CC 203

CONCRETE TECHNOLOGY

TOPIC 2

CONCRETE WORKS AT THE

CONSTRUCTION SITE

Lecturer name:

SAPLIZA BIN MAHMUD

SUB TOPIC 2

Factor that cause of occurrence material

segregation and bleeding.

Fresh concrete.

Concept of hard concrete

Preparation of concrete at site.

Concept of transporting

concrete

Method of compaction concrete

Concept of casting and placing concrete.

Method of curing concrete.

Standard laboratory testing on concrete

mixture

SUB TOPIC 2.1

Factor that cause of occurrence material segregation and

bleeding.

- Describe the segregation and bleeding of

concrete.

- Show the segregation and bleeding of

concrete mix.

- Effect of segregation and bleeding on

quality of concrete.

Segregation Separation of the constituent materials of concrete.

Source - over limit of compacting

Can divide by 3 types cause :

a) Coarse aggregate separating out or settling down from the concrete mix.

b) Paste separating away from coarse aggregate.

c) Water separating out from the material being of lowest specific gravity.

Coarse

aggregate

falls to

bottom

Segregation on site

Bleeding

Bleeding in concrete is sometimes referred as water gain

It is a particular form of segregation, in which some of the water from the concrete comes out to the surface of the concrete, being of the lowest specific gravity among all the ingredients of concrete.

Source – over limit of compacting

Bleeding on site

Effect of segregation and bleeding on

quality of concrete

a) Strength of concrete

b) Surface cracking.

c) Bonding of material in concrete mix

SUB TOPIC 2.2

Fresh concrete

- Relate the workability of wet concrete with

water cement ratio.

- Testing on workability of concrete

a) Slump test.

b) Vebe test.

Relate the workability of wet concrete with

water cement ratio.

One of material in concrete mixture.

If volume of water are over limit the strength and density of concrete is reduced.

If volume of water are low limit the concrete will be difficult to do and the hydration process is not perfect.

Slump Test

Purpose :

To determine the workability of concrete at site. ( BS 1881: Part 102:1983 )

Apparatus :

Cone and base plate.

Circular tamping rod 12mm.

Ruler steel and concrete mixing equipment.

Concrete.

Procedure:

Prepare materials for the concrete mix according to the mix ratio as directed.

Clean the surface of the cone, then lay on a flat

base plate.

Include the concrete mix into the cone of 3 layers and each layer compacted by 25 times using a tamping rod stuffing until full.

Flatten the top and clean the concrete surface.

Slowly and carefully remove the cone by lifting it vertically and place the cone next the concrete mix.

Place the rod horizontal at cone and concrete slump.

Measure and record the distance slumps the concrete.

Repeat the testing with add water cement ratio.

No. of testing 1 2

Mix Ratio 1 : 2 : 4 1 : 2 : 4

Mass of materials ( kg ) 28 kg 28 kg

Volume of water ( liter ) 2.3 L 2.8 L

Height of cone ( mm ) 300 mm 300 mm

Height of slumps ( mm ) 275 mm 245 mm

Distance of slumps ( mm ) 25 mm 55 mm

* Range BS( Limit slump 0 – 150 mm )

Data result:

b) Vebe test

Objective : to determine the workability of fresh concrete

Apparatus

Concrete

Vebe consistometer mechine

PROCEDURE i) A conventional slump test is performed, placing the slump cone inside the cylindrical part of the consistometer. ii) The glass disc attached to the swivel arm is turned and placed on the top of the concrete in the pot. iii) The electrical vibrator is switched on and a stop-watch is started, simultaneously. iv) Vibration is continued till the conical shape of the concrete disappears and the concrete assumes a cylindrical shape. v) When the concrete fully assumes a cylindrical shape, the stop-watch is switched off immediately. The time is noted.

SUB TOPIC 2.3

Concept of hard concrete.

- Criteria of hard concrete

a) Compression strength

b) Tensile / bending strength

c) Durability ( Ketahanlasakan )

d) Permeability ( Resapan )

e) Resistant to chemical reaction

f) Changing of shape

g) Shrinkage

h) Creeping ( rayapan )

i) Fire resistant

a) Compression strength

Most important property of hard concrete.

Generally considerad for the design.

For ordinary construction compressive strength in a range of 20-40 MPa.

Low range (cast in-situ), High range (precast)

Testing concrete prepared in standard cube 100 or 150mm for 7,14,21 and 28 days.

b) Tensile / bending strength.

Should be high to resist cracking from shrinkage and temperature changes.

Measured using one or more for

i) Direct tension test

ii) Split cylinder test

iii) Flexural test.

c) Durability ( Ketahanlasakan )

Concrete should be design to serve its purpose

Factors that influence durability can be divided into:

the environment and usage factors (outside control)

the composition and production factors

d) Permeability ( Resapan )

The low permeability of concrete is essential to increase the resistance properties of cooled action, chemical action and protects the place of steel corrosion occurs.

To obtain a low permeability, the concrete should be fully compacted and the preservation of good done.

Permeability can also be reduced by using low water content and increase the cement content.

e) Resistant to chemical reaction

Portland cement concrete is often attacked by acids such as organic acids and other food processing, especially when the work performed.

Chemical elements are commonly attacked concrete sulphate salt present in the soil and in sea water.

Resistance properties of concrete sulphate attack depends on the type of cement used, the cement content and water cement ratio.

f) Changing of shape

Elastic deformation that occurs at that time and depending on the value of applied stress.

Drying shrinkage that occurs in a long time and does not depend on the stresses in the concrete.

Creep occurred in the long run, but depending on the value of the stresses on the concrete.

As other materials, the concrete will also experience expansion and contraction due to temperature changes.

g) Shrinkage

Shrinkage of concrete depends on the amount of drying that occurs.

Thus it is affected by humidity and ambient air temperature, air flow rate on the surface and the rate of surface area and volume of concrete.

In normal conditions, contraction was influenced by the amount of available water in the concrete during the mixing and the cement content.

For a small concrete, usually assumed that half of the drying occurs in one of the first month and the balance within six months thereafter.

h) Creeping ( rayapan )

Creep of concrete depends on the value of the stresses in it.

For stresses up to 1 / 3 of the cube strength is considered to be directly proportional to the creep stress.

Creep is affected by temperature and moisture environment, the cement content in concrete, water cement ratio, cement type and nature of aggregates.

The mass of concrete also has some influence on the creep, but much less than its effect on contraction.

The parameters used to evaluate the creep is the ratio of applied stress on concrete cubes at one point of time.

i) Fire resistant

The combustion temperature is below 400[degrees] F (200[degrees] C). At this temperature, only the free water in the concrete will be lost, and the strength loss is minimal. Mineralogical changes in hydrated cement occur at higher temperatures. The most important factor is consider is the selection of aggregate. The differential thermal movement between the cement paste and the aggregate is what can cause damage. Quartzite aggregate is the most prone to fire damage by cracking through the quartzite aggregate and bond failure between the cement paste and the aggregate. Limestone aggregate exhibits better fire resistance when exposed to low temperature fire. Lightweight aggregate also performs well. For this situation, a 6 1/2-sack lightweight concrete should provide all the heat resistance needed.

At higher temperatures (above about 800[degrees]F), high-heat-resistant concrete is generally needed. Light-weight aggregates have proven to be more heat resistant and so that is the first step--use lightweight. When temperatures get extremely high, above 1000[degrees]F, calcium aluminates cement produces a fire-resistant concrete.

SUB TOPIC 2.4

Preparation of concrete at site.

- The batching method of concrete.

a) Volume method.

b) Weigh method.

- The mixing method of concrete.

a) Manually.

b) Machinery.

The batching method of concrete.

a) Volume method.

using the measuring box and the conventional method, which has long been used to work at construction site.

This method can not be used for work that requires accuracy because of the volume of concrete depends on its density.

The ratio of each substance is calculated based on the volume of concrete.

volume of cement is calculated based on the volume of one bag / weight of cement and sand as well as for coarse aggregate measured by volume of the same bag

Density of cement ( BS = 1442kg/m2 ) to determine the measure of box :

1bag cement = 50kg/ 1442kg/m3

= 0.035 m3

The measuring of box is

( 0.33m x 0.33m x 0.33m = 0.035m3 )

b) Weigh method.

using the principles of the scales.

suitable for control purposes in the laboratory for testing purposes.

Example:

Mix ratio (1:1.9:3.8 ) and W/C = 0.5

1 bag cement x 50kg = 50kg

1.9 x 50kg fine agg. = 95kg

3.8 x 50kg coarse agg. = 190kg

Volume of water = 25L

The mixing method of concrete.

a) Manually.

Mixing by hand.

Can’t to used or efection for the high workability.

Mixing for are small quantity mixture.

Manual method on site

b) Machinery.

i )Types T – drum are lean

This machine are used drum for mixing the materials of concrete.

When mixing are finish the concrete will be throut.

The shape of machine is cone and spin in one direction.

The angle of casting is 400 to 600 .

ii )Types NT – drum are not lean ( Cylinder )

This machine shape like cylinder and same are machine Types T.

The drum spin in vertical part.

The concrete mixing well throut at bottom part of machine.

SUB TOPIC 2.5

Transporting method of concrete.

- Definition of ready mix concrete.

- Transporting method of concrete on site.

a) Wheelbarrow.

b) Chute.

c) Pump.

d) Crane.

Ready mix concrete

Ready-mix concrete is a type of concrete that is manufactured in a factory or batching plant, according to a set recipe, and then delivered to a work site, by truck mixers.

Transporting method of concrete on site.

a) Wheelbarrow

Used for concrete transporting in small quantity.

Used to transport the concrete materials such as cement and aggragate.

b) Chute.

Tools to transporting concrete from height to the lower place.

Made from PVC pipe , wood and other materials.

Chute limited for 3m and diameter is 300mm.

c) Pump.

fast method and economical if careful planning.

using pipe ɸ 6” to pumped from the mixing place.

used for large quantities concrete.

can carry as long 30m – 50m vertically and 300m – 500m horizontal direction.

have two type is plunger pump and press pump.

used in difficult to utilise lorry and trasport machine.

d) Crane.

Have two types:

i) mobile crane.

ii) tower crane.

i) Mobile crane

Used for project large and in the construction of high structures less than 30m.

The concrete include in large container have a trap door at bottom.

ii) Tower crane

Used for building higher than 30m

Use skrip depend on load of size

Size of skrip is 1394kg @12.57m3

If the concrete placing is high, a large of skrip should be used to make it more economical.

SUB TOPIC 2.6

Casting and placing of concrete.

- Method of casting and placing for

concrete.

- Procedures of casting and placing on site.

Method of casting and placing

Casting

Space to casting the concrete in formwork at each structure member much be less than 1.2m for avoid the segregation cause.

Must be casting the concrete layer by layer.

Placing

Must be placing early after mixing with water.

Maksimum times is 11/2 hour.

Procedures of casting and placing on site.

Concrete mix must be near at site.

Must be placing in a uniform layer by layer for avoid separation materials.

For structure concrete wall and column must be placing in layer thickness 450mm.

Concrete must be quickly as possible to prevent from hardening.

Make sure every layer must be compact before the next layer.

Avoid for cold joint.

Avoid bar reinforcement move during the casting of concrete work.

51

Structural Assessment & Repair

Cold Joints

Cold joints are places of discontinuity within a member where concrete may not tightly bond to itself (old and new layer)

To achieve proper bond and water-tightness, the surface of hardened concrete must be free of dirt, debris, and laitance

New Placement on

Hardened (or Initial

Set) Concrete.

Cold

Joint

Laitance

Hardened

Concrete

SUB TOPIC 2.7

Compacting method of concrete.

- Method of compacting for concrete.

a) Steel rod.

b) Vibrator machine. ( internal , external

and platform )

- The advantages and disadvantages of

compacting concrete using vibrator and

steel rod.

Method of compacting for concrete.

a) Steel rod

Using for laboratory testing.

Diameter steel rod 12mm.

b) Vibrator machine. (internal , external

and platform)

Internal vibrator

Widely used by electric or diesel power.

Compaction of this type has different sizes of vibrating pins 10mm to 25mm.

Must be done carefully so that the reinforcement in the mold is not move or damaged.

Internal vibrator

External vibrator

is done outside the mold side of the concrete until the mixture became compressed.

Usually it is used in workshops and performed by the power to make beam, piling, box culvert.

External vibrator

Platform vibrator

is a table supported by a number of springs that run by electricity which allows the face of the vibrating table.

usually used in workshops to make the precast concrete.

Concrete parts shall be made in formwork and placed on the table to vibrator for compact.

Platform vibrator

SUB TOPIC 2.8

Method of curing concrete.

- Purpose of curing.

- Method of curing concrete.

a) Covered with wet materials( sacks, plastic, wood dust ) .

b) Flooding or spraying with water.

c ) Sweeping with chemicals.

d ) Using a vapor steam.

61

Methods of Curing

Water Spraying/Ponding

Must be carried out thoroughly. A continuous fog spraying or sprinkling of water is required

Ponding is accomplished by building earth or sand dikes around the perimeter of the concrete slab to retain a pond of water within the enclosed area

The water needs to be at a temperature close to that of the concrete surface

Maintain presence of water

62

Wet Covering

63

Maintain presence of water Wet Covering

Surfaces can be effectively cured by using hessian or other absorbent materials, providing they can be kept continually wet and are uniformly positioned

Waterproof Paper

Can also be used on unobstructed horizontal surface

Polythene Sheeting

Can be used similar to waterproof paper

White sheeting will reflect the sun’s rays and will be useful in warm weather

Polythene sheeting can cause discoloration or mottling of the hardened concrete

64

Plastic Sheets

65

Curing of Concrete

Gunny Sack

66

67

68

Prevent loss of mixing water

Spray-on-membranes Liquid membrane-forming materials containing

waxes, resins, etc. are applied by spraying, brushing or rolling on horizontal or vertical surfaces to retain moisture or retard evaporation

The material should be sprayed evenly as soon as the free water has evaporated

Curing compounds may prevent a bond between the hardened concrete and any screed or wet concrete applied later

69

Accelerate Hydration Use of Heat

Curing time can be reduced in a number of different ways, including space heating, electrical heating and steam curing

Space heating

area ready for concrete can be temporary enclosed by the use of screens, tarpaulins or plastic sheeting.

This enables oil or electric space heaters or fires to provide and maintain in this limited area a temperature which will keep the concrete at least above 5oC

70

Accelerate Hydration Electrical curing

Replacing braziers as a form of accelerated curing, primarily in precast factories, to assist in the reduction in the number of moulds

Low voltage electricity is used and by embedding wire heater elements between plywood laminations, thermal efficiency is high and the method clean and easy to use

Steam curing

Favourable method of curing for precast and concrete masonry block

Structural members are covered with tarpaulins and pipes fed underneath conveying moist hot steam

71

Curing in Hot Weather

In hot weather, exposed concrete must be kept cool, both during mixing and throughout the curing period

Problem in hot weather concreting due to the faster rate of hydration and increased rate of evaporation (concrete temperature, relative humidity, wind velocity, and air temperature)

When evaporation is not controlled, concrete are prone to thermal cracking

Lowering the temperature of coarse agg. decrease the concrete temperature

72

Cont.

The common methods for cooling concrete in hot weather application are:

Cooling the mixing water with chipped ice

Shading the aggregate storage and water tank

Painting the truck mixers white

Avoidance of overmixing

Keeping the humidity high and air temperature low with fog sprays around the area where concrete is placed

Use retarder in the mix

73

Curing Period

Curing period cannot be prescribed in a simple way but,

If the temperature is above 10oC, ACI 308-92 lays down a minimum of

3 days : RHPC (type III)

7 days : OPC (type I)

14 days : LHPC (type IV)

BS 8110:Part 1:1985 lays down the normal curing periods for different cements and exposure conditions