CE 3700

Engineering Materials Laboratory

“Determining the Compressive Strength of Hydraulic Cement Mortars”

Performed By:

Group 3 Section 3

Submitted By: Chase Andrew Bowman

Date Performed: Date Submitted: September 11th & 18th, 2014 September 25th, 2014

Department of Civil and Environmental Engineering Louisiana State University and A&M College

Fall 2014

Purpose In accordance with ASTM C 109, Standard Test Method for Compressive Strength of Hydraulic Cement Mortars, and ASTM C 150-02a, Standard Specification for Portland Cement, tests were performed on 2” by 2” cubes of mortar fabricated by 3 groups. The purpose of this lab exercise was to understand how different water/cement ratios affect the compressive strength of a hydraulic cement mortar. By doing this lab exercise we can better understand the fundamental characteristics of Portland Cement Concrete mixtures. Portland Cement when mixed with water creates a chemical reaction called hydration. During hydration the mixture will first display a plastic form before hardening. The higher the W/C ratio, the higher the workability a mixture will have during the plastic stage. However with a higher W/C ratio, the compressive strength will decrease. In a professional setting one might need a certain strength requirement and choosing a perfect W/C ratio is key to meeting this requirement while still achieving a high workability. It is important that ASTM C 150-02a sets standard and optional physical requirements regarding compressive strength because it gives us a reference for our mixtures. Please note: certain requirements for ASTM C 109 standards were not meet; this will be explained later in “Test Procedure.” Significance and Use Portland Cement is hydraulic cement that is composed of calcium silicates. Hydraulic cement undergoes a chemical reaction called Hydration, in which that cement particles absorb water and create a gel. This gel is what glues individual particles together and creates Portland Cement Concrete. Different amounts of water added to a mixture affect its workability and compressive strength. By performing ASTM C 109, we can determine a W/C ratio to meet certain specifications or requirements required for a mixture. If a sample of mortar cubes fail at compressive strength lower than required, we would know to add less water to the next sample. If a mortar cube greatly surpasses the required compressive strength, we know to add more water to the next sample to increase its workability. Equipment The following devices listed below were used in the fabrication and compressive strength tests on 2” by 2” Mortar cubes. These Devices are required to comply with ASTM C 109 testing standards.

• Weighing Devices – used for measuring the samples’ weights • Measuring Cylinder – used for measuring the water needed for the mixture • Sample Molds – a device made from brass that held 3 2”x2”x2” molds • Mixing Tray – a plastic tray in which the samples were mixed • Tamper – a piece of seasoned oak wood that removed air from the molds in two

different stages • Hammer – used for removing air once the molds have been filled

• Trowel – a steel blade with straight edge used for creating a flat/equal top of the molds

• Moisture Room – a room of high humidity where the samples are kept for 20-72 hours after being molded into cubes.

• Hydraulic Compressor Testing Machine – used for determining the compressive strength of the molds

Sample Identification ASTM C 109, Standard Test Method for Compressive Strength of Hydraulic Cement Mortars, states that for the standard mortar there shall be 1 part cement to 2.75 parts sand. The lab exercise used a 50/50 sand to cement ratio, therefore it did not meet the standard mortar ratio requirement. ASTM C 109 also states that a water/cement ratio for all Portland cement should be .485, however the water/cement ratio for lab was .400. The following test samples were used for preparing the mortar:

• 907.2 grams of type I Portland cement that is composed of minerals: 1. Lime (CaO) – main component (60-65%) 2. Silica (SiO2) 3. Alumina (Al2O3) 4. Iron Oxide (Fe2O3) 5. Gypsum (CaSO4.2H2O)

• 907.2 grams of course sand 1. 6.1.1 of ASTM C 109, Standard Test Method for Compressive

Strength of Hydraulic Cement Mortars, states, “The sand (Note 4) used for making test specimens shall be natural silica sand conforming to the requirements for graded standard sand in Specification C 778.”

2. Being that the sand used for lab was not Graded Standard Sand, it did not meet the requirements of ASTM C 109 and C 778.

• 362.9 grams of drinkable water 1. Please note that 362.9 grams of water is specific to group 3’s lab

In table 1 you can see the weights and W/C ratios of groups 1,2 and 3.

Table 1. Shows Groups 1,2, and 3’s starting points for preparing the mortar

Group S/C ratio W/C ratio Water Wt. (g) Cement Wt. (g) Sand Wt. (g) 1 50/50 0.5 453.6 907.2 907.2 2 50/50 0.45 408.2 907.2 907.2 3 50/50 0.4 362.9 907.2 907.2

Pleasenote:Thatallgroupshadthesamesandtocementratio,(907.2/907.2),buthaddifferentwaterweights.ThisgaveeachgroupadifferentW/Cratio.Wecanpredictthatgroup1’smortarwillhavethelowestcompressivestrengthduetoitshigherW/Cratio.

Test Procedure The test procedure was in accordance with ASTM C 109, Standard Test Method for Compressive Strength of Hydraulic Cement Mortars, with a few exceptions. ASTM C 109 states that a 2.75/1 ratio of sand/cement is to be used for a standard mortar, however a 1/1 ratio was used in lab. The water/cement ratio of .400 that was used for this lab did not meet the standard of .485 set for Portland cement by 10.1.1 of ASTM C 109. Also 6.1.1 under “Materials” states that a graded standard sand meeting the requirements for ASTM C 788 is to be used, however course sand was used for this lab exercise. Prior to our lab session it is assumed that our instructor completed all steps under “Preparation of Specimen Molds” of ASTM C 109. These steps are 9.1,9.2, and 9.3 and involve properly preparing the brass molds so that they create a watertight seal and do not expose the mortar to any unwanted additions that could compromise its strength or dimensions. Upon arriving to lab we discovered that our lab instructor for time saving reasons conducted 10.2, (Preparation of Mortar) under “Procedure.” 10.2.1 regards to finding the proper weights of the cement and the water. It is important to determine if the cement is air-entrained or not. Performing this step wrong could lead to an inaccurate amount of water needed for the mixture. After completion of 10.2 our instructor informed us of our samples’ weights. Group 3’s first step was 10.4.1 of ASTM C 109. The 907.2 grams of sand and roughly half of the Portland cement were placed into the mixing tray. The tray was mixed until a constant mixture was reached. The remaining Portland cement was poured into the tray and mixed again. Once the mixture reached a consistent spread of both sand and cement, half of the water was introduced into the tray. After thoroughly mixing by hand, the remaining water was added and mixed again. Image 1 displays mixing the samples thoroughly before and after the addition of water.

Image 1: Represents Procedure 10.4.1 in ASTM C 109

Theleftimageshowsmixingofthesampleswithoutwater.Therightimageshowsmixingafterwaterwasintroduced.Pleasenote:Thesamplesarethoroughlymixedbeforeandaftertheadditionofwatertoachievemaximumconsistency.

Please note that procedure 10.4.2 of ASTM C 109 was not performed because a duplicate batch was not needed given that we were only constructing 3 mortar samples. 10.2.1 indicates that this lab exercise involves 6 or 9 mortar samples. After completing the mixture we began procedure 10.4.3. The plastic mixture was then scooped from the tray and placed into the brass molds at an interval of 1” deep. After a mold was filled with mortar up to 1” we began to tamper to remove any possible air bubbles within the mixture. Each mold was filled to 1” and tampered separately using a 5” long piece of seasoned oak. Then we filled the brass molds so that they had extra mortar extruding from the top. They were tampered again using the same method as before. After tampering, a trowel was used to create an even surface on the top of the mortar. 10.4.3 of ASTM C 109 states that the mortar needs to be tampered 32 times in 4 rounds making sure that the tamper is always at a right angle to the adjacent wall. In this lab we did not meet this requirement being that we only tampered 25 times in a relatively unorganized manor. This led to air bubbles being present in our final mortar cube and a decrease in compressive strength. This will be explained later in “Analysis of Results.” After tampering, a trowel was used to create an even surface on the top of the mortar. Image 2 displays how the mortar extended pass the top of the mold and tampered. Image 3 shows how the trowel was used to create a flat, even surface.

Image 2: Represents procedure 10.4.3 in ASTM C 109

Theimageshowstamperingusingtheseasonoakwood.Thiswasafterthemoldswerefilledoverthetopwithmortar.Pleasenotethatimpropertamperingtechniquewasused,whichcausedairbubblestobepresentinthecube.Thisresultedinadecreaseofcompressivestrength.

Image 3: Represents Procedure 10.4.3 of ASTM C 109

After the mortar cubes were set in their mold, a flat surface was placed on top of them to make sure undesired moisture did not compromise the cubes. In accordance with procedure 10.5 of ASTM C 109, the mortar cubes were immediately placed in a moisture closet with high humidity. The mortar cubes would be removed from the high humidity after a period of 24 hours and were left to harden for another six days. Upon returning to lab on September 18th, procedure 10.6.2 of ASTM C 109 was done prior by the instructor. Please note that procedure 10.6.1 states that each cube once removed from the moisture closet is kept track of and is to be broken at different times such as 24 hours, 3 days, 7 days, and 28 days. In this lab exercise all 3 mortar cubes were broken 7 days following the molding.

Theimageshowshowthetrowelwasusedtodefinethetopedgeofthemortarcube.Pleasenotethatextramortarwasaddedduringthissettofillingapsalongtheedgesofthemold.Alsothelabinstructorassistedtoconfirmthatwehadaneventop.

The lab instructor prior to the second lab performed procedure 10.6.3 of ASTM C 109 on two out of the three mortar cubes. He recorded the load at which the cubes failed using a hydraulic compressor and passed them on to us. Once recording the previous two cubes’ load at failure we proceeded to perform the compressive strength test on the third. The third cube was carefully placed under the upper bearing block of the testing machine. The machine exerted a force on the top mortar until it failed. The third cube’s load at failure was then recorded. Image 4 displays the cube after if has reached its failing point.

Image 4: Procedure 10.6.3 of ASTM C 109

Theimagerepresentsthethirdmortarcubeafterfailingunderaloadof24,655Lbs.pleasenotethatthelabinstructorperformedthistestonthefirsttwomortarcubes.

Analysis of results Table 2 shows the area, load at which the cube failed, compressive strength, the age of the sample, and Type break. It also includes the average compressive strength of all samples, standard deviation, and % coefficient of variation. All three samples after testing displayed an hourglass shape. This tells us that all three mortar cubes were well dimensioned and that the bearing load during testing was consistent through out the entire sample. Sample 1 and 2 both showed a similar compressive strength averaging at 6343.25 PSI. The standard deviation for the first and second sample was about 22.5 PSI. This STD is an expected value for testing. However sample 3 had a compressive strength of only 6,163.42 PSI and when averaged with the other cubes the STD is raised and the average strength is decreased. It is assumed that the lower compressive strength of sample 3 is due to poor tampering technique mentioned previously in “Test Procedure.” The improper tampering technique allowed air bubbles to reside in the third sample. The higher standard deviation raises the coefficient of variation %. Starting with a water/cement ratio of .4, it was expected that the mortar samples would have a higher average compressive strength. This could have been a human error related to 10.4.1 of ASTM C 109, which involves mixtures the samples. Even though the samples had a lower strength than expected, they still achieve standard and optional physical requirements set by ASTM C 150-02a, Standard Specification for Portland Cement. ASTM C 150-02a states that a minimum of 4060 PSI is required for optional physical requirements.

Table 2: Displays Test Results for Group 3

Sample#

Area(WxH)

LoadatFailure(LBS)

CompressiveStrength(PSI)

AgeatTest(Days)

TypeBreak

1 4" 25,463 6,365.75 7 HourGlass

2 4" 25,283 6,320.75 7 HourGlass

3 4" 24,655 6,163.75 7 HourGlass

Average,PSI 6283.42 StandardDeviation,PSI 106.05 %CoefficientofVariation 1.69%

Graph 1 shows the average compressive strengths for each group. It was expected that the higher the water/cement ratio the more the strength would decrease. Observing the chart below this is expectation was met, however the difference between .4 W/C and .45 W/C is noticed. With a W/C ratio of .4, the mortar cubes were expected to have a much higher compressive strength then the mortar cubes with a .45 W/C ratio. This could have been a human error involved with mixing the test samples in ASTM C 109 procedure 10.4.1. Group 1’s mortar with a W/C ratio of .5 met expectations being that it was much lower than the other two groups.

Graph 1: Displays the average CS for each group

Pleasenote:ThereisanobviousdifferencebetweenthecompressivestrengthoftheWater/Cementratioof.5and.45.Thisisexpected,howevertheWater/Cementratioof.4didnotdisplaythislargeofadifference.Thiscouldbeblamedonmultiplehumanerrorssuchastamperingandmixing.

Table 3 and Graph 2 illustrate the estimated 14 and 28-day compressive strength of each group’s samples. The 28-day compressive strength was found by dividing the average compressive strength at 7 days by .07. The equation for this was f28=f7/.7. The 14-day strength was found by multiplying the estimated 28-day strength by .085. The equation for this was as follows f14=f28x.085. These equations were developed by researching different techniques involves converting 7 day strength to 28 day strength. Most techniques state that the 7 day compressive strength it between .65 and .75 of the 28 day compressive strength. By averaging out the range, .7 was determined to be the best factor to use to properly estimate.

Table 3: Displays Group’s estimated CS for 14 and 28 Days Compressive Strength (PSI)

Days Group 1 Group 2 Group 3

7 3926.58 4767.99 5609.4

14 6062.75 7361.91 8661.07

28 6283.42 7629.35 8975.71 Pleasenote:All9samplesfromallthreegroupsmetthestandardphysicalrequirementssetbyASTMC150-02a.Allestimatedvaluesof28-daystrengthalsomeettheOptionalPhysicalrequirementinTable4ofASTMC150-02a

Graph 2: Represents Table 3 in a visual setting

Pleasenote:Group1and2’sestimatedcompressivestrengthswereexpected,howeverGroup3’saveragecompressivestrengthwaslowerthanexpected.

Findings After reviewing the results of the lab, it is clear that there were a sign of possible human error regarding the procedure. These human errors are most likely the improper tampering techniques. Procedure 10.4.3 of ASTM C 109 was not followed, which likely let to the presence of air bubbles in the mortar. Also mixing the test samples before and after the introduction of water maybe has also played a role in this. Both group 1 and 2 showed expected results, which correctly portrays the known importance of the water/cement ratio in mortar. Group 1 had the highest W/C ratio and in turn it showed to have the lowest compressive strength. Group 2 with a W/C ratio of .45 also showed expected results. Groups three’s average compressive strength was noted as unexpected. Its assumed compressive strength was much lower than the tests results showed. Not only was the average lower than expected, but the third sample also broke at a significantly lower bearing load than the others. This was most likely due to a build up of air bubbles in the mortar cube. The Lab exercise properly displayed how the Water/Cement ratio affects the compressive strengths of mortar samples. Even with the factor errors all 9 mortar cubes of all three groups met Standard and Optional Physical Requirements of ASTM C 150-02a.