A Comprehensive Guide to Cement, Aggregates and Fresh Concrete Properties.

This article covers various aspects of concrete including an overview of its ingredients, the mixing process, pouring techniques, properties of fresh concrete, mechanical strength, durability, and the mix design for normal concrete. Additionally, it delves into the ACI 318-19 criteria for concrete compressive strength at different stages and outlines the stages for the rejection of concrete. These topics are particularly beneficial for site engineers or QA/QC engineers.

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Concrete

Concrete is a blend of cementitious material, water, Fine aggregates, Fine aggregates and sometimes we add different admixture for different purposes. It results in two main components:

• Two main components of concrete are aggregates (form the sturdy structure) and paste (cement mixed with water, binds aggregates).
• Ideal concrete requires optimal aggregate gradation and correct amount of cement paste to fill empty spaces.
• Quality of concrete depends on good quality aggregates, quality paste, and strong bonding between the two phases.
• Concrete is widely regarded as the most useful material in the construction industry.
• Some experts even suggest that concrete is the second most utilized material globally, after water.
• Despite its importance, concrete has its share of drawbacks, which will be discussed in upcoming topics.

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Cement

Cement is a type of binder used in construction. It's a chemical substance that sets, hardens, and sticks to other materials to bind them together. The most common type is Ordinary Portland Cement, which reacts with water to form a hard substance.

The reactivity of cement depends on several factors:

·??????? The quantity, quality, and distribution of minerals in the cement.

·??????? The fineness of the cement particles.

·??????? The level of alkalis present.

·??????? The interaction between minerals and water.

·??????? The temperature during hydration.

?Hydration reaction of cement:

The hydration reaction of cement is when water is added to cement, causing a chemical reaction that releases heat. This reaction is exothermic because it gives off heat

Heat of hydration:

The heat of hydration is the heat produced when water and Portland cement mix together. It mostly depends on how much C3S and C3A are in the cement, but it can also be affected by how much water is used, how finely the cement is ground, and the temperature during curing. When any of these things increase, the heat of hydration also goes up.

The Heat of Hydration of Hydraulic Cement can be determined through ASTM C 186

?Formation of bogues compounds and C-S-H Gel

When the hydration reaction occurs in cement, it forms various compounds known as bogues compounds, including C3S, C2S, C3A, and C4AH. These compounds play a crucial role in the chemical reactions that take place during hydration.

• Calcium silicate hydrate (C-S-H), which is a key component for strength and durability, is formed as a result of the reaction between the silicate phases of Portland cement and water. This C-S-H is primarily formed by C2S and C3S.
• Both C3S and C2S react with water to produce Calcium-Silicate-Hydrate Gel (CSH) and Calcium Hydroxide (Ca(OH)2). However, C3S reacts faster and produces more Ca(OH)2 compared to C2S.
• In typical cement, C2S produces about 99% CSH (by molecular weight), while C3S produces about 70% CSH (by molecular weight). This means that C3S contains more Ca(OH)2 compared to C2S.
• Ca(OH)2 is a compound that can easily leach out with water, leaving void spaces in the concrete. These voids can reduce the strength of the concrete and increase the likelihood of chemical ingress, such as sulfates or chlorides, which can deteriorate the concrete over time.
• Therefore, a lower amount of Ca(OH)2 indicates better quality concrete. C2S is more effective in the later stages of hydration as it produces more CSH gel, although this process takes time. On the other hand, C3S contributes to early-age strength but loses its strength over time due to the high amount of Ca(OH)2 it produces.
• CSH is essential for concrete strength and durability, while Ca(OH)2 has little contribution to concrete performance but can stabilize the hydrate product matrix and absorb some CO2 to form calcium carbonate over time.
• As per studies, the Calcium-Silicate-Hydrate Gel (CSH) is responsible for about 65-70% of the strength of concrete. This highlights the critical role that CSH plays in determining the overall strength and durability of concrete structures.

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Water requirement for the Hydration of Cement and gel formation

For proper hydration and the formation of Bouges Compounds, around 23% of water by weight of cement is needed. Additionally, about 15% of water by weight of cement is necessary for the formation of a proper gel structure or to fill the voids within the cement. Therefore, the total water required for complete hydration and workability is approximately 38% by weight of the cement.

Calorimetry Curve of cement:

Calorimetry is a method that accurately measures the heat produced during the initial hydration of cementitious materials. Unlike traditional tests like set time or compressive strength tests, calorimetry provides continuous and real-time data. This allows for a deeper understanding of the behavior of cement paste, concrete, or mortar.

By analyzing the timing and shape of temperature or heat curves obtained through calorimetry, we can gauge the relative performance of different cementitious mixes. Additionally, we can identify potential adverse interactions between materials used in the mix. This insight is crucial for optimizing the quality and durability of concrete structures.

The curve is drawn laterally to represent the relationship between the rate of heat release and the duration of time.

1.???? Initial Dissolution Phase:

?Upon mixing cement with water, rapid hydration of cement compounds begins, generating a significant amount of heat initially.

This intense heat generation quickly slows down within 8-10 minutes, marking the initial dissolution stage.

2.???? Induction Period:

Following the initial dissolution, there's a second phase characterized by a slow but non-zero reaction rate known as the induction period.

This phase is particularly valuable in practice because the material remains workable and castable due to its sustained plasticity.

The induction period typically lasts for about 30 minutes to a few hours before transitioning to the acceleration period.

3.???? Acceleration Period:

The acceleration period marks a phase where the formation of Calcium Silicate Hydrate (CSH) and Calcium Hydroxide (CH) occurs at an increasing rate, reaching a peak rate.

After the peak rate, the reaction gradually decelerates, with approximately 50% of the cement having reacted after about a day at temperatures between 20 and 25 degrees Celsius.

Over the following days, weeks, and even months, the reaction rate continues to decelerate.

4.???? Evolution of Cement Paste:

As the reaction progresses, the cement paste goes through different stages, gradually losing its plasticity and gaining rigidity and strength.

The setting of the cement paste occurs at the onset of the formation of the CSH gel.

When plotted on a graph as a calorimetric curve, the rate of heat liberation from the hydration reaction shows distinct phases. The setting of the cement paste typically aligns with the inflection point preceding a specific phase of the curve. This understanding of the hydration process is fundamental in optimizing the properties and performance of cementitious materials in various applications.

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Aggregates

Let's explore aggregates: types, properties, tests, and ASTM standards—all in an easy-to-understand format.

• Fine Aggregate:

·??????? Description: Aggregate with a size below 4.75 mm (3/16 inches), used in concrete for fine workability and surface finish.

·??????? ASTM Definition: Material passing Sieve No.4 and retained on Sieve No.200 is termed as fine aggregate.

• Coarse Aggregate:

·??????? Description: Aggregate with a size larger than 4.75 mm (3/16 inches), providing strength and stability to concrete.

·??????? ASTM Definition: Material that predominantly retains on Sieve No.4 is called coarse aggregate.

·??????? Practical Usage: In concrete, coarse aggregate can range up to 1.5 inches or 2 inches in rare cases, contributing to structural integrity and durability.

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Properties of Aggregates:

1.Roundness:

·??????? Measures particle sharpness, influenced by parent rock strength and abrasion resistance.

·??????? Rounded particles result from attrition or water shaping.

·??????? Examples: windblown sand, river gravel.

Sub-classification:

·??????? Well rounded: No original faces left.

·??????? Rounded: Original faces almost gone.

·??????? Sub-rounded: Faces reduced but still exist due to wear.

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1. Angularity in Aggregates:

·??????? Definition: Measures deviation from a round shape, indicating the sharpness of edges and corners, and degree of wear.

·??????? Expression: Angularity Number = 67 - percentage of solid volume of a compacted sample.

·??????? Calculation: The number 67 represents solid volume in a sample of completely round aggregates when compacted.

·??????? Relation to Void Proportion: Higher angularity means a greater percentage of voids compared to completely round aggregates (33% voids).

·??????? Range: Angularity numbers range from 0 to 11 for practical aggregates.

·??????? Types:

·??????? Angular: Well-defined edges from roughly planar surfaces, minimal wear.

·??????? Sub-angular: Some wear, surfaces mostly untouched.

·??????? Examples: Crushed rocks, crushed slag.

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2.Sphericity in Aggregates:

Definition: Ratio of particle surface area to volume, affecting workability and durability.

Types:

Flaky Particles: Thickness < 0.6 times mean sieve size of its size fraction.

Elongated Particles: Length > 1.8 times mean sieve size of its size fraction.

Mean Sieve Size: Arithmetic mean of sieve sizes where particle is retained and where it passes.

Sieving Series: 2.5, 2, 1.5, 1, 0.75, 0.5, 3/8, and ? inches sieves are typically used.

Assessment: Excess of 10-15% weight of coarse aggregate in elongated/flaky particles is undesirable.

Indices: Flakiness Index = (Weight of flaky particles / Weight of sample) x 100%.

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3. Particle Surface Texture in Aggregates:

• Definition: Refers to the pattern and roughness/smoothness of aggregate particles' surfaces.
• Factors Influencing Texture:

Hardness, grain size, and polar characteristics of the parent material.

Forces acting on particle surfaces.

• BS Classification:
• Glassy: Conchoidal fractures (e.g., Black flint, vitreous slag).
• Smooth: Fracture of laminated/fine-grained rock (e.g., gravel, marble).
• Granular: Uniform rounded grains fracture (e.g., sandstone).
• Rough: Fracture of medium-grained rocks without visible crystalline constituents (e.g., Basalt, limestone).
• Crystalline: Contains visible crystalline constituents (e.g., Granite).
• Honeycombed: Visible pores and cavities (e.g., Brick, Pumice).
• Role in Concrete:

·??????? Affects bond with cementing material, influencing concrete strength.

·??????? Rough texture provides better grip for cement paste, creating a stronger bond.

·??????? Shape and texture affect water requirements and workability of concrete.

• Impact on Fresh Concrete:

·??????? Shape and texture have a significant impact on fresh concrete properties.

·??????? Smooth and rounded aggregates enhance workability compared to rough and angular ones.

·??????? Crushed stone produces more angular and elongated aggregate, requiring more cement paste for workability.

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4. Bond of Aggregate:

·??????? Importance: Crucial for concrete strength, influenced by the bond between aggregate and cement paste.

·??????? Mechanism: Bond results from the interlocking of aggregate and paste due to surface roughness.

·??????? Impact of Surface Texture: Rough surfaces, like those of crushed particles, promote better bonding.

·??????? Observable Effect: Good bond is indicated by crushed concrete specimens containing broken aggregate particles throughout.

?5. Strength of Aggregate:

·??????? Impact on Concrete: The crushing strength of aggregate directly influences concrete properties.

?·??????? Limiting Factor: Compressive strength of concrete cannot surpass the strength of the aggregate.

·??????? BS Tests for Crushing Strength:

?·??????? Crushing Value Test

·??????? Ten Percent Fine Value Test

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6.Grading of Aggregates:

Importance: Crucially impacts concrete properties by controlling particle size distribution.

·??????? Fine Aggregates Grading (ASTM C 33): Determined by sieve analysis.

?·??????? Coarse Aggregates Grading (ASTM C 33): Also determined by sieve analysis, crucial for concrete quality control.

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• Fineness Modulus in Aggregate Grading:
• ?Definition:

Sum of percentages of aggregate retained on standard sieves divided by 100.

Indicates the fineness of the aggregate; higher values imply coarser aggregates.

• Utility:

Useful for concrete mix design, aiding in determining the proportion of fine and coarse aggregates.

• Calculation Example:

Given sieve analysis results of aggregate:

Sieve No.4: 20% retained

Sieve No.8: 30% retained

Sieve No.16: 25% retained

Sieve No.30: 15% retained

Sieve No.50: 8% retained

Sieve No.100: 2% retained

Fineness Modulus = (20 + 30 + 25 + 15 + 8 + 2) / 100 = 1.5 (example value)

Interpretation:

A fineness modulus of 1.5 indicates a moderately fine aggregate.

7.Hardness of Aggregate:

Definition: Resistance to wear and abrasion, a crucial quality index for aggregates used in concrete, especially for pavements and heavy-duty surfaces.

Testing Methods: Several methods used to check hardness, including:

Abrasion: Rubbing of foreign material against the aggregate.

Attrition: Rubbing of aggregate particles against each other.

Impact: Wear caused by dropping weight over the aggregate.

Combined Actions: Hardness may be determined by a combination of abrasion, attrition, and impact tests.

8.Resistance to Freezing and Thawing:

·??????? Importance: Vital for concrete durability in cold climates, related to absorption, porosity, and pore structure of aggregate.

·??????? Factors Affecting Resistance:

·??????? Aggregate absorption: Excessive absorption leads to poor resistance.

·??????? Porosity and pore structure: Insufficient pore space for water expansion can reduce resistance.

·??????? Assessment Methods:

·??????? Past experience: Historical performance of the aggregate.

·??????? Freezing-thaw test: Concrete specimens undergo cycles of freezing and thawing to measure deterioration, often assessed by reduction in dynamic modulus of elasticity.

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9. Chemical Stability in Aggregates:

·??????? Importance: Essential to prevent harmful reactions with cement and external influences.

·??????? Alkalis-Aggregate Reaction (Cement-Aggregate Reaction):

·??????? Types:

a)???? Alkali-Silica Reaction.

b)??? Alkali-Carbonate Reaction.

·??????? Alkali-Silica Reaction (ASR):

a)???? Most common, occurs between active silica in aggregate and alkalis in cement.

b)??? Forms alkali-silica gel, leading to expansion, cracking, and disruption of cement paste.

c)???? Influenced by particle size, porosity, alkalis content in cement, non-evaporable water, permeability, wet-dry cycles, and temperature.

·??????? Detection: Testing necessary for aggregates with no service record.

·??????? Tests for Alkalis-Aggregate Reaction (ASTM):

1.???? Mortar Bar Test (ASTM C 227)

2.???? Rock Cylinder Test (ASTM C 586)

3.???? Quick Chemical Test (ASTM C 289)

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10. Reduction of Alkalis-Aggregate Reaction:

Method: Addition of reactive silica in finely divided form to the mix can reduce or eliminate expansion due to alkalis-aggregate reaction.

Mechanism:

Increased surface area due to added silica reduces alkalis concentration per unit area, leading to less alkali-silica gel formation.

Higher Calcium hydroxide/alkalis ratio at the aggregate boundary forms non-expanding calcium-alkalis silicate product.

Pozzolanic Materials:

Addition of pozzolanic materials like fly ash, silica fume, etc., also reduces expansion by increasing surface area and altering chemical ratios.

Recommendation:

Generally recommended to add 20 g of reactive silica for each gram of alkali in cement exceeding 0.5% of cement weight.

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11.Bulk Density of Aggregate:

Definition: Weight of aggregate required to fill a container (mold) of known volume, accounting for both particles and voids.

ASTM C 29 Standard:

Utilizes cylindrical molds of various capacities (1/10, 1/3, 1/2, and 1 cubic foot) based on aggregate size.

Aggregates are filled in three equal layers and compacted using rodding, jigging, or shoveling.

Methods of Compaction:

a)???? Rodding: Using a rod to compact each layer.

b)??? Jigging: Mechanical vibration or jolting to compact the aggregate.

c)???? Shoveling: Manual compaction using a shovel.

Application: Bulk density measurement aids in assessing the compactness and weight characteristics of aggregates.

12. Specific Gravity of Aggregate:

Definition: Ratio of the unit weight of an aggregate to the unit weight of water.

Purpose: Essential for mix design calculations, although not indicative of aggregate quality.

Typical Range: Most normal aggregates have specific gravity values between 2.4 and 2.9.

Terms Used:

a)???? Bulk Specific Gravity (BSG)

b)??? Bulk Specific Gravity (Saturated Surface Dry = SSD)

c)???? Apparent Specific Gravity (ASG)

d)??? Average Specific Gravity (Ave.SG)

Usage in Mix Design: Specific gravity of saturated surface dry aggregates is commonly used in concrete mix design calculations.

13. Absorption and Surface Moisture in Aggregates:

Absorption:

·??????? Definition: Water in the voids of an aggregate particle expressed as a percentage of its oven-dried weight.

·??????? ASTM Standards: ASTM C 127 for fine aggregates and ASTM C 128 for coarse aggregates.

·??????? Formula: % Absorption = [(B - A) / A] x 100 (where A = weight of oven-dried sample, B = weight of sample in SSD condition).

·??????? Greater absorption indicates larger voids in the aggregate.

Surface Moisture:

·??????? Definition: Moisture present on the surface of the aggregate in addition to that absorbed up to its SSD condition.

·??????? Impact on Mix Proportion: Correction of mix proportion is necessary to account for surface moisture.

·??????? ASTM Procedure: Determined according to ASTM C 70.

·??????? Correction: Weight of water added to the mix should be decreased by the weight of surface moisture in the aggregate.

?14. Soundness of Aggregate:

·??????? Definition: Ability of the aggregate to resist excessive volume changes due to physical conditions like freezing-thawing, temperature fluctuations, wetting-drying cycles, etc.

·??????? Unsound Aggregate: Volume changes lead to deterioration of concrete, ranging from scaling to extensive cracking.

·??????? ASTM C 88 Test Procedure:

1.???? Subject aggregate sample alternately to immersion in saturated sodium or magnesium sulfate solution and drying in oven to constant weight.

2.???? Constant weight: Loss in weight < 0.1% of sample weight in 4 hours at 110-150°C.

3.???? Formation of salt crystals disrupts aggregate particles, simulating ice formation action.

4.???? Loss in weight after cycles indicates degree of unsoundness.

·??????? Test Purpose: Predict behavior under actual site conditions, not for acceptance or rejection of aggregate.

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Properties of Fresh concrete

Workability of Concrete:

Definition: difficulty of mixing, transporting, placing, and consolidating (compacting) of the concrete. Workability affects the degree of compaction to be achieved with a given compaction effort and hence strength. A concrete should have such workability that its compaction to achieve maximum density is possible by providing minimum amount of work.

·??????? Factors Affecting Workability:

·??????? Water Content: Higher water content increases workability.

·??????? Grading and Shape of Aggregate: Well-graded and rounded aggregates improve?? workability.

·??????? Maximum Size of Aggregate: Larger size aggregates improve workability.

·??????? Aggregate-Cement Ratio: Lower ratios decrease workability.

·??????? Admixtures/Additives: Plasticizers and mineral admixtures can enhance workability.

·??????? Time and Temperature: Workability decreases with time and at elevated temperatures.

·??????? Importance: Proper workability ensures maximum compaction with minimal effort, leading to dense and strong concrete.

?Measurement of Workability:

1. Slump Test (ASTM C 143):

Equipment: Frustum cone mold (top diameter: 4 inches, bottom diameter: 8 inches, height: 12 inches), tamping rod (5/8 inches diameter, 24 inches long).

Procedure:

·??????? Fill the mold in three equal layers, compacting each layer with 25 blows from the tamping rod.

·??????? Finish the top surface of the concrete.

·??????? Lift the mold and measure the vertical distance between the mold and the displaced top surface center.

Types of Slump:

·??????? True Slump: Even slump all around, indicates good workability.

·??????? Shear Slump: One half slides down, shows lack of cohesion.

·??????? Collapse Slump: Specimen settles or flows, depth varies from 6 to 10 inches.

Interpretation:

·??????? True slump indicates proper workability for assessment.

·??????? Shear slump suggests cohesion issues in the mix.

·??????? Collapse slump indicates excessive water or poor mix consistency.

2. ?Compacting Factor Test (BS 1881: Part 103):

Definition: Compacting factor is the ratio of partially compacted concrete weight to fully compacted concrete weight, indicating workability.

Apparatus:

·??????? Two hoppers with trap doors (one above the other).

·??????? Cylindrical mold below the lower hopper.

Procedure:

1.???? Fill upper hopper with concrete to the brim.

2.???? Open trap door to let concrete fall into lower hopper, guiding with a rod.

3.???? Let concrete from lower hopper fall into cylindrical mold.

4.???? Remove excess, level, and weigh the partially compacted concrete (W1).

5.???? Refill the mold, compacting each layer.

6.???? Level top surface, clean, and weigh the fully compacted concrete in the mold (W2).

7.???? Weigh the empty clean cylinder (W).

8.???? Calculate compacting factor: (W1 – W) / (W2 – W).

The compacting factor indicates the workability of the concrete: higher values suggest better workability.

?3. Remolding Test (ASTM C 124):

Objective: Measure workability based on the energy required to change the shape of a concrete specimen from a frustum of a cone to a cylinder.

Apparatus:

Standard slump cone

Cylinder (12 inches diameter, 8 inches height) mounted on a flow table

Disc-shaped rider

Procedure:

1.???? Place the slump cone in the cylinder on the flow table.

2.???? Fill the slump cone with concrete in a standard manner.

3.???? Lift the mold, leaving the concrete in the cylinder.

4.???? Place the disc-shaped rider on top of the concrete specimen.

5.???? Start jolting the table at one jolt per second, lifting it by 1/4 inch and dropping it.

6.???? Continue jolting until the concrete specimen changes completely from a frustum of a cone to a cylinder.

Note the number of jolts required to achieve remolding.

Interpretation:

1.???? More jolts needed for very dry concrete, indicating lower workability.

2.???? The test is conducted in a laboratory setting to assess concrete workability.

?4. Vebe Test (BS 1881: Part 104/BS EN 12350-3):

Objective: Measure workability by determining the time (Vebe-seconds) needed to change the shape of a concrete specimen using vibration.

Apparatus:

·??????? Vibrating table with a speed of 300 revolutions per minute.

Procedure:

1.???? Prepare the concrete specimen in a standard manner.

2.???? Place the specimen on the vibrating table.

3.???? Start the vibrating table at a rate of 300 revolutions per minute.

4.???? Observe the time (Vebe-seconds) required for the specimen to change shape under vibration.

Interpretation:

1.???? Longer Vebe-seconds indicate lower workability (more effort required for remolding).

2.???? This laboratory test assesses concrete workability using vibration instead of jolting.?

5. Ball Penetration Test (ASTM C360):

Objective: Determine the consistency of fresh concrete by measuring the depth of penetration of a 6-inch hemisphere metal ball weighing 30 lbs.?

Procedure:

1.???? Ensure the depth of concrete to be tested is at least 8 inches, and the least lateral dimension is 18 inches.

2.???? Place the 30 lb metal ball gently on the surface of the fresh concrete.

3.???? Allow the ball to penetrate slowly under its own weight.

4.???? Measure the depth of penetration of the ball into the concrete.

Interpretation:

1.???? Deeper penetration indicates higher workability and lower consistency.

2.???? The test is simple and suitable for routine checking of consistency in fresh concrete for control purposes.?

6. Visual Inspection (Experience):

Method: Skilled mason or concrete expert assesses concrete by patting it with a trowel and evaluating the ease of finishing.

Procedure:

1.???? Use a trowel to pat the concrete surface.

2.???? Assess the ease of finishing based on personal experience and expertise.

3.???? Consider factors like workability, smoothness, and ability to achieve desired surface finish.

Interpretation:

1.???? Skilled individuals can gauge workability, consistency, and finishing characteristics of concrete through visual inspection and tactile feedback.

2.???? This method is subjective but valuable in real-time assessment during concrete operations.

?2. Segregation in Concrete:

Definition: Separation of concrete constituents, leading to non-uniform distribution, caused by particle size differences, specific gravity variations, and improper handling.

?Types of Segregation:

·??????? Coarse Particle Segregation: Coarser particles separate from finer ones due to settling or movement along slopes. Common in dry lean mixes, but can be mitigated by adding water for improved cohesion.

·??????? Grout Segregation: Grout separates in overly wet mixes, causing uneven distribution.

Causes:

·??????? Particle size discrepancies

·??????? Specific gravity variations

·??????? Improper water content

·??????? Inadequate mix handling

Effects:

·??????? Compromised uniformity

·??????? Reduced strength in affected areas

·??????? Aesthetic issues in finished concrete

Prevention:

·??????? Proper mix design and water-cement ratio

·??????? Careful handling during transportation and placement

·??????? Minimize free fall heights during pouring

·??????? Use of appropriate vibration techniques for consolidation

Segregation undermines concrete quality and structural integrity, highlighting the importance of meticulous handling and mix control.

?Factors Affecting Segregation:

• Grading of Aggregates:

·??????? Finer grading reduces segregation tendencies.

• Method of Handling:

·??????? Direct transfer from mixing to final position minimizes segregation.

·??????? Dropping from height, passing through chutes, or discharging against obstacles increases segregation.

• Excessive Vibration:

·??????? Prolonged vibration leads to coarse particles settling at the bottom and finer particles rising to the top, causing weak concrete and laitance formation.

• Air Entrainment:

·??????? Air entrainment reduces segregation risk.

• Difference in Specific Gravities:

·??????? Significant differences in specific gravities between coarse and fine aggregates increase segregation.

• Detection of Segregation:

1.???? On-Site Observation: Segregation is visually detectable during concrete handling.

2.???? Flow Test: Conducting a flow test provides insights into segregation tendencies.

3.???? Vibration Test: Vibrate a concrete cube and observe coarse aggregate distribution to detect over-vibration-induced segregation.

Bleeding of concrete

Bleeding in fresh concrete refers to the rising of mixing water to the surface after placement. This happens because the solid components of concrete cannot retain all the mixing water, preventing it from moving upward freely.

??Factors Influencing Bleeding in Concrete:

1.???? Fineness of Cement:

• Finer cement particles reduce bleeding due to increased surface area holding water.

2.???? Alkali Content of Cement:

• Higher alkali content in cement decreases bleeding.

3.???? C3A Content:

• Higher C3A content leads to less bleeding, possibly due to rapid water reaction.

4.???? Calcium Chloride Addition:

• Adding calcium chloride reduces bleeding in concrete.

5.???? Temperature:

• Elevated temperatures increase bleeding rate but don't affect total bleeding capacity.

6.???? Fine Aggregate Properties:

• Certain physical properties of fine aggregate, especially particles smaller than 150 μm, may reduce bleeding.

7.???? Cement Quantity:

• Rich mixes with higher cement content experience less bleeding due to increased surface area.

8.???? Pozzolan Addition:

• Incorporating finely divided pozzolan reduces bleeding by providing more surface area to retain water.

9.???? Air Entrainment:

• Air entrainment effectively reduces bleeding in concrete.

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Measurement of bleeding

The bleeding capacity and rate of bleeding in concrete can be measured using the ASTM C 232 test procedure. Here's a breakdown of the steps involved:

?1.???? Preparation of Specimen:

• Fill a mold with concrete in three equal layers, compacting each layer with 25 blows of a standard tamper.

Level and smooth the top surface of the concrete using a trowel.

2.???? Initial Recording:

• Immediately after finishing, record the time and weigh the mold with the concrete.

3.???? Collection of Bleed Water:

• Place the specimen on a level platform and cover it to prevent evaporation of bleed water.
• At 10-minute intervals, collect the water that has accumulated on the concrete surface using a pipette or similar device.
• Tilt the specimen slightly before collecting water to facilitate drainage.
• Continue this process at regular intervals (20, 30, and 40 minutes, then every 30 minutes) until bleeding ceases.

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4.???? Calculating Bleeding:

• Calculate bleeding volume per unit area using the formula V = Vw / A, where Vw is the volume of bleeding water collected and A is the surface area of the concrete.
• Calculate bleeding as a percentage of net mixing water using C = (Ww / W) x S and % Bleeding = (D / C) x 100, where Ww is the net water weight, W is the total batch weight, S is the specimen weight, and D is the volume of bleeding water.
• This procedure allows for quantitative assessment of bleeding in concrete samples, providing valuable information about the water migration within the material.

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Effects of bleeding in concrete

1.???? Excess Moisture and Weakness: The top layer may become overly wet, leading to porous and weak concrete when trapped beneath subsequent layers.

2.???? Weak Wearing Surface: Remixed bleeding water during finishing can result in a weak surface prone to damage.

3.???? Poor Bonding: Bleeding water under coarse aggregates or bars can create zones of poor bond, reducing strength and increasing permeability.

4.???? Laitance Formation: Fine cement particles carried by bleeding water can form a porous and dusty surface, weaken bonds between concrete lifts, and create planes of weakness.

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4. Plastic shrinkage cracking

Plastic shrinkage cracking happens when the surface of freshly placed concrete dries too quickly, leading to cracks. These cracks typically form on horizontal surfaces and occur when the rate of water evaporation from the concrete surface is higher than the rate at which bleeding water rises to the surface.

?Causes and Precautions

Causes:

• High concrete and air temperatures
• Low humidity levels
• Intense sunlight exposure
• Strong winds

Precautions to Minimize Cracking:

• Dampen subgrade and forms before pouring concrete
• Use windbreaks to reduce wind speed over the surface
• Install sunshades to shield from direct sunlight
• Moisturize dry aggregates before mixing
• Lower concrete temperature with cool water and aggregates
• Cover the surface with wet material during delays
• Minimize time between pouring, finishing, and curing
• Protect the surface in the initial hours after placement

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Type of Water Used in Concrete (ACI: 26.4.1.3)

• Mixing water shall conform to ASTM C1602, which specifies the quality requirements for mixing water used in concrete.
• Water used in mixing concrete shall be clean and free from injurious amounts of oils, acids, alkalis, salts, organic materials, or other substances that may adversely affect the setting, strength, or durability of concrete.
• The water used should be potable and suitable for drinking, ensuring that it does not contain impurities that could compromise the performance or properties of the concrete.
• Adhering to these standards helps maintain the integrity and quality of the concrete mix, ensuring optimal performance and longevity of the finished concrete structures.

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