Week 5 – Materials and Systems: Concrete

Week 5 – Materials and Systems: Concrete Opus caementicium  Concrete is a conglomerate, a mixture of elements which cannot be found in nature  The romans developed a building technique called ‘opus caementicium’, where stone rubbles are alternated with dense mortar and confined with course bricks acting as framework  Romans perfected their knowledge on mortar and concrete by developing Pozzolana, a cement obtained combining volcanic rocks with lime and sand which has the property to harden under water Hydraulic lime  The term identifies different types of lime mortar, which set through hydration Cement and concrete  Today’s concrete is obtained through the admixture of sand and aggregates (coarse and fine), water and cement (the bonding agent)

Cement - Ordinary Portland Cement (OPC)  Cement is the most important concrete ingredient  Cement used in the concrete industry derives from Aspdin’s Portland Cement  OPC is a complex energy intensive industrial product obtained by burning limestone slurry and subsequently combining it with silica, iron and alumina  OPC is a hydraulic type of cement, it reacts chemically with water by hardening and it provides a durable and chemically stable bond with the aggregates Cement production process – 1  Cement is typically obtained from limestone and clay  These materials are crushed and blended, then heated Cement production process – 2  Limestone/clay  blending  kiln  klinker  cement mill  cement

Supplementary cementitious materials  Basic formula or ordinary cement can be enriched by the addition of supplementary materials and by creating cement ‘blends’  These materials are generally added in quantities not exceeding 10% for economy, to improve the workability of cement without affecting its strength Fly Ash Material extracted from the flue gases of a boiler fired with pulverized coal

Slag Slag is a granulated material consisting of silicates and calcium aluminosilicate, a byproduct of steel making in a blast furnace

Cement types in Australia Code Type GP General purpose Portland

GB

General purpose blend

GL

General purpose limestone

HE

High early strength

LH

Low heat

SR

Sulphate resisting

SL

Shrinkage limited

Amorphous Silica A very fine pozzolanic material composed mostly of non crystalline silica

Applications - Most form of ordinary concretes - May contain additives up to 5% (fly, ash, slag) - Most form of ordinary concretes - May contain addivies more than 5% and up to 10% (slag, fly, ash, silica) - May contain limestone alone or limestone with additives for more than 8% and up to 20% - 3 day strength is of the same order of 7 day strength of GP cement - Early formwork removal, rapid strength - Used in large concrete pours (dams) to limit rate of heat generation - Intended primarily for use where resistance to ground waters containing sulfates in solution is required - Does not change in volume owing to drying shrinkage - Used for roads / pavements / bridges

White Portland Cement  Used principally for architectural purposes  Type: GP cement  White cements are characterised by a different chemical composition than ordinary Portland  Used for o Exposed concrete o Grout o Mortar o Render  Forms the base of pigmented cement when combined with coloured iron oxides Water  Is the reacting agent of cement to form a paste that holds the aggregate together  Potable water is generally acceptable  Water is added to the mix in order to make it more workable Water/Cement ratio  The water to cement ratio in the mix is a critical factor and determines concrete strength, durability and workability  The higher percentage of cement means the higher it is in compressive strength and durability of concrete  It is preferable to have a low water/cement ratio to improve the durability of concrete. However, water plays an important role by making the mix fluid and workable Aggregates  Aggregates are sand (fine and coarse) stone or gravels that are added to the concrete mix to provide bulk, reduce cost, improve durability and affect workability, colour and texture of concrete  Defined in two groups depending on their size: o Fine: Concrete sand is different from plasterers or mortar sand o Coarse: Stone, gravel, crushed rock screenings Aggregates grading  Concrete aggregates must be clean and chemically inactive  Aggregates must also be graded, the grade influences the water demand Admixtures  Chemicals that enhance specific properties of the mix o Accelerators: Used to speed up set times for example in cold/humid weather o Retarders: Used in hot weather to delay set times o Slag or fly ash: Used to reduce cement content and improve workability o Plasticisers: Used to reduce water content and improve strength o Pigments: Used to colour the concrete

Reinforced concrete  Combination of concrete with steel reinforcement generates a material that is capable of having both compressive and tensile strength Concrete reinforcement  Uses steel to react to tensional stress and relies on concrete for compressional stress  Concrete and steel are a perfect pair as they have the same coefficient of thermal expansion  Concrete cover protects steel from fire and corrosion due to ‘carbonation’ Concrete defects  Degradation of the concrete surface can be induced by the corrosion of steel reinforcement  Corrosion of reinforcing bars may be due to external sources such as o Exposure to sulphate attacks o Chlorides, carbonation o Effects of stress and moisture  ‘Concrete cancer’ traditionally refers more specifically to ‘internal’ causes of degradation Corrosion defects and cracks Cracking

Spalling

Delamination

Corner effects

Rebars and mesh designation  L – Low ductility – LD bars or mesh, not suitable for high deflection applications  N – Normal ductility – Bars or mesh for general purpose  E – Earthquake – Earthquake resistant bars or mesh

Common mesh designations  S – Square – Square rib mesh at typically 100mm centres – bar sizes vary  R – Rectangular – Rec rib mesh at typically 100x200mm centres – bar sizes vary  TM – Trench mesh – 3 to 6 bars running lengthwise typically at 100mm centres held together by cross bars. For ground beams

Concrete Properties  Four main properties: o Workability o Cohesiveness o Strength o Durability  Three different states o Plastic (fresh) o Setting o Hardening  The four main properties of concrete change considerable depending on which state the concrete is in Workability  Workability means how easy is it to place, handle, compact and finish a concrete mix  Workability is affected by: o The amount of cement paste More paste means more workable mix is o The aggregate grading Well-graded, rounded aggregate improve workability of the mix Slump test  Used to measure the workability of concrete before concrete is poured o 30-80mm – Mass concrete o 50080mm – Unreinforced footings, caissons, substructure walls, slabs o 50-100mm – Beams, reinforced footings, columns o 80-120mm – Reinforced walls