Black River Vocational Training Centre
General Construction Department
Carry out concrete work
Information Sheet
Preparing and mixing concrete
Concrete can be considered a mixture of water cement (cement paste) and aggregates (fine and coarse) in a definite proportion which undergoes a process of hydration which allows the mixture to become solid taking a rock like form. Concrete since its use in the early days of the Egyptian has been used for several application in the building and construction industry. Every where the work carried out with the use of concrete is evident as seen in our: bridges, airplane run ways, pairs, some road ways houses side walks and the list goes on.
The question might be asked how is concrete mixed and placed to form the various products built with concrete. Concrete is mixed using one of two means. Firstly concrete is mixed manually (by hand). The process involved in mixing concrete by hand is rather simple and straight forward and follows the following listed steps below in the flow chart.
Preparing and mixing concrete
Concrete can be considered a mixture of water cement (cement paste) and aggregates (fine and coarse) in a definite proportion which undergoes a process of hydration which allows the mixture to become solid taking a rock like form. Concrete since its use in the early days of the Egyptian has been used for several application in the building and construction industry. Every where the work carried out with the use of concrete is evident as seen in our: bridges, airplane run ways, pairs, some road ways houses side walks and the list goes on.
The question might be asked how is concrete mixed and placed to form the various products built with concrete. Concrete is mixed using one of two means. Firstly concrete is mixed manually (by hand). The process involved in mixing concrete by hand is rather simple and straight forward and follows the following listed steps below in the flow chart.
Procedure for Mixing concrete by Hand
Step 1
Make concrete bed
Make concrete bed
Step 2
Place fine aggregate on concrete bed
Place fine aggregate on concrete bed
Step 3
Add cement and blend with aggregate
Add cement and blend with aggregate
Step 4
Add coarse aggregate
Add coarse aggregate
Step 5
Add water and combine all dry ingredients using a shovel
Add water and combine all dry ingredients using a shovel
Step 6
Mix ingredient until the desired concrete is achieved
The steps above shows the Procedure for mixing concrete by hand
Where there is a high demand for concrete in a relatively short period of time it should be noted that concrete con be mixed using machine. When concrete however is mixed with machine the process is altered as the sequence of operations alters significantly from that mixing done with the hand. The procedure for mixing concrete by mechanically is as follows:
Steps in mixing Concrete With Machines
Mix ingredient until the desired concrete is achieved
The steps above shows the Procedure for mixing concrete by hand
Where there is a high demand for concrete in a relatively short period of time it should be noted that concrete con be mixed using machine. When concrete however is mixed with machine the process is altered as the sequence of operations alters significantly from that mixing done with the hand. The procedure for mixing concrete by mechanically is as follows:
Steps in mixing Concrete With Machines
Add ½ the required water in mixer
Add ½ coarse aggregate required
Add ½ cement
Add all fine aggregate
Add remainder of coarse aggregate
Add remainder of cement and water until desired mix is achieved
N.B. It should be noted that the mixer must be thoroughly clean within 30 min after use
Types of concrete
Concrete can be produced of several different types. These types are as follows:
Premix / ready mixed: this form of concrete is where concrete has been mixed offsite generally in a very controlled environment and then transported to site in specially designed trucks with rotating buckets. This form of concrete generally used with pumps which have the ability to concrete up tubes near 100 feet without segregation. This form of concrete is generally of high standard as the condition used for mixing is generally more through than on site mixing.
In-situ cast concrete: this form of concrete is mixed and placed on site using either mechanical or manual means. This is most advantageous as alterations can be made to the amount of concrete being used and concrete is conveniently mixed close to where it will be used. This type of concrete is however limited as large amounts concrete generally take a lot of time and or man power to be mixed.
Pre-cast concrete: this form of concrete is basically concrete that has been mixed and formed into structure and then placed into potion as a unit on site. This form concrete is very useful when constructing several units with similar type members in each unit.
Terms and definitions
Like most trades concrete operators have some basic terminologies within their trade which are listed below
Aggregates A hard rock like mass of sand, grvel or crushed rock. These generally come in two form fine (will pass through a number 4 sieve) and coarse aggregate that is larger than ¼ in 6mm)
Air-entraining Material added to concrete allow small air pockets to be embedded in concrete to prevent cracking due to freezing of concrete which result in the expansion and contraction of concrete
Bleeding This is the condition in freshly placed concrete where the water rises to the top of the surfaces
Cement A material which is one of the active ingredients in concrete which is used as the binding agent for aggregate.
Cement Paste The active part of wet concrete and generally denotes the strength of the concrete. This paste is formed when water and cement interacts in a concrete mix
Compressive Strength This is the ability of concrete to resist land load being imposed against the surface of concrete or the ability of concrete to resist crushing loads.
Tensile strength The ability of concrete to resist being pulled apart or twist. Concrete in general has low tensile strength hence steel reinforcement is used to improve on the strength of the concrete
Consistency This is the degree to which concrete ingredients have been combined together in an even proportion and is generally determined by the wetness or dry ness of concrete mix
Cubic This is the unit measure for concrete. This is generally determined by length of concrete multiplied by width of concrete multiplied by the thickness of the concrete
Water-tightness This is the ability of concrete to resist the passage of water through its membranes
Cement ratio This is the comparison of the amount of cement in a given concrete mix to amount of water in the mix. It used be noted that less water in the mix the greater the strength of the concrete within the allowed limits for mixing of concrete.
Slump A test administered on wet concrete to determine the workability (stiffness) or plasticity of the wet concrete
Hydration A chemical process which allows the concrete mix to loose water and become solid.
Admixture Mixtures added to concrete to add to the qualities of the concrete.
Accelerator A admixture that reduces the setting time for concrete.
Retardant A admixture which facilitate the extension of the setting time of concrete
Placing of concrete
Concrete placement is both a skill and an art the concrete work must me meticulous in his approach towards finishing the concrete surface as there is room for creativity in his approach towards his finish.
N.B. It should be noted that the mixer must be thoroughly clean within 30 min after use
Types of concrete
Concrete can be produced of several different types. These types are as follows:
Premix / ready mixed: this form of concrete is where concrete has been mixed offsite generally in a very controlled environment and then transported to site in specially designed trucks with rotating buckets. This form of concrete generally used with pumps which have the ability to concrete up tubes near 100 feet without segregation. This form of concrete is generally of high standard as the condition used for mixing is generally more through than on site mixing.
In-situ cast concrete: this form of concrete is mixed and placed on site using either mechanical or manual means. This is most advantageous as alterations can be made to the amount of concrete being used and concrete is conveniently mixed close to where it will be used. This type of concrete is however limited as large amounts concrete generally take a lot of time and or man power to be mixed.
Pre-cast concrete: this form of concrete is basically concrete that has been mixed and formed into structure and then placed into potion as a unit on site. This form concrete is very useful when constructing several units with similar type members in each unit.
Terms and definitions
Like most trades concrete operators have some basic terminologies within their trade which are listed below
Aggregates A hard rock like mass of sand, grvel or crushed rock. These generally come in two form fine (will pass through a number 4 sieve) and coarse aggregate that is larger than ¼ in 6mm)
Air-entraining Material added to concrete allow small air pockets to be embedded in concrete to prevent cracking due to freezing of concrete which result in the expansion and contraction of concrete
Bleeding This is the condition in freshly placed concrete where the water rises to the top of the surfaces
Cement A material which is one of the active ingredients in concrete which is used as the binding agent for aggregate.
Cement Paste The active part of wet concrete and generally denotes the strength of the concrete. This paste is formed when water and cement interacts in a concrete mix
Compressive Strength This is the ability of concrete to resist land load being imposed against the surface of concrete or the ability of concrete to resist crushing loads.
Tensile strength The ability of concrete to resist being pulled apart or twist. Concrete in general has low tensile strength hence steel reinforcement is used to improve on the strength of the concrete
Consistency This is the degree to which concrete ingredients have been combined together in an even proportion and is generally determined by the wetness or dry ness of concrete mix
Cubic This is the unit measure for concrete. This is generally determined by length of concrete multiplied by width of concrete multiplied by the thickness of the concrete
Water-tightness This is the ability of concrete to resist the passage of water through its membranes
Cement ratio This is the comparison of the amount of cement in a given concrete mix to amount of water in the mix. It used be noted that less water in the mix the greater the strength of the concrete within the allowed limits for mixing of concrete.
Slump A test administered on wet concrete to determine the workability (stiffness) or plasticity of the wet concrete
Hydration A chemical process which allows the concrete mix to loose water and become solid.
Admixture Mixtures added to concrete to add to the qualities of the concrete.
Accelerator A admixture that reduces the setting time for concrete.
Retardant A admixture which facilitate the extension of the setting time of concrete
Placing of concrete
Concrete placement is both a skill and an art the concrete work must me meticulous in his approach towards finishing the concrete surface as there is room for creativity in his approach towards his finish.
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