Standing Committee on Concrete Technology Annual Concrete Seminar 2011

Concrete in building construction – An historical perspective from Hong Kong

Daniel Brown

23rd March 2011

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Historical Perspective on Concrete Structures

• Brief loo k a t th e historica l u se of concrete

• Concret e code s an d standard s from th e past

• Material s used

• Durabilit y Issues

• Structura l Issues

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Galilee, Israel - 7000BC

• Concrete Floor 7000BC found in 1985

• Lime concrete from burning limestone to make quicklime, mixed with water and stone.

• Concrete is Designer Rock and let man dictate shape

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Ancient Egypt, 2000BC

• 1st Picture of Use of Concrete

• Workmen filling earthenware jars with water mixed with lime used as mortar for stone work and to form concrete wall.

• Site Foreman & QC

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Coliseum, Ancient Rome AD 82

• Lightweight pozzolanic concrete in arches. Faced with stone.

• 50,000 spectators

• HK Stadium has 45,000 spectators

• Stonework limestone

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Hardrian’s Wall Roman Britain, AD 100-150

• Lime Cement originally used in mortar.

• Core is formed of concrete.

• 75 Miles long

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Portland Cement - 1824

• Patented by Joseph Aspin in 1824

• Leed’s Builder

• Son moved to London with patent and worked with Brunel

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Reinforced Concrete - 1860’s

• Joseph Tall’s concrete cottages in Kent.

• Amongst earliest surviving RC structures.

• Various applications in boats and flower pots during and after this period

• 120 Year Design Life

• Marine Exposure Conditions

• Over 100 intrinsic an d inferred performanc e criteria for concrete .

• In 100 years this will be a Historical Structure

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Reinforced Concrete AD 2000

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Concrete codes and standards

• London County Council by laws

• 1915, 1938, 1952, 1964

• Hong Kong Building (Construction) Regulations

• 1956, 1964, 1975, 1976… 2004

• 1934 - publication of first UK National Code, permissible stress design, concrete specified by mix proportions

• 1957 - Load factor design approach introduced

• 1969 – Minimum shear reinforcement introduced

• 1972 – Limit state design

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Major differences between old and new design codes

• No requirement for nominal shear reinforcement in the old codes and shear loads often taken by the concrete with no reinforcement provided

• This under provision for shear will have significant effect in the case of change of use/ increased loads where insufficient shear capacity may be provided

• Building design took advantage of this by over sizing concrete elements – no effect on requirements for shear reinforcement

• Concrete specified by mix design with assumed strengths for different mix designs rather than on required concrete strength alone.

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Differences/ similarities between old and new design codes (1)

• Loading requirements have changed little over the years, office loading has remained fairly constant

• Design for shear and punching have changed significantly over the years

• Requirements for shear in pre-stressed concrete construction have also developed similarly over the years. No significant use of pre stressed or post tensioned construction in Hong Kong pre 1980.

• Fire resistance through design has changed over the years where concrete was once considered to be fire proof and more guidance has been introduced through successive codes. The early LCC codes started to introduce cover requirements for fire resistance periods

• Bond and anchorage requirements changed where in the earlier codes bond stresses were stated for each of the nominal mixes.

• Serviceability has changed with codes and building designs where in older buildings spans were limited codes were concerned with strength rather than serviceability.

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Differences/ similarities between old and new design codes (2)

• Guidance on span/ depth ratios was first introduced followed by methods for calculating crack widths.

• Requirements for robustness were introduced following a collapse of a multistory building in the UK in 1968 to help accommodate accidental loading including internal ties and connections.

• Foundations started to include precast concrete piles towards the end of the 19th century with bored piles being more generally used from the 1950’s onwards.

• Future changes to accommodate identified concerns – seismic effects, blast damage, environment change, sustainability, etc.????

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Concrete specification by mix design

• 1915 LCC Reinforced Concrete Regulations

• RC Design Handbook 1932 contained 6 mixes. Cement:fine aggregate:course aggregate by volume:

• Mix A 1:3:6 (8N/mm2)

• Mix B 1:2.5:5 (10N/mm2)

• Mix C 1:2:4 (14N/mm2)

• Mix D 1:1.6:3.3 (16N/mm2)

• Mix E 1:1.5:3 (17N/mm2)

• Mix F 1:1:2 (18N/mm2)

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Expected concrete strength assessment

Age Band Allowable Stress (Psi) Expected Value for Concrete Strength (MPa)

Beam & Slab Column Beam & Slab Column

Pre1946 600 600 12.4 12.4

1951-55 600 600 12.4 12.4

1956-60 750 975 15.5 20.2

1961-65 750 975 15.5 20.2

1966-70 750 975 15.5 20.2

1971-75 760 1140 15.7 23.6

1976-80 760 1140 15.7 23.6

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Concrete Materials

• Cement/ binder

• Fine Aggregate

• Course aggregate

• Mixing water

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Cement/ binder

• OP C (limestone, clay, sili ca an d gypsum)

• Hig h alumin a cement (HAC) 195 0 t o 1970

• Calciu m chlorid e a s a n accelerato r – u p t o th e mid 1970’s

• Fineness/ surfa ce are a clinke r groun d wit h gypsu m afte r kiln

• Standards, develope d ove r th e last 10 0 years

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Aggregates

• Source/ quarrying

• Crushing

• Concrete design codes assume to be sound

• Grade, shape, size

• Development of standards

• Reactivity

• Storage

• Transportation

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Water and Additives

• Hydration of cement – chemical reaction

• Source/ quality

• Temperature

• Lubrication

• Availability issues in Hong Kong, water shortages, etc. in the 1950’s and 1960’s

• Pre- 1970 use of Brackish or saline water

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Durability Considerations

• Uncertainty on materials used and quality of construction

• Uncertain repair history (human intervention)

• Time, 4th Dimension – Aging Process – Environmental Loading – Durability Performance – Service Life ?? – Repair Philosophy ??

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Construction influence – Older concrete building construction

• Site batching/ mixing of concrete with inherent quality uncertainties;

• Uncertainty on material supply and testing;

• Potential for corruption;

• Concrete compaction/ permeability;

• Joints;

• Skill/ experience/ training/ supervision;

• Smaller, older buildings easier to control construction processes and site management.

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HK - Harsh Exposure Conditions

Building Deterioration

• 10 Years befor e corrosion starts

• After 20 years extensive spalling and cracking

• Chloride, carbonation

• Moisture availability

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CONCRETE

Structural Chemical Physical

Alkali Aggregate Reactivity (External Moisture) Chloride and/or Sulphate Reaction

Other Hostile Chemical Attack

Impact Salt Crystallisation Overload Moisture

Movement (e.g. Settlement) Abrasion Explosion Erosion

Thermal Freeze/Thaw

Concrete Deterioration

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Electrochemical Corrosion

Carbonation Corrosive Contaminants e.g. chloride

Stray Currents

Internal External

e.g. From Mixing From the Environment Contaminated Aggregate

Contaminated Water e.g Sea Splash/Spray Admixture Groundwater

(Require External Moisture) Industrial Chemicals

Steel Corrosion

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Fe Fe++e- eeee­­­­

CathodeAnodeAnode

Cl-Cl-

Rebar

SaltSalt

Oxygen & Moisture

+ve Ions+ve Ions Carbonation

CO2

Cathode

Reinforcement Corrosion

Anodic Reaction (Oxidation) Cathodic Reaction (Reduction) O + 2 H O + 4 e- 4OH­

2+2Fe 2Fe + 4e­ High pH Maintained2 2+Fe + 2H O Fe(OH) + 2H+2

pH reduced

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The Behaviour of Materials

All materials are reactive chemicals - their physical properties are completely dependent on their environment

• Material behavior not well understood in the past and potentially reactive/ deleterious materials used in concrete construction

• How long can the material sustain the properties for which it has been designed? On a deemed to suit basis in the past

• Service life often not clearly defined in the past but believed to be more than 10,000 Building structures in Hong Kong over 50 years old.

• Collapses / failures

• Intern al deterioration

• Environment

• Change o f use

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Structural Issues

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Structural Failures

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Structural Failures

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Structural Failures

Thank You

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