Div 03 Concrete PDF

DIV. 03 CONCRETE BTTB1027 - Cast-in-place Concrete Agenda Water Content Concrete properties Concrete States Portland cement Aggregates for concrete  CONCRETE Structural material produced by mixing predetermined amounts of Portland cement, aggregates, and water, and allowing this mi...

DIV. 03 CONCRETE BTTB1027 - Cast-in-place Concrete Agenda Water Content Concrete properties Concrete States Portland cement Aggregates for concrete  CONCRETE Structural material produced by mixing predetermined amounts of Portland cement, aggregates, and water, and allowing this mixture to cure.  QUALITY OF CONCRETE Aggregate should consist of particles that have adequate strength and resistance to exposure conditions. Concrete depends greatly on the quality of the paste. Each particle of aggregate is completely coated with paste and all the space between aggregate particles is filled.  Water content Strength of concrete is determined by the amount of water used water required for complete hydration is about 40% of the weight of Portland cement – Water-cement ratio = 0.40 Water content Advantages of reducing water content: – Increased compressive strength – Increased water tightness – Increased resistance to weathering – Better bond between concrete and reinforcement – Less volume change States of concrete Concrete has three different states: – PLASTIC – SETTING – HARDENING Plastic State When the concrete is first mixed it is like bread dough. It is soft and can be worked or molded into different shapes. In this state concrete is called PLASTIC. Concrete is plastic during placing and compaction. The most important properties of plastic concrete are workability and cohesiveness.  Setting State Concrete then begins to stiffen. The stiffening of concrete, when it is no longer soft, is called SETTING. Setting takes place after compaction and during finishing. Concrete that is sloppy or wet may be easy to place but will be more difficult to finish. Hardening State After concrete has set it begins to gain strength and harden. The properties of hardened concrete are strength and durability. Hardened concrete will have no footprints on it if walked on. Concrete Properties 3 main properties of concrete are: – WORKABILITY – STRENGTH – DURABILITY Workability Workability means how easy it is to: – Place – Handle – Compact – Finish a concrete mix. Workability is affected by? The Water-cement ratio The cement paste is the soft or liquid part of the concrete mix. The more paste mixed with the coarse and fine aggregates, the more workable a mix. The Aggregate grading Well-graded, smooth, rounded aggregates improve the workability of a mix. Make a more workable mix Add more cement paste Use well graded aggregates. Use an admixture Never try to make a mixture more workable by just adding more water because this lowers the strength and durability of concrete.  Strength and Durability Strength and Durability are affected by the density of the concrete. Denser concrete is more watertight (or less permeable). Concrete durability increase with strength. Strength of concrete in the hardened state is usually measured by the compressive strength using the Compression Test.  Strength and Durability are affected by? – Compaction is removing the air from concrete. – Curing is keeping concrete damp for a period, to allow it to reach maximum strength. – Warmer weather will cause concrete to have a higher early strength. – Different types of cement will affect concrete properties: ie how quickly or slowly concrete gains strength. – Water cement ratio too much water and not enough cement means concrete will be weaker and less durable.  PORTLAND CEMENTS Most common type of cement. Ingredient of concrete, mortar, stucco, and non-specialty grout. Developed in England (1824) Fine powder, produced by heating limestone and clay minerals in a kiln to form clinker, grinding the clinker, and adding 2 to 3 percent of gypsum Portland cement is caustic, so it can cause chemical burns.  Portland Cement Types TYPE 10, NORMAL – General-purpose cement. Its uses include pavements and sidewalks, water pipe, masonry units. TYPE 20, MODERATE – Cement is used where precaution against moderate sulphate attack is important. Its uses include large piers, heavy retaining walls. TYPE 30, HIGH EARLY STRENGTH – Cement provides high strength at an early period, usually a week or less. It is used when forms are to be removed as soon as possible, and in cold weather to reduce the controlled curing time.  Portland Cement Types TYPE 40, LOW HEAT OF HYDRATION – Cement is used where the rate and amount of heat generated must be minimized. It developed strength at a slower rate than Type 10. It is used in massive concrete structures. TYPE 50, SULPHATE RESISTING – Cement is used only in concrete exposed to severe sulphate action. It is used principally where soils or groundwater have a high sulphate content. Sulphate Attack on Concrete Sulphate ions attack components of the cement paste – crystals occupy empty space, and as they continue to form, they cause the paste to crack, further damaging the concrete Internal Sources – Rare, originates from such concrete-making materials External sources. – more common and usually are a result of high-sulphate soils and ground waters Water for Concrete Almost any natural water that is drinkable and has no pronounced taste or odour is satisfactory as mixing water for making concrete. Aggregate for Concrete Aggregate generally occupy 60% to 80% of the volume of concrete; they strongly influence the properties of concrete, its mix proportions, and cost.  Characteristics of Aggregate for Concrete Resistance to Freezing and Thawing – Important characteristic in exterior concrete. If the problem particle is near the surface it can cause a pop out Particle Shape and Surface Texture – Bond between cement paste and aggregate generally increases as particles change from smooth and rounded to rough and angular Therefore higher compression strength Grading – Determined by a sieve analysis. – Grading affect aggregate Size proportions, cement and water requirements, workability, pump ability, shrinkage, and durability  Characteristics of Aggregate for Concrete Bulk Density – The weight of a material per unit volume including voids Surface Moisture – The amount of water used for the concrete mixture must be adjusted for the moisture conditions of the aggregate. weighing the aggregate and adjusting for moisture content when proportioning concrete. Compressive Strength – The strength of an aggregate is rarely tested and does not influence the strength of normal-strength concrete as much as paste strength and paste- aggregate bond. – But does become important in high-strength concrete  DIV. 03 CONCRETE BTTB1027 - Air-entrained concrete Agenda Air-entrained concrete Concrete Tests Air-Entrained Concrete One of the greatest advances in concrete technology was the development of air-entrained concrete in the mid- 1930’s. Contains billions of microscopic air cells per cubic foot. Produced by using an air-entraining cement or an air-entraining admixture Air pockets relieve internal pressure on the concrete by providing tiny chambers for water to expand into when it freezes. Properties of Air-Entrained Concrete Freeze-Thaw Resistance – As the water in moist concrete freezes, it produces pressures in the pores of the cement paste and aggregate Resistance to De-icers – The entrained air voids in concrete allow space for salt crystals to grow, thus relieving internal stress similar to freezing water. Workability – The ease of placing, finishing freshly mixed concrete Sulphate Resistance – Advantage is taken of the reduction in the water-cement ratio Properties of Air-Entrained Concrete Strength – Advantage is taken of the reduction in the water-cement ratio Abrasion Resistance – Little effect, increased strength increases abrasion resistance Water tightness – Increases slightly, reduction in the water-cement ratio Air-Entrained Concrete Air entrained versus VS Entrapped air air entrained air entrained admixture Concrete test- Slump Slump test is used to check workability of freshly made concrete. Can be an indicator of an improperly mixed batch Test is popular due to the simplicity slump test video Concrete test- Slump Air content in concrete measurement of air content in fresh concrete is typically performed using the pressure method Begin the test within 15 minutes after obtaining the composite sample. Measured as % of air content test video ASTM C 231  Compression Strength Test Used to determine the compressive strength of concrete Cast test cylinders from the concrete received at the job site. Concrete test cylinder in position in the testing machine, and compressed to failure Compression Strength Test ASTM C39 Concrete Cylinder Compression Testing https://www.youtube.com/watch?v=llNpeF7hayo