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Compressive Strength Test

The result of concrete test will, if plotted, assume the “normal distribution” – that is, the familiar bell-shaped curve. The curve can be described in two characteristics:

o The MEAN or the AVERAGE, ( )

Mean,

o The STANDARD DEVIATION,

Standard Deviation,

(The smaller the standard deviation, the steeper the curve, indicating results grouped tightly around the mean: the higher the standard deviation, the shallower the normal distribution curve, indicating widespread test results.)

Yield Determinations

Unit Weight Determinations

- are important in that the volume of concrete being produced for a given batch can be checked.

- is often used as guide for air contents.

- it is also an indicator of strength for a given class of concrete.

ASTM C31-69

- It is the test procedure for making compressive test cylinders.

o For standard test cylinder - is 6 inches in diameter by 12 inches high for aggregates up to 2 inches.

o For larger aggregates - the diameter should be at least three times the aggregates size and the height at least twice the diameter.

- The molds used are generally waxed cardboard or plastic.

• Fill the test cylinders with three equal layer rodded 28 times.

- Concrete of 1 – 3 inch slump - may be rodded or vibrated

- Concrete under 1 inch slump - must be vibrated

- Concrete over 3 inch slump - must be rodded

• Stuck of level and cover with glass or steel plate or damp burlap and allow setting for 24 hours.

• Move the cylinders to proper curing facilities where moist curing will take place for the required time.

The strengths of the cylinders are generally taken 7 to 28 days.

Testing Concrete

Concrete Slump Test

- is an empirical test that measures the workability of fresh concrete.

- more specifically, it measures the consistency of the concrete in that specific batch. This test is performed to check the consistency of freshly made concrete.

Consistency - denoted by the fluidity of the concrete.

- It refers to the ease with which the concrete flows. It is used to indicate the degree of wetness.

– are seldom made, but a convenient, reliable test to determine indirectly the tensile strength of concrete is in use today.

Workabilty - to denote the ease with which concrete can be mixed, transported, placed, and finished w/o segregation.

Direct test for Tensile Strength

Air-Content Test

Flexure Strength or Modulus of Rapture

Splitting Tensile Strength

– is determined by a formula based on the theory of elasticity.

– is determined with 6 by 6 concrete beams specimens.

a. The Pressure Method - cannot be used with lightweight or porous aggregates since the water will be forced into the aggregate's pore structure and erroneous results will be obtained.

b. The Volumetric Method - can be used with all types of aggregates and is recommended for lightweight or porous aggregates

c. Free-Air Method

• Obtaining Raw Materials

ASTM C94-71a Specification: Weight Measurements

– is the measured maximum resistance to axial loading, expressed as force per unit of cross-sectional area in pounds per square inch(psi).

• The designer selects a desired compressive strength (f’c).

• f’c is reduced by an appropriate safety factor.

(Safety factors are determined and published by organizations such as American Concrete Institute (ACI).)

• The concrete that is placed in the structure is tested at 28 days and evaluated upon ASTM or ACI criteria to determine that it has the required compressive strength (f’c).

Compressive Strength

Mixing Equipments

a. Cement: When the quantity of cement in a batch exceeds 30 percent of scale capacity, the quantity indicated by scale shall be within 1 percent of the required weight. For small batches, the quantity of cement shall be not less than neither required or more than 4 percent in excess.

Stationary Mixer

Examples on the effects of common impurities in mixing water:

Ready Mix

- can be found on jobs which require large amounts of concrete at steady rates.

- the required mixing time of a stationary mixer may vary, but generally 1 minute is required for the first cubic yard and 15 seconds for each additional cubic yard or fraction of a cubic yard.

- most of the stationary mixers have timing devices which can be set and locked to prevent the discharge of the concrete before proper mixing time has elapsed.

- ASTM C94 specifies mixing time based on drum revolutions. Generally 70-100 revolutions at rotation rate designated by manufacturer of the mixer is required to produce uniform concrete. No more than 100 revolutions mixing spped can be made.

-ASTM C94 also specifies that discharge of the concrete shall be completed withing 1 1/2 hours or before the drum has revolved 300 revolutions.

b. Aggregates: 2 percent in a cumulative aggregate batch; 1 percent when scale is used in excess of 30 percent of its capacity. For cumulative weights less than 30 percent scale capacity, the tolerance shall be 0.3 percent of scale capacity or 3 percent of the required cumulative weight.

*The salt of manganese, tin, zinc, and lead may cause reduction in strength and variations in setting times.

*Sodium iodate, sodium phosphate, sodium arsenate, and sodium borate when present in amounts as little as 1 percent by weight of cement greatly retards its set and strength development.

*Sugar in concentration of as little as 0. 15 percent by weight of cement will usually retard the setting time of the cement.

*Vegetable or animal oils, when concentration are greater than 2 percent by weight of cement, an approximate of 20 percent or more strength loss will occur.

a. Central Mix b. Transit Mix c. Shrink Mix

- is concrete delivered to the job site ready for placement.

Concrete Mixing

Portable Mixer

c. Water: Mixing water shall consist of added water, ice, surface moisture on aggregates, and water added in the form of admixtures, and shall be measured by weight or volume to an accuracy of 1 percent of total mixing water required.

Transit Mix Trucks

- the capacities of these mixers are generally in cubic feet, and they are used for small concrete placement or where time of placement must be controlled.

- are made to transport and mix concrete up to the construction site. They can be charged with dry materials and water, with the mixing occurring during transport. They can also be loaded from a "central mix" plant, with this process the material has already been mixed prior to loading. The concrete mixing transport truck maintains the material's liquid state through agitation, or turning of the drum, until delivery. The interior of the drum on a concrete mixing truck is fitted with a spiral blade.

d. Admixtures: Powdered admixtures shall be measured by weight, paste or liquid admixtures by volume or weight. Accuracy for weighing shall be 3 percent; volumetric measurement shall be within an accuracy of 0.3 percent of the total amount required, or plus and minus the volume of dose required for one sack of cement, whichever is greater.

Paving Mixer

Shotcrete

- are concrete mixers mounted on crawler treads. The materials are fed into the mix from dry batch trucks, and the machine travels along the finish grade and deposits fresh concrete behind itself to be screeded and finished by the rest of the paving train.

- is a non-proprietary term used to describe mortar or concrete that is placed by high-velocity compressed air and adheres to the surface on which it is projected.

Coefficient of variation

– used to measure the degree of uniformity of concrete production at a given concrete plant.

- used as a rating for the degree of control for concrete plant over production variables.

- used to determine over design requirements for concrete mix designs.

(If V is low, it indicates a fairly uniform product; conversely, if the value of V is high, the product will not be very uniform.)

Impurities in water may also affect setting time and volume stability, and may cause efflorescence(the leaching of free lime), discoloration, and excessive reinforcement corrosion.

CONCRETE MANUFACTURING

Accelerated Tests

- Are being developed to allow earlier acquisition of strength information.

o 2-day break – have been cured in boiling water or in autogenous curing boxes

o 5-day break – have been cured under heat and pressure and a system that will chemically analyze concrete before placement for its potential strength

The compressive strength of the concrete is determined by loading cylinders to failure.

S = P/A ; S = compressive strength

P = load in lb A = area in sq in.

– the specimens made under controlled curing conditions are generally the specimens used to judge the quality of the concrete for the job.

Record Cylinders

Concrete: Mixing Water

CONCEPT

Duff Abrams found sea water to be adequate in producing concrete having 3.5 percent salt content.

Industrial waste water and sanitary sewage can be used in producing concretes.

OY! OY! OY! AMAZING OOOHHHH!

It is generally accepted that any potable water can be used as mixing water in the manufacture of concrete.

BUT

If the water available is questionable, the comparative test should be run.

Typical Analysis of City Water Supplies (parts per million)

Courtesy of Portland Cement Association

The upper limit of total dissolved solids is usually 2000 ppm.

Higher than this limit should run comparative test. Although higher concentration are not always harmful, certain cement may react adversely.

*Resistance to Abrasion- most common abrasion test called “Los Angeles Rattler Test”

*Resistance to freezing and thawing- Past Performance Records and Freeze-Thaw Test

*Gradation and Particle Shape- Sieve Analysis

*Bulk Unit Weight- weight of the aggregate per unit volume

*Specific Gravity- ratio of the weight of a substance to the weight of an equal volume of water.

*Absorption and Moisture content- can be computed

Characteristics can be obtained from:

When the sum of the dissolved salts of Carbonates and bicarbonates of sodium and potassium exceeds 1000 ppm, tests for setting time and 28- day strength should be made.

Concentration of magnesium sulfate and magnesium chloride up to 40,000 ppm have been used without harmful effects on concrete strength.

EXAMPLE

Polymerization

- a chemical process that combines several monomers to form a polymer or polymeric compound.

Polymer-impregnated concrete (PIC) – consists of performed, cured concrete that is impregnated with a liquid monomer or resin system and subsequently polymerized.

Polymer-cement concrete (PCC) – consists of Portland cement concrete to which a monomer or resin system has been added during mixing.

Polymer-concrete (PC) – is a system utilizing only graded aggregates and a monomer or resin system.

PROPORTIONING CONCRETE INGREDIENTS

Mix Design or Proportioning

Factors to be considered when proportioning concrete:

- is the determination if the relative amounts of materials required to produce a concrete that will be economical and workable in the plastic state and that will have the required properties in the hardened state.

- Mixing apparatus

- Concrete handling and transportation

- Finishing

- Curing

- Strength requirements

o Coring with diamond – used to remove specimen for strength testing.

o Ultrasonic – nondestructive; by correlating density to strength

o Impact Hammer – measures the rebound of a spring-loaded plunger after hitting a smooth concrete surface.

– used to increase the durability of concrete by protecting it against freeze-thaw cycle damage.

AIR-ENTRAINING AGENTS

To test compressive strength of in-place of concrete, various destructive and nondestructive methods can be used:

- causes an increase in the rate of hydration of the hydraulic cement and thus shortens the time of setting, increase the rate of strength development.

ACCELERATORS

Concrete: Aggregates

Aggregates must possess certain characteristics to produce a workable, strong, durable and economical concrete.

– can be used to reduce cold joints, allow smaller crews to finish flat work and permit later control joint sawing

RETARDERS

Most commonly used admixtures:

– usually retarders that have been modified to suppress or modify the retarding effect

WATER REDUCERS

Characteristics of aggregates (Courtesy Portland Cement Association)

Concrete: Admixtures

- defined as any material other than Portland cement, aggregates and water added to a concrete or mortar mix before or during mixing.

Admixtures are generally used for one or more of the following reasons:

- To improve workability of the fresh concrete

- To reduce water content, thereby increasing strength for a given water-cement ratio

- To increase durability of hardened cement

- To retard setting time or increase it

- To impart color to concrete

- To maintain volume stability by reducing or offsetting shrinkage during curing

- To increase concrete resistance to freezing and thawing

Trial mixes or small sample batches duplicate job conditions as close as possible so that the admixture dosages and results will be close to job expectations.

It will also allow study of admixtures’ compatibility if more than one admixture is to be used in the concrete.

Trial Mixes

- Concrete economy is basically a matter of reducing cement content, since it is usually the most expensive ingredient, without sacrificing the service requirements.

Concrete Economy

Another method used is to base designs on past experiences.

A batch of concrete mixed to determine the water-cement ratio that will produce the required slump and compressive strength; from a trial batch, one can also compute the yield, cement factor, and required quantities of each material.

Trial Batching