Tests on Concrete

Tests on Concrete

The following are some of the important tests conducted on concrete:

1.Slump test.

2.Compaction factor test.

3.Crushing strength test.

1.Slump Test: This test is conducted to determine the workability of concrete. It needs a slump cone for test. Slump cone is a vessel in the shape of a frustum of a cone with diameter at bottom 200 mm and 50 mm at top and 300 mm high. This cone is kept over a impervious platform and is filled with concrete in four layers. Each layer is tamped with a 16 mm pointed rod for 25 times. After filling completely the cone is gently pulled up. The decrease in the height of the concrete is called slump. Higher the slump, more workable is the concrete. The desired values of slumps for various works.

2.Compaction Factor Test: This is another test to identify the workability of concrete. This test is conducted in the laboratory. The test equipment consists of two hoppers and a cylinder fixed to a stand, the dimensions and the distances between the three vessels being standardized.

3.Crushing Strength Test: Metallic moulds of size 150 mm × 150 mm × 150 mm are used for casting concrete cubes. Before filling mould, it is properly oiled on its inner surfaces, so that cubes can be easily separated. Fresh cube is filled with concrete to be tested in 3 layers and kept in the room. After24 hours, cube is removed from the mould and kept under water for curing. After 28 days of curing cubes are tested in the compression testing machine. In this test cubes are placed over the smooth surface which is in contact with side plates of mould. The crushing load is noted and crushing strength his found as load divided by surface area (150 × 150 mm2).

Code specify the desirable strength of concrete for 3 days and 7 days for quick assessment of strength of concrete.

Properties of Concrete

Properties of Concrete

What Are The Properties of Concrete?

Most people commonly use the words cement and concrete interchangeably. It's important to note that cement is only one component of three concrete properties and the term "concrete" is a mixture of the three.

Concrete properties consist of three basic components: water, aggregate (rocks and sand), and portland cement. Cement, usually in powder form, acts as a binding agent when mixed with water and aggregates.

This combination or mixture, when poured, hardens into this durable material called concrete that we are all familiar with.

The Three Basic Components of Concrete

1. Portland Cement - The cement and water form a paste that coats the aggregates and sand in the mix. The paste hardens and binds the aggregates and sand together.

2. Water - Water is needed to form a chemical reaction with the cement (hydration) to form heat. This starts the drying and hardening process. Water is also used to provide workability with the concrete.

The amount of water in the mix in pounds compared with the amount of cement is called the water/cement ratio. The lower the w/c ratio the stronger the concrete.

3. Aggregates - Sand is the fine aggregate and crushed stone and gravel are the coarse aggregate. These two materials help strengthen the concrete.

Some Desired Properties of Good Concrete

1. A workable mix that is easy to place and consolidate.

2. A high quality concrete using the lowest water-cement ratio possible, using the proper size coarse aggregate practical for the job you're doing, using the optimum ratio of fine to coarse aggregate.

3. For hardened concrete resistance to freezing and thawing, deicing chemicals, wear resistance, strength, and low permeability (water tightness).

These are the basics of concrete properties to produce a concrete mix. There are other admixtures that are added to improve workability, durability, and set times.

Curing of Concrete

Curing of Concrete

Curing may be defined as the process of maintaining satisfactory moisture and temperature conditions for freshly placed concrete for some specified time for proper hardening of concrete. Curing in the early ages of concrete is more important. Curing for 14 days is very important. Better to continue it for 7 to 14 days more. If curing is not done properly, the strength of concrete reduces. Cracks develop due shrinkage. The durability of concrete structure reduces.

The following curing methods are employed:

(a)  Spraying of water

(b)  Covering the surface with wet gunny bags, straw etc.

(c)  Ponding

(d)  Steam curing and

(e)  Application of curing compounds.

(a) Spraying of water: Walls, columns, plastered surfaces are cured by sprinkling water.

(b) Wet covering the surface: Columns and other vertical surfaces may be cured by covering the surfaces with wet gunny bags or straw.

(c) Ponding: The horizontal surfaces like slab and floors are cured by stagnating the water to aheight of 25 to 50 mm by providing temporary small hunds with mortar.

(d) Steam curing: In the manufacture of pre-fabricated concrete units steam is passed over the units kept in closed chambers. It accelerates curing process, resulting into the reduction of curing period.

(e) Application of curing compounds: Compounds like calcium chloride may be applied on the curing surface. The compound shows affinity to the moisture and retains it on the surface. It keeps the concrete surface wet for a long time.

Preparing and Placing of Concrete

Preparing and Placing of Concrete

The following steps are involved in the concreting:

1.Batching

2.Mixing

3.Transporting and placing and

4.Compacting.

1. Batching: The measurement of materials for making concrete is known as batching. The following two methods of batching is practiced:

(a) Volume batching

(b) Weight batching

(a) Volume Batching: In this method cement, sand and concrete are batched by volume. A gauge box is made with wooden plates, its volume being equal to that of one bag of cement. One bag of cement has volume of 35 litres. The required amount of sand and coarse aggregate is added by measuring on to the gauge box. The quantity of water required for making concrete is found after deciding water cement ratio. For example, if water cement ratio is 0.5, for one bag of cement (50 kg), water required is 0.5 × 50= 25 kg, which is equal to 25 litres. Suitable measure is used to select required quantity of water.

(b) Weight Batching: This is the recommended method of batching. A weighing platform is used in the field to pick up correct proportion of sand and coarse aggregates. Large weigh batching plants have automatic weighing equipments.

2.Mixing: To produce uniform and good concrete, it is necessary to mix cement, sand and coarse aggregate, first in dry condition and then in wet condition after adding water. The following methods are practiced:

(a) Hand Mixing

(b) Machine Mixing.

(a)  Hand Mixing: Required amount of coarse aggregate for a batch is weighed and is spread on an impervious platform. Then the sand required for the batch is spread over coarse aggregate. They are mixed in dry condition by overturning the mix with shovels. Then the cement required for the batch is spread over the dry mix and mixed by shovels. After uniform texture is observed water is added gradually and mixing is continued.

(b) Machine Mixing: In large and important works machine mixing is preferred. A typical concrete mixer. Required quantities if sand and coarse aggregates are placed in the drum of the mixer. 4 to 5 rotations are made for dry mixing and then required quantity of cement is added and dry mixing is made with another 4 to 5 rotations. Water is gradually added and drum is rotated for 2 to 3 minutes during which period it makes about 50 rotations. At this stage uniform and homogeneous mix is obtained.

3. Transporting and Placing of Concrete. After mixing concrete should be transported to the final position. In small works it is transported in iron pans from hand to hand of a set of workers. Wheel barrow and hand carts also may be employed. In large scale concreting chutes and belt conveyors or pipes with pumps are employed. In transporting care should be taken to see that seggregation of aggregate from matrix of cement do not take place.

Concrete is placed on form works. The form works should be cleaned and properly oiled. If concrete is to be placed for foundation, the soil bed should be compacted well and is made free fromloose soil.

Functions of Various Ingredients

Functions of Various Ingredients

Cement is the binding material. After addition of water it hydrates and binds aggregates and the surrounding surfaces like stone and bricks. Generally richer mix (with more cement) gives more strength. Setting time starts after 30 minutes and ends after 6 hours. Hence concrete should be laid in its mould before 30 minutes of mixing of water and should not be subjected to any external forces till final setting takes place.

Coarse aggregate consists of crushed stones. It should be well graded and the stones should be of igneous origin. They should be clean, sharp, angular and hard. They give mass to the concrete and prevent shrinkage of cement. Fine aggregate consists of river sand. It prevents shrinkage of cement. When surrounded by cement it gains mobility enters the voids in coarse aggregates and binding of ingredients takes place. It adds density to concrete, since it fills the voids. Denser the concrete higher is its strength.

Water used for making concrete should be clean. It activates the hydration of cement and forms plastic mass. As it sets completely concrete becomes hard mass. Water gives workability to concrete which means water makes it possible to mix the concrete with ease and place it in final position. More the water better is the workability. However excess water reduces the strength of concrete. The variation of strength of concrete with water cement ratio. To achieve required workability and at the same time good strength a water cement ratio of 0.4 to 0.45 is used, in case of machine mixingand water cement ratio of 0.5 to 0.6 is used for hand mixing.

Concrete

Concrete

 Plain concrete, commonly known as concrete, is an intimate mixture of binding material, fine aggregate, coarse aggregate and water. This can be easily moulded to desired shape and size before it loses plasticity and hardens. Plain concrete is strong in compression but very weak in tension. The tensile property is introduced in concrete by inducting different materials and this attempt has given rise to RCC, RBC, PSC, FRC, cellular concrete and Ferro cement.

Plain Concrete

Major ingredients of concrete are:

1.Binding material (like cement, lime, polymer)

2.Fine aggregate (sand)

3.Coarse aggregates (crushed stone, jelly)

4.Water.

A small quantity of admixtures like air entraining agents, water proofing agents, workability agents etc. may also be added to impart special properties to the plain concrete mixture.

Depending upon the proportion of ingredient, strength of concrete varies. It is possible to determine the proportion of the ingredients for a particular strength by mix design procedure. In the

absence of mix design the ingredients are proportioned as 1:1:2, 1:11/2 :3, 1:2:4, 1:3:6 and 1:4:8, which is the ratio of weights of cement to sand to coarse aggregate.

In proportioning of concrete it is kept in mind that voids in coarse aggregates are filled with sand and the voids in sand are filled with cement paste. Proportion of ingredients usually adopted for various works

How to test for a good brick?

 

How to test for a good brick?

Drop a brick vertically from a height of 1 m. A good quality brick will not break.

Strike two bricks against each other. Good quality bricks will produce a clear ringing sound on contact.

Perfect Mix

For a 4"thick wall (partition walls in the middle of the house), keep mortar proportion as =1:4 • (cement: sand)

For a 9"wall (outer wall), keep mortar proportion as = 1:6 (cement: sand)

Good Practice

Begin work at the corners, first to a height of 3 or 4 layers with base extending in steps.

Place all bricks on their bed. The depression on top provides space for the mortar to bond well. Use line-string, plumb bob, and spirit level for checking alignment, and to keep vertical and horizontal lines straight.

Soak your bricks in water for 8 hours at least before use, else it will absorb moisture from mortar.