INSTRUCTION FOR CONCRETE-MIXING LAB

Dr. Sophia Hassiotis

In this presentation

Laboratory set up Personal Protective equipment Directions for mixing Slump test Cylinder casting Cleaning up your station Mix Design

Laboratory Set up

EQUIPMENT AND MATERIALS

Materials (cement, sand, aggregate, water, admixtures)

Field scale, slump cone Four buckets per station Mixing hoes and trough Two 4x8 cylinders Scoops, trowel, mallet,

ruler, rods, cleaning tools

Personal protective equipment

Boots or closed leather shoes

Safety glasses Apron Gloves Respirator mask

Directions for mixing

Add 80% of coarse and fine aggregate, and 80% of water --- Mix

Add air-entrainment admixture (use: 1ml for each pound of cement)

Add ALL cement--- Mix until cement is blended in

Directions for mixing (cont)

Add the rest of the coarse and fine aggregate

Add the rest of the water. Mix for a couple minutes,

let it rest for a couple minutes, mix again and perform the slump test

Add more water if needed to obtain a workable slump.

Record the actual amount of water used.

Slump Test

Dampen inside of cone and place on smooth, moist, non-absorbent level surface

Fill cone to 1/3 by volume (about 2 ½ inches.) Rod 25 times with 5/8”x24” tamping rod. Distribute rodding evenly over

entire cross section.

Fill cone to 2/3 by volume (about 6 inches). Rod the second layer 25 times with rod penetrating into but not through

the first layer. Distribute rodding evenly.

Fill cone to overflow. Rod the third layer 25 times with rod penetrating into but not through the second layer. Distribute rodding evenly.

Remove the excess concrete from the top of the cone using tamping rod. Clean overflow from base of cone

Immediately lift cone vertically with slow, even motion. Do not jar the concrete or tilt the cone during this process.

Invert the withdrawn cone and place next to, but not touching the slumped concrete

Lay the tamping rod across the top of the slump cone and measure the slump from the bottom of the rod to the top of the slumped concrete at the point of the original center of the base

NOTE 1: The slump test should be completed in a maximum of 2 and ½ minutes.

NOTE 2: Do not remove your feet from the sides of the cone till the test is completed.

NOTE 3: Make sure rod penetrates into the layer below

Cylinder Casting Place the 4x8 casting molds on the floor. Fill the mold to 1/2 full and rod the bottom layer with 25 strokes evenly

spaced. Consolidate the lift 10 to 15 times with a rubber mallet, your open hand or the

tamping rod. Fill to the top of the mold and rod the top layer with 25 strokes penetrating

the top of the second layer. Consolidate the lift 10 to 15 times with a rubber mallet, your open hand or the tamping rod.

Strike off the top surface of the concrete using a sawing action with the steel trowel. Take special care to smooth the surface.

Mark the cylinders with your group number and date.

Carefully move, using a trowel or hand spade, the cylinders to temporary storage.

Cover the cylinders with its cover or a plastic bag. After 20 to 48 hours remove the molds and place them in the water

bath. Transfer your identifying marks from the molds to the top of the

cylinders before placing them in the water bath.

Clean up your station

Clean up your station (cont)

NOTE: Do NOT use sink to wash. Use the minimum water possible and clean into the TWO COMMON buckets provided for ALL groups. Use one bucket to dump the concrete. Use the other as a cleaning bucket.

Procedure for Mix DesignACI 211.1-09 Standard Practice for Selecting Proportions for Normal,

Heavyweight, and Mass Concrete

Absolute Volume of a Material = Weight of Material / ( S.G. x 62. 4 lb/cf )

S.G. is the specific gravity of a material and 62.4 lb/cf is the Unit Weight of water.  

A bag of cement (94 lbs.) in a bulk state occupies approximately 1 cubic foot of volume.

If the cement was without voids it would occupy only 0.48 cubic feet.

Therefore, the volume the cement will actually occupy in a batch of concrete will be its absolute volume of 0.48 cubic feet.

EXAMPLE: Design a batch of 1.0 cubic yard (cy) of concrete for a continuous spread footing.

Design for a strength of 4000 psi.

Step 1: Choice of Slump

Use Table 6.3.1 to find an appropriate value. In this example, for footings the slump should be between 1 and 3 inches. Assume a slump of 3 for a mix that is more workable.

Step 2: Choice of max aggregate size

Use the largest that is economically available and consistent with the dimensions of the structure.

ACI recommends that maximum aggregate size be limited to 1/3 of the slab depth and 3/4 of the minimum clear space between reinforcing bars. 

In this example, we will assume a maximum size of 1.0 inch.

 

Step 3:Estimate mix water and air content

Step 3 (cont)

For our example, for a slump of 3 inches and a maximum aggregate size of 1 inch, use the air-entrained part of the table to find that you will need 295 lbs of water per cubic yard of concrete.

The fine print under the table specifies that the water should be reduced by 25 lbs if you are using well rounded aggregates (ex. Alluvial rock.) Therefore, in this example we will use 270 lbs of water.

The same table also shows that the recommended percent of air content is 6% for severe exposure

Step 4: Select w/c ratio

In this example, for a 4000psi, air-entrained concrete you should pick w/c = 0.48

Step 5: Calculate Cement Content

Weight of Water / Weight of Cement = 0.48

Water weight = 295 lbs - 25 lbs = 270 lbs

Therefore,

Weight of Cement = 270 lbs / 0.48 = 560 lbs

Step 6: Estimate Coarse Aggregate Content

Weight of coarse aggregate = 0.69 cy x (98.5 lbs/cy) x (27 cf/cy) =

1,840 lbs/cy

Using the definition of unit weight: Unit weight=Weight/unit volume

Step 7: Estimate fine aggregate content

MATERIAL VOLUMES VOLUME AND WEIGHT OF SAND

Remember:Absolute Volume of a Material =

Weight of Material / ( S.G. x 62. 4 lb/cf )

Definition of moisture Content

Solving for the Wet Weight

OR :

 

Adjusting for Aggregate Moisture

The weights that we calculate above are dry weights. To create the batch we will add into the mix wet aggregate weighing:

Coarse aggregate, Wwet = 1840 lbs x 1.0195 = 1875 lbs Fine aggregate, Wwet = 1135 lbs x 1.063 = 1206 lbs

  Absorbed water does not become part of the mixing water. However,

any free water will add to the water content so we must account for it.  The surface water contributed by the coarse aggregate is 1.95% -

1.0% = 0.95% The surface water contributed by the fine aggregate is 6.3% - 1.1% =

5.2%  The estimated requirement for added water, therefore, becomes

270 lbs – 1840 lbs x (0.0095) - 1135 lbs x (0.052) = 193 lbs

Instructions for your mixBefore you come into the lab you should design your mix for 0.25 cubic feet of concrete.

Use the following materials.

Instructions for your mix (cont)

EACH PERSON in the group must calculate the required mix. Then, please check your answer with all others in the your group. Come to the lab with ONE answer that represents the lab mix that your group will create in the lab. You will NOT have the time to review this mix while you are in the lab. COME TO THE LAB PREPARED. CHECK YOUR MIX THE NIGHT BEFORE

MAKE SURE YOU SIGN THE ATTENDANCE SHEET BEFORE YOU LEAVE THE LAB