BT - Concrete

Ar. Rino D.A. Fernandez, uapClass Instructor

TRIVIA:

Concrete is the most widely used construction material.

Present consumption of concrete in the world is 11 billion metric tonnes every year

REASONS WHY CONCRETE IS THE MOST WIDELY USED MATERIAL:

Concrete possesses excellent resistance to water


Concrete can be formed into a variety of shapes and sizes


Concrete is one of the cheapest and most readily available materials

Concrete is made by mixing

Cement

Aggregates (sand & gravel)

Admixtures

Water

CONCRETE MIXTURE

Cement – 4 parts calcined limestone plus 1 part clay which are mixed, burned and pulverized, sold in 40 kilos per bags

Sand – or fine aggregates, natural sand or crushed stone well graded with size from 0.003-¼ inch (0.076 - 6.4mm )

Gravel – or course aggregates, crushed stone or blast furnace slags with size from ¼ inch - 1 ½ inches (6.4mm - 37.5mm)

Water – clean, potable & free from harmful substances

Admixture – or additive as required by situations, ingredients added to concrete or mortar modify its properties immediately before or after mixing.

CONCRETE PROPORTIONING:

Class

Cementper bag

Sandper M3

Gravel

Per M3

WaterPer gallon

StrengthPSI

Uses

AAA 1 1 2 6 4,500 Pre- stresses andPost - tensioned

AA 1 1 1/2 3 6 4,000 Underwater retaining wall, shear wall and elevator core walls

A 1 2 4 6 3,500 Footings, pier, columns, girders, beams, joists and slabs

B 1 2 1/2 5 6 3,000 Slabs on fill and non – load bearing wall or 4” CHB

C 1 3 6 6 2,500 Concrete plant boxes and parapet walls

D 1 3 1/2 7 6 2,000 Plant boxes, footpaths, walkway and lean concrete

CONCRETE CHARACTERISTICS:

Inherently strong in compression but weak in tension (steel reinforcement is required to handle tensile and shear stresses)

Capable of being formed into any shape with variety of surface finishes, textures and patterns

Relatively low-cost and inherently fire-resistant

Liability is its own weight (150 lbs/cu.ft)

The provision for forming and molding required before placing for setting and curing.

CEMENT:

o Initially used by the ancient Egyptians & was developed not later the late 19th century, open a new era for modern architecture particularly of the Brutalist period

o Portland Cement – named after it resembles a unique limestone found in the Isle of Portland, England (hydraulic cement made by burning a mixture of clay and limestone in rotary kiln resulting into clinker, further pulverized into very fine powder)

Pozzolan – named after a natural cement from Puzzuolli, an ancient Roman town near Mt. Vesuvius

CEMENT TYPE:

Type I : Normal - Portland cement for general construction

Type II : Moderate - sulfate resistant, where heat build-up can damage the structure

Type III : High Early Strength ( HES ) - cures faster ( 3 - 7 days) and gains earlier strength

Type IV : Low Heat - generates less heat hydration where heat build-up can significantly damage the structure and used for massive concrete structures

Type V : Sulfate Resistant - used where resistance to severe sulfate action is required that damage the structure when it leach out from the soil.

OTHER USES FOR CEMENT:

Mortar = cement + sand + water

Plaster = cement + lime + sand + water

Grout = cement + sand + considerable amount of water

Paste = cement + water

ADMIXTURES:

ADMIXTURES:

Pozzolanic Admixture – used as substitute for cement (35%) for structures in high temperatures and seawater installation or as additives to decrease the weight of a concrete per cubic foot

Water Reducing Admixture – an agent or super plasticizers that improves workability and maintain consistency of concrete

Waterproofing – integrated type in powder form added during mixing . Render surface in decks, basements and pools water impervious.

ADMIXTURES:

Coloring Agent – are pigments or dyes mixed into topping to render/alter color evenly to concrete surface

Surface Sealing Agents – liquid waxes sprayed over the surface that is easily removed after curing. Prevents evaporation of water into a new concrete allowing hydration and seal the pores of concrete surface after it has hardened

Set Inhibiting Agent – in liquid form (applied immediately before pouring of concrete) or powder form (applied directly after pouring). Inhibits the setting of cement paste avoiding bonding to surface aggregates and left exposed for architectural effect.

ADMIXTURES:

Dispersal Agents- prevents bleeding of concrete from concrete.

Bonding Agent – either metallic aggregate (bonding thru oxidation and expansion) or synthetic latex emulsion (sprayed or painted to coalesce and bond) to improve the bond between old and new concrete.

Air – Entraining Agent – can be inter-ground during manufacturing of Type I,II and III cement. Enables the concrete to be highly resistant against corrosion.

Gas Forming Agent – develops the potential strength of a concrete

ADMIXTURES:

Non-Skid Surfaces - use abrasive material in topping to produced unslippery surface for pavement construction

Hardener – chemical/fine metallic aggregate improve the density of concrete surface subject to impact and wear.

Accelerator – Speeds up the setting of concrete to reduce the whole curing period or for early removal of forms.

Retarder – delays or extend the setting time of concrete especially during hot weather condition (hydration accelerates curing) allowing more time to place, consolidate and finish the concrete.

QUALITIES OF GOOD CONCRETE:

Strong – proper selection of materials.

Durable – correct proportion and mixing process

Uniform Quality – careful transportation and placement

Thoroughly Sound – timed curing and provision for protection

Different Process of Mixing Concrete:

Manual – flat surface with shovels and buggy

Small Power – a manual mixing rotating drum

Bagger Mixer – equipped with diesel engine and pump operated mechanical mixing drum (1 or 2bags) or rotating mixing drum at the back of a truck.

Method of Transporting a Concrete:

Ready Mixed- concrete mixed at batch plant for delivery by an agitator to construction site.


Shrink Mixed – concrete partially mixed at the batch plant then mixed completely in a truck mixed en route to construction site.

Transit Mixed – concrete dry batch at a batch plant & mixed at the truck mixer en route to construction site.


Gunite – or “shotcrete” for lightweight construction, where concrete mix is pumped through a hose and sprayed at high velocity over reinforcement until desired thickness is reached.

Water – Cement Ratio:

Water cement ratio controls the strength, durability and water tightness of hardened concrete.

Based on Abram’s Law (D.A. Abrams, 1919) “the compressive strength of concrete is inversely proportional to the ratio of water to cement”

Too much water will weaken concrete after curing.

Little water is dense but causes difficulty in placement and workability of concrete.

Water – Cement Ratio:

The Average water-cement ratio is 6 gallons per 40 kg. of cement bag.

Excessive water causes bleeding and laitance.

Bleeding (emergence of excess mixing water on the surface of newly placed concrete caused by settlement of solids within the mass)

Laitance (milky deposit containing cement and fine aggregate on the surface of new concrete combined with bleeding, overworking of mix or improper finishing).

CONTROL OF CONCRETE MIXES:

Concrete is normally specified according to the compressive strength it develops depending on the type of cement used. If ordinary Portland (7 - 28 days) or high early strength (3 – 7 days) after placement

Two methods of testing concrete’s compressive

strength:

1. Slump test

2. Compressive Cylinder Test


1. Slump test – to measure the consistency of freshly mixed concrete including mortar and stucco .Where a concrete is placed at a slump cone ( 12” high with a respective top diameter and bottom is 4” and 8”) and tamped in a prescribed manner then lifted to determine the decrease in height expressed by vertical settling in inches.

allowable slump : beams and columns (3”) slabs, walls parapets and tops (2”) side walls and arches (4”) heavy mass of concrete (2”)


2. Compressive Cylinder Test – a quasi-quality control test based on 7–28 days curing period to determine the compressive strength of a concrete specimen. A metal cast cylinder (12” high and 6” diameter ) is poured with concrete and taken out within 24 hours while still wet and dated. Sent to laboratory check if it can support axial compression before fracturing.

Consolidation of Concrete:

The process of eliminating voids other than entrained air within newly placed concrete and ensuring close contact of the concrete with form surfaces and embedded steel reinforcement.

Though : vibration, spading and rodding

Excessive vibration causes segregation (separation of coarse aggregate from mortar causing excessive horizontal movement making a free fall mix)

Excessive vibration also causes stratification (separation combined with excessive wetting into horizontal layers where lighter material migrate towards the top)

Methods of Construction:

Cast-In Place – concrete deposited, formed and cured and finished in its final position as a part of structure.

Slip Form – a form that can be move slowly and continuously as concrete is being placed during the construction of concrete pavement or building.

Lift Slab Construction - structural members are sequentially lifted into final position

Tilt-Up Construction – structural members are custom pre-casted on site then hoisted into final position

Pre-Stressing – pre-tensioning and post-tensioning

Tools in Concrete Construction:

Curing Blanket – built covering made of sacks, matting, burlap, wet earth, sawdust , straw and plastic.

* curing – maintaining the humidity and temperature of freshly placed concrete during some definite period following, placing, casting or finishing to assure satisfactory hydration of cementitious materials and proper hardening of concrete.

Scaffolding – or staging a temporary platform structure of wooden poles & planks providing platform for working men to stand while erecting or repairing buildings and provide access to work areas above ground

Formwork – temporary construction to contain wet concrete in the required shape while it is cast & setting

Concrete Finishes: Beton brut – concrete left in natural state after

formwork is removed reflecting the texture, joints and fasteners of board form.

Finish – broom, float, exposed aggregate, swirl, bush hammered.

Topping – a layer of high quality concrete or mortar placed to form a floor surface on a concrete base.

Properties of Concrete:

Creep - long duration stress produces stress over time and consequently causes permanent deformation. This phenomenon can be reduced through the use of reinforcement in the compression zone.

Fire Resistance – concrete is incombustible and somewhat insulative, but long exposure to fire can be damaging. Density of the concrete and type of aggregate affect fire resistance.

Shrinkage – ordinary concrete shrinks amount during curing process. This may account for potential movement over 100ºF temperature differential causing cracks. This must be controlled by providing expansion joints and steel reinforcements.

Properties of Concrete:

Hardness – relative to durability and appearance, accomplished thru troweled or floated finish to draw more paste to the surface. But harder superficial layer has a tendency to spall, use gas forming agent

Porosity – Sizes of pores left during hydration process or created by excessive evaporation and shrinkage cracks affecting the watertightness of concrete. Apply compaction techniques.

Steel Reinforcement:

Steel bars, strands and wires is required to absorb concrete’s weakness in tensile and shear stress.

Steel reinforce is required to tie vertical and horizontal elements, reinforce openings, minimize shrinkage cracking and control thermal expansion and contraction.

Although, architects can design reinforce concrete, a competent structural engineer must be tapped to investigate and ultimately incorporated into final design.

Steel Reinforcement:

Reinforcement bars are steel sections hot-rolled with ribs or other deformations for better mechanical bonding to concrete.

E.g. No.4 = ½” or 12mm diameter

Welded wire fabric consist of grid of steel wires welded together at all points of intersection. Use to provide temperature reinforcement for slabs while heavier gauges can be used for concrete walls. Designated by the size of a grid in inches followed by a number indicating the wire gauge or cross sectional area.

E.g. 0.15x0.15 – Ga.W1.4xW1.4

Effectivity of reinforced concrete depends on the holding power between the surface of steel bar and the surrounding steel called bond.

Function of Reinforcing Steel in Concrete: Tensile strength for bending members

Resistance to shrinkage

Ductility

BAR CODE FOR REINFORCED CONCRETE DESIGN:

Bar numberBar number Diameter in INCHDiameter in INCH Diameter in Diameter in MILLIMETERMILLIMETER

#2#2 ¼”¼” 4MM4MM

#3#3 3/8”3/8” 10MM10MM

#4#4 ½”½” 12MM12MM

#5#5 5/8”5/8” 16MM16MM

#6#6 ¾”¾” 20MM20MM

#7#7 7/8”7/8” 22MM22MM

#8#8 1”1” 25MM25MM

#9#9 1 1/8”1 1/8” 28MM28MM

#10#10 1 ¼”1 ¼” 32MM32MM

CONCRETE PRODUCTS:Made from lightweight and heavyweight materials for interior and exterior load-bearing walls, firewalls, curtain and panel walls.C

1. Concrete BlockHollow load-bearing - concrete hollow blocks 40-50 lbs.Solid load-bearing block - core area not more than 25% of

gross sectional cross areaHollow non-load bearing block - core area more than 25% of

gross-sectional cross areaConcrete Tile - a surfacing unit of slate or of some other

impervious compositionConcrete Brick - solid or hollow masonry unit of clay or shale

molded into rectangular shape while plastic & burn in a KILNCommon Size : 4”x8”x16” – non load-bearing

6”x8”x16” – load bearingQuality – handmade, machine cured and steam cured

(Ramrod, Jackbilt, Permanent and Superior)

2. Cast StoneUsed to simulate stone from concrete methods by splitting

solid concrete block to expose two rough surfaces with either :

High quality silica mortar cast in mold with face shaped to imitate chipped sand stone, shale or slate mixed with color additives.

White cement mortar cast in mold with hard and smooth surface ground off to expose some stone on a surface mixed with granite or marble chips.


3. Pre – Cast Facing Slabs Units custom made to any size to fit a particular structure.

Made from lightweight aggregates and are hoist over the building.

Fixed by pins or dowels to a steel frame or metal strap anchors to a concrete back up wall.


4. Cellular Concrete Blocks Lightweight block with outstanding thermal & sound insulation

qualities Made from silica rich sand & lime water mixed with aluminum

powder. Easily cut/sawed into any desired shape w/ woodworking tools Stucco & plaster can be applied directly to the face of a block Other materials may be nailed directly to them

5. Decorative Blocks Used for fences, sun baffles, tiles, pipes, balusters and

trellises


Aggregate is the granular material, such as sand, gravel, crushed stone,crushed blast-furnace slag, or construction and demolition waste that is usedwith a cementing medium to produce either concrete or mortar. The term coarseaggregate refers to the aggregate particles larger than 4.75 mm (No. 4 sieve), andthe term fine aggregate refers to the aggregate particles smaller than 4.75 mmbut larger than 75 μm (No. 200 sieve). Gravel is the coarse aggregate resultingfrom natural disintegration by weathering of rock. The term sand is commonlyused for fine aggregate resulting from either natural weathering or crushing ofstone. Crushed stone is the product resulting from industrial crushing of rocks,boulders, or large cobblestones. Iron blast-furnace slag, a by-product of the iron industry

TYPES OF CONCRETE:

general categories based on compressive strength:■ Low-strength concrete: less than 20 MPa (3000 psi)■ Moderate-strength concrete: 20 to 40 MPa (3000 to 6000 psi)■ High-strength concrete: more than 40 MPa (6000 psi).Moderate-strength concrete, also referred to as ordinary or normal concrete,is used for most structural work. High-strength concrete is used for special application

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BT - Concrete