Saturday, 19 Jun 2021

CONCRETE

Is an artificial stone made out from the mixture of cement, sand, gravel and water or other inert materials: this is known as Solid mass or plain concrete.

CEMENT

– A powder of alumina, silica, lime, iron oxide and  (Magnesia, ) burned together in a (kiln) and finely (pulverized) and used as an ingredients of (mortar and cement.)??

AGGREGATE

–  Are classified as fine and coarse aggregate that forms into concrete when bounded together into a conglomerate mass by a matrix or cement paste.

* Fine aggregate – the materials smaller than 9 mm. in diameter.

* Coarse aggregate – the materials over 9 mm.  in diameter.

WATER

–  For use in concrete mixing shall  be clean and free  from injurious amounts of oils, acids, alkali, salts, organic materials or other substances that may be deleterious to concrete or steel.

TYPES OF CONCRETE ANDTHIER WEIGHT

  1. Lightweight concrete
    1. Medium stone concrete
    1. Heavyweight concrete
  1. Lightweight concrete – is classified into three types depending upon the kind of aggregates used in mixing, which predetermines their weight.

a. Low density concrete – is employed for insulation purposes whose unit weight rarely exceeds 50 pounds per cubic foot or 800 kg/m3.

b. Moderate-strength concrete – used to fill over light gage steel floor panels.

c. Structural concrete – same characteristics as that of medium stone concrete and weighs from 90-120 pound per cubic foot.

2. Medium stone concrete – known as structural concrete weighing from 145-152 pounds per

    cubic foot generally assumed to be 150 pounds per cubic foot or 3300 kg/cum.

3. Heavyweight concrete – used for shielding against gamma radiation in nuclear reactor

and other similar structure. This is also used as counter weight for a lift bridge. The contents of heavyweight concrete are cement, heavy iron ores, crushed rock, steel scraps, punching’s or shot  (as fine) is also used.

WEIGHT OF HEAVYWEIGHT CONCRETE

-Depends upon the kind of aggregate used in mixing.

  1. Heavy rock aggregate – weighs 200 – 300 pounds per cu. ft. or 3,200 kg/cu.m.
  2. Iron punchings added to high density ores – 4,325 kg/cum
  3. Ores and steel – 330 lb/cu. ft or 5,300 kg/cum

MIXING OF CONCRETE

The process of mixing concrete for building construction is done in two different ways

   either on site job mixing or ready mixed concrete.

Job mixed concrete – shall be done in a batch mixed of approved type. The mixer shall be rotated at  

a speed, mixing shall be continued for at least 1 ½ minutes after all materials are in drum. Mixing of concrete shall be done until after a uniform distribution of the materials has been attained and that the mixture shall be discharged completely before recharging the mixer.

Ready mixed concrete – the concrete mixture batching plant, is most preferred, because the pro- 

portion of the materials such as cement, and aggregates are controlled by weight meter. This use of ready mixed concrete is suitable and convenient for constructions done in a congested city condition.

PREPARATION OF EQUIPMENT AND DEPOSITING:

– Before concrete is placed, all equipment for mixing and transporting of concrete shall be cleaned.

– Water shall be removed from the place of deposit before concrete is placed unless a tremie is to be used or unless otherwise permitted by the building official. Building construction in a place where ice fall is not known or encountered. Preparation of the site for pouring of concrete only embraces the removal of water, debris, mud, dirts, laitance, and other unsound materials that will adversely affect the strength and durability of concrete.

DEPOSITING of concrete – shall be made as early as practicable in its final place to avoid segregation of particles due to re-handling  or flowing.

“ After the concreting started, it shall be carried on as a continuous operation until the placing of the panel or section is completed”.

SEGREGATION

– Is the separation of sand and stone from the matrix or paste that causes inferior     quality of concrete. The cause of separation or segregation of aggregation are:

  1. Transferring of the concrete from the mixture of the forms.
  2. Dropping of the concrete mixture from a high elevation
  3. Improper tamping and spading
  4. The use of long chutes
  5. Excess amount of tamping, vibrating or puddling in the forms.
  6. Concrete particles tend to segregate because of their dissimilarity.
  7. Gravel tends to settle and the lighter materials and water also tend to rise inside a container when delayed in the delivery to the forms.
  8. Lateral movement such as the flow within the form tends to separate the particles.

REQUIREMENTS FOR A GOOD QUALITY CONCRETE

  1. STRENGTH AND DURABILITY right proportion of cement, aggregates and water.

2.  WORKABILITY concrete mixture must be in  plastic form and could readily be  

      placed in the form.

3. DENSE AND UNIFORMITY IN QUALITY compact with uniform distribution of  

    particles in order to be water tight.

4. CURING requires time, favorable temperature, and continuous presence of water or

     moisture in concrete after pouring.

FACTORS that regulate the strength of concrete

  1. Correct proportion
  2. Suitability or quality of the materials
  3. Proper methods in mixing
  4. Proper placement or depositing of concrete inside the form.

5. Adequate protection of concrete during the period of curing

CURING

  • Hardening of concrete depend upon the chemical reaction between the cement and water. The protection of concrete from loss of surface moisture is 7 days when ordinary portland cement is used and 3 days for an early high strength portland cement.

THE METHODS OF CURING SURFACE CONCRETE ARE:

            1. Covering of the surface with burlap continuously wet for the required period.

2. Covering the slab with a layer of wet sand or saw dust 1 inch or 25 mm. thick.

3. Wet straw or hay on top of the slab continuously wet.

4. Continuous sprinkling of water on the slab surface.

5. Avoid early removal of forms;

ADMIXTURE

  • Is a material other than Portland cement, aggregate or water added to concrete to modify its properties.
  •  

CONCRETE PROPORTION AND WATER CEMENT RATIO

  • Water and cement ratio plays an important role in the strength and durability or concrete. There are two methods being adopted in proportioning concrete mixture; it is their volume or by weighing measure.

Table 4-1 CONCRETE PROPORTION

Class of MixtureCement Bag 40 kg.Sand cu. ft.      cu. mGravel cu.ft.        cu.m.
AA11 ½          .0433              .085
A12              .0574              .113
B12 ½          .0715              .142
C13               .0856              .170

            To create a solid mass where cement paste enters the voids, concrete proportion shows that sand is always one half the volume of gravel, for instance,1: 2: 4 means 1 bag cement, 2 parts sand, and 4 parts gravel is the proportion for class A concrete.

Another way of expressing such proportion us 1 : 6 which simply means that every bag cement, 6 parts of fine and coarse aggregate forms a class A mixture. Such idea does not necessarily fix the volume of the fine aggregate to be always ½ the volume of gravel. verify if the proportion you are adopting, say 1 : 2 : 4 mixture has enough paste to cover the gravel and the reinforcing bars including the pipes and other materials to be embedded in the concrete.

Aggregate proportion could be adjusted say from 1 : 2 : 4 to 1 : 2 ½ : 3 ½ which is also equivalent to 1 : 6 mixture, this will reduce a little to gravel volume and at the same instance increase the paste to cover both the gravel and the steel bars.

the concrete proportion where fine aggregate is always ½ the volume of the coarse aggregate is effective on a massive structure with less reinforcement and also on concrete slabs with considerable thickness like roads.

Table 4-2 MAXIMUM PERMISIBLE WATER – CEMENT RATIOS FOR CONCRETE

Specified Compressive strength    Non-air entrained concrete        Air entrained Concrete
      F’s-Psi         –       kg/cmAbsolute                  Liters per Ratio by wt.           Bag cementAbsolute                 Liters per Ratio by wt.         Bag cement
2500             1750.65                         27.60.54                      23.1
3000             2100.58                         25.00.46                      19.7
3500             2450.51                         22.00.40                      17.0
4000             2800.44                          19.00.35                     15.1
4500             3150.30                          16.30.30                     12.9

Air –entrained concrete – is used extensively in the pavement of road construction, it resists frost action and cycles of wetting or freezing. It also provides higher immunity to surface scaling caused by chemicals.

When made with normal weight aggregate, concrete that is intended to be watertight shall have a maximum water cement ratio to 0.48 for exposure to fresh water and 0.44 for exposure to sea water.

Table 4-3 CONCRETE AIR CONTENT FOR VARIOUS  SIZES OR COARSE AGGREGATE

Nominal maximum size of coarse:  AggregateTotal air content percent by volume
3/8                            9.5                                  6 to 10
½                             12.7                                   5 to 9
¾                             19.0                                   4 to 8
1                              25.4                               3.5 to 6.5
1 ½                          38.0                                   3 to 6
2                              51.0                              2.5 to 5.5
3                              76.0                              1.5 to 4.5

Image: Pexels

YOU MAY ALSO LIKE:

Leave a Reply

Your email address will not be published. Required fields are marked *