Form-work is required to support the concrete until it attains sufficient strength to support itself.
A good form-work should have the following properties.
- It should be strong enough to resist the pressure or the weight of the fresh concrete and superimposed loads, if any.
- It should be rigid enough to retain the shape without excessive deformation.
- It should be economical in total cost.
- It must be easily removable.
Materials for Formwork
The main items which affect the cost of formwork are materials and labour in fabricating, erecting and removing the formwork. Materials used for formworks are decided by economy, requirement or a combination of both. most commonly employed materials are timber, plywood, Steel and Aluminium (nails, bolts and form connectors, etc). If the materials can be used more than one time, the cost per use may be relatively low.
Removal of Formwork
The formwork may be removed after the expiry of following periods under normal conditions when ordinary Portland cement has been used when rapid hardening cement is used, the above period is greatly reduced.
Formwork for Columns
Column form work may be out of timber, steel or fiber-board. The four sides of the Box are held in position by the use of wooden Blocks, bolts and yokes. A wash of water should be given to the inside of the formwork just before starting the placing of concrete.
The beam formwork
Consists of a three-side box which is supported by column forms or cross members called head trees.
Wall Formwork
The wall formwork consists of timber sheeting supported by vertical post and horizontal Wales. Small pieces of timber known as spacers may be used and they are to be removed as the concrete
Formwork for Staircase
It consists of stringers, sheets, joists, bearers and vertical posts. Stair is designed to be self supporting longitudinally from floor to landing or across wall to wall. The sloping slab is supported by planking which is further supported on battens and posts. To give shape to the stringers, inclined planks are fixed.
STEEL
Steel is a combination of Fe and C (Carbon).Carbon Percentage is around 0.2%-2.14%.Fe is a very flexible material and Carbon will reduce the flexibility. Zn,Na,Al also can be mix with Fe but Carbon is the cheapest one.
- Recycling
• Steel components generate minimum waste and all light gauge steel construction materials are 100% recyclable.
- Strength and durability
• Steel framing material is significantly stronger than wood. No other construction material can match the superior strength and durability of steel. Steel has the highest strength-to-weight ratio. Strength and non-combustible qualities of steel enable it’s framed structure to resist fires.
- Termites
• Steel is simply impervious to termites and other damage-causing bugs and pests. Steel studs are impervious to insects, rot, and mildew.
- Climate and condition or dimensionally stable
• Steel will not crack, shrink, splinter, creep, split, warp, or swell.
• Individual members of a wood-framed building react differently to climate changes. This wood movement may include expansion, contraction, warping, twisting, etc.
- Cost effective
• Steel studs are non-flammable, and won't add fuel to a fire. Insurance rates for steel homes are often cheaper than wood framed homes.
- Weight
• Steel components weigh 60% less than wood. A 2000 square foot home requires only 6 tons of steel compared to 20 tons of timber.
• Steel is light in self weight and very strong; transportation and site handling are easier.
- Always straight
• Unlike lumber, light steel components are always straight, this allows for a flat wall every time.
• Steel stays straight and true, while wood may warp or crack.
- High structural efficiency
• Steel components allow for longer spans with less post supports.
• Steel has one of the highest weights to strength ratios of all construction materials. This strength advantage means better design flexibility, wider spans and better material usage.
- Steel is a “green” structure environmentally friendly material
• Steel is environmentally friendly.
• Steel-framed housing dramatically reduces the amount of trees consumed for residential construction, thus conserving one of nature's most precious resources. The only wood products used for light gauge steel construction are plywood sheathing and door and window trim components.
- Only simple site assembly is needed.
- Steel construction is fast. There is virtually no "wet" work.
- Design is easy in seismic conditions.
Disadvantages of Steel Buildings
- In many areas it is difficult to find framing crews that are trained in light steel construction. This disadvantage usually raises the overall project cost.
- Because Steel studs are a poor insulator, they suffer from thermal bridging which reduce the overall wall insulation value.
- Even though steel components will not burn, they will actually fail (collapse) before a wood component in a house fire.
- Steel is an excellent thermal conductor requiring additional exterior insulation or thermal breaks to overcome this disadvantage. Thermal conductivity is probably the most serious of steel's disadvantages.
- Finding steel roof trusses is difficult in most areas so it is common to use wood roof trusses (and sometimes floor trusses).
- In addition to the labor training required to convert to the use of steel, builders and laborers must obtain some new tools not presently used by carpenters. The combination of additional training and tools translate to increased costs of construction for a short period of time.
Failure of Steel
Ductile failure Creep
Fatigue failure Impact failure
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
- Cement – react with water to form a “glue” to hold the concrete (primary binder to joint aggregate into a solid mass)
- Water – react with cement to gain the strength (hydration)
- 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
- 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
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
- 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
- Mixing
- Transporting
- 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
- Compacting The purpose of compacting the concrete is to achieve maximum density by removing air voids from the concrete.
- 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
- 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.
- Workability Workability is often considered to be a measure of the work needed to compact the wet concrete
- Uniformity Degree of homogeneity or state of distribution within the mix
- 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
- Bleeding The Bleeding is the development of the layer of water at the top or surface of freshly placed concrete
è Strength
è Durability
è Fire Resistance
è Acoustic Property
è Alkali Resistance
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
No comments:
Post a Comment