Mixing and handling fresh concrete
Lecturer: Rafah Rasheed Abdul Majeed.
ERBIL PLOYTECHNIC UNIVERSITY
ERBIL TECHNICAL ENGINEERING COLLEGE
#Concrete Technology
2. Mixing and handling fresh concrete
2
Mixing and handling fresh concrete
• Batching.
• Mixing .
• Transporting.
• Placement .
• Finishing.
• Curing.
• Batching:- Batching is the process of measuring
concrete mix ingredients by either mass or volume and
introducing them in to the mixer. Most specifications
require that batching be done by mass rather than by
volume. Equipment should be capable of measuring
quantities within these to learners for the smallest
batch regularly used as well as for larger batches.
Liquid chemical admixtures should be charged into
the mixture as aqueous solutions.. Admixture
dispensers should be checked frequently since errors
in dispensing admixtures, particularly overdoses, can
lead to serious problems in both fresh and hardened
concrete.
3. Mixing and handling fresh concrete
3
• Mixing:- Concrete should be mixed completely in a
mixer or by hand until it becomes uniform in
appearance .
Mechanical mixers include on-site mixers and central
mixers .The capacity of these mixers varies from (1.0 m3
---
9m3
). Mixing time and number of revolutions vary
the size of the mixer .
*Specification require minimum of (1min) of mixing for
stationary mixers of up to (0.75)m3
of capacity , with an
increase of 15 second for each additional (0.75)m3
of
capacity .
*Mixers are usually charged with (10%) of water ,
followed by uniform additions of solids and (80%) of
water finally the remainder of the water is added to the
mixer.
4. Mixing and handling fresh concrete
4
#Three mixing methods con be used for reading
mixed concrete:-
1. Central – mixed concrete :- Concrete is mixed
completely in a stationary mixer and delivered in
the truck (2 rpm to 6 rpm).
2. Shrink – Mixed concrete :- Concrete is partially
mixed in a stationary mixer and completed in truck
mixer (4 rpm to 6 rpm ).
3. Truck mixed concrete :- Concrete is mixed
completely in truck mixer (4 rpm to 16r
*The concrete should be discharged at the job site
with in 90 minute from the start of mixing
• Transporting:-There are three occurrence that
occur during handling and placing , could seriously
affect the quality of the finished work:-
Delays. The objective in planning any work schedule is
to produce the fastest work . Machines for transporting and
handling concrete are being improved all the time, and
reduce the delay time during concrete placement.
5. Mixing and handling fresh concrete
5
Early Stiffening and Drying Out. Concrete begins to
stiffen as soon as the cementations materials and water are
mixed, but the degree of stiffening that occurs in the first
30 minutes is not usually a problem; concrete that is kept
agitated generally can be placed and compacted within 11⁄2
hours after mixing unless hot concrete temperatures or high
cement contents speed up hydration excessively.
Segregation. Segregation is the tendency for coarse
aggregate to separate from the sand-cement mortar. This
results in part of the batch having too little coarse aggregate
and the remainder having too much. The former is likely to
shrink more and crack and have poor resistance to abrasion.
6. Mixing and handling fresh concrete
6
Methods and Equipment for Transporting and
Handling Concrete:-
Pumps Used to convey concrete directly from central
discharge point at jobsite.
Tremies For placing concrete under water.
Truck mixers Used to transport concrete for uses in
pavements, structures , and building .
Choosing the Best Method
The first thing to look at is the type of job, its physical size,
the total amount of concrete to be placed, and the time
schedule. Studying the job details . This aids in choosing
the concrete handling equipment necessary for placing
concrete at the required levels. Concrete must be moved
from the mixer to the point of placement as rapidly as
possible without segregation or loss of ingredients
7. Mixing and handling fresh concrete
7
Placing and Finishing concrete :-
Preparation to placing concrete for pavements or
slabs on grade includes compacting, trimming, and
moistening the sub grade erecting the forms; and setting the
reinforcing steel and other embedded items. old hardened
concrete usually requires mechanical cleaning and
roughening prior to placement of new concrete. Forms
should be accurately set, clean, tight, adequately
braced, should be moistened before placing concrete,
Reinforcing steel should be clean and free when concrete is
placed. Concrete should be deposited continuously as near
as possible to its final position without objectionable
segregation .In general, concrete should be placed in walls,
thick slabs, or foundations in horizontal layers of uniform
thickness.
Placing Concrete Under Water:-
Methods for placing concrete underwater include the tremie
is a smooth, straight pipe long enough to reach the lowest
point to be concreted from a working platform above the
water. Mobile concrete pumps with a variable radius boom
makes easy work of placing concrete underwater
.
8. Mixing and handling fresh concrete
8
Special Placing Techniques:-
Concrete may be placed by methods other than the usual
cast-in-place method. These methods, such as shotcreting.
Vibration:-
Vibration, either internal or external, is the most widely
used method for consolidating concrete. When concrete is
vibrated, the internal friction between the aggregate
particles is temporarily destroyed and the concrete behaves
like a liquid; it settles in the forms under the action of
gravity and the large entrapped air voids rise more easily
to the surface.
Internal Vibration. Internal are commonly used to
consolidate concrete in walls, columns , beams, and slabs.
Flexible-shaft vibrators consist of a vibrating head
connected to a driving motor by a flexible shaft. Inside the
head.
9. Mixing and handling fresh concrete
9
External Vibration. External vibrators can be form
vibrators ,vibrating tables, or surface vibrators such as
vibratory screeds, plate vibrators, vibratory roller screeds,
or vibratory hand floats or trowels. Form vibrators,
designed to be securely attached to the outside of the forms,
are especially useful (1) for consolidating concrete in
members that are very thin or congested with
reinforcement,(2) to supplement internal vibration, and (3)
for stiff mixes where internal vibrators cannot be used.
• Curing:-
Curing is the maintenance of a satisfactory moisture
content and temperature in concrete for a period of time
immediately following placing and finishing so that the
desired properties may develop . Curing has a strong
influence on the properties of hardened concrete proper
curing will increase durability, strength, water tightness,
abrasion resistance, volume stability, and resistance to
freezing and thawing and deicers.
10. Mixing and handling fresh concrete
10
Methods of curing:-
1. Ponding or Immersion.
2. Spring or Fogging.
3. Wet covering.
4. Impervious plastic sheets.
5. Steam curing.
Curing period :-
Curing period depend on several factors
1. Types of cement.
2. Mixture proportion .
3. Required strength.
4. Ambient weather.
5. Size and shape of structure.
6. Future exposure conditions.
7. Method of curing.
The most concrete curing should be min. 7 days ,the
curing period can be reduced to 3 days if high early
strength concrete is used.
