ASTM C1019: How to Make Grout Prisms for Compressive Strength Testing

Updated on February 8, 2019
Lissa Clason profile image

Melissa has a bachelor's degree in geology, and is an ACI-certified field and lab technician at Froehling and Robertson.

Grout is a fluid mixture of cement and aggregate that has a higher water content than concrete to make it easier to place. It is used between masonry units to hold them together and fill joints. The grout prisms made from this test are designed to help select proportions used in the grout mix at the beginning of that particular job by seeing how they break compared to other mixes with slightly different proportions, or to test the uniformity of grout preparation throughout each phase of construction. The goal is to establish a baseline strength for all grout used on a construction project and keep it consistent for each location where grout is used.

Grout prisms in the moisture room.
Grout prisms in the moisture room.

Equipment Needed to Make Grout Prisms

Grout box – must be officially approved for use on that site. This approval is made based on comparative tests at the beginning of the project where they compare grout prisms made in your box with grout prisms made the traditional way, with a pinwheel of masonry blocks. If the grout box is not approved you may need to make your grout prisms the old fashioned way. These boxes must be rigid and able to hold their shape through the whole process of pouring and tamping the grout.

Tape – Tape is necessary to hold the grout box together and helps to keep it rigid and stable.

Scoop – Scoops must be clean and large enough for you to get a representative sample but not too big or you might spill excess grout into the other holes in the box.

Tamping Rod – use the same tamping rod that you would use for 4x8 cylinders. It must be 3/8 ± 16 inches in diameter, 12 ± 4 inches long, and have a rounded hemispherical tip. As with all other equipment, it must be clean and free of excess concrete before use.

Sampling Receptacle – must be large enough to hold ½ ft³ of grout for all of your testing (assuming you are doing slump and making test specimens).

Straightedge – must be flat and plane.

You will also need a bucket of water and a rag or sponge to clean your equipment, and another rag to keep your grout prisms damp.

Putting Your Grout Box Together

To put together your grout box, follow these steps.

Step 1

The grout box comes in 3 parts, the outer box in the top picture and two of the inserts in the bottom picture.

Step 2

First fold the outer box into the shape in the picture, and then fold the bottom tabs, folding the labeled part in first.

Step 3

Tape along the jagged line in the picture, making sure to keep it flat and in line.

Step 4

For the inserts, fold each one along the two perforated lines in the center.

Step 5

Then, once you have folded them, slide the centers together so that you have an x-shaped piece of cardboard.

Step 6

Next, fold the outer portions of the cross into the shape of a smaller box.

Step 7

Eventually you will have it looking like a box with 4 cells inside.

Step 8

Slot the finished insert inside the outer box, making sure it’s flush with the bottom.

Step 9

Label the side of the box with your name, the date you are casting it, the project name and number, the set number, and any remarks.

Step 10 (AFTER Grout is Poured)

After you have poured your grout, close the labeled top flaps, and then the unlabeled ones, and then tape shut, making sure they are flat.

Grout Prism Procedure

Step 1

Mix the grout you have sampled thoroughly. You will need to begin filling your molds within 15 minutes of sampling your grout, which depends on the type of grout you are working with.

If you are working with self-consolidating grout, fill each cell in the box in one layer. There is no need to rod self-consolidating grout.

If you are working with normal grout, fill each cell in the box up to the halfway mark and rod that layer 15 times to the bottom depth of the layer for each cell. Then fill the second layer, slightly overfilling it, and rod each cell 15 times, penetrating ½ an inch into the first layer.

NOTE: All rods MUST be evenly distributed across the surface of the mold, so be sure to rod each blow in a different area.

Step 2

Strike off the top surface of each prism with a straightedge, sweeping once across the top and then using a back and forth sawing motion until it is even with the top of the mold.

Step 3

Immediately cover the top with a damp absorbent material such as a wet cloth or a paper towel. Keep the top surface damp by wetting the absorbent material and then covering it with a nonabsorbent, nonreactive material. Do not disturb the grout box in any way.

Step 4

Between 15 and 30 minutes after filling the mold, add grout without rodding to fill the depression on the top surface of each prism, which is caused by initial water loss. Strike off the top surface again to make a flat surface that is even with the top of the mold.

Step 5

Close the grout box, wetting the top surface touching the grout to keep it wet, and store it somewhere safe and relatively cool (out of the sun) until pickup.

Step 6

It is preferable to label the box before you go out on the job, but if you have not labeled it yet you should make sure it has your name, the date you are casting it, the project number and name and any remarks you may have. Now you will need to enter the information into Metafield. After you put it into the system, be sure to put the Metafield sample number on the box.

Pickup and Removing the Grout Box

When you pick the grout box up, store it in a cool place out of direct sunlight and cushion the box so that it doesn't roll or bounce as you drive.

Upon your return to the lab, strip the cardboard out of the grout boxes with a rubber mallet and a stripping tool. It is much easier to remove the cardboard if you wet it, so you might want to put the intact box in the moisture room for a couple of hours before you strip it. When you have removed the cardboard from the surfaces of your prisms, you should have 4 prisms available.

Label the grout prisms with their sample number, their number in the set, and the date that they are meant to be broken, and file them in the moisture room under the date that they are meant to be broken. Typically you have one grout for a seven day break and 3 for a 28 day break. If one of the 28 day breaks fails to meet the required strength, the third one will be broken on the 56 day mark instead.

© 2019 Melissa Clason


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