CIV2235 Structural Materials Lecture Notes PDF

CIV2235 Structural Materials Lecture #4 (Week 4): Properties of Hardened Concrete Dr Fatemeh Azhari (Clayton Campus) Email: [email protected] Department of Civil Engineering  Today’s lecture understand re...

CIV2235 Structural Materials Lecture #4 (Week 4): Properties of Hardened Concrete Dr Fatemeh Azhari (Clayton Campus) Email: [email protected] Department of Civil Engineering  Today’s lecture understand relevant properties of hardened concrete & why they are relevant to engineering design —compressive/tensile strength —modulus of elasticity —creep and shrinkage learn where to look for properties of concrete in AS3600 learn methods used in AS3600 to estimate some concrete properties 1. Australian Standard, AS3600, Section 3 Design Properties of Concrete 2. Hardened Concrete, Chapter 5 of Advanced Concrete Technology 3. Properties of hardened concrete, Chapter 14 of Civil Engineering materials  1. Structural performance strength: the ability of the structure to withstand load. serviceability: the ability of the structure to provide a comfortable, aesthetic environment when used for intended functions. durability:the time for which the structure is serviceable, and the maintenance required for it to remain serviceable. Key hardened concrete properties Strength Modulus of Elasticity Creep Shrinkage Coefficient of thermal expansion Durability Materials Science [of Concrete] Structural Design  Three stages of concrete  Water to cement ratio vs permeability Permeability (in fluid mechanics & earth sciences) is a measure of the ability of a porous material to allow fluids to pass through it. ↑ w/c = ↑ permeability permeability ~ porosity (pores) PORES CONNECTIVITY porosity=measure of void spaces, ratio of void volume/total volume  Volume changes in cement hydration Vec Empty Capillary pores Capillary Capillaries water Vcw Vw Water Vgw Gel water Hydrated Solid cement or products of Vp cement hydration gel Vc Ceme nt Unhydrated Vuc cement before hydration during hydration  Lower w/c =  porosity = greater strength (only w/c = 0.27 is needed to fully hydrate the cement; surplus free water leads to void formation) Low w/c High w/c Cement Particles Suspended in mixing Water Air and/or Fully Water-filled Hydrated Cement voids  2. Compressive strength age for strength: 28 days cylinders: 150 (D) and 300 mm (H) → → Compressive strength: major factors of influence water/cement ratio (w/c): w/c ↓ strength ↑ age: age ↑ strength ↑ type of curing degree of compaction: good compaction (less voids): strength ↑  < 10 > Compressive strength The most common test preformed on concrete is for compressive strength. There are several reasons for this: 1. The most important properties of concrete→ directly related to the compressive strength; 2. Concrete has little tensile strength and is used primarily in compression 3. structural design codes are based on compressive strength The test is relatively simple and inexpensive to perform.  < 11 > Compressive strength D standard specimen H 30° The larger the diameter (D) the lower compressive strength region unaffected by H/D = 2: to reduce the effect of lateral forces developed lateral forces between the end surfaces of the concrete specimen and the adjacent steel platens of the testing machine  < 12 > Capillary porosity and strength High capillary porosity leads to lower strength  < 13 > Compressive strength – w/c ratio ↑ w/c ↑ w/c = ↓ strength But do not forget this… ↑ w/c = ↑ workability  < 14 > Compressive strength – w/c ratio & age 1 age=constant: w/c ↓ strength ↑ 2 w/c=constant: age ↑ strength ↑ age ↑ : more reaction between cement and water, thus reducing capillary porosity → strength ↑ What’s curing? < 15 > ▪ Curing means to cover the concrete so that it stays MOIST ▪ By keeping concrete moist the bond between the paste and the aggregates gets stronger ▪ Concrete doesn’t harden properly if it is left to dry out When to cure? ▪ Curing is done just after finishing the concrete surface, as soon as it will not be damaged  < 16 > Curing is the maintenance of satisfactory moisture content and of favourable temperature of concrete immediately after compaction and until concrete has developed the desired strength Concrete must be properly cured to develop optimum properties Compressive strength of properly cured concrete is 80–100 % greater than the strength of concrete which has not been cured at all. Properly cured concrete surfaces wear well. Drying, shrinkage, cracking is reduced. Cause: excessive loss of water by evaporation can delay or prevent adequate hydration. surface The surface is particularly susceptible to insufficient hydration because it dries first  < 17 > Curing: minimize moisture loss for hydration of cement Wet Hessian Mats Wet Curing (flooding) Plastic Sheet (reduces evaporation) Standard Laboratory Curing (Fog Standard Laboratory Sprayed Curing Membrane room @ 25 degC and 100% RH) Curing (Bath @ 25 degC)  < 18 > Curing and strength Curing is the process of controlling the rate and extent of moisture loss from concrete during cement hydration curing ↑ strength ↑  < 19 > Curing temperature effect on strength  < 20 > Compaction  Compaction = less voids =  strength ▪ 5% voids 25% loss of strength  < 21 > Compressive strength- Characteristic strength (f’c) Value of the material strength, as assessed by a standard test at 28 days, below which not more than 5% of the test results are expected to fall. AS3600, Section 3.1.1  < 22 > Stress-strain relationship what is going on at the microscale… understand this → better engineered materials  < 23 > Stress-strain relationship slope is steeper for higher strength concrete normal concrete, strain at the peak stress =0.2%  < 24 > Stress-strain relationship elastic materials linear elastic nonlinear elastic rubbers… inelastic materials permanent deformation  < 25 > Stress-strain curve-AS3600 Concrete – an isotropic material < 26 > Mechanical behaviour is described by TWO material constants: E and 𝞶 Anisotropic materials: fibres 3.Modulus of elasticity – AS3600 (3 methods) < 27 > ρ:density 3 fcmi = mean value of in situ compressive strength of methods concrete at the relevant age (see Clause 3.1.1.2 and Table 3.1.2)  < 28 > Tensile strength  < 29 > Tensile strength and compressive strength Tensile strength is required e.g. in the calculation of deflection. AS3600  4. Creep < 30 > instantaneous deflection Deflection after a long time Creep = Increase of deformation with time while under load —increases deflection of concrete beams — reinforced concrete columns, redistribution of stress in concrete & steel —tall buildings, the shortage of reinforced concrete columns may cause the final height of the building to be significantly shorter, and has to be taken into consideration in design and construction  < 31 > Creep creep=increase of strain, in time, under a sustained constant stress constant stress creep strain initial strain concrete exhibits creep even at very low stress & under normal env. conditions steel creeps only at high stresses (normal conditions) creep deformation is of the same magnitude as the elastic deformation creep develops in a concrete rapidly at the beginning & gradually decreases with time; continues over a long period of time (more than 30 years!!!) cause: removal of adsorbed water from hydrated cement paste, and growth of microcracks  < 32 > Factor that influence creep ▪ level of stress ▪ relative humidity ▪ strength of concrete at the time of loading ▪ average path length for moisture movement ▪ duration of applied load  < 33 > Creep is not fully reversible elastic recovery = approximately of the same order as the initial elastic strain the elastic recovery is followed by a gradual decrease in strain (creep recovery) creep recovery occurs more rapidly than creep concrete unloaded Creep: AS3600: 2018, Section 3 < 34 > Creep: AS3600: 2018, Section 3 < 35 > Creep: AS3600: 2018, Section 3  < 37 > Figure 3.1.8.3: coefficient k2 k2 (four sides exposed) TIME AFTER LOADING, t 5. Shrinkage < 38 > plastic shrinkage autogenous shrinkage shrinkage = volume REDUCTION drying shrinkage caused by loss of water thermal shrinkage concrete ‘I cannot live without shrinkages and cracks’ volume change induced cracks load induced cracks  < 39 > Factors influencing shrinkage ▪ aggregate content ▪ w/c ratio ▪ relative humidity (exposure environment) ▪ surface-to-volume ratio ▪ age of concrete  < 40 > Plastic shrinkage ▪ When cement paste is plastic, it undergoes a volumetric contraction due to loss of water by evaporation from the surface ▪ rarely impair strength/durability  < 41 > Drying shrinkage Contraction of a hardened concrete due to the loss of capillary water inevitable unless completely submerged in water or in environment with 100% relative humidity a phenomenon that routinely occurs hydrated cement paste shrinks considerably more than concrete KEY FACTORS: aggregate content: ↑ aggregates, ↓ shrinkage aggregate’s elasticity modulus ↑ : ↓ shrinkage water to cement ratio: ↑ w/c, ↑ shrinkage relative humidity (RH): rate of shrinkage is lower at higher RH size of members: large, small shrinkage  < 42 > Autogeneous shrinkage This is the shrinkage associated with the withdrawal of water from the capillary and gel pores for the hydration of the unhydrated cement. It is especially severe at low w/c ratios, where a great degree of unhydrated cement is present. → At low w/c ratios, all water content is rapidly drawn into the hydration process and the demand for more water creates very fine capillaries. The surface tension within the capillaries causes autogenous shrinkage → cracks Autogenous shrinkage can be largely avoided by keeping the surface of concrete continuously wet It should be considered specifically when new concrete is cast against hardened concrete.  < 43 > Shrinkage – AS3600 drying shrinkage autogenous strain shrinkage strain Shrinkage: AS3600: 2018, Section 3 < 44 > Shrinkage: AS3600: 2018, Section 3 < 45 >  Shrinkage: AS3600: 2018, Section 3 theoretical thickness A: Cross Section Area P:Perimeter  < 47 > Thermal expansion coefficient – AS3600 ❑ materials expand/contracts when temperature increases/decreases ❑ cement paste and aggregates have dissimilar thermal coefficients ❑ the thermal coefficient of concrete is a function of the ones of mortar/ aggregates (detail can be found in page 246, Neville’s book ‘Concrete Technology’, 2nd Edition)  < 48 > In massive structures (thickness > 1m): thermal shrinkage dominates  < 49 > Restrained shrinkage (a) initial state (e) net tensile stress exceeds tensile strength Crack relieves tension (b) dried/cooled without restraint Contraction, but no stress (d) dried/cooled with ends restrained long term effect Creep reduces stress developed (c) dried/cooled with ends restrained short term effect Tensile stress develops  < 50 > 6. Testing, Specification, Construction Operations Relations between Australian Standards Specification of concrete: normal class and special class Testing Construction operations: mixing/transporting/placing/finishing 1. Australian Standard, AS 3600, Section 3 Design Properties of Concrete 2. Australian Standard, AS 1379, Specification and Supply of Concrete  Relationships Between the Australian Standards < 51 > Structural Design AS3600 Concrete Structures Specification of Concrete AS1379 Supply of Concrete Specification of Sampling and Testing Constituent Materials Specifications Portland & Blended Concrete Testing Cements AS1012 Methods of Testing Concrete AS3972 Cement Testing Admixtures AS2350 AS1478 Aggregate Testing AS1141 Methods for Sampling Aggregates & Testing Aggregates AS2758  < 52 > Specifying concrete AS1379 Specification and Supply of Concrete ▪ Site-mixed, factory-mixed and truck-mixed concretes ▪ Specification of materials ▪ Plants and equipments ▪ Specifying and ordering of concrete ▪ Sampling testing for compliance  AS1379—Specification of concrete < 53 > Two Classes of Concrete: Normal Class : bulk of the concrete supplied Special Class: special requirements for a particular project Strength grade: e.g. N30 normal AS1379 N# strength in MPa  Normal Class Concrete < 54 > applies to most concrete strength grades: N20, N25, N32, N40 or N50 slump: 40, 60, 80 or 100 mm max aggregate size: 10, 14, or 20 mm density: 2100 to 2800 kg/m3 shrinkage not exceeding 1000 x 10-6 no lightweight aggregates other requirements by AS1379 (Section 1.5.3) Special Class Concrete – AS1379 (Section 1.5.4) < 55 > “Special Class” is concrete which is specified to have certain properties or characteristics different from, or additional to, those of Normal Class concrete. performance specification: i.e., particular mechanical/durability properties prescription specification : e.g. the minimum Woronora Bridge cement content shall be 400 kg/m3  < 56 > Project Assessment On a regular testing basis: volume slump strength (compressive, flexure or tensile) air content chloride and sulphate content drying shrinkage Sampling and testing (AS1379,Section 5) < 57 > slump test air content test temperature test chloride and sulfate content drying shrinkage (AS1379, 5.6) etc. compressive strength test tensile strength test permeability test  < 58 > Slump and Tolerance Specified slump Tolerance (mm) 110 30  compressive strength test < 59 > a large number of test samples!!! Strength varies between batches frequency distribution curve/bell curve (normal or Gaussian distribution) histogram of strength values  < 60 > n samples Standard deviation Mean value  < 61 > Probability very good Good Poor Strength f’c f’c f’c Target (poor) (good) (very good) kS  Strength Assessment < 62 > ▪ specifically for a plant or a group of plants ▪ Production assessment ▪ continuous sampling ▪ Project assessment ▪ one selected grade Supplier is obligated to provide the records, if requested by the purchaser Other Standard Requirements < 63 > (Wide basic requirements for all concrete) Density 2100-2800 kg/m Acid soluble chloride and sulphate limits Shrinkage (max 1000 microstrain) 7 day strength 50% of grade strength (up to N50) Cement complying with AS3972 alone or plus ‘one or more supplementary cementitious materials’  Drying Shrinkage < 64 > Sampled at the point of discharge Casting, curing, and testing carried out in accordance with AS 1012.13 by registered laboratory Drying shrinkage: measured after 56 days drying (50% RH & 23oC), in accordance with AS 1012.13 Normal Class Concrete: shrinkage ≤ 1000 µε Special Class : shrinkage specified to an acceptable limit Construction Sequence < 65 > Batching Mixing Transport (to the project site) Placement in the formwork Compaction Finishing Curing Formwork removal  Concrete Plant < 66 > A concrete plant (or batch plant or batching plant ) is equipment that combines various ingredients to form concrete. Some of these inputs include water, air, admixtures, sand, aggregate, fly ash, silica fume, slag, and cement. Storage of materials - Aggregates < 67 > Free drainage Clear identification Prevent uncontrolled intermingling of sizes/types Storage of materials - Cement < 68 > Kept dry Prevent contamination that will have an adverse effect on the performance of the concrete Prevent uncontrolled intermingling or mixing of different types of constituents Mixing Equipment – Uniformity of Mixing < 69 > Batch Mixers are designed to: uniformly distribute ingredients throughout the volume of mixed concrete with the minimum mixing time or number of revolutions necessary. have variable speed for mixing, discharging and agitating have a rated mixing capacity not more than 65% of the gross internal volume of the mixing chamber unless proven otherwise Concrete Placement < 70 > The main objective in placing, is to deposit the concrete as close as possible to its final position as quickly and efficiently as you can, so that segregation is avoided and it can be fully compacted. Workability considerations ☞ type of structural members (slab, column, etc.) ☞ size & geometry of formwork ☞ density, size & spacing of reinforcement ☞ type of placement equipment Concrete Placement - methods < 71 > Barrow Chute Crane and kibble Pump Barrow Tremie Slip-form Cran e Chute Tremi e Pump  Concrete Placement - methods < 72 > Barrow free fall of concrete should not exceed 2m without Chute additional end controls Crane and kibble Pump ideal for: strip footings, floor slabs, road pavements Tremie Slip-form Chute ✅ Concrete Placement - methods < 73 > Barrow Chute Crane and kibble Pump Tremie Slip-form versatile—place concrete vertically/horizontally up to 200 m height, 1000 m horizontal require little space continuous distribution ( → no cold joints) low labour required short set-up time Compaction < 74 > Concrete is compacted to: remove voids in concrete (5% voids lower strength by as much as 30%) complete contact between concrete and the formwork and the surface of reinforcing steel make fresh concrete conform to the formwork entrapped air bubbles  < 75 > COMPACTION: EFFECTS OF VOIDS 50% ↑ compaction = less voids = ↑ strength Types of Compaction < 76 > immersion vibrators: also known as internal/poker/needle vibrator surface vibrators formwork (external) vibration ‘self-compacted’ concrete manual compaction: rodding, ramming, tamping external vibrator surface vibrator internal vibrator  Roller compacted concrete (RCC) < 77 > RCC takes its name from the construction method used to build it RCC has the same basic ingredient as conventional concrete Unlike conventional concrete, it's a drier mix—stiff enough to be compacted by vibratory rollers RCC: the most important development in concrete dam and pavement technology.  Screening (strike off) < 78 > The process of cutting off excess concrete to bring the top surface of a slab to proper grade  < 79 > Summary understand relevant hardened properties of concrete — isotropic material: Young’s modulus and Poisson’s ratio — creep: time-dependent behaviour — shrinkage: * drying shrinkage: loss of water → reduction in volume * thermal shrinkage: temperature drops → reduction in volume properties of concrete in AS3600 Concrete Structures methods used in AS3600 to estimate some concrete properties  < 80 > Survey Announcement  < 81 > Test 1 Announcement Test 1 on practical class of week 5: Friday, 23 Aug 2022, 5 PM Closed book, formula sheets will be given The contents of concrete technology from week 1 to week 4 The solution of week 4 worksheets will be uploaded on Moodle next Monday.  < 82 > LAB Classes ❑ Please read the safety instructions carefully in Concrete Laboratory Classes (Clayton) before you attend the Concrete Laboratory Classes. ❑ No Safety Footwear and Glasses, No Lab Classes ❑ Correct your calculation of week 2 concrete mix design before the class ❑ Please be punctual and attend the concrete laboratory class  < 83 > Capping of specimens: Please watch the following videos: https://www.youtube.com/watch?v=6ferJ1OncnQ https://www.youtube.com/watch?v=0UCoYGfPLoo – Sulphur Capping https://www.youtube.com/watch?v=Qy9jGoSwgik – Gypsum capping