University of New Mexico

Civil Engineering Department

Civil Engineering Materials Laboratory, CE 305L

COMPRESSIVE STRENGTH OF HYDRAULIC CEMENTS MORTARS

ASTM C109

Scope

This test method covers the determination of the compressive strength of hydraulic cement mortars using 2-inch (50 mm) cube specimens.

References

ASTM C109 Compressive Strength of Hydraulic Cement Mortars

ASTM C778 Specification for Standard Sand

ASTM C150 Specification for Portland Cement

ASTM C305 Mechanical Mixing of Hydraulic Cement Pastes & Mortars of Plastic Consistency

ASTM C230 Specification for Flow Table for Use in Tests of Hydraulic Cements

Apparatus

Scale (2000 g)

Specimen Molds

Mixer, Mixing Bowl, Mixing Paddle, & Scraper

Flow Table and Flow Mold

Tamper

Trowel

Testing Machine, Forney 400 kip testing machine

Specimen Molds

Mixer and Mixing Bowl Used

Materials

Graded Standard Sand - Natural silica sand, form Ottawa, Illinois, conforming to the requirements of ASTM Specification C778.

Portland Cement per ASTM C150

Mold Release Agent

Procedure

Download the procedure for Compressive Strength of Cement Mortars

Download the Excel form for Compressive Strength of Cement Mortars

Mortar Composition - The proportions of materials for the standard mortar shall be one part of cement to 2.75 parts of graded standard sand by weight. Use a water-cement ratio of 0.485 for all portland cements and 0.460 for all air-entraining portland cements. The water-cement ratio for other than portland and air-entraining portland cements shall be such as to produce a flow of 110 +/- 5.

Batch the following, which is sufficient for 6 samples:

Cement, 500g

Sand, 1375g

Water, g

Specimen Mold Preparation - Apply a thin coating of mold release to the interior surfaces of the molds and base plates. Wipe surfaces with a cloth to remove any excess.

Mortar Mixing Procedure (ASTM C305) - Place dry paddle and dry bowl in the mixing position of the mixer. Introduce the materials into the bowl in the following manner:

1) Place all the mixing water in the bowl.

2) Add the cement to the water; then start the mixer and mix at slow speed (140 rpm) for 30 s.

3) Add sand slowly over a 30 s period, while continued mixing at slow speed.

4) Stop the mixer, change to medium speed (285 rpm), and mix for an additional 30 s.

5) Stop the mixer and let the mortar stand for 1.5 minutes. During the first 15 s, quickly scrape down into the batch any mortar that may have collected on the side of the bowl; then for remainder of the interval, cover with the lid.

6) Finish mixing for 1 minute at medium speed (285 rpm).

7) Determine flow of mortar as follows:

     a) Wipe table clean and dry and place flow mold at center.

b) Place a layer of mortar about 1 inch thickness in the mold and tamp 20 times.

     c) Then fill the mold and tamp this second layer 20 times.

     d) Cut mortar flush with top of mold with a trowel, held perpendicular to the mold, using a sawing motion.

     e) Wipe table around mold clean of all mortar and dry; then remove mold.

     f) Drop table through ½ inch height 25 times in 15 s.

     g) Use calipers to measure the diameters along the 4 scribed lines on the table. The sum of the four readings is the flow (the percent increase in the original diameter). Record this flow value.

8) Following flow test, return all mortar to the mixing bowl. Scrap down the sides and remix for 15 s at medium speed (285 rpm).

Molding Test Specimens - Start molding within 2 minute and 30 s after completion of the original mixing of the mortar.

1) Place a layer of mortar about 1 in. (25 mm) (approximately one half of the depth of the mold) in all of the cube compartments.

Figure 1. Order of Tamping in Molding of Test Specimens.

2) Tamp the mortar in each cube compartment 32 times in about 10 s in four (4) rounds, each round to be at right angles to the other and consisting of eight adjoining strokes over the surface of the specimen (see Figure 1).

3) Fill the compartments with the remaining mortar and tamp as specified for the first layer. During tamping of the second layer, bring in the mortar forced out onto the tops of the molds after each round of tamping using gloved finger and the tamper. On completion of the tamping, the tops of all cubes should extend slightly above the top of the mold.

4) Trowel mortar of each cube both laterally and longitudinally. Cut off mortar to a plane surface with the top of the mold by drawing the straight edge of the trowel, held perpendicular to the mold, with a sawing motion over the length of the mold.

5) Place molded specimens in a moist room for 24 hours. Keep specimens in their molds for this initial curing period. After 24 hours remove specimens from the molds and immerse in a saturated lime water curing tank.

Compressive Strength Determination - Test all specimens according to the specified testing schedule. Three, 7, and 28 day strengths will be obtained to ascertain the strength gain as a function of time.

1) Remove specified test specimen from the curing tank. Wipe to a surface dry condition and remove any loose sand grains or incrustations for test surfaces. Determine the unit weight of the specimen by carefully weighing, and measuring the dimensions of, each cubical specimen.

2) Apply the load to specimen faces that were in contact with the true plane surfaces of the mold. Check the straightness of these faces with a straight edge. Note that grinding is required if the surfaces have appreciable curvature. Select opposing surfaces which have the straightest profiles.

3) Place specimen below the center of the upper bearing block of the testing machine. Ascertain that this spherically seated block is free to tilt. Test at a loading rate such that the peak load will be reached in a period of 20 s to 80 s. Make no adjustments in the controls of the testing machine while specimen is yielding prior to failure.

4) Schematically show how the specimen failed.

5) Record the total maximum load as indicated by the testing machine. Calculate the compressive strength of the specimen in pounds per square inch (psi). Express this strength in Pascals (Pa).

6) Calculate the average of all like specimens and report to the nearest 10 psi (70 kPa).

Testing 2"x2" mortars in Tinius Olsen UTM

Report

After testing at 1, 3, and 7 days, generate curves, using all group data for each cement type, of compressive strength vs. time of testing. Show all data points and plot an average curve. Plot the ASTM C150 Specifications.

Questions

1) What can you infer from this data? Does the data make sense? Is it consistent with the knowledge you've gained in the lecture? How so?

•  Do the mortar cube results satisfy the strength requirements of ASTM C150?

Sample Data from CE 305 Fall 2003

Section 2

Section 1

The above chart displays the compressive strengths for all replicates from section 1 and section 2. The ASTM specification for 7 day compressive strength is 2200 psi. All 8 cubes tested exceeded the 2200 psi as shown above.

Shape of Fracture

As seen in the photo to the right, an hour-glass shape is common for a 2in. X 2in. cement specimen after fracture.

Statement of Test Results

We were unable to test the mortar cubes at 1, 3, and 28 days. If we would have been able to test at the other days listed, we would be able to plot up the data and see how the strength increases with time.

The data we achieved from the 7 day tests meet the ASTM C150 specifications of 2200 psi.

Conclusion

The cement mortar cubes that were prepared and tested in lab sufficiently met the ASTM C150 specifications. I would have liked to have ran the 1, 3, and 28 days tests to see the strength-time relationship we would have achieved. I would recommend having volunteers run the tests if the classes are not in lab to get the data.