2.2. Concrete

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Questions and Answers

What is the main benefit of using fly ash in Portland cement concrete?

Reduces the amount of fine aggregates needed

Increases the heating temperature of the mix

Acts as a direct replacement for water

Substitutes for cement and enhances durability (correct)

Which of the following is NOT a characteristic of clean water used in concrete mixing?

Free from oils and organic materials

Clear of injurious salts and acids

Contains beneficial minerals for strength (correct)

Should not have high levels of alkalinity

What does the slump test primarily measure?

Quality of aggregates used in the mix

Setting time of the concrete

Strength of hardened concrete

Fluidity and workability of fresh concrete (correct)

Which proper range is suggested for the water-cement ratio to ensure a workable concrete mix?

0.4 to 0.5 (D) Signup and view all the answers

What is the maximum slump allowed for slabs, beams, thin reinforced walls, and building columns?

150 mm (D) Signup and view all the answers

What effect does excess water have on the concrete mix?

Reduces the holding power of the paste (C) Signup and view all the answers

Which of the following by-product aggregates is created by treating molten blast furnace slag?

Expanded Slag (C) Signup and view all the answers

How does the strength of concrete generally change with respect to the water-cement ratio?

Strength decreases as the ratio increases (A) Signup and view all the answers

Which characteristic is NOT a requirement for good quality concrete?

Highly porous (D) Signup and view all the answers

What is the primary purpose of proper curing in concrete construction?

To protect the concrete after it is placed (B) Signup and view all the answers

Which of the following types of aggregates is smaller than ¼ inch?

Fine aggregate (C) Signup and view all the answers

What is a suitable property for aggregates used in concrete?

Free from injurious amounts of dust (C) Signup and view all the answers

Which of the following is NOT classified as a lightweight aggregate?

Crushed gravel (A) Signup and view all the answers

Which aggregate is derived from volcanic origin and is known for its insulative properties?

Pumice (A) Signup and view all the answers

What should be avoided when selecting fine aggregate for concrete?

High silt content (D) Signup and view all the answers

Which concrete mix class is most suitable for underwater applications?

Class AA (B) Signup and view all the answers

What is the primary drawback of the arbitrary selection method for proportioning concrete?

It is not suitable for large-scale projects. (B) Signup and view all the answers

Which method of mixing concrete is recommended for reinforced concrete work?

Machine Mixing (A) Signup and view all the answers

How is the water-cement ratio determined in the water-cement ratio and slump test method?

By selecting the amount necessary for desired strength. (B) Signup and view all the answers

What does the fineness modulus method help determine in concrete mixing?

The ratio of sand to gravel in the mix. (B) Signup and view all the answers

Which class of concrete has the lowest specified compressive strength?

Class D (D) Signup and view all the answers

What is a characteristic of portable mixers?

They are typically classified as gravity or pneumatic mixers. (C) Signup and view all the answers

Which mixing method is characterized by the delivery of wet mortar or concrete using compressed air?

Wet-Mix Process (B) Signup and view all the answers

What distinguishes central yard mixing from other concrete mixing methods?

It involves mixing concrete at a central location before transporting it. (C) Signup and view all the answers

What unique function does the shotcrete method serve in concrete applications?

It allows concrete to be placed at high velocities. (A) Signup and view all the answers

What is the primary purpose of agitation in concrete delivery?

To prevent material segregation and loss of plasticity (D) Signup and view all the answers

Which method involves mixing concrete in a stationary mixer before delivery?

Central Mix (D) Signup and view all the answers

What is a common limitation for pouring concrete in rainy conditions?

Concrete must be covered while still fresh (A) Signup and view all the answers

What occurs after the initial set of concrete?

The concrete can no longer be manipulated without risk (B) Signup and view all the answers

What is one method used for curing concrete?

Sprinkling with water until hard enough to walk on (C) Signup and view all the answers

Which of the following is the role of a water-reducing agent in concrete?

To decrease the water needed for desired workability (D) Signup and view all the answers

What happens if concrete is dropped from a height greater than the specified limits?

It can cause segregation and reduce quality (C) Signup and view all the answers

What characteristic does a surfactant impart to the mixing water in concrete?

Reduces the surface tension of the water (C) Signup and view all the answers

How long should newly poured concrete be kept completely wetted after placement?

10 days (C) Signup and view all the answers

What is the effect of re-tempering concrete after it has reached its initial set?

Alters the water-cement ratio (A) Signup and view all the answers

What is the primary function of an air entraining agent in concrete?

To improve workability and resistance to cracking (B) Signup and view all the answers

Which of the following best describes a core test?

A test on a concrete structure using a core drill (B) Signup and view all the answers

What characteristic do retarders or decelerators impart to concrete?

Slowing down the setting process (B) Signup and view all the answers

Which agent is used to improve the bond between two concrete surfaces?

Bonding agent (A) Signup and view all the answers

What is the purpose of concrete waterproofing compounds?

To make concrete more impermeable (D) Signup and view all the answers

Which type of test primarily measures the compressive strength of a concrete batch?

Compression test (B) Signup and view all the answers

What is a common ingredient in accelerators that promote faster setting of concrete?

Calcium chloride (A) Signup and view all the answers

Which component is primarily used to create a non-skid surface in concrete?

Non-skid surface agent (C) Signup and view all the answers

What size is a test cylinder used in the compression test?

152 mm diameter and 305 mm length (D) Signup and view all the answers

Which type of admixture prevents the cement paste from bonding to surfaces?

Set inhibiting agent (A) Signup and view all the answers

Study Notes

Qualities of Good Concrete

Good concrete should be strong, durable, uniform in quality, and thoroughly sound.
Achieving this requires careful selection of materials, correct proportioning, thorough mixing, careful transportation and placing, and proper curing.

Cement

High-grade Portland cement conforming to ASTM C150 is commonly used in reinforced concrete construction.
Key tests for cement include: soundness, setting time, fineness, and tensile strength.

Aggregate

Aggregate is a hard, inert material like sand and gravel added to cement to produce concrete or mortar.
Inert fillers must be durable, strong, clean, hard, uncoated, and free from harmful substances like dusts, lumps, soft particles, shale, alkali, organic matter, loam, or other deleterious substances.

Fine Aggregate

This type of aggregate is smaller than ¼" (6.4mm) and passes through a 3/8" (9.5mm) sieve.
Examples include sand, stone screenings, or other inert materials.
Siliceous quartz sands are preferred, but sands from durable rocks are also suitable.
A good sand is well-graded from fine to coarse, has minimal voids, and has a relatively coarse appearance.
Beach sand and lahar should not be used.

Coarse Aggregate

This type of aggregate includes crushed stones, gravel, or blast furnace slag with a size exceeding 1/4" (6.4mm).

Lightweight Aggregates

These aggregates are used to make lightweight, nailable concrete or to control its thermal insulation qualities.
They are grouped into four categories:
- Aggregates of Volcanic Origin
  - Pumice: A natural lightweight aggregate that originates from the rapid cooling of molten volcanic matter, useful where insulating properties are needed.
  - Perlite: A volcanic glass expanded by heat into a lightweight aggregate, suitable where maximum strength is not required, like in precast slabs, blocks, floor fill, fireproofing, and plaster.
- Micaceous Minerals
  - Vermiculite: Mica expanded by heat into very light threads, used as nonstructural lightweight aggregate and in insulation.
- Diatomaceous earth and Expanded Shale
  - Expanded Shale (Expanded Clay): Exfoliated clay or shale, used for lightweight aggregate, such as "AIROX", "ROCKLITE", and "HAYDITE".
  - Expanded Slate: Exfoliated slate, used as a lightweight aggregate.
  - Diatomite: Tiny aquatic plant skeletons heated to melting point, producing a lightweight aggregate.
- By-product Aggregates
  - Fly Ash: Fine material particles created by burning coal, used as a substitute for cement.
  - Cinder: Used as aggregates fused into lumps by coal combustion, not softer ashes from lower-temperature combustion.
  - Expanded Slag: Made by treating molten blast furnace slag with controlled amounts of water or steam.
  - Foamed Slag: Used for precast blocks, walls, and panel filling in steel-framed buildings.

Water

Water used in concrete should be clean and free from harmful substances like oils, acids, alkalis, salts, and organic materials.
Seawater or brackish water should not be used.
Water combines with cement to create a paste that coats the aggregates.
The strength of this paste determines the strength of the mixture.
Excess water weakens the paste's holding power.
The water-cement ratio influences the consistency of the mix.
The ideal mix should be plastic and workable.

Slump Test

This test measures the consistency of fresh concrete before it sets, indicating the mix's fluidity and workability.
Helps identify improperly mixed batches and assess the mix's tendency to "slump."
An apparatus with a cone with a bottom opening of 8" and a top opening of 4" is used.

Consistency (Slump)

There are various slump ranges for different materials and applications:
- Reinforced foundation walls and footings: 125 mm (max), 50 mm (min)
- Plain footings, caissons, and substructure walls: 100 mm (max), 25 mm (min)
- Slabs, beams, thin reinforced walls, and building columns: 150 mm (max), 75 mm (min)
- Pavements and floor laid on the ground: 75 mm (max), 25 mm (min)
- Heavy mass construction: 75 mm (max), 25 mm (min)

Proportioning of Concrete

Concrete strength depends on the water-cement ratio (water volume to cement volume).
Lower water-cement ratios result in stronger concrete.
Consistency is determined by the specific use case, aiming for a dry mix whenever possible.
Aggregate is added to the cement and water mix to achieve the desired consistency.
The combination of fine and coarse aggregates aims to maximize the aggregate quantity while maintaining a workable mix.

Concrete Proportioning and Mixing Methods

Proportioning Methods

There are several methods for proportioning concrete:
- Arbitrary Selection: The oldest and least accurate approach, using loose volume measurements for aggregates. Not recommended for large projects.
- Water-Cement Ratio and Slump Test: Water amount is determined based on required strength, and aggregates are added for desired consistency (slump).
- Water-Ratio, Slump, and Fineness Modulus: Similar to the water-cement method, but with the fine and coarse aggregate proportions determined by the fineness modulus method.

Concrete Mix Classes

Class "AAA" (1:1:2): Ideal for post-tensioning, pre-tensioning, dams with a compressive strength of 4,500 psi.
Class "AA" (1:1.5:3): Suitable for concrete underwater, retaining walls, shear walls, with a strength of 4,000 psi.
Class "A" (1:2:4): Used for floor slabs, beams, columns, arches, foundations, and has a compressive strength of 3,500 psi.
Class "B" (1:2.5:5): Suitable for slabs on fill, retaining walls, piers, building walls, and sidewalks, with a compressive strength of 3,000 psi.
Class "C" (1:3:6): Used for stone masonry, filling, parapet walls, and plant boxes with a compressive strength of 2,500 psi.
Class "D" (1:3.5:7): Ideal for footpaths, walkways, lean concrete, with a compressive strength of 2,000 psi.

Concrete Mixing Methods

Hand Mixing: Suitable for small jobs or when a watertight platform is available.
Machine Mixing: Recommended for reinforced concrete work, typically more uniform and cost-effective.
- Portable Mixers: Used for smaller projects, including drum, trough, gravity, and pneumatic mixers.
- Stationary Mixers: Employed by ready-mix producers.
Other Methods:
- Central Yard Mixing: Concrete mixed at a central yard and transported using transit mix trucks.
- Paving Mixers: Mounted on crawler treads, commonly used in road construction.
- Shotcrete: Mortar or concrete placed with high-velocity compressed air to adhere to the surface, used in two ways:
  - Dry-Mix Process: Dry materials are mixed, and water is added at the nozzle.
  - Wet-Mix Process: Wet mortar or concrete is forced through a delivery hose using compressed air.
- Pumped Concrete (Pumpcrete): Concrete is transported through hoses or pipes using a pump.

Minimum Required Ultimate Compressive Strength

Determined through tests on concrete cylinders or cubes after 28 days, specifying the desired compressive strength of each concrete class.

Ready-Mix Concrete

Ready-mix concrete production delivers concrete to job sites using various methods.
- Central Mix: Concrete is mixed in a stationary mixer and delivered to the job site using a transit mixer.
- Transit-Mixed Concrete: Concrete is entirely mixed in a transit truck.
- Shrink-Mixed Concrete: A combination of central mix and transit mix.

Agitation

Continuous gentle motion in an agitator truck to prevent material segregation and loss of plasticity, ensuring concrete doesn't set during transit.

Setting Time

The initial set of concrete takes place between 1 to 3 hours after mixing.
Manually, concrete shouldn't stand for more than 30 minutes.
Re-tempering or re-gaging (adding water) after the initial set is prohibited as it alters the water-cement ratio.

Pouring Concrete

Pouring concrete in the rain is acceptable while it's still fresh (2-4 hours), but covering is necessary.
Minimal effects from rain are seen after hardening (4-8 hours).
Delivery from the mixer to the forms should be continuous and uninterrupted, ideally within 30 minutes.
Dropping concrete from heights is limited to 5 feet for unexposed work and 3 feet for exposed work.
Exposed concrete surfaces need protection from premature drying (using canvas, burlap, etc.) and must be kept wet for at least 10 days after pouring.
Shrinkage cracks at joints between fresh and hardened concrete are expected.

Curing of Concrete

Maintaining humidity and temperature for a specified period after placement ensures proper hydration and hardening.
Common curing methods include:
- Covering concrete with moist materials like paper sacking, burlap, sand, or earth.
- Sprinkling concrete with water until it's hard enough to walk on.

Admixtures

Substances (other than cement, water, or aggregates) added to concrete or mortar to alter their properties.
Also known as additives.

Additives

Materials in small quantities used to modify specific properties (used in plaster, paints, mortars, concrete).
Admixtures improve these concrete properties:
- Workability: Water-reducing agents decrease water needed for desired workability, leading to higher strength (also called superplasticizer).
- Surface Sealing: Prevents water evaporation, promoting hydration.
- Surface-active agent (Surfactant): Lowers surface tension, improving water wetting and penetration.
- Dispersal Agent: Separates cement particles using electrostatic charges, preventing water bleeding on the surface.
- Inert powders: Finely divided powders such as powdered glass, silica sand, stone dust, hydrated lime.

Concrete Testing Methods

Compression Test: Determines the compressive strength of a concrete batch by measuring the maximum load a cylinder can withstand using a hydraulic press.
Test Cylinder: A concrete cylinder of 152 mm (6") diameter and 305 mm (12") length, cured under controlled conditions, is used for compressive strength tests.
Core Test: A compression test using a cylinder cut from the hardened concrete structure, usually done with a core drill.

Concrete Agents

To improve durability by entrainment of air:
- Air Entraining Agent: Disperses entrained air in the mix, improving workability, freeze-thaw resistance, and resistance to de-icing chemicals.
- Gas Forming Agent: Causes slight expansion in plastic concrete or mortar, reducing or eliminating voids.
To accelerate setting or hardening:
- Accelerators: Hasten the setting and strength development of the mix, with calcium chloride being a common ingredient.
To retard setting:
- Retarders/Decelerators: Slow the setting process for extended working time.
To improve wear resistance:
- Concrete Hardener: Improves the density of surfaces subject to rolling and impact loads (e.g., FLOORHARD with zinc or magnesium fluorosilicates).
- Bonding Agent: Improves the bond between two concrete surfaces.
- Set Inhibiting Agent: Prevents cement paste from bonding to a surface, inhibiting setting.
- Non-Skid Surface Agent: An abrasive material used as a topping to prevent slippery surfaces.
Additional Concrete Materials:
- Pozzolanic Admixture: Sometimes used in structures exposed to high temperatures, seawater, or sulfate-containing water.
To impart water-repellant or waterproofing qualities:
- Concrete Waterproof: Makes concrete impermeable.
- Waterproofing (permeability-reducing) compounds: Various materials used to reduce permeability.
Coloring Agent: Added to a concrete mix for color.

Forms for Concrete Construction

Lumber Forms :
- Lumber sizes vary based on formwork type:
  - 2" thick for columns, beams, and girder bottoms
  - 1" thick for floor panels and beam and girder sides
  - 2x4s for struts, posts, shores, and uprights
  - 1" or 2" thick for cleats
- Preservation: Crude oil and petrolene prevent damage from weathering.
Plywood Forms: Used for smooth surfaces, should be waterproof, Grade "A," and at least 12 mm (½") thick.
Steel Forms: Can be utilized as pans for concrete joist construction or steel decking for concrete slabs and slab-and-joist construction.

Metal Reinforcement for Concrete

Steel Bars or Rods

Mainly in the form of steel bars and rods, round or square cross section.
Bars can be plain or deformed (with lugs for better bonding).
Billet-steel bars: Made using open hearth or acid Bessemer furnaces with fixed chemical compositions, rolled from billets, available in three grades (structural, intermediate, hard). Intermediate grade is commonly used in architecture.
Rail-steel bars: Rolled from standard T-rails, come in only one grade.
Bars range in size from 1/4" to 1-1/4" and come in 20 or 30 ft. lengths.

Wire Fabric / Welded Wire Fabric

Cold-drawn steel wire fabric used to reinforce slabs, floors, and stuccoed work.
A grid pattern of welded wires with varying sizes and spacings (e.g., 4"x4" - 6/6, 6" x 6" - 8/8).
First pair of numbers refers to wire spacing, and the second pair indicates wire gauge.
Available in rolls of 5 or 6 ft. wide and 150, 200, and 300 ft. long.

Building Materials - Processed Concrete

Triangle-Mesh Wire Fabric

Constructed from longitudinal wires with diagonally running cross wires or bond wires.

Expanded Mesh

Made from solid steel sheets shaped into various forms (diamond, crimp, herringbone, Z-rib), like STEELCRETE.

Laths

Flat or segmental sheets with solid ribs, either stamped, perforated, or deformed into open mesh.
Available in painted, galvanized, or different steel grades.
Examples: RIBPLEX, HY RIB.

Processed Concrete Types

"AEROCRETE"
- Expanded structural concrete made by adding aluminum powder to a cement and sand/cinder mixture.
- Water creates gas, expanding the mixture and forming air cells.
- Used in flooring, roofing, partitions, soundproofing, insulation, and lightweight fill.
"GUNITE"
- Sand and cement mixture deposited using high pneumatic pressure and a CEMENT GUN.
- Water is added just before the dry constituents emerge from the nozzle.
- Used for encasing steel, reinforcing slabs, and curtain walls, also ideal for pools.
**"PORETE" **
- Portland cement concrete with chemical foam to generate gases.
- Produces lightweight precast units (hollow or solid) used for roofs and industrial building siding.
"HAYDITE"
- Concrete with lightweight aggregate added.

Cement Brands

Several cement brands are mentioned with images of their packaging: Northern Cement, Rizals, APO, Republic Cement, RapidSet, Fortune, Holcim, Excel, WallRight, and Eagle Cement.

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Module 1 - Materials of Construction - Building Technology (Atlas)