Compressive Test For Paste Cement Cubes 50mm With Different w/c Ratio "Report"

INTRODUCTION

This test method coves determination of the compressive strength of cement mortars , using  2 in (50 mm ) cube specimens .

 APPARATUS

In this test we will use equipment  as follow :

1) weights or weighing device .

2) glass graduate .

3) specimens molds : thre cubes of (50 mm) side .

4) mixer ( electrically driven mechanical mixer of the type equiped with paddle and mixing bowl ) .

5) testing machine .

6) tamper and trowel .

 And the following pictures shows this apparatus 

MATERIALS

graded standard sand should be used ( c778 ) with cement in the proportion  ( 1 cement : 2.75 sand ) by weight .

in this report we used water-cement ratio  as follow :

 0.45  , 0.5 , 0,55  , and  0,6  w/c  ratio .

PROCEDURES

 

 A- preparation of morter

1) weight (300)gm of cement and prepare the corresponding weights of standard sand and water.

2) place the dry paddle and the dry bowl in the mixing position in the mixer. then introduce the materials for abatch into the bowl and mix in the following manner .

3) place all the mixing water in the bowl and add the cement to the water , then start the mixer and mix at the low speed (140+or- 5  r/min ) for 30 second. add the entire quantity of sand slowly over a(30 s) period , while mixing as slow speed.

stop the mixer , change to medium speed (285+10 r/min) and mix for 30 second . stop the mixer and let the morter stand for 1.5 min . during the first (15 s) . of this interval , quickly scrape down into the batch any morter that may have collected on the side of the bowl .and finish by mixing for (1 min ) at medium speed .

B-molding test specimens

1) thinly cover the interior faces of the specimen molds with oil .

2) start molding the specimens within atotal time of not more than 2.5 min after completion of mixing.

3) place alayer of morter about 25mm (half the depth of the mold ) in all the cube specimens .

4) tamp the morter in each cube 32 times (4*8) , about 4 rounds < each round to be at right angles to the other .

5) the 4 rounds of tamping shall be completed in one cube before going to the next .

6) when the tamping of the first layer in all cubes is comleted , fill the molds with the remaining morter and tamp as specified for the first layer .

7) cut off the morter to a plans surface with astraight edge.

8) keep the molds in a moist room for 20-24 houres then open them and keep the specimens in a water basin for aweek .

c- testing specimens

1) after 7 days ( +3 hours), take the specimens out of the basin , dry them with a clean cloth , put them , one after the other in the testing machine.

2) the cubes must be put on one side , using extra steel plates up and down the specimen.

3) start loading in speed of 1.4 kn/sec or (350 kg/cm^2 ) in  a- minute .

4)when failure , record load and the compressive strength .

CALCULATIONS

 

now we will determine the compressive of every sample different w/c ration .

-The first sample with  0.45  w/c ratio  .

NO.CUBE	LOAD (KN)	COMP.STRENGTH        (MPa)_____	STRESS  WITH (kg/cm^2)	Weight (g)
1	51.7	20.68	210.805	283
2	50.6	20.24	206.32	281

- Average  comp -stress = 210.805+206.32 /2  = 208.563  kg/cm^2   for  0.45 w/c .

    The second sample  with  0.55 w/c ratio :-

NO.CUBE	LOAD (KN)	COMP.STRENGTH        (MPa)_____	STRESS  WITH (kg/cm^2)	Weight (g)
1	54.262	21.505	221.25	280
2	49.907	19.963	203.49	275

Average  comp- stress =  221.25+203.49  / 2   =  212.37 kg/cm^2  , for  0.5 w/c

-The third sample with 0.55 w/c ratio :-

NO.CUBE	LOAD (KN)	COMP.STRENGTH        (MPa)_____	STRESS  WITH (kg/cm^2)	Weight (g)
1	55.467	22.187	226.165	280
2	37.196	14.879	151.665	275

-  Average comp-stress = 226.165+151.665   /2  =  188.915  kg/ cm^2  , for 0.55 w/c ratio

The forth sample with 0.6 w/c ratio :-

NO.CUBE	LOAD (KN)	COMP.STRENGTH        (MPa)_____	STRESS  WITH (kg/cm^2)	Weight (g)
1	10	4	40.774	285
2	34.486	13.794	140.615	275

- Ave .comp- stress=  40.774+140.615  / 2  =  90.6945  kg/cm^2  , for 0.6 w/c ratio.

and this picture from excel ratio average comp-stress and w/c ratio .

COMMENTS

1) The comp-strength of sample 0.5 w/c is bigger than strength of 0.45 w/c

2) the value of comp-strength 0.6 w/c is the least of every samples strength.

3) the cement used is portland cement .

4) when increase in w/c ratio m the comp-strength decrease.

5) two sample for one w/c ratio is not enough to judge comp-strength.