Superstructure Construction Material Suitability

 

EXECUTIVE SUMMARY

This project involves the selection criteria of the material used to construct a three story college building superstructure at the brown field site. Generally superstructure is the above foundation/ground level part of the building. The main goal of this report is to analyze several materials and the suitability for superstructure construction. We will analyze the following materials; reinforced concrete, steel, timber and then select one most suitable material for our project. The main goal of the report is to analyze three of the possible materials suitable for the superstructure and select the most suitable materials for different elements while considering the sites generic and specific implications.

Components of a superstructure

Before we analyze and give a recommendation of the best suitable materials for a superstructure it is important to understand the various components that make up a superstructure. This includes; columns, slabs, walls, roofs and beams. In details

1. Columns

Columns are the perpendicular load bearing elements of the superstructure and can be either for architectural or engineering purposes. Architectural columns are majorly for aesthetics while engineering columns are for support/load bearing and are used to transmit loading to the footing then to the ground. In some cases structural columns are decorated to improve its aesthetics. Columns can be made in either reinforced concrete, steel or timber based on several suitability factors like the size of the structure, materials availability etc.  For this case we will use steel frames encased in concrete cover for columns. Column construction will involve; Colum formwork, Colum reinforcement, concrete pumping and formwork lifting.

2. Slabs

These are the flat and horizontal components of the building where the weight due to different elements like occupancy act on. Slabs can be made of either reinforced concrete, steel or timber. For this case we will use concrete slabs. . Slab works consists of the following steps: Slab formwork, concrete pumping and finally the formwork lifting

3. Beams

Beams are the horizontal elements used to transfer the structures weight to the columns which then transmit the weight to the footing then to the ground. They can either be made of reinforced concrete, steel or wood. For this project steel beams will be used. The construction of the steel beams which will then be encased in concrete. Beam construction will involve Beam reinforcement, beam formwork, concrete pumping and lifting of the formwork.

4. Walls

Walls are the building enclosing structures and can either be load bearing or non-load bearing part of the superstructure. Walls can either be made of reinforced concrete for load bearing walls and either timber, masonry bricks, straw, etc. for non-load bearing walls.

5. Roofs

Roofs are the overall top part of the structure mostly used a shade and protection of the building from excessive rain and sun, The roof structure design is influenced by many factors like, the span of the building, the types of roof cover to be used, roof cover and weather load, cost, aesthetics

MATERIALS REVIEW

It takes a combination of many different materials to complete a building construction project

1. Reinforced concrete

Reinforced concrete a mix of concrete and steel reinforcements. Concrete is one of the most ancient construction materials and it was employed by romans as early 100 BC and has extreme compressive strength. To achieve much higher tensional strength while constructing, concrete is mixed with steel bars to form reinforced concrete that has an overall greater strength than either steel or concrete alone, Benham, 1983.

Reinforced concrete is widely used in the construction of various elements of a superstructure of buildings; columns, beams, slabs and shear walls. Concrete is normally poured around steel formwork and the shape is determined by the encasements used to limit the movement of concrete. The concrete is then left to dry to achieve the various structural elements and maximum strength. (Abdulla 708477)

2. Steel.

There are several metals used in construction but steel is the most popular of them all and it has been used over centuries now. Steel has got numerous applications that range from decorative to structural support to reinforcements.

Steel applied independently and not in concrete is known as structural steel. Structural steel is mostly used to refer to hot rolled steel sections, plates and shapes. The different structural steel shapes are as a result of passing heated steel strips through successive rollers with respective molds. (Ali 708389)

3. Wood

Wood is among the earliest materials ever used in construction and is still being widely used despite the popularity of concrete and steel. Wood is mostly used for structural framing (finish carpentry), cabinetry, trim, floors. Wood has high strength in compression, tension and bending in relation to its weight. Wood also has extremely good impact resistance. In current times there is an increased use of timber as plywood (Glue laminated timber) to achieve even much great strength and lengths. In this glue laminated timber wooden frames are arranged at right angles to each other. This maximized the strength of the wood. (Mohammed 729291)

 

MATERIAL SELECTION (REINFORCED CONCRETE FOR THE SLAB WHILE STEEL FRAMES FOR SLABS AND COLUIMNS)

Our material of choice for this case is reinforced concrete and steel. Concrete is the most used construction material world-wide and its application is very ancient with very few changes introduced along the years. Due to this few variabilities in development, concrete execution technique in constructions are well established so as to achieve perfect structures. Steel follows closely with it being popular with multi story structures.

Concrete is an artificial mixture made from Portland cement, aggregates and water. Cement has been in use around the world for numerous years but the most commonly used cement today (Portland cement) was invented in Britain in the year 1824. Portland cement is produced by mixing ground limestone, shale or clay, sand and iron ore then the mix is heat to 1600 degrees Celsius in the rotary kiln.

Concrete is formed by mixing the Portland cement and aggregates and water whereby a chemical process called hydration occurs. The hydration process forms concrete which is in plastic state and it transforms into solid state in about 2 hours. The concrete continually gains strength on cooling with maximum strength at the 28^th day given the curing is done correctly,

SUITABILITY OF CONCRETE FOR THE CONCRETE SLABS

The following properties of concrete makes them the best suited materials for construction of slabs. Concrete is a mix of several materials formed through solidification of cement, water and aggregates (either fine or course aggregates or both). Reinforced concrete is steel rod enhanced concrete to achieve a much higher strength.

1. High Compressive and Tensional strength

The strength of concrete is generally higher than most of the other construction materials. The strength of concrete is determined by the water – cement ratio and the lower the ratio the higher the final strength of the given concrete. There are two components used in classifying the strength of concrete as a suitable construction material, Neville, 2011

Compressive strength of concrete

This the maximum resistance ability of concrete to axial loading at an age of 28 days after mixing, placement and curing. Compressive strength of concrete is measured in Pounds per square inch (psi). During the first 28 days, concrete achieves about 90 % of its strength and it is important to ensure sufficient curing disallowing drying/freezing so as to achieve this strength. The concrete’s compressive strength is dependent on quality and proportions of the concrete’s ingredients and the curing environment. Neville 2011,

Tensional strength

Reinforced concrete derives its tensional strength from using steel reinforcement bars. Steel has extremely good tensional strength with relatively low compressive strength as compared to concrete. The combination of the two harness the compression and tensional strength of each other to achieve higher overall strength. Mtallib, 2010.

2. Workability

It is much easier to work with concrete in construction of the super structure. This is due to its plastic state within the 0 – 2 hour range. Workability can be termed as the ease with which it can be mixed, placed, compacted and finished. There are however several factors that influence the workability of concrete which include; Characteristics and quantity of the cementing materials, the slump (concrete consistence), duration and the method of transportation, the percentage of the entrained air, the aggregate grading, surface texture and shape, water content, ambient and concrete air temperature, presence of admixtures

The workability of concrete can be determined using a test called slump test that is basically the indication of the water content of a mix. The water content to a great extent determine the workability of any given concrete.

3. Good adhesion to reinforcements

One of the major factors that make concrete the most preferred materials in construction over ages is its great adhesion ability with different materials and more so with steel reinforcements. The bond strength between concrete and steel reinforcement bars are as a result of mechanical interactions, chemical reactions and thermodynamics reaction that happen upon placement of concrete to steel formwork. Kendall, 1983

4. Corrosion resistance

The corrosion resistance ability of concrete makes reinforced concrete materials best suited against corrosion as compared to other construction material like wood and timber. Corrosion mostly affect the steel reinforcement and least concrete. Steel reinforcements are normally encased within concrete covering to reduce corrosive action of the environment and other materials to the steel reinforcements, Neville, 2011.

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5. Fire and Heat resistance

Concrete has very high heat resistance compared to steel and wooden construction material. Reinforced concrete materials failure due to heat is mostly due to the failure of steel bars under extreme heat. However to counter this the concrete cover provides a good insulation of the covered reinforcement steel bars in extreme fire conditions. This provides more time for fire extinguishing and rescue in case of a fire hazard.

6. Corrosion Resistance

The corrosion resistance of concrete is very high hence can survive numerous types of environment. On the contrary reinforcement steel react to numerous aspects of our environments including humidity causing rust. Reinforced concrete is designed such that the more resistant concrete protects the much delicate steel bars.

7. Sufficient density

The density of reinforced concrete is basically the measure of its unit weight which is average weight density of 150 lb/ft³ (pcf).. This density is sufficient enough to protect the building from nature forces and other natural or induced forces that can affect the given structure

SUITABILITY OF STEEL FRAMES FOR BEAMS AND COLUMNS

Steel was selected for construction of the beam and column structural members due to the following reasons.

1. Steel is easy to assemble

It is much easier to assemble steel as the different steel frames can easily be fabricated to the required shape and size in the factory and then be carried to the given site for assembly. It is also easier and faster to fabricate steel at site as compared to concrete and wood making it cheap to build the given substructure. All this speed up the construction process while maintaining the required tight construction tolerance

2. Steel is strong in both compression and tension hence high tensional and compressive strength.

Steel has relatively very high compressive and tensional strength making it very suitable for construction of beams and columns. More and more stronger structural steel has been developed along the way and the yield strength of most structural steel in use today is more than 50,000 psi.

3. Steel is more reliable and predictable

Most steel is fabricated in the factories where its properties are closely monitored and controlled conditions by using very modern and recent technology in quality assurance

4. Use of steel is more efficient.

Construction of buildings using steel can be optimized by use of slender columns maximizing the floor space available for other things. Typical steel column occupies 75 % less space as compared to a concrete column

References

1. ACI committee, roller compacted mass concrete, part 1, ACI manual of concrete practice
2. Benham, P.P, Warnock, F.V, 1983, Mechanics of solids and structures, Pitman publishing Limited, London UK
3. British Standards, BS4449, British standards for reinforcing steel
4. EN 10025; European structural steel code of practice.
5. Kendall, K. Howard, A.J, 1983, the relation between Porosity, microstructure and strength, and the approach to advanced cement based materials.
6. Materials data book, Cambridge University Engineering department
7. Mtallib, M.O.A, Marke A.I, 2010, Comparative evaluation of flexural strength of concrete, Nigerian Journal Technology, 2013
8. Neville, A.M, 2011, Properties of concrete, Department of Civil Engineering, University of Leeds, England.
9. Neville, A.M, Brooks, J.J, Concrete Technology, Department of Engineering, University of Leeds, England.