PART 5
CONCRETE
Concrete is a building material which solidifies and hardens after mixing with water and placement due to a chemical process known as hydration. The water reacts with cement, which bonds the other components fine aggregates (sand) and coarse aggregates (gravel) together; and eventually produces a hard stone-like material.
Concrete is used to make pavements, pipe, engineering structures, foundations, roads, bridges, walls, footings, etc.
CLASSIFICATIONS OF CONCRETE
General Classification of Concrete
Plain Concrete: A structural concrete with no reinforcement or with less reinforcement than the minimum amount specified for reinforced concrete.
Reinforced Concrete: A structural concrete reinforced with no less than the minimum amounts of reinforcing bars, pre-stressing tendons or non-pre-stressed reinforcement.
Classifications According to Proportion
Class AA. A rich concrete mixture that can be used for columns of reinforced concrete building and for construction where a very strong and dense concrete is required. (1:1.5:3)
Class A. A good concrete mixture that can be used for reinforced concrete works of all kinds and best suited for general concrete works. (1:2:4)
Class B. A medium concrete mixture that can be used for plain concrete foundations, walls, floors, etc. and for not much strength of impermeability is required. (1:2.5:5)
Class C. A lean concrete mixture that can be used for heavy masses. (1:3:6)
PROPORTION OF INGREDIENTS OF CEMENT CONCRETE.
PROPORTIONS FOR NOMINAL MIX CONCRETE
OTHER CLASSIFICATION OF CONCRETE
Lime Concrete. The main ingredient is slaked lime and used as a binding material.
Cement Concrete
Special Cement Concrete. Produced to suit a variety of special requirements of environmental conditions.
Reinforced Cement Concrete. Obtained by embedding steel bars in tension zones of the structural member to offset tension weakness of plain cement concrete.
Pre-Stressed Cement Concrete. Obtained with high graded steel wires or tendon wires.
Aerated Concrete. Manufactured from calcareous and siliceous materials.
Heavy Weight Concrete. Produced by using special heavy weight aggregates and compacting well by mechanical means.
Pre - packed Concrete. Obtained by injecting cement sand mortar under pressure to fill voids already packed and fully compact coarse aggregates.
Special Types of Concrete
Air Entrained Concrete. Contains billions of microscopic cells per cubic ft and produced by the use of air entraining Portland cement.
High-Early Strength Concrete. Produced by using high-early strength cement.
Light weight Concrete. Made from light weight aggregates.
PROPORTIONING INGREDIENTS FOR CONCRETE
Mix proportioning Method
Weight Method. Fairly simple and quick for estimating mix proportion using an assumed or known weight of concrete per unit volume.
Absolute-Volume Method. More accurate method involves use of specific gravity values for all ingredients to calculate the absolute volume and each will occupy in a unit volume of concrete.
Fineness Modulus Method. The term fineness modulus is used to indicate an index number which is roughly proportional to the average size of the particle in the entire quantity of aggregates.
Arbitrary Standard Method. Ratio of fine aggregates and coarse aggregates to develop a dense mix that lies between 1:1/2 and 1:2/1/2.
Minimum Voids Method. The quantity of fine aggregate used in the mixed is about 10% more than the voids in the coarse aggregates and the quantity of cement is kept about 15% more than the voids in the fine aggregates.
Maximum density Method. In this method a box of fixed volume is filled with varying proportions of fine and coarse aggregates.
Water Cement Ratio. The amount of water in mixing concrete is the most important factor affecting the strength of a given proportion.
PROPERTIES OF CONCRETE
Properties of concrete are divided into two parts:
Properties of Fresh Concrete or Plastic Stage and
Properties of Hardened Concrete.
Plastic Stage Consistency.
Consistency of a concrete mix is a measure of the stiffness or sloppiness or fluidity of the mix. For effective handling, placing and compacting the concrete; and consistency must be the same foreach batch. It is therefore necessary to measure the consistency of concrete at regular intervals.
Slump Test. Commonly used to measure consistency of concrete.
Slump Loss. From the time of mixing, fresh concrete gradually loses consistency. This gives rise to the problems only if the concrete becomes too stiff to handle, place and compact properly. Slump loss in concrete is caused due to the following reasons: hydration of cement (generating more heat), loss of water by evaporation, absorption of water by dry aggregates and absorption of water by surfaces in contact with the concrete.
Workability. The ease with which concrete can be compacted fully without segregation and bleeding. The workability of a concrete mix is the relative ease with which concrete can be placed, compacted and finished without separation or segregation of the individual materials. Workability is not the same thing as consistency. Mixes with the same consistency can have different workabilities, if they are made with different sizes of stone, the smaller the stone the more workable the concrete. It is not possible to measure workability but the slump test, together with an assessment of properties like stone content, cohesiveness and plasticity; it gives a useful indication.
Segregation. Implies the separation of the coarser particles from the mix which results in no homogeneity of the concrete mix.
Bleeding. The appearance of water along the cement particles on the surface of freshly laid concrete on compaction and finishing.
Settlement and Bleeding. Cement and aggregate particles have densities about three times that of water. In fresh concrete they consequently tend to settle and displace mixing water which migrates upward and may collect on the top.
Setting. The hardening of concrete before its hydration is known as setting of concrete.
Hydration. Concrete derives its strength by the hydration of cement particles. The hydration of cement is not a momentary action but a process continuing for a long time.
Air entrainment. Air entrainment reduces the density of concrete and consequently reduces the strength.
Hardened stage
Strength. The characteristics strength of concrete is defined as the compressive strength of 150 mm size cubes after 28 days of curing below which not more than 5 % of the test results are expected to fail. When we refer to concrete strength; we generally talk about compressive strength of concrete. Because concrete is strong in compression but relatively weak in tension and bending. Concrete compressive strength is measured in pounds per square inch (psi) or newton per square millimeter (MPa). Compressive strength mostly depends upon the amount and type of cement used in concrete mix. It is also affected by the water-cement ratio, mixing method, placing and curing. Concrete tensile strength ranges from 7% to 12% of compressive strength. Both tensile strength and bending strength can be increased by adding reinforcement.
Durability. Environmental forces such as weathering, chemical attack, heat, freezing, and thawing mat deteriorate concrete. The period existence of concrete without getting adversely affected by these forces is known as durability. Durability might be defined as the ability to maintain satisfactory performance over and extended service life. The design service life of most buildings is often 30 years, although buildings often last 50 to 100 years. Most concrete buildings are demolished due to obsolescence rather than deterioration. Different concretes require different degrees of durability depending on the exposure environment and properties desired. Appropriate concrete ingredients, mix proportions, finishes and curing practices can be adjusted on the basis of required durability of concrete.
Impermeability. The resistance of concrete to the flow of water through its pores. Excess water during concreting leaves a large number of continuous pores leading to the permeability.
Dimensional changes. Concrete shrinks with age and the total shrinkage depends upon the constituents of concrete, size of the member and the environmental conditions. Total shrinkage is approximately 0.0003 of original dimension.
Shrinkage. The volume decreases of concrete caused by drying and chemical changes. In another word, the reduction of volume for the setting and hardening of concrete is defined as shrinkage.
Creep. Deformation of concrete structure under sustained load is defined as concrete creep. Long term pressure or stress on concrete can make changes in shape. This deformation usually occurs in the direction the force is applied. The permanent dimension changes due to loading over a long period is termed as creep.
Modulus of Elasticity. The modulus of Elasticity of concrete depends on the Modulus of Elasticity of the concrete ingredients and their mix proportions. As per ACI code, the modulus of Elasticity to be calculated using following equation:
Ec= 33γc√(fc′)
Where:
Ec= modulus elasticity of concrete, γc= unit weight of concrete,
fc′= compressive strength
Water Tightness. Another property of concrete is water tightness. Sometimes it is called impermeability of concrete. Water tightness of concrete is directly related to the durability of concrete. The lesser the permeability; the more the durability of concrete.
Thermal Conductivity. Concrete has moderate thermal conductivity, much lower than metals, but significantly higher than other building materials such as wood, and it is a poor insulator. A layer of concrete is frequently used for 'fireproofing' of steel structures. However, the term fireproof is inappropriate, for high temperature fires can be hot enough to induce chemical changes in concrete, which in the extreme can cause considerable structural damage to the concrete.
MBV-CMT 5
Unit Weight. The unit weight of concrete depends on percentage of reinforcement, type of aggregate and number of voids.
PREPARATION OF CONCRETE SPECIMENS FOR TESTING (ASTM Designation C192/C 192 – 02)
Procedures in preparing concrete specimens for testing:
Mix the concrete mixture in room temperature condition.
Cement shall be thoroughly mixed to aggregates to provide uniform supply throughout the tests and passed through an 850-μm (Sieve No. 20) to remove all lumps.
Segregation of aggregates into individual size fractions and for each batch combine in proper proportions to produce the desired grading or uniform sizes.
Mix concrete into a suitable mixer or by hand and it is important to mix consistently the mixture.
Measure the slump of each batch of concrete immediately after mixing in accordance with the slump test method (ASTM Designation C143/C143M - 03).
Determine the air content when required in accordance with test method for air content (ASTM Designation C173/C231).
Start making specimens in a clean and dry cylindrical mold. Select portions of the batch of mixed concrete in tests for molding specimens as the representative of the actual proportions and condition of the concrete.
Place the concrete mixture in the cylindrical mold using a scoop, trowel or shovel in the required number of layers of approximately equal volume.
Rod uniformly each layer with the rounded end of the rod using the number of strokes and size of rod specified.
It is necessary to remix the concrete mixture to prevent segregation during the molding of the specimens.
Ensure a symmetrical distribution of the concrete and to minimize segregation of the coarse aggregates within the mold.
Distribute the concrete mixture by the use of a tamping rod prior consolidation.
In placing the final layer add an amount that will exactly fill the mold after compaction.
After consolidation, strike off the surface of the concrete by float or trowel and finish the surface with a wood or float.
Remove the specimens from the mold 24 ±8 hours after casting.
Curing of Concrete Test Specimens Curing of concrete is a method used to maintain a satisfactory moisture content and temperature in some concrete for a period of time immediately after placing and finishing to develop the desired properties for its intended use.
Procedures in curing of concrete test specimens:
For Initial Curing:
Cover the concrete specimens immediately after finishing preferably with a non-absorptive, nonreactive plate or a sheet tough durable impervious plastic to prevent evaporation of water from unhardened concrete.
Concrete specimens shall be stored immediately after finishing until the removal of the molds to prevent loss of moisture from the specimens.
Select an appropriate procedure or combination of procedures that will prevent moisture loss and is non absorptive and nonreactive with the concrete.
Remove the concrete specimens from the molds 24 ± 8 hours after casting and apply the appropriate identification.
For Final Curing:
Final curing for all concrete specimens must be started 30 minutes after removal from the cylinder mold and temperature shall be maintained at room temperature of 23oC ± 2oC.
Curing of concrete specimens may be done in a water tank conforming to AASHTO M 201 and free moisture must be maintained on specimen surfaces at all times until tested.
Curing period for the concrete specimens is 28 days to reach its probable compressive strength.
PART 6
UNIVERSAL TESTING MACHINE
WHAT IS UNIVERSAL TESTING MACHINE (UTM)
To test mechanical properties of a material such as compressive, tensile, shear and flexural must use this machine called Universal Testing Machine (UTM). This testing machine can make a stress strain diagram that we used to compute yield strength, tensile strength, and others.
COMPONENTS OF UNIVERSAL TESTING MACHINE
Two main parts of UTM:
Loading Unit
Control Unit
LOADING UNIT
Where the test specimen takes place and the load that must be exerted to the material.
Consists of following components:
Load Frame – It consists of a table (to place the specimen for compression test), upper crosshead, and lower crosshead.
Upper/Lower crosshead: Upper crosshead is to clamp the specimen needs to be tested from top or its one end; Lower crosshead is the movable crosshead whose screws can be loosened for height adjustment and tightened.
Elongation Scale – The relative movement of the lower and upper table is measured by an elongation scale which is provided along with the loading unit.
CONTROL UNIT
Where the load is applied and get the corresponding test result. The load is applied with a control valve and released by a release valve. The load is applied with the help of hydraulic pressure.
Consists of following components:
Hydraulic Power Unit – This unit consists of an oil pump that provides non-pulsating oil flow into the main cylinder of the load unit. This flow helps in the smooth application of load on the specimen. The oil pump in a hydraulic power unit is run by an electric motor and sump.
Load Measuring Unit – This unit has a pendulum dynamometer unit that has a small cylinder with a piston which moves with the non-pulsating oil flow. The pendulum is connected to the piston by a pivot lever. The pivot deflects based on the load applied to the specimen. This deflection is converted to the load pointer and displayed as the load on the dial. The range of load application can be adjusted by means of a knob in the load measuring unit (0-100 kN; 0-250 kN; 0-500 kN and 0-1000kN). The accuracy of measuring unit controls the overall accuracy of the machine.
Control Devices – It can be electric or hydraulic. Electric control devices make use of switches to move the crossheads and switch on/off the unit. A hydraulic control device consists of two valves, Right Control Valve and Left Control Valve or Release Valve. A right control valve is used to apply load on the specimen. The left control valve is used to release the load application.
FUNCTIONS OF UNIVERSAL TESTING MACHINE
To test the mechanical properties of materials is the main function of UTM. The following are some standard tests performed by UTM:
Tensile Test – Determine the force needed to pull the specimen apart and along with how much the material stretches before it breaks.
Procedure: Clamp a single piece of anything (re-bar) on each end and pull it apart until it breaks. This measures how strong it is (tensile strength) , how stretchy it is (elongation), and how stiff it is (tensile modulus).
Compression Test – This is opposite of tensile test where you compress an object between two level plates until a certain load or distance has been reached or the product breaks. The compressive strength of a material is calculated as the stress required to rupture the specimen or deform the specimen to a given percentage of its height.
Bending Stress – This is where you support a length of material by spanning it across two supports on each end. There is nothing supporting the middle portion underneath it. Then you press down from above directly in the middle of the span of material until the supported material breaks or reaches a specific distance. This test measures how strong the material in flexure (flexural strength) and how stiff it is (flexural modulus).
Peel Test – This test pulls apart two materials that have been bonded together.
Procedure: One clamp holds one material, and the other clamp holds the other materials. Then you separate them apart for a few inches.