Briefly explain and label the following :
Arrises-edge
formed by the
intersection
Bed-bottom surface
Frog-face depression
Header-end surface
Stretcher-side
surface, visible in
elevation
Setting and hardening process of cement
On first contact with water, calcium ions and hydroxide ions are rapidly released
from the surface of each C3S grain.
The induction (dormant) period is caused by the need to achieve a certain
concentration of ions in solution before crystal nuclei are formed for the hydration
products to grow from.
At the end of dormant period, the thickness of the hydrate layer increases and
reach the growing crystals
Initial set of cement paste is defined as the time when the paste has gained enough
rigidity to no longer be in a fluid state. In another word initial set indicates the
beginning of forming of gel or beginning of solidification.
The final setting time is when rigidity has increased to a point that the paste
becomes a solid of very low strength.
In general, cement exhibit initial set in 2 to 4 hours and final set in 5 to 8 hours.
Final set is then followed by substantial increases in strength, referred to as the
hardening stage
‘Hardening’ refers the process of strength growth and may continue for weeks or
months after the concrete has been mixed and placed. Hardening is due largely to
the formation of calcium silicate hydrate as the cement hydrates.
Contraction Joints
All concrete, once placed, will contract slightly during the curing process; this is the
primary cause of small surface cracks that appear during the curing process. When set,
concrete will expand and/or contract slightly with ambient temperature.
Contraction joints are the simplest of joints in that they are basically a break in the
concrete and the reinforcement created to allow the natural shrinkage of concrete
(because of curing and/or temperature change) to take place without generating crack-
inducing tensile forces within the slab.
They are formed by embedding a 25×25mm timber or plastic joint-former into the still wet
concrete, then removing it once hardening has taken place, and filling the void with a
suitable sealant.
Sealed movement joint
Steel dowels in edge of concrete slab
Causes Honeycombs In Concrete
Honeycombs are hollow spaces and cavities left in concrete mass on surface or inside
the concrete mass where concrete could not reach. These look like honey bees nest
Honeycomb is due to non-reaching of concrete to all places due to which cavities and
hallow pockets are created, main reasons are:
1) Improper vibration during concrete.
2) Less cover to reinforcement bars
3) Use of very stiff concrete (this can be avoided by controlling water as per slump
test).
4) Places like junction of beam to beam to column and to one or more beams are
the typical spots where honey combs are observed. This is due to jumbling of
reinforcement of beams and column rods at one place; special attention is
required at such place during concreting and vibrating.
5) Presence of more percentage of bigger size of aggregate in concrete also
prevents concrete to fill narrow spaces between the reinforcement rods.
Preparations of the formwork before placing the concrete
1. It should be strong enough to withstand all types of dead and live loads.
2. The formwork must be erected in accordance with the approved drawings
3. It should be rigidly constructed and efficiently propped and braced both
horizontally and vertically, so as to retain its shape and avoid distortion of
concrete elements.
4. The joints in the formwork should be water-tight against leakage of cement grout.
5. Erection of formwork should permit removal of various parts in desired
sequences without damage to the concrete.
6. The formwork should be set accurately to the desired line and levels (correctly
aligned and leveled). It should have plane surface.
7. Clean and free from rubbish
8. The release agents been applied on surface of formworks
