CONCRETE

As a Quantity Surveyor he must have some knowledge about Materials that have been used in Construction Industry.Concrete is one of them. It's Consisting with,

• Cement
• Water
• Fine aggregate (sand)
• Course aggregate (stone chip or boulders)
• Admixtures

Roles of each Compositions

1. Cement – react with water to form a “glue” to hold the concrete (primary binder to joint aggregate into a solid mass)
2. Water – react with cement to gain the strength (hydration)
3. Aggregate – Make concrete stronger, more durable, less cost and reduce shrinkage of  Concrete                                                                                                                                              40mm = Mass Concrete in Road Construction                                                                               20mm = General Concrete work                                                                                                     10mm = Thin Sections, Screed over 50mm thk                                                                                5mm   = Screeds 50mm thk or less                                              
4. Admixture–modify the properties of the concrete such as increase setting time, durability, Workability of fresh concrete,give colour to Concrete

Types of concrete

•   Light weight concrete

 Light weight aggregate concrete

 Cellular concrete

 No fine concrete

•  Aerated concrete

•  Polymer and polymer cement                                                                                                

Light weight concrete

Its advantages are less need for structural steel reinforcement , smaller foundation           requirements,better fire resistant and insulation properties.

      it can be produced in three ways;

1.       By using light weight aggregate – Light  weight aggregate concrete

2.       By introducing gases or air bubbles- Cellular concrete

3.       By removing the fine aggregate- No fine concrete

            Aerated concrete

            It is an environmentally friendly material. These include lime, gypsum, water and a                            expansion agent

            Polymer and polymer cement concrete

            Polymer concrete is a composite material in which the binder consists entirely of synthetic                   organic polymer.There is a substantial increase of cost because of the use of polymer 

             instead of Portland cement.

Stages of Concrete Preparation

1. Batching                                                                                                                                Before concrete can be mixed, the components have to be measured in their correct proportions.This can be done either by volume or weight        
2. Mixing                                                                                                                               
3. Transporting                                                                                                                           
4. Placing                                                                                                                                         It is essential that concrete should be placed in the form-work as near as practicable to its final position and before it becomes  too stiff to work. Concrete is placed in even layers. A concrete layer is placed over before the previous has set           
5. Compacting                                                                                                                                 The purpose of compacting the concrete is to achieve maximum density by removing air voids from the concrete. 
6. Curing                                                                                                                                         After placing and compacting , the concrete should be allowed to set and harden for a particular period under controlled humidity & temperature. This process is termed as ‘Curing’

Properties of Fresh Concrete

1. Consistency                                           The slump test, is the most generally accepted method used to measure the consistency of concrete. The test equipment consists of a slump cone (a metal conical module 300mm high, with a 200mm- diameter base 100mm-diameter top) and a steel rod 16mm in diameter and between 450mm and 600mm long with a hemi spherically shaped tip.                    
2. Workability                               Workability is often considered to be a measure of the work needed to compact the wet concrete             
3. Uniformity                                                                                                                          Degree of homogeneity or state of distribution within the mix                                             
4. Segregation                                                                                                                     Segregation is the tendency for coarse aggregate to separate from the sand-cement mortar.This results in part of the batch having too little coarse aggregate and the remainder having too much.The former is likely  to shrink more and crack and have poor resistance to abrasion         
5. Bleeding                                                                                                                                 The Bleeding is the development of the layer of water at the top or surface of freshly placed concrete  

Properties of Hardened Concrete

è Strength

è Durability

è Fire Resistance

è Acoustic Property

è Alkali Resistance

Factors Affecting Strength of Concrete

è Curing conditions, humidity, temperature

è w/c , (inversely related)

è Aggregate characteristics, roughness, grading,

è Cement type

è Cement content (directly related)

è Mixing water