hydration of cement

-Setting time

1. The time taken from additionn of water to initial set and final set
2. Vicast Test-to test cement setting time

Hardening

1. Hardening also means useful strength development

hydration of cement

Hydration=the chemical combination of cement and water

1. Will produce a very had and strong binding medium for aggregate particles in concrete
2. Hydration may continue over a period of a month
3. The heat generated during hydration will affect the durability of concrete due to micro cracking

Learning outcome

-Upon completion of this chapter , you should be able to:

1. Describe hydration of cement
2. discuss factors affecting cement setting
3. Outline the Vicat Test

Concrete

Cement
-Hydration of cement

Moisture

-Saturated , surface dry

1. Ideal state
2. Do not require alteration to mixing water

-Wet

1. Surface moisture is pressent
2. Most common
3. Require deduction of water when mixing

Moisture

-4 states which aggregate may exist in:
-oven-dry

1. on heating to 105 celcius , no loss of weight
2. seldom occur
3. require extra water when mixing

-Air-dry

1. No free moisture and surface are dry
2. Occur at upper part of stockpiles in dry wheather
3. Require extra water when mixing

Strength

-Strength of aggregate generally higher than compressive strength of concrete:
-Factors affecting strength of concrete:

1. Aggregate size
2. Aggregates shape and texture
3. Cleanliness

Sieve Analysis

1. To establish the grading of an aggregate
2. Steps:-

• Air-dried sample passed through series of sieves  of decreasing dimensions
• Use mechanical vibrator/carried out shaking by hand
• Materials retained on each sieve is weighed and recorded
• The % passing through by weight is calculated
• Gradings is determined by comparing to grading requirement (BS 882)

well-graded aggregate

-consist of aggregate with different sizes
-Fill as much as possible of the total space
-Voids between ;arge particles can be filled efficiently with slightly smaller ones.

Grading

-Purpose:to assist in producing concrete with:

1. Satisfactory plastic properties

(workability , cohesion , resist bleeding)

1. Satisfactory hardened properties (strength , durability , surface finish)

Grading

-Particle size distribution of aggregate is called grading
-Coarse aggregate-size bigger than 5mm(3/6 in)
-Fine aggregate -size smaller than 5mm
-Sand-size as low as 0.07 mmm
-Silt -0.06-0.02 mm
-Clay-smaller than 0.02mm

Bulking Test

-To determine% bulking of a damp sand sample
-Steps:

1. Fill in container with damp sand sample (Vd)
2. Add water until saturated .With further addition of water , films of water merge , water moves into voids . This volume is the same as volume of dry sample(V)
3. Calculate %of bulking