SPECIAL CONCRETES [PART 03]

Transcription

1 SPECIAL CONCRETES [PART 03] Fahim Al-Neshawy & Esko Sistonen Normal concrete HPC & HSC SCC Underwater concrete Fast drying concrete Shotcrete Concrete types Fiber reinforced concrete Self- Healing concrete Lightweight concrete Mass concrete Polymer concrete Colored concrete

2 MASS CONCRETE Definition Mass concrete is defined as Any volume of concrete with dimensions large enough to require that measures be taken to cope with (resist) the generation of heat from hydration of cement and attendant volume change to minimize cracking.

3 Why study of mass concrete? Why study of mass concrete? generation of heat from hydration of cement

4 Why study of mass concrete? When the temperature in the core of the concrete increases, the surface of concrete is cooled in comparison with the core, due to thermal expansion The respective volume changes in the concrete causes compressive forces to develop in the core, and tension forces to develop at the surface Factors affecting temperature rise 1. Geometry Pours (cast) with a large volume: surface area ratio are more susceptible to thermal cracking! Research has been performed to produce more accurate dimensions for when temperature rise should be considered. 2. Cement Composition Cements used for Mass Concrete should have a low C3S and C3A content to reduce excessive heat during hydration. Most Mass Concrete structure do not require early strength, so slower hydration is usually not harmful to construction.

5 Factors affecting temperature rise 3. Cement Fineness Cement with a lower fineness with slow hydration, and reduce temperature rise. 4. Cement Content Mass Concrete mixtures should contain as low of a cement content as possible to achieve the desired strength. This lowers the heat of hydration and subsequent temperature rise. Can be as low as 100 kg/m³ Factors affecting temperature rise 5. Aggregate Content Coarse Aggregate should be have an MSA (max. size of agg.) of 15 cm if possible. A higher coarse aggregate content (70-85%) can be used to lower the cement content, reducing temperature rise.

6 Factors affecting temperature rise 5. Coarse Aggregate Coefficient of Thermal Expansion The CTE of the coarse aggregate is the main influence on the CTE of the concrete. Choosing an aggregate with a low CTE can cut thermal stresses in half. Factors affecting temperature rise 6. Supplementary Cementitious Materials (SCMs) SCMs such as Fly Ash, and Slag can greatly reduce the heat of hydration. Pozzolans such as FA (class F is best > slower hydration) and Slag will produce between 15-50% of the heat of normal Portland Cement. Highly reactive SCMs such as Silica Fume and Metakaolin do not substantially lower the heat of hydration.

7 Factors affecting temperature rise 6. Supplementary Cementitious Materials (SCMs) Use of SCMs such as FA and GGBFS is commonly up to 60-75% cement replacement. Factors affecting temperature rise 7. Placement Temperature Casting at lower temperatures will reduce the thermal stresses; o Slows hydration > lowers heat of hydration o Lowers temperature differential between the core and the outer surface

8 Factors affecting temperature rise 8. Water cement ratio W/C has a large effect on temperature rise W/C = is common 9. Workability o Most Mass Concrete mixtures have a ~0-60 mm slump o Water reducer WRs or Superplasticizers may have to be used to retain workability Materials and Mix Proportions The heat of hydration of a cement is a function of its compound composition and fineness cement contents as low as 100 kg/m3 o substitution of 20 percent pozzolan 4 to 8% entrained air water-reducing admixtures the extent of reduction in water content by the use of entrained air and the largest possible size of aggregate o At a given water-cement ratio and consistency, as the maximum aggregate size is increased, both the water and the cement contents are reduced.

9 Applications SELF-HEALING CONCRETE

10 Self healing (bacterial) concrete What is S.H.B.C? A new concrete technology that autonomously repairs cracks. How does it work? Alkaliphilic bacteria added to concrete matrix. Bacteria react to the water and metabolize crystals, which close the crack and protect the steel within. Self healing bacterial concrete Bacterias used Bacillus cohnii Bacillus filla Bacillus parturii Cement and water have a ph value of up to 13 when mixed together, usually a hostile environment for life, most organisms die in an environment with a ph value of 10 or above. Microbes that survive in alkaline environments can be found in natural environments, for example: o alkali lakes in Russia, o carbonate-rich soils in desert areas of Spain o soda lakes in Egypt.

11 Preperation of bacterial concrete Self healing bacterial concrete can be prepared in two ways. By direct application By encapsulation in light weight concrete. By the method of direct application bacterial spores and calcium lactate are added directly while making the concrete and mixed. Here when the crack occurs in the concrete bacterial spores broke and bacteria comes to life and feed on the calcium lactate and limestone is produced which fill the cracks. Preperation of bacterial concrete By encapsulation method the bacteria and its food, calcium lactate, are placed inside treated clay pellets and concrete is made About 6% of the clay pellets are added for making bacterial concrete When concrete structures are made with bacterial concrete, when the crack occurs in the structure and clay pellets are broken and bacterial treatment occurs and hence the concrete is healed. Minor cracks about 0.5mm width can be treated by using bacterial concrete Among theses two methods encapsulation method is commonly used, even though it s costlier than direct application. Bacillus bacteria are harmless to human life and hence it can be used effectively.

12 What is happening inside bio concrete (mechanism) 1. The cracks are formed on the surface of concrete due to many reasons like shrinkage, Inadequate water for hydration etc. 2. The water is deliberately forced into the crack and the precursor is activated What is happening inside bio concrete (mechanism) Ca(C 3 H 5 O 2 ) 2 + 7O 2 CaCO 3 + 5CO 2 + 5H 2 O CO 2 + Ca(OH) 2 CaCO 3 + H 2 O (carbonation) 3. The activated precursor intern induces the bacteria to react with that precursor and form a base of calcium carbonate called as limestone, the chemical equation is given above.

13 BEFORE HEALING AFTER HRALING 20x

14 40x Advantages The self healing bacterial concrete helps in reduced maintenance and repair costs of steel reinforced concrete structures. Oxygen is an agent that can induce corrosion, as bacteria feeds on oxygen tendency for the corrosion of reinforcement can be reduced. Self healing bacteria can be used in places where humans find it difficult to reach for the maintenance of the structures. Hence it reduces risking of human life in dangerous areas and also increases the durability of the structure. Formation of crack will be healed in the initial stage itself thereby increasing the service life of the structure than expected life.

15 Disadvantages If the volume of self healing agents (bacteria and calcium lactate) mixed becomes greater than 20%, the strength of the concrete is reduced. Preparation of self healing concrete needs bacteria and calcium lactate. Preparation of calcium lactate from milk is costlier. Hence preparation of self healing concrete costs double than conventional concrete. Applications Self healing bacterial concrete can be used for sectors such as tunnel-lining, structural basement walls, highway bridges, concrete floors marine structures. Marine structure Tunnel lining

16 Applications Concrete flooring Underground retaining walls Highway bridge SHOTCRETE

17 A concrete placing process where concrete mixtures are conveyed through a hose then - with the help of pressure - projected at high velocity onto a surface to achieve high quality in-place compaction. It produces high quality dense concrete, with a low w/cm ratio, low permeability, and a high cementitious material content. Shotcrete is today a term that describes spraying concrete or mortar with either a dry or wet mix process. Gunite is a trademarked name that is incorrectly used to describe the dry-mix shotcrete process Shotcrete emerged as the only acceptable industry term to correctly describe "pneumatically applied concrete.

18 Shotcrete, high performance product consisting of Cement + aggregates + water + admixture + non-alkaline accelerator was invented in the early 1900s by American taxidermist Carl Akeley. used to fill plaster models of animals. In 1911, he was granted a patent. Until the 1950 s, the wetmix process was devised, only the dry-mix process was used.

19 Sprayed concrete is reinforced by conventional steel rods, steel mesh, and/or fibers. Fiber reinforcement (steel or synthetic) is also used for stabilization in applications such as slopes or tunneling.

20 conventional concrete is first placed and then compacted in the second operation. shotcrete undergoes placement and compaction at the same time. Shotcrete is more dense, homogeneous, strong, and waterproof. It can be impacted onto any type or shape of surface, including vertical or overhead areas 1. Dry process 2. Wet process 1. Dry process: Step1: Pre blended, dry or semi-dampened materials are placed into shotcrete equipment and metered into a hose. Step2: Compressed air conveys materials at high velocity to the nozzle where the water is added. Step3: Then the material is consolidated on receiving surface by high impact velocity.

21 Advantages of Dry process: Easy start up, shutdown and clean up. Control of materials is on site. Nozzle can be up to: 300 m horizontally or 150 mm vertically from the gun. Wet process: Step 1: All ingredients, including water, are thoroughly mixed and introduced into the shotcrete equipment. Step 2: Wet material is pumped to the nozzle where compressed air is introduced Step 3: Mostly wet-process shotcreting is done with premixed mortar or small aggregate concrete.

22 Wet process: Advantages of Wet process: Little or no formwork is required. Cost effective method for placing concrete. Ideal for irregular surface applications Allows for easier material handling in areas with difficult access

23 Rehabilitation of subway tunnels construction of domed roofs Swimming pool construction lining construction UNDERWATER CONCRETE - UWC

24 Introduction There are often situations such as : Port and harbor installations Bridge piers in rivers Water industry structures Metro systems Deep shafts in unstable ground, in which concrete is to be placed underwater. Performance requirements for UWC Workability & self compaction Cohesion against washout & segregation Low heat of hydration Controlled set time Compressive strength Adequate bond Problems faced Segregation of fine aggregates from coarse aggregates Water pollution Increased w/c ratio washout

25 Materials used : Composition (Example 0 32 mm aggregate): Aggregate Use an aggregate suitable for pumped mixes Fines including cement > 400 kg/m³ Cement and Powder Additives Minimum cement content 350 kg/m³ Limestone can be added to the fines content in the mix design Admixtures Superplasticizer for the reduction of free water in the mix Mix stabilizer to minimize washout effect of fines and cement (especially in running water conditions) Materials used : Components Description Example formula Aggregates Cement Any quality aggregates possible Any cement meeting local standards All aggregate sizes are possible Target cement paste volume according pumping concrete recommendations: > 350 kg/m³ Powder additives Limestone, fly ash or ground granulated blast furnace slag Sufficient fines content by adjustment of the binder content: Fines including cement > 400 kg/m³ Water content Fresh water and recycling water with requirements regarding fines content w/c-ratio according to standards with regard to exposure class: < 0.48

26 Materials used : Components Description Example formula Concrete admixtures Superplasticizer Type dependent on placement and early strength requirements Stabilizer for stagnant water Stabilizer for running water Superplasticizer % * Cement Stabilizer: % * Cement Installation requirements Most often used today is pumping a suitably modified mix through a standard concrete pump. The end of the delivery pipe must be kept deep enough in the fresh concrete. Concrete laying techniques Tremie method Bucket Placing Pump method

27 Tremie method A Tremie is a watertight pipe Generally 250mm in dia. Funnel shaped hopper at its upper end and a loose plug at the bottom. It is supported on a working platform above water level. Tremie method

28 Specifications of concrete to be used in Tremie method: Coarse Aggregate: Gravel of 20 mm max. size. Use % of the total aggregate by weight. Sand, 45-50% of the total aggregate by weight Water/Cement Ratio: 0.42 (0.45 Maximum). Water-Reducing Admixture (preferably it is also plasticizer): Do not use super plasticizers. Air-Entrainment Admixtures: To give 6% total air. Retarding Admixture: To increase setting time to 4-24 hours, as required. Slump: 160 mm ± 25 mm This mix will develop compressive strength in the range of MPa at 28 days. Bucket placing The buckets used for underwater placement of freshly mixed concrete should have drop-bottom or roller- gate openings. The gates should be able to be opened from above water. If air is used to open the bucket, the air should discharge through a line to the surface to prevent water disturbance. The top of the bucket must be covered to prevent water from washing the surface of the freshly mixed concrete.

29 Bucket placing drop-bottom opening roller- gate opening Pump method: Pumping concrete directly into its final position, involving both horizontal and vertical delivery of concrete. Pumping concrete has the advantage of operational efficiency with potential savings of time and labour. Pump method of laying concrete

30 References 1. ACI 207.1R-96 (1996). Mass Concrete Reported by ACI Committee S. B. Abdul Wahab, (2014). Self healing bacterial concrete. Online at: 3. Jürg Schlumpf and Jürgen Höfler (2006). Shotcrete in Tunnel Construction - Introduction to the basic technology of sprayed concrete. Online at: bf68-599a e/Fact%202_shotcrete%20handbook.pdf 4. Underwater concrete - mix design and construction practices ftp://dfi.org/onemine/marine%20foundations%20book%20- %20individual%20papers/29-5.4%20Underwater%20Concrete%20- %20Mix%20Design%20and%20Construction%20Practices.pdf Next lecture: Mon Permeability of concrete: Permeation defines the ease with which fluids, both liquids and gases, can enter into, or move through concrete. Durability of concrete: Is the ability to resist o weathering action, o chemical attack, o abrasion, o or any process of deterioration.