine the unit weight as per the directions on the data sheet.

3 University of Florida ­ Department of Civil and Coastal Engineering

CGN3501 Civil Engineering Material Lab Preparation and Evaluation of a Concrete Mixture

Volumetric Air Content Test for Concrete (Roller Meter) (ASTM C 173) Record all data on Data Sheet, page 8 1) Fill the bowl of the roller meter with fresh concrete in 3 layers of equal depth. Rod each layer with 25 strokes of the tamping rod uniformly distributed over the cross section of the layer. Rod the bottom layer throughout its full depth. Rod the second and the top layer throughout their depth so that the strokes just penetrate into the underlying layer. After each layer is rodded, tap the sides of the measure 10 to 15 times with the rubber mallet to close any voids left by the tamping rod and to release any large air bubbles that may have been trapped. 2) Strike off the top surface of the concrete and finish it as smooth and level as possible with a flat plate or the tamping rod. Wipe the flange of the bowl clean. 3) Place the top section of the meter onto the bowl and clamp it in place. 4) Insert the funnel and add water until it appears in the viewing window on the neck of the meter. Remove the funnel and adjust the water level using a rubber syringe until the bottom of the meniscus coincides with the zero mark. Attach and tighten the screw cap. 5) Invert and agitate the roller meter for about 60 sec. or until the concrete settles free from the base. With the neck elevated, roll and rock the unit for about 60 sec. with the neck tilted about 45º or until the air appears to have been removed from the concrete. Set the apparatus upright, shake it lightly, and allow it to stand until the air rises to the top. Repeat the operation until no further drop in the water column is observed. 6) Determine the air content by reading to the bottom of the meniscus to the nearest 0.1%. 7) If there are bubbles evident in the viewing window, add sufficient isopropyl alcohol (in 1 cup increments) to dispel the bubbles on the surface of the water. The amount of alcohol used is then added to the measured air content, with each cup corresponding to the volume for 1% air. Cleaning After the test specimens have been prepared, make sure all floor areas and equipment are cleaned. Place all excess concrete in wheel barrow and discard in the proper manner.

4 University of Florida ­ Department of Civil and Coastal Engineering

CGN3501 Civil Engineering Material Lab Preparation and Evaluation of a Concrete Mixture

Part B : Hardened Concrete Properties Strength tests on moist cured concrete must be performed while the specimen is in a fully saturated condition. All specimens can be removed from the ponding bin at the beginning of the lab period but should be covered with damp burlap or plastic to prevent excessive drying prior to testing. Splitting Tensile Strength (ASTM C 496) Record all data on Data Sheet, page 9 1) Select three specimens from the 0.40 W/C ratio group. Number each specimen and measure its dimensions (length and diameter). 2) Place a test specimen in the loading machine as indicated in Figure 1. Make sure the specimen is accurately centered under the loading head. 3) Slowly load the specimen until failure. Use a loading rate of 125 lbf/sec. Record the ultimate load attained. 4) Repeat Steps 1 - 3 for the remaining two specimens.

Load

Loading Platen Steel Bar Bed Plate of Test Machine

1/8" x 1" Plywood 4"x8" Concrete Cylinder Failure Plane 1/8" x 1" Plywood

Figure 1: Splitting tensile strength test for concrete. Reduction of Data 1) Calculate the splitting tensile strength of each specimen using the formula:

T=

Where: T = Splitting Tensile Strength (psi) P = Ultimate Load (lbf) L = Specimen Length (in)

2P LD

5 University of Florida ­ Department of Civil and Coastal Engineering

CGN3501 Civil Engineering Material Lab Preparation and Evaluation of a Concrete Mixture

D = Specimen Diameter (in) 2) Calculate the mean splitting tensile strength and the coefficient of variation for the entire group of three specimens. 3) Sketch the failure modes and note any irregularities. Compressive Strength (ASTM C 39) Record all data on Data Sheet, pages 9 1) Select three specimens from the 0.40 W/C ratio group. Number each specimen and measure its dimensions (length and diameter). 2) Place a neoprene cap on each end of a test specimen. Place the specimen in the loading machine as shown in Figure 2. Make sure the specimen is accurately centered under the loading head. 3) Load the specimen at a rate of loading of 500 lb/sec until failure. Record the ultimate load attained. 4) Repeat Steps 2 and 3 for the remaining specimens (0.40 W/C ratio) 5) Select other three specimens from the 0.40 W/C ratio group. Number each specimen and measure its dimensions (length and diameter). 6) Repeat Steps 2 and 3, but with a rate of loading of 1000 lb/sec, for this 0.40 w/c ratio group. 7) Select the three specimens with 0.70 W/C ratio group. Number each specimen and measure its dimensions (length and diameter). 8) Repeat Steps 2 and 3 for this group, using a rate of loading of 1000 lb/sec.

Load

Loading Platen Steel Bar

Neopren Cap 4"x8" Concrete Cylinder

Bed Plate of Test Machine

Neopren Cap

Figure 2: Compressive Strength Test for Concrete

6 University of Florida ­ Department of Civil and Coastal Engineering

CGN3501 Civil Engineering Material Lab Preparation and Evaluation of a Concrete Mixture

Reduction of Data 1) Calculate the compressive strength of each specimen using the following formula:

=

P A

Where: = Compressive Strength (psi) P = Ultimate Load (lbf) A = Area of the cross section of the specimen 2) Calculate the mean compressive strength and the coefficient of variation for each group of three specimens (i.e. for each W/C ratio and rate of loading).

CV =

Where: CV = Coefficient of Variation S = standard deviation of the sample = mean of the sample

S

3) Sketch the failure modes and note any irregularities. 4) Comment on the relationship between W/C ratio and strength and between rate of loading and strength. Note: you can bring a photo camera to the lab and certainly you can include those pictures in your report instead of the sketches, but make sure that the pictures are relevant in terms of showing failure modes and the scheme of the loading test. Also any picture included in the report has to be along with comments about it.

7 University of Florida ­ Department of Civil and Coastal Engineering

CGN3501 Civil Engineering Material Lab Preparation and Evaluation of a Concrete Mixture

Properties of Aggregates Used: Coarse Aggregate Dry Bulk Spec. Grav. ______________ Absorption ______________ Determination of Natural Moisture Content: Mass of pan Mass of pan + aggregate Before drying Mass of pan + aggregate After drying Mass of moisture % Natural Moisture ______________ ______________ ______________ ______________ ______________ _____________ _____________ _____________ _____________ _____________ Fine Aggregate _____________ _____________

Calculation of Actual w/c of Concrete: First Batch (Before addition of more mixing water) Computed w/c = (free mixing water)/ cement = _______________ Actual Mass Added (lbs) Cement Water Coarse Agg. Fine Agg. Adjusted Mass (lbs) (Agg. in SSD condition) Absolute Volume (ft3)__________

__________ ______________ _____________ (Free mixing water) __________ ______________ _____________ __________ ______________ _____________ __________ ______________ _____________ Sub total: _____________ Vol. of Air: _____________ Total volume: _____________

Volume of nine 4" X 8" cylinders: _____________ Volume of concrete (used in air content test) discarded: _____________ Volume of concrete left in mixer before addition of more water: _____________ After making of 9 cylinders Mass (lbs) (before addition of more water) Cement Free mixing Water ____________ ____________ Mass (lbs) (after addition of more water) ______________ ______________

Computed new w/c: _____________

8 University of Florida ­ Department of Civil and Coastal Engineering

CGN3501 Civil Engineering Material Lab Preparation and Evaluation of a Concrete Mixture

Fresh Concrete Properties Slump and Air Content W/C Ratio 0.40 0.70 Determination of Volume of Air-Content Test Container Mass of the Air Content container Mass of Air Content container filled with water Volume of Air Content container = = = lb lb ft3 Slump (ASTM C 143) Air Content (ASTM C 173)

Unit Weight Test Mass of the Unit Weight bucket (M) = Mass of bucket filled with water Volume of Unit Weight bucket (V) = = Unit Weight (ASTM C 138) Mass of Concrete and Bucket (lb) (B) Mass of Concrete (lb) W=B-M Unit Weight of Concrete (lb/ft3) = W/V lb lb ft3

W/C Ratio

0.40 0.70

9 University of Florida ­ Department of Civil and Coastal Engineering

CGN3501 Civil Engineering Material Lab Preparation and Evaluation of a Concrete Mixture

Hardened Concrete Properties Splitting Tensile Strength (0.40 W/C Ratio, 125lb/sec rate of Load)(ASTM C 496) Length Diameter Ultimate Load Tensile Strength Specimen (in) (in) (lbf) (psi) ID [L] [D] [P] [T] 1 2 3 ____ ____ ____ ____ ____ ____ ___ ___ ___ ___ ___ ___ ___ ___ ___ ____________ ____________ ____________ Mean Standard Dev. Coeff. of Var. ______________ ______________ ______________ ______________ ______________ ______________

Compressive Strength (0.40 W/C Ratio, 500lb/sec rate of Load) (ASTM C 39) Ultimate Load Compressive Strength Diameter Area Specimen (lbf) (psi) (in) (in2) ID [fc'] [D] [A] [P] 1 2 3 ____ ____ ____ ____ ____ ____ _______ _______ _______ ____________ ____________ ____________ Mean Standard Dev. Coeff. of Var. ____________ ____________ ____________ ____________ ____________ ____________

Compressive Strength (0.40 W/C Ratio, 1000lb/sec rate of loading) (ASTM C 39) Ultimate Load Compressive Strength Diameter Area Specimen (lbf) (psi) (in) (in2) ID [fc'] [D] [A] [P] 1 2 3 ____ ____ ____ ____ ____ ____ _______ _______ _______ ____________ ____________ ____________ Mean Standard Dev. Coeff. of Var. ____________ ____________ ____________ ____________ ____________ ____________

Compressive Strength (0.70 W/C Ratio, 1000lb/sec rate of loading) (ASTM C 39) Ultimate Load Compressive Strength Diameter Area Specimen (lbf) (psi) (in) (in2) ID [fc'] [D] [A] [P] 1 2 3 ____ ____ ____ ____ ____ ____ _______ _______ _______ ____________ ____________ ____________ Mean Standard Dev. Coeff. of Var. ____________ ____________ ____________ ____________ ____________ ____________

10 University of Florida ­ Department of Civil and Coastal Engineering

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Laboratory Exercise #2

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