1. Concrete

2. Concrete Main Primary Ingredients
Cement
Aggregate
Water

3. Cement Been around for at least 2 thousand yrs Portland Cement
Romans used them – found cements from natural mixtures of limestone and clay
Portland Cement
1824 the first modern step was taken by Joseph Aspdin – produces a powder from limestone and clay
Called it Portland Cement because it resembled a stone quarried near Portland, England
Been improved since then and produces under strict guidelines today

4. Portland Cement Ingredients
Lime, silica, alumina and iron
4 parts limestone to 1 part clay

5. Portland Cement Types 10 Normal – general work
20 Moderate – better resistant to sulfate – lower temp rise
30 – high early strength – fast curing (cold weather)
40 – low heat – lower heat – slower curing
50 – sulfate resistant – subject to attack from sulfate (salt)

6. Portland Cement Packaging
Bags – 94 pounds per bag – equal to one cubic foot volume
Or by the truck load

7. Cement Admixtures
Air entraining – contains small amounts of additives that case microscopic air bubbles to form the concrete
2 to 8 percent finished volume
Give improved workability
Gives better resistance to freeze and thaw cycle
With same structural strength

8. Aggregates Makes up ¾ of concrete mixtures Qualities
Strong
Resistant to freeze thaw cycle
Chemically stable
Properly sized
In general aggregate size must be no greater then ¾ size of the opening it needs to be placed in including reinforcement

9. Typical Aggregated sizes
¾” –thin slabs
¾” to 1 -1/2” – slabs structural work
Up to 6 in – Dams and other large projects

10. Making and placing Concrete
Rules for making high quality concrete
Use clean sound ingredients
Mix them in the correct proportions
Handle the wet concrete properly to avoid segregation
Cure the concrete carefully
Water to cement ratio – major factor in strength of concrete – less water stronger
But water is needed to hydrate the mixture – gives necessary fluidity and plasticity for placing and finishing
Absolute water-cement ratio under .60 – weight of water in mixture

11. Handling and Placing Concrete
Wet concrete is not a liquid – but a slurry – unstable mixture of solids and liquid
Segregate – move horizontally any distance in formwork
Dropped from height
Vibrated excessively

12. Curing Concrete Concrete cures through hydration Takes a long time
Must be kept moist until its required strength is achieved
Takes a long time
28 days typically
If allowed to dry during the time it will lose strength

13. Concrete Weights Normal weight concretes Structural lightweight
135 to 160 lb/ft3
Made with sand and gravel and crushed stone
Structural lightweight
85 to 135 lb/ft3
Made with expanded clay shale slate and slag
Insulating concrete
15 to 85 lb/ft3
Made with prumice, scoria, prelite, vemiculite dhatomite
Heavy weight concrete
175 to 400 lb/ft3
Made with hernalite, barite, limonite, iron, steel

14. Concrete Testing Slump test Compressive Strength Test
Slump cone 12 in high 8 in dia. At bottom
The cone is filled in three equal layers each being tampered 25 times with a 5/8 in bullet nose tampering rod
Cone is filled and level off it is then lifted and carefully – the amount of slump is then measured
Compressive Strength Test
Concrete specimens are usually cylindrical with a length equal to twice its diameter
The mold is filled in three equal parts and tampered 25 times with standard tamping rod between layers
When mold is full it is struck off and level and covered with a glass or metal plate to prevent evaporation
The specimens are removed from mold after 24 hours and place in the curing location
At the end of the curing period the specimens are subject to a compression test

15. Concrete Admixtures
An admixture which is used to speed up the initial set of concrete is called an accelerator
Most common is calcium chloride
The function of a retarded is to delay or extend the setting time of the cement paste in concrete
Used when concrete is set in hot, low humidity, and winding conditions
Most common retarders are carbohydrate derivatives and calcium liginsulfonate
The introduction of controlled amounts of air into a mix has proved to be an important advance in concrete engineering
A proper amounts of air improved workability, easier placing, increased durability, better resistance to frost action, improve flow in concrete pump lines and reduction in bleeding and water gain.
Air entrained concrete contains microscopic bubbles of air formed with the aid of group of chemicals called surface active agents, material which have property of reducing the surface tension of water
Air entrainment may have adverse effects on the strength of concrete – the decrease in strength is proportional to the amount of air contents
Super plasticizer – is a substance that disperse the clumps by coating each cement particle thus separating and dispersing the cement in the water.
When cement and water are mixed the cement particles tend to gather in clumps known as cement agglomerates, this clumping action prevents through mixing of cement and water producing a loss in the workability of the concrete mix as well as preventing complete hydration of the cement producing a less then full potential of strength
To prolong the life expectancy of the concrete surface, in addition to proper curing and finishing concrete hardeners are added to concrete mix

16. Reinforcing Concrete
Concrete has no useful tensile strength shear and bending
Steel and concrete change dimension nearly at the same rate
Steel is protected from corrosion by the alkaline chemistry of concrete plus concrete bonds strongly to steel
Theory of concrete reinforcing is extremely simple
Put steel where there is tension in the concrete member
Bars are rolled into 11 standard diameters in numbers of 1/8”
Bar size 3 to 18
Bars come in 3 grades
40,000 psi
50,000 psi
60,000 psi
Reinforcing steel is also produced in sheets or rolls of welded wire fabric
Used to reinforce slabs on grade
Principal advantage is economy of labor (easy to place)

17. Precast Concrete Advantages of Precast
Casting and curing conditions as well as concrete design can be rigidly controlled, resulting in consistently high quality concrete
Close supervision and control of materials and a specialized work force in a centralized plant also help to produce a high quality product
Finishing work on concrete surfaces can be done more easily and efficiently in a plant than in position on the site
The cost of form work is reduced since it can be set up on the ground rather than begin suspended or supported in position
Because of superior reinforcing techniques the load carrying ability of a member may be increase while at the same time reducing its dead load by reducing the amount of concrete required
Plant production is not normally subject to delays due to adverse weather conditions as are job site operations

18. Precast Concrete
Precast structural members fall into two general classifications
Normally reinforced
Prestressed
Prestressed broken down into two categories
Pretensioned
posttensioned

19. Pre stressed Concrete
A prestressed concrete unit is one into which engineered stresses have been introduced before it has been subjected to a load
Pretensioning
High strength steel strands are stretched tightly then concrete is pour concrete bonds together with the steel after it has cured the concrete unit is place in compression
Posttensioning
High strength steel strands are stretched tightly then concrete is poured , cured then the steel is tensioned usually done in place

20. Pre stressed Concrete
There are some distinct advantages to producing architectural precast units in a central plant
Design freedom
High degree of quality control of both the ingredients and the finished techniques
The shapes, sizes and configurations of units for are building may be as diversified or similar as the designer wishes
The possibility of including in the precast units some or all of the services which are normally installed after erection
The speed at which a building can be closed in reducing cost and allowing earlier access by the service trades.