Lecture 2 Design of Concrete Mixes and Properties of Fresh Concrete Dr Magnus Currie ENG-1010: Reinforced Concrete Design
Overview To understand how to design fresh concrete and the properties of fresh concrete Understanding shrinkage Understanding curing Understanding bleeding
Significance The first 48 hours are very important for the structural performance of the concrete structure It controls the long term behaviour It controls the long term behaviour, influence fc, Ec, creep and durability
Properties at Early Age Workability Slump loss Segregation/bleeding Plastic shrinkage Time of Set temperatures Temperature Curing
Properties of Fresh Concrete Workability - ease of placement - resistance to segregation - homogenous mass Consistency - Ability to flow
Setting and Hardening
Slump test Inverted Cone Fill it up with three layers of equal volume Rod each layer 25 times Scrape off the surface
Slump Test
Slump Test Results Slump class (European)Slump in mm S1 (stiff) S S3 (medium) S S5 (fluid)≥220
Factors affecting slump/paste content Constant water cement ratio - increase paste content - increase slump = NOT GOOD Constant cement content - increase water content - increase slump = NOT GOOD
Factors affecting Slump-Water Content Add water at the constant cement content, w/c increases, slump increases Add water at a constant water cement ratio, have to increase cement as well, slump increases.
Factors affecting slump Aggregates - grading the larger the particle size, the higher the slump for a given paste content
Bleeding and Segregation Segregation: coarse aggregate tends to segregate due to gravity. Typical of dry mixes Bleeding: water rising to the surface. Typical of wet mixes. When excessive, cement particles and water go to the surface porosity goes up – paste can easily be abraded -> dusting
Segregation in Fresh Concrete
Bleeding and its control Creates problems -Poor pumpability -Delays in finishing -High w/c at the top -Poor bond between 2 layers
Bleeding and its control Causes -Lack of fines -Too much water content Remedies -More fines -Adjust grading -Entrained air -Reduce water content
Air Content
Early Age Volume Change
Free shrinkage Causes volume change, but no stresses
Restrained shrinkage This creates stresses which may cause cracking
Restrained shrinkage cracking
Causes of Plastic Shrinkage Cracking Water evaporates faster than it can reach the top surface Drying whilst plastic Cracking
Plastic Shrinkage Cracking - Remedies Control the wind velocity (using hoardings) Reduce the concrete’s temperature - ice can be used as mixing water Increase the humidity at the surface - fogging - cover with polythene - curing compound (admixture) Fibre Reinforcement
Curing The time needed for the chemical reaction of Portland cement with water Glue is being made Concrete after 14 days of curing has completed only 40% of its potential 70% at 28 days
Curing Tips Ample water Do not let it dry Dry concrete = dead concrete – all reactions stop Can not revitalise concrete after it dries Keep temperature at a moderate level Concrete with flyash requires longer curing
Temperature and Curing The higher the temperature the faster the curing Best temperature is room temperature Strongest concrete is made at temperature around 40F (not practical) If concrete freezes during first 24 hours it may never be able to attain its original properties
Temperature Affects on Curing Real high temperatures above 120F cause serious damage since cement may set too fast Accelerated curing procedures produce strong concrete, but durability might suffer Autoclave curing (may not be practical)
Summary To understand how to design fresh concrete and the properties of fresh concrete Understanding shrinkage Understanding curing Understanding bleeding
Questions? The Shard, London