Concrete Ingredients and Mixing of Concrete
Definitions of Terms Associated with the Materials used in Concrete
A.Portland Cement: a dry powder made by burning limestone and clay, and then grinding and mixing to an even consistency.
B.Concrete: a mixture of stone aggregates, sand, portland cement, and water that hardens as it dries.
C.Masonry: refers to anything constructed of brick, stone, tile or concrete units set or held in place with portland cement.
D.Mortar: a mixture of sand, portland cement, water and finishing lime.
E.Finishing Lime: a powder made by grinding and treating limestone.
F.Fine Aggregate: sand and other small particle of stone.
G.Coarse Aggregate: gravel; large particles of stone used in concrete.
H.Clay: the smallest group of soil particles.
I.Sand: small particles of stone.
J.Silt: a substance composed of intermediate size soil particles.
K.Gravel: particles of stone larger than sand; also called coarse aggregate.
L.Washed sand: sand flushed with water to remove clay and silt.
M.Air-entrained concrete: ready mix concrete with tiny bubbles of air trapped throughout the mixture to strengthen it.
How does the bonding of aggregates form concrete?
- A cement and water mixture produces a paste that coats the surface of each of the pieces of aggregates.
- After a few hours after mixing, a chemical reaction starts between the cement and water called hydration.
- When this chemical reaction begins, the cement paste hardens gradually and the concrete sets.
- Upon the completion of the chemical reaction, the cement and water paste will harden much like glue and binds the aggregates together to form the solid mass of concrete.
How do you select the ingredients for concrete?
- Portland Cement
- Chemical combination of calcium, silicon, aluminum, iron, gypsum and small amounts of other ingredients.
- Portland cement is not a trade name, but is used to distinguish this group of cement from other kinds.
- Most cement will pass through a sieve of 40,000 openings per square inch.
- The cement manufacturing process includes several chemical reactions.
- The result is a hydraulic product which sets and hardens after reacting with water.
Types of Portland Cement are manufactured to meet physical and chemical requirements for special application.
Type I: General Purpose Cement
Type II: Modified Portland Cement: has a lower heat of hydration than Type I.
Type III: High/Early Strength Cement
Type IV: Low Heat Cement
Type V: Sulfate Resistant Cement
Air entraining Cement: designated as Type Ia, IIa, and IIIa and basically correspond to Types I, Type II, and Type III.
- lowers the water and sand requirements per cubic yard.
- can be worked more easily
- tends to reduce the segregation of the aggregates from the mix and improves uniformity
- may be finished earlier than the non-air entrained
- improves the resistance to freeze/thaw action
- it is effective in preventing serious surface scaling caused by the preventing the use of chemicals to melt snow and ice
- it is more watertight than air entrained
- Uses of each type.
- Used in structures of considerable size, such as large piers, heavy retaining walls.
- Used where sulfate may attack concrete
- Used when strengtheners are desired
- Used in cold weather construction
- Development of strength is at a slower rate
- Used in mass concrete such as large gravity dams where temperature rise resulting from the heat generated during hardening is a critical factor
- Used only in construction exposed to severe sulfate action
- Slower rate of strength gain than normal portland cement
Air entrained Cement: used for the same type construction as Type I, Type II, and Type III.
- Sand and other small particles of stone that will pass through a 1/4 inch mesh screen
- Clean and free of clay, silt and chaff
- Gravel, pebbles or crushed rock ranging in size from 1/4 inch up.
- Size of coarse aggregate to use depends on the thickness of concrete slab being poured.
- In thin slabs or walls the coarse aggregate should not exceed 1/3 inch the thickness of the concrete being placed.
- To make good concrete, aggregates of various size should fit together to form a fairly solid mass.
- Stone particles must be clean and free of clay, silt, chaff or any other material.
Light weight aggregate: (clay, slag or shale)
Light weight insulating materials may be used to produce concrete which weigh 15 to 90 lbs. per cubic foot.
Test for aggregates
Organic matter test
- Fill a 12 ounce prescription bottle with sand up to the 1 2 ounce mark.
- A 3% solution of caustic soda (sodium hydroxide) is added to fill the bottle to the 7 ounce mark.
- Shake the bottle thoroughly and let stand for 24 hours.
- If the liquid is darker than a straw color, too much organic matter is present.
- Fill a one quart glass jar to a depth of 2 inches with the sand to be tested.
- Add water until the jar is 3/4 full
- Screw on a lid and shake the mixture vigorously for one minute to mix all particles with the water
- Shake the jar sideways several times to level the sand
- Place the jar where it will not be disturbed for one hour for a silt test or 12 hours for a clay and silt test
- After one hour measure the thickness of the silt layer on top of the sand
- If the layer is more than 1/8 inch thick, the sand is not suitable for use in concrete unless the silt is removed by washing
- If the layer is not 1/8 inch thick in 1 hour, let the mixture stand for 12 hours. Then, remeasure the layers that have settled on the sand.
- If the silt plus clay layer exceeds 1/8 inch, wash the sand before using it in concrete
Water should be:
- Free of oil
- Free of acid
- Free of alkali
- Free from harmful amounts of dirts
Should be free of excessive impurities which might effect:
- Setting time
- Concrete strength
- Volume stability
- Surface discoloration
- Corrosion of steel
Drinking water generally is suitable for mixing with concrete
Types Of Portland Cement
- Normal Portland Cement
- Modified Portland Cement
- High/Early Strength
- Low Heat
- Oil Well
Advantages of Air Entrained Concrete
- Mixing concrete may reduce water and sand
- Plastic concrete
- Reduced segregation and
- surface bleeding
- Improved workability
- May be finished sooner
- Increased water tightness
- Resists freezing and thawing
- Resists surface scaling
- due to deicers