EXAMPLE QUESTIONS According to their sizes, aggregates are divided into 2 main groups, write down the names of these groups and give one example for each group. Explain briefly the main characteristics of aggregates. Write down the names of the three main binding materials.

MANUFACTURED STONES Manufactured stones are produced from natural stone fragments by using a binding element. Also ,colour pigments can be added in order to obtain various colours.

Examples: Mortar Concrete Concrete Blocks Terrazzo Ytong

CONCRETE Difference between cement and concrete; Cement is the basic ingredient of the concrete. Concrete is a chemical compound made-up of different materials like cement, sand and gravel by mixing them with water.

General Introduction for Concrete Poured concrete ( In-situ Concrete) is used in structures like single homes, apartment houses, schools, offices, industrial buildings etc. Concrete structure serves as footings, foundations, slabs, steps and walls.

Reasons behind why concrete is used: It resists decay, moisture, fire, weather conditions and corrosion from acids. But the main reason is that before it hardens, it can be moulded to any shape.

In order to shape the poured concrete forms are used. Some of the common materials used to construct forms are: - wood, - plywood, - metallic sheets and frames.

Concrete can be reinforced (made stronger) with steel rods/bars or wire mesh. The reinforcement material is laid in the forms before the concrete is poured into the forms. Therefore it’s buried in the concrete while the concrete batch poured into the forms.

PREPARATION OF CONCRETE Concrete is prepared by two methods: - Mixed on the site with materials stockpiled nearby -Ready mixed in central plants and transported to building site in huge mixer trucks.

Mixing on the construction is less popular. Because it is not practical as the preparation and transportation and placement of concrete into forms takes long time. In addition, stockpiling and mixing takes a lot of space at the construction site.

INGREDIENTS OF CONCRETE Concrete is mixture of 3 basic materials, in proper amounts: - Portland cement - Water - Aggregate (Coarse and Fine Aggregate)

WATER RATIO The water- cement ratio of given concrete determines its strength. If too little water is used in the mix, some particles of cement won’t get wet; therefore they won’t be chemically changed. If too much water is used, the excess water may be trapped in the mixed. In either case, the concrete will be weakened. A 50kg. Bag of cement requires 10-11 lt. of water for a complete chemical combination of materials. The use of exact amount is not practical for field conditions. Usually 17,5 to 35 lt. must be used for each bag of cement.

The moisture and/or wetness of the aggregate in the mixture of the concrete is an important factor in determining the water ratio. The aggregate normally found in three conditions: damp, wet, very wet. Certainly, the volume of water required for damp/wet/very wet aggregate is less than dry aggregate.

A test for determining the wetness of the fine aggregate is very simple: Press some sand in your hand. If it falls apart after you open your hand, it is damp. Make a ball of a few centimetres in diameter, in your hand, and if the ball holds its shape, the material is wet. If the material sparkles and wets your hand, it’s very wet.

Adjustments for certain amounts of water could be as follows: Water per cement bag Aggregate Damp Wet Very Wet Cold Weather 20 Lt. 19 Lt. 18 Lt. 17 Lt. Normal Weather 25 Lt. 21,5 Lt. 20,5 Lt. Hot Weather 30 Lt. 27 Lt. 23 Lt. 20 Lt.

PROPORTIONS OF THE MIX A common mix by volume: 1:3:4 This mix contains one part cement, three parts sand and four parts coarse aggregate i.e. gravel, crushed stone.

Nominal volume mix by different types of uses: 1:8 all-in 1:3:6 Foundations/site concrete 1:2:4 Minimum quality plain structure concrete 1:1,5:3 Reinforced concrete

MIXING OF CONCRETE An important factor in the workability and strength of concrete is the method used to mix the ingredients. It is essential to mix thoroughly for small jobs, sometimes concrete is mixed on the site using equipment called mixer. An other case the concrete is mixed at a central ready mix plants and transport to the site in a ready mix truck. In most ready mixed operations, the materials are dry batched at the central plant and than mixed enroute to the site in truck mixers.

Another method is to mix the concrete in a stationary mixer at the central plant just enough to intermingle the ingredients. The mixing is then completed in a truck mixer enroute to the job site. Most truck mixers have a capacity of 9 cubic meters (m3). And carry their water supply.

MIX CONSISTENCY/ SLUMP TEST The mix consistency or degree of stiffness of plastic concrete is called slump (akicilik). Very fluid (wet) mixes are called high slump concrete while stiff (dry) mixes are called low slump concrete. Slump is related primarily to water-cement ratio. High water content causes a high slump. Generally a low slump concrete will produce a better concrete product. A slump test may be performed in the field By using a sheet metal slump cone of 20 cm in diameter .

At the bottom, 10 cm in dia at the top and 30,5 cm. Height. After it’s filled with the sample concrete, the concrete is rotted 25 times it a metal rod. Then it’s filled and rotted 25 more times, and later the same process is repeated once more. Finally the concrete is taken out by lifting it straight up: Naturally the concrete is slumped and stays still at some height. The distance between the height of the come and the concrete paste is called slump rating of the concrete batch.

CURING The strength of the concrete grows quite rapidly for the first twenty-eight days (The period of time compression tests are based on), then levels off and continues increasing in strength for an indefinite period of time. The main purpose of curing is to have the exposed surface of concrete kept moist and warm so that the chemical process of hydration can take place

An excessive water loss (through evaporation) will cause the concrete to shrink rapidly and crack. The cracks will appear as tensile stresses are relieved due to shrinkage. With a temperature at or near 0°C hydration is slowed to a point where little or no strength is acquired. It’s therefore important to maintain a balance between moisture loss and the proper temperature to ensure proper curing during the initial setting.

METHODS OF THE CURING CONCRETE Water is added to the surface of the concrete. This can be done by flooding the area, using earth or sand as a retainer around the edge of the concrete. Water can be sprinkled over by using a hose at some periods of once, twice a day, for about 3-4 weeks.

2. Sealing the surface. This may be accomplished by covering the exposed area with waterproof paper or polyethylene sheet, or by spraying on a sealing compound. The use of waterproof paper or polyethylene sheet is common on large areas. Sealing compounds are used immediately after the finish on the slab is complete. Sprayer applies them and care should be taken to ensure complete coverage of the exposed area.

3 Steam and heat curing. This method is used generally under the controlled conditions of precasting plant. Steam and heat accelerates the strength of the concrete by shortening the curing time.