CONSTRUCTION LAB REPORT: Concrete Mixing

 CONSTRUCTION LAB REPORT

Concrete Mixing

Summary:

This blog post is a report of laboratory concrete mixture, followed by a slump test to ensure good workability of the concrete.

Introduction:

Concrete is a mixture of cement, sand, aggregate, water, and possibly an admixture.

Proportions of each ingredient are adjusted to produce a well-balanced mix. Concrete sets in as few as 10 hours and continues to harden and cure as long as moisture and unhydrated cement are present. However, most of the increase in strength occurs within a few weeks.

Experimental Procedure:

1.    For strength requirements, select the water to cement ratio by weight.

2.    Before mixing concrete, be sure that the mixer has been "buttered" with a mixture of cement, sand, and water.

3.    Divide your water into two buckets, one with about 3/4 of the water.

4.    Put about half the coarse aggregate and the 3/4-bucket of water.

5.    Start the mixer.

6.    Add about half the fine aggregate.

7.    Carefully add all the cement with the mixer running. Try not to make a lot of dust!

8.    Mix until all the cement is blended in.

9.    Add the rest of the coarse and fine aggregate.

10. Mix for a while.

11. Add enough water from the final quarter of the water to produce a workable mix.

12. Mix for three minutes, followed by a three-minute rest, followed by a two-minute final mixing.

 

Manual results:

Mass of cement = 10.855 Kg

Mass of fine aggregates = 15.065 kg

Mass of coarse aggregates = 22.816 kg

Mass of water = 5800 kg

 

The concrete mixture is finally added in cylindrical plastic molds after painting them with oil from inside.

Slump Test

Introduction:

WORKABILITY is the relative ease or difficulty of placing and consolidating concrete. When placed, all concrete should be as stiff as possible, yet maintain a homogeneous, and void less mass.

Too much stiffness makes it too difficult or impossible to work the concrete into the forms and around reinforcing steel. On the other hand, too fluid a mixture is also detrimental.

The slump test is performed on newly mixed concrete.

Apparatus:

To perform the test, you need a slump cone and a tamping rod. The slump cone, is 12 in. in height, with a base opening 6 in. Both the top and bottom openings are perpendicular to the vertical axis of the cone.

Cylindrical plastic molds are used to cast the concrete specimens.

Procedure:

·       The molds should be filled in three approximately equal layers.

·     Each layer should be ridded (poked with a bullet-nosed steel rod) 25 times to eliminate unwanted air bubbles. The top of the molds should be struck off with the rod and then with a wooden float.

·     Once the specimens have been struck off, the outside of the molds should be cleaned as well as the mixer and tools. Fresh concrete is a lot easier to clean than when it has set.

·   The cylinders should now be placed in the lab fog room for curing. After 24 hours a member of your group must strip off the mold and carefully mark your specimen with your group number.

·       If the slump is less than required, return the concrete to the mixer, add the remaining water, and mix for one minute.

Results of the slump test:

Depending on the water-cement ratio of the mix, the concrete slump will fall under one of four categories:

·       True slump:

The concrete largely retains its cone shape, demonstrating that the mix is cohesive and its workability isn’t too high.

·       Zero slump:

The concrete retains its shape completely. This shows that the mix is very dry (this kind of concrete is best used in road construction).

·       Shear slump:

The top half of the concrete subsides dramatically, leaning to one side, meaning the mix has workability but low cohesion. The mix may have too much water content and can be retested after being amended

·       Collapse:

The mix doesn’t retain its shape at all and completely collapses. This means the water-cement ratio is too high and needs to be fully amended.

 

The concrete cylinders are finally cured in water after 24 hours of the concrete mixture to have stronger concrete.