Concrete Material

Light, Flexible Concrete|
Concrete Material

Introduction

Concrete	Cement Water Fine aggregates (0.15~5mm) Coarse aggregates (5~25~40mm) Admixtures
Mortar	Cement Water Sands (0.15~5mm)
Grout	Cement Water
Relative advantages	Excellent water resistance Excellent fire resistance Ability to be cast into any desired shape & size Raw materials are cheap & readily available on site, except cement Durability Energy conservation Thermal protection Resource conservation Aesthetics
Relative disadvantages	Brittle material (sudden failure in unreinforced concrete) Very low tensile strength, 1/10 of max compressive strength Low ductility Low strength-to-weight ratio Volume instability, shrinkage & creep

Portland Cement

Major chemical compounds	C₃S C₂S C₃A C₄AF Gypsum, CSH₂	C: CaO S: SiO₂ A: Al₂O₃ F: Fe₂O₃
C₃S	Strength a C₃S content & cement fineness Early strength component Fast hydration	Most reactive, much heat released
C₂S	Long term strength Slow reaction, little heat
C₃A		Undesirable Fastest reaction Insignificant strength
C₄AF	Sulfate-resisting Slow reaction, little heat	Insignificant strength
CSH₂	Presence in cement prevents porous hydrogarnet Prevent significant temperature increase	Undesirable in large proportions More ettringite formed, causing microcracking & unsoundness
Types of Portland Cements	Ordinary: OPC	C₃S:C₂S:C₃A:C₄AF =3:2:1:1
	Rapid Hardening, RHPC	4.5:2:1:1
	Low Heat, LHPC	2:4.5:1:1
	Sulfate Resisting, SRPC	3:2:0.5:1.5

Water

Uses	Mixing water: hydration + lubrication Curing water	Non-evaporable water Evaporable water: gel pore water
Quantity of water	High	Segregation Bleeding Porous, weak concrete
	Low	Low workability Poor compaction Incomplete hydration
	Water movement	Drying shrinkage Creep Swelling
Effects of impurities on concrete	Set-accelerating	Cl^-, CO₃^2-, HCO₃^-, NaOH, KOH
	Set-retarding	Organic materials
	Strength-reducing	SO₄^2-, organic matter
	Corrosion	Cl^-, acids Seawater
	Staining & efflorescence	Clay, silt Cl^-, Fe⁺
	Increased shrinkage	Clay, silt
Usable water	Typical water supply	<1,000 ppm
	Seawater	31,000~39,000 ppm
	Curing water	Not liable to: Sulphate attack Chloride attack Acid (low pH) attack

Aggregates

Significance

Classification (BS 812)

Strength

Factors:

Assessment tests:

Admixtures

Definition	Materials other than cement, aggregates & water, added just before or during mixing to modify concrete properties	Alter: workability setting strength durability permeability set acc. or retarding
Classification	Air-entraining agents	Increase frost resistance
	Chemical admixtures	Alter: Setting Hardening Water requirements
	Mineral admixtures	Materials of low reactivity Rock dust, hydrated lime, ground limestone Increase workability & durability Cementitious materials: Natural cement, blast furnace slug, hydraulic lime Increase strength Pozzolanic materials: Pulverized fuel ash, silica fume, ground granulated blast furnace slag Increase strength Increase impermeability

Fresh Concrete

Long term requirements	Strength Durability Volume stability
Short term requirements	Uniformity Workability Compactability Finishable Not segregate Minimum bleeding
Criteria	Compromise between long term & short term requirements
Sequence of main operations	Batching of concrete components Mixing Conveying Placing Compacting Finishing Moist curing Removal of formwork
Segregation	Causes: Too much water: movement Too little water: less cohesion Lack of fine particles Improper handling
Bleeding	Causes: Too much water Lack of fines Over-vibration Excessive bleeding: Plastic settlement cracking Bleeding rate < Evaporation rate: Plastic shrinkage cracking
Measures	Increase cement fineness Increase alkali content Increase air entrainment Decrease water content Double-mixing method
Tests	Vebe test: slump ~ 0mm Compacting factor test: slump 0~75 mm Slump test: slump 50~100 mm Flow table test: slump >150mm

Concrete Mix Design

Strength, f_cu	Compressive strength at 28 days, below which not more than 5% of all tests fails
Workability	Slump >100mm: add plasticiser
Durability	Max W/C ratio Min C Adequate concrete cover
Mix design steps	Std deviation, k_s W/C ratio Water content C = W / W/C Total aggregate content %fines Water absorption of aggregates
Concrete	(cement, water, coarse agg., fine agg.,) Proportion by weight: 1: 2: 4: 2.5 Proportion by volume: 1: 1.5: 4.5: 3

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