Preparation of Aligned Steel Fiber Reinforced Cementitious Composite and Its Flexural Behavior

This method produces a steel fiber reinforced cementitious composite that is superior to those made with previous techniques. The main advantage of this technique is that short steel fibers need fresh cementitious composites aligned by electromagnetic field, which significantly increases ability to reinforce samples. Several people will help demonstrate the procedures.

Associate Professor Xiaowei Wang Professor Jian Zhou Graduate Student Luansu Wei and Doctoral Student Hui Li Knowing the mortar workability is key to preparing specimens. Have ready a sinking depth meter to characterize the mortar. In addition, have a coaxial rotational mortar rheometer ready for workability testing.

The elements for mortar mix should be near a mortar mixer. The same amount of water, cement and sand will go into each mix. Testing also requires a source of superplasticizer.

To make the mortar, first add sand to the mixer sand-holder. Then, put water into the mixer. Next, add the cement.

Let the water and cement mix for 30 seconds. Start the slow, automated addition of sand and continue mixing over another 30 seconds. Mix for 60 seconds longer before stopping.

At this point, get the sinking depth meter and place fresh mortar into the sample container. Adjust the cone over the sample. Fix the cone at a position that just contacts the surface.

Zero the meter on the device, then release the cone to complete the test. Check the meter to see if the sinking depth is between 50 and 100 millimeters. If the result is out of range, prepare to mix another batch.

Start with water in the mixer and then add plasticizer to adjust the sinking depth. Continue to add cement and sand as before. When the new mixture is ready, perform the sinking depth test again.

The mix is acceptable if the result is in the 50 to 100 millimeter range. Next, test the viscosity of the mixture. Transfer about 300 milliliters of fresh mortar to the rheometer sample container.

Proceed with the viscosity test. The results will help determine the necessary magnetic field strength. Preparation of the experiment sample requires a magnetic field.

This solanoid is on a compacting table. It can apply a uniform field over a sample mold. The plastic mold for the mortar should fit easily into the interior of the solanoid.

Take the mold to a 30-liter mortar mixer and set it aside. Near the mixer, have the raw materials for 15 liters of the previously determined mortar mix. The cement and sand are combined, as are the water and superplasticizer.

Steel fiber for the desired steel fiber volume fraction is the new component for the aligned steel mixture. Create the mortar by first adding the cement and sand to the mixer. Mix and confirm by visual inspection that these are well-mixed before continuing.

Then, add the water and the superplasticizer and mix for about one minute. Finally, add the steel fibers and mix for two more minutes. When done, pour the fresh mortar into the plastic mold quickly.

Move the mold onto a compacting table. Turn on the compacting table for 30 seconds. Ensure the mold is completely filled before continuing.

Now, place the mold into the solanoid chamber. Turn on both the solanoid and the compacting table for 50 seconds. When the table has stopped completely, turn off the solanoid and gently remove the mold.

Use a trowel to smooth the top surface but avoid disturbing the steel fibers. Prepare three electromagnetically-treated specimens and three standard fiber-reinforced specimens for each steel fiber volume fraction. Keep the specimens indoors and in their molds for 24 hours.

Then, take the specimens to the fog room to demold them and leave them to cure. The mechanical tests are performed on a three-point bending test rig. Two rollers will support the sample.

A load will be applied from above. This schematic shows how a specimen will be mounted on the rig. Note the supports are 50 millimeters from either end of the 550 millimeter length.

The support points are labeled B.The load is applied at the midpoint of that length at a point labeled A.There are studs glued to the specimen on either side at the points labeled D to support a linear variable differential transformer, or LVDT, holder. The points labeled C are on plates attached to the specimen. Mark the specimen at points corresponding to the labeled points on the schematic.

The points C are where the mid-span deflection is measured. There are supports for the LVDT holder glued at the points D.Mount the specimen on the test rig. Use an LVDT holder and fix the LVDT to the mid-span on the sides of the specimen.

Connect the LVDT to a data logger and set the data acquisition frequency. At the rig, gradually raise the specimen, using the bottom supports. Stop when the upper loading cell is close, but not touching, the top surface.

Zero the quantities to be measured and set up the three-point bending load test. Start the test and record the full history of the loading and mid-span deflection. This is a sample load-deflection plot for the mid-span deflection.

Note when the load is beyond its peak value. This is how the current test specimen appears on the test rig when the load is just beyond its peak value. Once beyond the peak load, stop the test when the displacement is greater than 30 millimeters.

The test for the specimen has been stopped after the displacement reached 30 millimeters. At that point, remove the specimen and prepare to test those that remain. Here in red are the measured flexural strengths for aligned steel fiber reinforced composites with different steel fiber volume fractions.

The numerical values represent the average of three specimens and the error bars give the standard deviation. In blue are the flexural strengths for standard steel fiber reinforced composites. Note that in every case, the flexural strength for the aligned steel specimens is higher.

These are low-displacement plots of the mid-span displacements for mixtures with steel fiber volume fractions of 0.8%1.2%and 2%Each plot has data for three samples for aligned specimens and traditional specimens of the same steel fiber volume fraction, here 0.8%The aligned steel fiber specimens consistently have a higher peak load and area under the curve. The same holds for the 1.2%steel fiber volume fraction and the 2%steel fiber volume fraction. X-ray computed tomography tests give the spatial distribution of steel fibers in two 0.8%steel fiber volume samples.

At left is the aligned steel fiber specimen, providing evidence of the alignment. At right is the traditional steel fiber specimen, where the fiber directions are random. By applying an electromagnetic field treatment using the solanoid set-up developed in this study, the steel fibers in fresh mortar were highly aligned and the aligned steel fiber reinforced cementitious composite specimens were successfully prepared.

The ordinary efficiency factors of steel fibers in aligned steel fiber reinforced cementitious components exceeded 0.9 while those ordinary steel fiber reinforced cementitious components were around 0.6. The number of steel fibers read in the created sections of the aligned steel fiber reinforced cementitious composites was greater than several of the ordinary steel fiber reinforced cementitious composites. The flexural strength and flexural toughness of aligned steel fiber reinforced cementitious composites are significantly higher than those of ordinary steel fiber reinforced cementitious composites.

Although the protocol described in this paper was demonstrated with steel fiber reinforced cement mortar, it's also applicable to steel fiber reinforced concrete.