Class Day NineteenClass Day Nineteen

Introduction to Reinforced ConcreteIntroduction to Reinforced Concrete

Concrete is a mixture of graded aggregates held Concrete is a mixture of graded aggregates held together by a matrix.together by a matrix.

Concrete used in construction is a mixture of Concrete used in construction is a mixture of graded aggregates:graded aggregates:

Large aggregates:Large aggregates: Hard gravel ranging in Hard gravel ranging in size approximately from ¼” to ¾”. Clean and size approximately from ¼” to ¾”. Clean and free of dust & small particles.free of dust & small particles.

Small aggregates:Small aggregates: Clean washed sand free of Clean washed sand free of dust and small particles.dust and small particles.

Aggregates form a mixture of larger stones, Aggregates form a mixture of larger stones, with voids filled with smaller stones, with with voids filled with smaller stones, with voids filled with smaller stones, with voids voids filled with smaller stones, with voids filled with sand particles, all coated and held filled with sand particles, all coated and held in place by a properly mixed paste made from in place by a properly mixed paste made from portland cement and water.portland cement and water.

ConcreteConcreteSTRUCTURAL CONCRETESTRUCTURAL CONCRETE IngredientsIngredients

Portland CementPortland CementCourse AggregateCourse AggregateFine AggregateFine AggregateWaterWater

Admixtures are notAdmixtures are notstandardstandard and are and areused for specialused for specialpurpose, such as purpose, such as ease of placement orease of placement orprotection fromprotection fromfreezing.freezing.

Even though concrete is “rocklike”, the final result Even though concrete is “rocklike”, the final result can appear pliable, and assume most any shape.can appear pliable, and assume most any shape.

THE STRENGTH OF CONCRETE IS DEPENDENT THE STRENGTH OF CONCRETE IS DEPENDENT UPON FOUR CONDITIONS:UPON FOUR CONDITIONS:

1 1 The hardness of the aggregatesThe hardness of the aggregates. Obviously, . Obviously, the hardness of the aggregate is important to the hardness of the aggregate is important to strength in compression, since concrete could fail strength in compression, since concrete could fail by forces that crush the aggregates.by forces that crush the aggregates.

2 2 The degree of proportion of distribution of the The degree of proportion of distribution of the aggregate sizesaggregate sizes. The variation of aggregate size is . The variation of aggregate size is important in that proper distribution is important in that proper distribution is instrumental in eliminating voids of hard material, instrumental in eliminating voids of hard material, making the mix as homogeneous as possible. making the mix as homogeneous as possible.

33 The ratio of the amount of water to that of The ratio of the amount of water to that of cementcement. The hardening of concrete is not a drying . The hardening of concrete is not a drying process, but a chemical reaction called process, but a chemical reaction called HYDRATIONHYDRATION. It is the reaction between water and . It is the reaction between water and portland cement, and a concrete mix requires only portland cement, and a concrete mix requires only a small amount of water for hydration to occur. a small amount of water for hydration to occur.

Too much water dilutes the hydration Too much water dilutes the hydration process. Too little water limits the completeness process. Too little water limits the completeness of the process. of the process.

If the water in concrete freezes before If the water in concrete freezes before hydration is complete, the process stops and the hydration is complete, the process stops and the resulting concrete is weak and poor in wearability.resulting concrete is weak and poor in wearability.

44 The care given to the concrete after it has The care given to the concrete after it has been placedbeen placed. After concrete is placed, it must be . After concrete is placed, it must be given the conditions to “cure” which is the given the conditions to “cure” which is the completeness of the hydration process. completeness of the hydration process.

Concrete placed where the hot sun Concrete placed where the hot sun evaporates the water near the surface causes evaporates the water near the surface causes uneven hydration and results in poor durability of uneven hydration and results in poor durability of the outer surface. Concrete footings placed the outer surface. Concrete footings placed against dry earth lose water due to absorption against dry earth lose water due to absorption into the dirt, causing uneven curing.into the dirt, causing uneven curing.

PORTLAND CEMENTPORTLAND CEMENT

The manufacture of cement for concrete was The manufacture of cement for concrete was first developed in Portland, England in 1824. That first developed in Portland, England in 1824. That process has developed in all parts of the world process has developed in all parts of the world using the natural ingredients necessary for the using the natural ingredients necessary for the finished product. finished product. The designation, ‘portland’ is The designation, ‘portland’ is one of definition of ingredients rather than a one of definition of ingredients rather than a proper name.proper name.

Portland cement is a mixture, primarily of Portland cement is a mixture, primarily of ingredients that contain lime, iron, silica, and ingredients that contain lime, iron, silica, and alumina – all natural elements found in various alumina – all natural elements found in various formations of limestone, marble, marl, seashells, formations of limestone, marble, marl, seashells, clay, or shale. clay, or shale.

Portland cement is manufactured in seven types, Portland cement is manufactured in seven types, each containing admixtures for specific each containing admixtures for specific requirements such as high early strength, air requirements such as high early strength, air entraining, low heat of hydration, and resistance to entraining, low heat of hydration, and resistance to sulfates. sulfates. The general use of cement for concrete The general use of cement for concrete in construction is Type 1 – normal cement.in construction is Type 1 – normal cement.

AS A DEVELOPING ARCHITECTAS A DEVELOPING ARCHITECT, or AS SOMEONE , or AS SOMEONE WHO IS SUPPOSED TO POSESS INTELLIGENCE: WHO IS SUPPOSED TO POSESS INTELLIGENCE:

please do not ever refer to concrete items as please do not ever refer to concrete items as “cement walkways or cement footings or cement “cement walkways or cement footings or cement driveways or cement roads.” Cement is an driveways or cement roads.” Cement is an ingredient in the manufacture of concrete – just as ingredient in the manufacture of concrete – just as flour is an ingredient in bread – or just as sugar is flour is an ingredient in bread – or just as sugar is an ingredient in ice cream.an ingredient in ice cream.

MIXING AND PLACING CONCRETEMIXING AND PLACING CONCRETE

AA MIXINGMIXING: Concrete should be mixed, using a : Concrete should be mixed, using a reference of proportioning of aggregates by reference of proportioning of aggregates by weight. The condition of aggregates is important, weight. The condition of aggregates is important, in that the amount of moisture present in gravel or in that the amount of moisture present in gravel or sand contributes to the amount of water for the sand contributes to the amount of water for the mix, and excessive water is detrimental to the mix, and excessive water is detrimental to the strength of the mix. strength of the mix.

Concrete for construction is generally mixed at a Concrete for construction is generally mixed at a batching plant, set up for the scientific methods of batching plant, set up for the scientific methods of proportioning the mix. The ingredients are placed proportioning the mix. The ingredients are placed in a large rotating drum on a truck, so the mix is in a large rotating drum on a truck, so the mix is complete by the time it gets to the job site.complete by the time it gets to the job site.

CONCRETE TRANSIT TRUCK

You as a project architectYou as a project architect will specify a will specify a strength required for concrete. The contractor in strength required for concrete. The contractor in turn will order the concrete from a manufacturing turn will order the concrete from a manufacturing plant and hold them responsible for the strength plant and hold them responsible for the strength of the mix. You, the architect, will then get a copy of the mix. You, the architect, will then get a copy of the details of the concrete mix design. of the details of the concrete mix design.

Often the concrete batching manufacturer Often the concrete batching manufacturer will send a representative ( other than the truck will send a representative ( other than the truck driver) to the job site to see that the concrete is driver) to the job site to see that the concrete is placed properly, and placed properly, and that no additional amount of that no additional amount of water is added to the mix after it leaves the plant. water is added to the mix after it leaves the plant.

BB PLACEMENT OF CONCRETE PLACEMENT OF CONCRETE

The depositing of concrete at the site is extremely The depositing of concrete at the site is extremely important to its strength. important to its strength.

If concrete is placed in excavations that can If concrete is placed in excavations that can absorb water, then part of the water needed absorb water, then part of the water needed for hydration might be lost.for hydration might be lost.

Concrete placed in forms and around Concrete placed in forms and around reinforcing steel must be worked and tamped reinforcing steel must be worked and tamped into place to eliminate voids in the final mix, into place to eliminate voids in the final mix, and to assure a complete bond between the and to assure a complete bond between the concrete and reinforcing steel.concrete and reinforcing steel.

If concrete is allowed to free-fall for a long If concrete is allowed to free-fall for a long distance from the end of the chute, such as to distance from the end of the chute, such as to the bottom of a deep footing, the bottom of a deep footing, the aggregates the aggregates will separate and the mass will not have a will separate and the mass will not have a properly distributed mix of ingredients.properly distributed mix of ingredients.

In large placements, In large placements, the deposited concrete the deposited concrete must be worked into place with vibrators to must be worked into place with vibrators to fill voidsfill voids of cement paste around exposed of cement paste around exposed surfaces of the aggregates.surfaces of the aggregates.

Several methods of depositing concrete are Several methods of depositing concrete are employed:employed:

Where the truck can have ready access to the Where the truck can have ready access to the placement site, placement site, the material is deposited from the material is deposited from a curved, adjustable, rotateable, metal chutea curved, adjustable, rotateable, metal chute..

If the placement site is not accessible by the If the placement site is not accessible by the truck, concrete can be truck, concrete can be pumped through a pumped through a flexible tubeflexible tube to reach distant areas, or to to reach distant areas, or to locations on upper stories of buildings. locations on upper stories of buildings.

Large quantities of concrete that must be Large quantities of concrete that must be placed in remote areas can be transported in placed in remote areas can be transported in steel buckets equipped with trap doors, steel buckets equipped with trap doors, moved to the deposit site by overhead moved to the deposit site by overhead cranes.cranes.

If concrete is to be placed in a deep footing such If concrete is to be placed in a deep footing such as a drilled pier shaft, or if concrete is to be placed as a drilled pier shaft, or if concrete is to be placed underwater, the material may be conveyed through a underwater, the material may be conveyed through a tube called a “TREMIE.” tube called a “TREMIE.”

This type of placement assures that the mix will This type of placement assures that the mix will not be allowed to free-fall and separate the not be allowed to free-fall and separate the ingredients, and in the case of underwater, will prevent ingredients, and in the case of underwater, will prevent the mix from being diluted. the mix from being diluted.

Concrete Placement Direct From the Transit Mixer

Placement with a Concrete Pump

Placement of concrete using a Crane & Bucket

SAMPLING CONCRETE FOR TESTING TO VERIFY SAMPLING CONCRETE FOR TESTING TO VERIFY STRENGTHSTRENGTH

Two procedures are done at the jobs site – Two procedures are done at the jobs site – and are done each time for a specified quantity of and are done each time for a specified quantity of concrete that is placed. concrete that is placed.

You as the architect may direct samples to You as the architect may direct samples to be taken for a specified number of cubic yards of be taken for a specified number of cubic yards of concrete placed, and that each placement will be concrete placed, and that each placement will be documented as to specific location on the job.documented as to specific location on the job.

These samples are taken by a representative These samples are taken by a representative of a testing laboratory.of a testing laboratory.

The Slump Test: The Slump Test:

A cone shaped metal cylinder approximately A cone shaped metal cylinder approximately 12” tall, 4” open top diameter, 8” open bottom 12” tall, 4” open top diameter, 8” open bottom diameter, is placed on a clean, level surface. diameter, is placed on a clean, level surface. Concrete is inserted into the top and tamped full Concrete is inserted into the top and tamped full with a steel rod. The top surface is struck smooth. with a steel rod. The top surface is struck smooth.

The slump cone is then lifted upward, The slump cone is then lifted upward, allowing the concrete to remain. The concrete allowing the concrete to remain. The concrete mass, still being wet, will slump down due to its mass, still being wet, will slump down due to its weight. The distance from the top of the cone to weight. The distance from the top of the cone to the top is the mass is measured. Too much slump the top is the mass is measured. Too much slump indicates too much water – a clue to see if the mix indicates too much water – a clue to see if the mix has inferior strength. Slumps should be 6” or has inferior strength. Slumps should be 6” or less.less.

Sample collected

Slump MeasuredCone Removed and Concrete Allowed to ‘Slump’

Slump Cone Filled

THE SLUMP TEST

Concrete Test Cylinders:Concrete Test Cylinders:

You the architect will specify that three test You the architect will specify that three test cylinders will be taken for each specific batch of cylinders will be taken for each specific batch of concrete placed.concrete placed.

A cardboard or plastic cylinder 6” in A cardboard or plastic cylinder 6” in diameter, 12” tall is filled with concrete, tamped diameter, 12” tall is filled with concrete, tamped solid and struck smooth at the top. Three of these solid and struck smooth at the top. Three of these will be done for each batch.will be done for each batch.

The cylinders are allowed to “set”, then The cylinders are allowed to “set”, then placed in plastic bags, identified and sent to an placed in plastic bags, identified and sent to an independent testing laboratory, where they will independent testing laboratory, where they will remain for your specified length of time.remain for your specified length of time.

At age 7 days one cylinder will be tested in a At age 7 days one cylinder will be tested in a machine to determine its strength. It should attain machine to determine its strength. It should attain approximately ½ to 2/3 of the compressive approximately ½ to 2/3 of the compressive strength you specified. strength you specified.

At 21 days another cylinder will be tested for At 21 days another cylinder will be tested for strength. The results should be in the vicinity of strength. The results should be in the vicinity of 80% to 90% of specified strength.80% to 90% of specified strength.

At 28 days (the age of fully cured concrete) At 28 days (the age of fully cured concrete) the last cylinder will be tested to ascertain its the last cylinder will be tested to ascertain its matured strength.matured strength.

If strengths are high after the 7 day test, you If strengths are high after the 7 day test, you may choose to skip the 21 day test. But in any may choose to skip the 21 day test. But in any case, the 28 day test should be made.case, the 28 day test should be made.

Failures are rare, but the consequences are Failures are rare, but the consequences are left to you, the architect’s specification. If a failure left to you, the architect’s specification. If a failure occurs after 28 days, You may have required by occurs after 28 days, You may have required by your specification that on-site samples of the your specification that on-site samples of the material be taken by the laboratory of the failed material be taken by the laboratory of the failed batch, then tested for matured strength. These will batch, then tested for matured strength. These will be samples, circular in shape, cut by a machine be samples, circular in shape, cut by a machine made for the purpose. made for the purpose.

If the cored samples do not test to the If the cored samples do not test to the required strength, the Contractor is required to required strength, the Contractor is required to remove all the quantity of the failed batch and remove all the quantity of the failed batch and replace it with new concrete. For these reasons, replace it with new concrete. For these reasons, documentation of batches of concrete placed is documentation of batches of concrete placed is important.important.

Obviously, that is why the Contractor requires Obviously, that is why the Contractor requires the concrete manufacturer to be responsible for the concrete manufacturer to be responsible for the strength of the mix.the strength of the mix.

Test Cylinders Filled with a Sampling of the Concrete

Test Cylinders in bags for curing

Preparation for breaking a test cylinder

CONCRETE REINFORCING STEELCONCRETE REINFORCING STEEL

Steel bars are made specifically to resist Steel bars are made specifically to resist tensile and shearing diagonal tension in concrete tensile and shearing diagonal tension in concrete structural members. They are made in several structural members. They are made in several strength grades, so identified, and have a strength grades, so identified, and have a deformed surface to aid in bonding with concrete.deformed surface to aid in bonding with concrete.

Steel bar size:Steel bar size:Eleven Standard DiametersEleven Standard Diameters

3, 4, 5, 6, 7, 8, 9, 10, 11, 14, 183, 4, 5, 6, 7, 8, 9, 10, 11, 14, 18 Number refers to eighths of an inchNumber refers to eighths of an inch

Steel bar yield strength grades:Steel bar yield strength grades:40,000 p.s.i.40,000 p.s.i.50,000 p.s.i50,000 p.s.i60,000 p.s.i60,000 p.s.i

Steel reinforcing is placed inside concrete Steel reinforcing is placed inside concrete forms according to details of placement forms according to details of placement requirements determined by structural requirements determined by structural calculations, and secured in place with wire ties.calculations, and secured in place with wire ties.

Sufficient room must be allowed between Sufficient room must be allowed between bars and between bars and forms to allow the bars and between bars and forms to allow the installation and consolidation of the concrete mix.installation and consolidation of the concrete mix.

Since steel is subject to weakening by fire, a Since steel is subject to weakening by fire, a concrete cover of specific thickness on the concrete cover of specific thickness on the outside of reinforcement is required for outside of reinforcement is required for maintaining the fire integrity of concrete maintaining the fire integrity of concrete structures. structures.

Reinforcing supports placed inside forms for Reinforcing supports placed inside forms for the purpose of holding steel bars in position are the purpose of holding steel bars in position are called ‘chairs’. They are made in numerous called ‘chairs’. They are made in numerous configurations for specific conditions, constructed configurations for specific conditions, constructed of wire electronically welded together. of wire electronically welded together.

Reinforcing steel for underground concrete Reinforcing steel for underground concrete footings is sometimes required to be suspended footings is sometimes required to be suspended from above in order to support the steel. A solid from above in order to support the steel. A solid connection such as a reinforcing steel bar stuck in connection such as a reinforcing steel bar stuck in the soil as a support would leave a channel the soil as a support would leave a channel through the concrete for moisture to enter and through the concrete for moisture to enter and deteriorate the reinforcing steel. deteriorate the reinforcing steel.

ORGANIZATIONS FOR REINFORCED CONCRETE ORGANIZATIONS FOR REINFORCED CONCRETE STANDARDS include:STANDARDS include:

CRSICRSI The Concrete Reinforcing Steel InstituteThe Concrete Reinforcing Steel Institute governs the development and standards for the governs the development and standards for the manufacture of steel reinforcing.manufacture of steel reinforcing.

ACIACI The American Concrete InstituteThe American Concrete Institute publishes the specifications for the design, publishes the specifications for the design, reinforcing, placing, and control of structural reinforcing, placing, and control of structural concrete. concrete.

Improperly consolidated Concrete

Segregation at the bottom of the pour (also note the trash at the bottom of the wall)

Extensive Reinforcing Can Make Placement and Consolidation Difficult

Reinforcing SupportsReinforcing Supports