Tests on Fresh Concrete

The procedure below first describes the mixing process and then the typical tests (slump, density, and air content) used in the field to determine workability, consistency and quality. The procedure described here has been found to work well with a small concrete mixer.

1. Mixing Concrete by the Trial Method

1. Weigh quantities of coarse aggregate and fine aggregate and store them in separate containers. Record the exact weights on the data sheet.
2. Weigh the quantity of cement given above in Table 1, and place it in a separate container.
3. Weigh the quantity of mixing water given above in Table 1, and place it in a container.
4. Dampen the inside of the mixer and all tools that will be used, so that they are wet but not with any standing water.
5. Put the coarse aggregate, fine aggregate, and about 1/5 of the water into the small concrete mixer and mix for about 2 minutes.
6. With the mixer still rotating, begin to add the cement and additional water in small increments (10% to 20% of the total in each step) and mix for an additional 5 minutes.
7. Stop the mixer and test the slump of the concrete mix. In testing the slump, dampen the slump cone and place in the mixing pan. Hold the slump cone down firmly against the pan. Fill the slump cone with concrete in three layers, each approximately one-third of the volume of the slump cone for each layer.
8. Rod each layer with 25 strokes, distributed uniformly over the cross-section of the cone. The rod should slightly penetrate into the previous layer. After the top layer has been rodded, strike off excess concrete with the tamping rod, so that the cone is filled exactly.
9. Immediately remove the cone from the concrete by raising it carefully in a vertical direction. Measure the "slump" of the concrete by determining the difference between the height of the mold and the height of the subsided concrete.
10. After the slump measurement is completed, tap the side of the concrete frustum gently with the tamping rod.
11. At this point the slump of the mixture should be in the 3 to 4 in. range. If the slump is too low, or the mix appears to be harsh, gradually add small quantities of fine or coarse aggregate (or both), thoroughly remix the concrete, and retest the slump. Repeat this process until the mix reaches the desired slump and consistency. Be sure that to keep track of the additional amounts of material used.
12. When the batch is judged to be satisfactory by the slump test, weigh the remaining aggregates and record in the data sheet. Compute the actual amount of coarse and fine aggregates used in the concrete mix from the initial weights.
13. Determine the concrete mix's unit weight by filling and weighing a 1 cubic foot container. The container should be filled and rodded in the same manner as the cone for the slump test.

2. Air-Entrainment Testing

If the concrete mix was designed for a region with freeze-thaw cycles, it is probable that an air-entrainment admixture content would have been specified to bring the total air content to a range of 6% to 8%. To demonstrate this effect, take the remaining concrete and remix it while adding the air-entrainment admixture. First, mix for about 3 minutes, and then conduct an air content test by using an air entrainment apparatus. Keep in mind that the procedure for conducting the test is device specific, so the following procedure refers exclusively to the device used in this video, or similar.

1. Fill the bottom container with concrete following the instructions for obtaining a unit weight.
2. Close the red-colored main air valve on the top of the air receiver.
3. Open both petcocks located on the top of the lid.
4. Place lid on the material container and close the four toggle clamps.
5. Pour water into the funnel until water comes out the petcock in the center of the lid.
6. Jar the meter gently until no air bubbles come out through the center petcock. Close both petcocks.
7. Close the main air valve and bleeder valve in the end of the air receiver.
8. Gently pump air into the receiver until gauge hand gets close to the red line. Make sure the hand passes the initial starting point. It does not matter whether the hand is on one side or the other side of the red line.
9. Tap the gauge gently with one hand. At the same time, crack the bleeder valve until the gauge hand rests exactly on the initial starting point.
10. Quickly close the bleeder valve. Open the main air valve between the air receiver and the material container.
11. Jar the container slightly after releasing the pressure to allow particles to rearrange. Tap the gauge gently until the gauge hand comes to rest. Record the reading as the percent of air entrained.

3. Concrete Test Cylinder Preparation

1. Fill the cylinder molds by placing the concrete into the cylinder mold in three layers of approximately equal volume.
2. Rod each layer with 25 strokes using a small tamping rod (1/4 in. diameter rod). Distribute the strokes uniformly over the cross-section of the mold.
3. After the top layer has been rodded, strike off the surface of the concrete with a trowel. Fill a total of ten concrete cylinder specimens. Measure the weight of all ten cylinder molds filled with concrete and record the weight on the data sheet.
4. Cover the concrete cylinders with a plastic bag to prevent evaporation of water from the concrete.
5. For the final mix, calculate the required weight of materials to make one cubic yard of concrete. Record these results in the data sheet.
6. After 24 + 8 hours, strip the disposable plastic molds from the concrete cylinder specimens. The cylinders are then placed in the curing environment listed in Table 1. Typical curing regimens are: (1) fog room curing at 73.5+3.5^oF (23+2^oC) and 100% RH, per ASTM C 192, (2) ambient curing in the lab, and (3) insulated box curing (i.e., cure box). We will use ambient curing in this experiment.

4. Adding Superplasticizers

1. To demonstrate the use of an admixture, return all concrete to the mixer and add a small amount of superplasticizer. Mix well for 3 minutes and conduct a flow table test.
2. Wet the table and the mold. Wipe off excess water.
3. While holding the mold firmly, fill the mold with concrete in two layers. Rod each layer 25 times, making sure that rodding is uniform across the cross-sectional area.
4. Strike off the top of the mold, so that the mold is filled exactly.
5. Remove the mold with a steady upward pull.
6. Using the handle, raise and drop the table from a height of 0.5 in, 15 times in about 15 seconds.
7. Take the average of six symmetrically distributed caliper measurements to the nearest ¼ in. This value will be the diameter of the spread concrete.