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Lecture 2 Design of Concrete Mixes and Properties of Fresh Concrete Dr Magnus Currie ENG-1010: Reinforced Concrete Design.

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Presentation on theme: "Lecture 2 Design of Concrete Mixes and Properties of Fresh Concrete Dr Magnus Currie ENG-1010: Reinforced Concrete Design."— Presentation transcript:

1 Lecture 2 Design of Concrete Mixes and Properties of Fresh Concrete Dr Magnus Currie ENG-1010: Reinforced Concrete Design

2 Overview To understand how to design fresh concrete and the properties of fresh concrete Understanding shrinkage Understanding curing Understanding bleeding

3 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

4 Properties at Early Age Workability Slump loss Segregation/bleeding Plastic shrinkage Time of Set temperatures Temperature Curing

5 Properties of Fresh Concrete Workability - ease of placement - resistance to segregation - homogenous mass Consistency - Ability to flow

6 Setting and Hardening

7 Slump test Inverted Cone Fill it up with three layers of equal volume Rod each layer 25 times Scrape off the surface

8 Slump Test

9 Slump Test Results Slump class (European)Slump in mm S1 (stiff) S S3 (medium) S S5 (fluid)≥220

10 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

11 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.

12 Factors affecting slump Aggregates - grading the larger the particle size, the higher the slump for a given paste content

13 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

14 Segregation in Fresh Concrete

15 Bleeding and its control Creates problems -Poor pumpability -Delays in finishing -High w/c at the top -Poor bond between 2 layers

16 Bleeding and its control Causes -Lack of fines -Too much water content Remedies -More fines -Adjust grading -Entrained air -Reduce water content

17 Air Content

18 Early Age Volume Change

19 Free shrinkage Causes volume change, but no stresses

20 Restrained shrinkage This creates stresses which may cause cracking

21 Restrained shrinkage cracking

22 Causes of Plastic Shrinkage Cracking Water evaporates faster than it can reach the top surface Drying whilst plastic Cracking

23 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

24 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

25 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

26 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

27 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)

28 Summary To understand how to design fresh concrete and the properties of fresh concrete Understanding shrinkage Understanding curing Understanding bleeding

29 Questions? The Shard, London


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