This document provides an overview of concrete, including its definition, properties, composition, testing, and uses. Some key points: - Concrete is a mixture of cement, aggregates (sand and gravel), and water that can be used for load-bearing construction. - Its properties depend on the mix proportions, water-cement ratio, and type of aggregates used. Good compaction and curing are important for strength. - Concrete has high compressive strength but low tensile strength, so it is often reinforced with steel bars or prestressed using steel tendons. - Aggregates make up the majority of a concrete mix by weight and influence properties like strength and durability. Proper testing of aggregates is

1. MANDIUDZA and
MAJURU T
PRESENTATION

2. CONSTRUCTION MATERIALS
CONCRETE

3. CONTENTS
 DEFINTION OF CONCRETE
 PHYSICAL PROPERTIES
 AGREGATES
 DURABILITY
 STRENGTH CLASSES
 SPECIFICATION FOR MIXES
 TESTS
 CONCRETE FINISHES
 COMPONENTS

4. DEFINITION
 Concrete is a mixture of cement, aggregates and water,
with any other admixtures which may be added to
modify the placing and curing processes or the ultimate
physical properties.
 Initially when mixed, concrete is a plastic material,
which takes the shape of the mould or formwork.
 When hardened it may be a dense load-bearing material
or a lightweight thermally insulating material,
depending largely on the aggregates used.
 It may be reinforced or pre-stressed by the
incorporation of steel.

5. PROPERTIES OF CONCRETE
 In it fresh state concrete should :
 1. Be composed of accurate batched (measured)
proportions of cement, fine and coarse aggregate and
water.
 2 . Have correct cement – aggregate ratio
 3. Have the required water –cement ration.
 4. Be well mixed
 5. Not segregate during transportation and placing
 .Be fully compacted since voids cause weakness
 7 be provided with the specific finish
 8. Be well cured

6. PHYSICAL PROPERTIES OF
CONCRETE
 Thermal movement:
 The coefficient of thermal expansion of concrete varies
between 7 and 14 x10-6 deg C, according to the type of
aggregate used, the mix proportions and curing
conditions.
 Moisture movement:
 During the curing process, concrete exhibits some
irreversible shrinkage which must be accommodated
within the construction joints. The extent of the
shrinkage is dependent upon the restraining effect of
the aggregate and is generally larger when smaller or
lightweight aggregates are used. High-aggregate
content mixes with low workability tend to have small
drying shrinkages. The reversible moisture movement
for cured concrete is typically 2–6 x10-4 deg C,
depending upon the aggregate.

7. PHYSICAL PROPERTIES OF
CONCRETE cont….
 Creep:
 Creep is the long-term deformation of concrete under
sustained loads.
 The extent of creep is largely dependent upon the
modulus of elasticity of the aggregate.
 Thus an aggregate with a high modulus of elasticity
offers a high restraint to creep.

8. AGGREGATES FOR CONCRETE
 Forms a major component of concretes, typically
approximately 80% by weight in cured mass concrete.
 Aggregate properties including crushing strength, size,
grading and shape have significant effects on the
physical properties of the concrete mixes and hardened
concrete.
 The appearance of visual concrete can be influenced by
aggregate LIME, CEMENT AND CONCRETE.
 Aggregates for concrete are normally classified as
lightweight, dense or high-density.
 Standard dense aggregates are classified by size as fine
(i.e. sand) or coarse (i.e. gravel).
 Additionally, steel or polypropylene fibres or gas
bubbles may be incorporated into the mix for specialist
purposes.

9. Water/cement ratio
 water/cement ratio = weight of free water
weight of cement

10. DURABILITY OF CONCRETE
 While good-quality well-compacted concrete with an
adequate cement content and a low water/cement
ratio is generally durable, concrete may be subjected to
external agencies which cause deterioration or, in
certain circumstances, such as alkali-silica reaction, to
internal degradation.
1. Sulphate attack
• Sulphates are present in soils, and rate of sulphate
attack on concrete dependent on the soluble sulphate
content of the groundwater.
• Soluble sulphates react with the tricalcium aluminate
(C3A) component of the hardened cement paste,
producing calcium sulfoaluminate (ettringite).
• The use of sulphate-resisting Portland cement or
combinations of Portland cement and fly ash
(pulverised-fuel ash [PFA]) or granulated blast furnace
slag (GGBS) reduces the risk of sulphate attack in well-
compacted concrete. In the presence of high soluble
sulphate concentrations, concrete requires surface
protection.

11. DURABILITY OF CONCRETE cont….
2. Frost resistance
• Weak permeable concrete is particularly vulnerable to
the absorption of water into capillary pores and cracks.
On freezing, the ice formed will expand causing frost
damage.
• The use of air-entraining agents, which produce
discontinuous pores within concrete, reduces the risk of
surface frost damage.
• Concrete is particularly vulnerable to frost damage
during the first two days of early hardening. Where new
concrete is at risk, frost precautions are necessary to
ensure that the mix temperature does not fall below
5°C until a strength of 2 MPa is achieved.

12. CONCRETE STRENGTH
CLASSES
 Concrete should be specified, placed and cured
according to BS codes. The preferred strength classes of
concrete are shown in Table3.10 , in which the numbers
refer to the test sample crushing strengths of a 150 300
mm cylinder and a 150 mm cube, respectively.

14. SPECIFICATION OF CONCRETE
MIXES
 There are five methods for specifying concrete
described in BS 8500–1:2002. All should conform to the
standards BS 8500–1: 2002 and BS EN 206–1: 2000.
 The five methods are:
• designated concrete
• designed concrete
• prescribed concrete
• standardised prescribed concrete
• proprietary concrete.

15. Designed concrete
 The producer is responsible for selecting a designed
concrete which will meet the performance criteria
listed by the specifier.
 The specifier must clearly indicate the required use,
curing conditions, exposure conditions, surface finish,
maximum aggregate size and any excluded materials. In
addition the compressive strength class, the maximum
water/cement ratio, the minimum cement content, the
consistence (slump) and permitted cement types should
be quoted.
 Within these constraints, the producer is responsible for
producing a concrete which conforms to the required
properties and any additional stated characteristics.

16. Prescribed concrete
 The purchaser fully specifies all the materials by weight
(kg/m3 ), including admixtures and the standard
strength class.
 The purchaser is therefore responsible for the
performance characteristics of the concrete.
 Prescribed concretes are used particularly for specialist
finishes such as exposed aggregate visual concrete.

17. Standardised prescribed
concrete
 Standardised prescribed concretes are a set of five
standard mixes, which may be mixed on site, with a
restricted range of materials.
 Standard mixes ST1 to ST5 may be made to S1, S2, S3 or
S4 slump classes, giving low, medium, high or very high
workability.
 The specification must record a maximum aggregate
size and whether the concrete is to be reinforced or
not.

18. Proprietary concrete
 Proprietary concrete must conform to the standards BS
8500–2: 2002 and BS EN 206–1: 2000 and be properly
identified. This category allows for a concrete supplier
to produce a concrete mix with an appropriate
performance but without indicating its composition.

19. MATERIALS FOR CONCRETE
Cement
 Each type produced is required to conform to a British
Standard Specification and is for use in a particular
situation. Manufactured by wet or dry process.
Aggregates
 Refers to the sands, gravels and crushed stones that are
mixed with the cement and water to produce concrete and
since usually at least 80 percent of the concrete consist of
aggregates it is essential that the correct material is
selected.
Water
 Clean and safe for human consumption

20. PROPERTIES OF AGGREGATES
 Clean
 Crushing strength should be at least equal to cement
paste
 Durable and not liable to shrinkage
 No organic impurities
TYPES OF AGGRERATES
Normal Heavy Light
Sand Barytes Clinker
Gravel Iron Purnice
Crushed stone Steel punching Foamed slag
Broken brick Expanded shale
Blast furnace slag

21. METHODS OF TESTING OF
AGGREGATES
 Sampling method
 Quartering
 The riffle box
 Sieve box
 Silt test
 Bulking of sand
 Organic impurities test for sand
 Test sand on site for impurities

22. ADMIXTURES
 They are materials other than cement, aggregates and
water that may be added to the concrete during mixing
to improve or modify its properties.
TYPES OF ADMIXTURES
 Air entraining agents
 Accelerators
 Plasticicers
 Retarders
 Damp proofers

23. CONCRETE PRODUCTION
Batching
 It means measuring out of materials.
By weight
 It is measuring aggregates by its mass.
 For example: If the ratio is 1:3: 6
 50kgs of cemment;150kgs of sand; 300kgs gravel/stone
By volume
 Gauge boxes of sand are used
 For example. A mix is required to be batched by volume
in the ratio of I cement , 2 sand and 4 stones.
 Since one bag of cement contains 0.035m3. the gauge
box should be big enough to accommodate 0.07m3.

24. CONCRETE PRODUCTION
 Mixing
By machine
 Large mixers are loaded in the following sequence:
 Coarse aggregate, cement, fine aggregate and water.
By hand
 Turn thrice dry, thrice wet or until concrete is of an
even colour. Mixing should be done on a clean hard
base.

25. CONCRETE PRODUCTION
Transporting
 It can be transported by wheelbarrow, dumper, crane,
and skip, conveyor belt, monorail ,pump or lorry
Placing
 Should be placed near to the final position. Water
should not added
Compacting
 It is done to remove voids.
 Internal compaction –poker vibrator is used and can
compact up to 600mm in depth.

26. EQUIPMENT AND TOOLS FOR
COMPACTION
External compaction
 Tamping beams fastened to vibrators for use in slabs.
 Vibrating table and Kango hammer can be used for
cubes.
Hand Compaction
 50x50mm timber
 Hand hammer
 Tamping board

27. CURING
 It is process of keeping the concrete damp, which is
carried out up to 28 days, during which the concrete
gains its strength. After this period the strength gain is
slow
CURING METHODS
 Cover as soon as possible with water proof sheeting tied
down at the edges.
 Cover the surface with damp, sacking, sawdust and
ensure it kept damp.
 Continuous spraying with water-the spray must not be
fine.
 Ponding- heaping sand around the periphery and filling
in with water.
 Spraying on a sealing membrane.

28. ADVANTAGES OF CURING
 Elimination of surface cracking
 Improved resistance to abrasion
 Greater final strength
 Less likelihood or surface dusting
 An increase in impermeability
 Improved weather resistance and durability

29. IN-SITU CONCRETE TESTING
 The compressive strength of hardened concrete may be
estimated in-situ by mechanical or ultrasonic
measurements. The Schmidt hammer or sclerometer
measures the surface hardness of concrete by
determining the rebound of a steel plunger fired at the
surface.
 In the pull-out test, the force required to extract a
previously cast-in standard steel cone gives a measure
of concrete strength. Ultrasonic devices determine the
velocity of ultrasound pulses through concrete. Since
pulse velocity increases with concrete density, the
technique can be used to determine variations within
similar concretes.
 The test gives a broad classification of the quality of
concrete, but not absolute data for concretes of
different materials in unknown proportions.

30. Concrete cube and cylinder
tests
 To maintain quality control of concrete, representative
test samples should be taken, cured under controlled
conditions and tested for compressive strength after the
appropriate 3-, 7- or 28-day period.
 Steel cylinder and cube moulds are filled in layers with
either hand or mechanical vibration. For hand tamping,
a 100 mm cube would be filled in two equal layers, each
tamped 25 times with a 25 mm square-end standard
compacting bar; mechanical vibration would normally
be with a vibrating table or pneumatic vibrator.
 The mix is then trowelled off level with the mould.
Cubes and cylinders are cured under controlled
moisture and temperature conditions for 24 hours, then
stripped and cured under water at 18–20°C until
required for testing.

31. Cylinder and cube

32. Slump test
 The slump test is used for determining the workability of a
mix on site. It is gives a good indication of consistency from
one batch to the next, but it is not effective for very dry or
very wet mixes. The base plate is placed on level ground and
the cone filled with the concrete mix in three equal layers,
each layer being tamped down 25 times with the 16 mm
diameter tamping rod. The drop in level (mm) is the recorded
slump, which may be a true slump, a shear slump or a collapse
slump. Typical slump values would be zero to 25 mm for very
dry mixes, frequently used in road making; 10–40 mm (low
workability) for use in foundations with light reinforcement;
50–90 mm (medium workability) for normal reinforced
concrete placed with vibration and over 100 mm for high-
workability concrete.
 Typically slump values between 10 mm and 175 mm may be
measured, although accuracy and repeatability are reduced at
both extremes of the workability range. The slump test is not
appropriate for aerated, no-fines or gap-graded concretes.

33. Slump Test apparatus

34. Slump Test Results shown

35. Slump Test apparatus

36. PRESTRESSED CONCRETE
 Concrete has a high compressive strength but is weak in
tension.
 Pre-stressing with steel wires or tendons ensures that
the concrete component of the composite material
always remains in compression when subjected to
flexing up to the maximum working load.
 The tensile forces within the steel tendons act upon the
concrete putting it into compression, such that only
under excessive loads would the concrete go into
tension and crack.
 Two distinct systems are employed: in pre-tensioning,
the tendons are tensioned before the concrete is cured;
and in post-tensioning the tendons are tensioned after
the concrete is hardened.

37. Pre-stressing Defined
 It is the introduction of a compressive force to the
concrete to counter the stresses that will result from
the applied load.

38. Pre-tensioning
 Done before the casting of the concrete
 Large numbers of precast concrete units, including
flooring systems, are manufactured by the pre-
tensioning process.
 Tendons are fed through a series of beam moulds and
the appropriate tension applied. The concrete is placed,
vibrated and cured.
 The tendons are cut at the ends of the beams, putting
the concrete into compression. As with precast
reinforced concrete it is vital that pre-stressed beams
are installed the correct way up according to the
anticipated loads.
 Examples include: foundation pile, railway sleeper,
electrical pole, etc.

39. Post-tensioning
 Done after concrete is casted.
 In the post-tensioning system the tendons are located in
the formwork within sheaths or ducts. The concrete is
placed, and when sufficiently strong, the tendons are
stressed against the concrete and locked off with
special anchor grips incorporated into the ends of the
concrete.
 Post-tensioning has the advantage over pre-tensioning
that the tendons can be curved to follow the most
efficient pre-stress lines. This enables long spans of
minimum thickness to be constructed.
 Popular in civil constructions eg, roads, bridges,
railways, tunnels, containment tanks, etc,

40. Visual concrete
 The production of visual concrete, whether precast or
in-situ, requires not only a high standard of quality
control in manufacture, but also careful consideration
to the correct specification and detailing of the
material to ensure a quality finish which weathers
appropriately.
 The appearance of visual concrete is affected by four
key factors:
• the composition of the concrete mix;
• the formwork used;
• any surface treatment after casting;
• the quality of workmanship.

41. DESIGN CONSIDERATIONS
 The satisfactory production of large areas of smooth
concrete is difficult due to variations in colour and the
inevitability of some surface blemishes, which can be
improved, but not eradicated by remedial work.
Externally smooth concrete weathers unevenly due to
the build-up of dirt deposits and the flow of rainwater.
 If concrete is to be used externally as a visual material,
early design considerations must be given to the use of
textured or profiled surfaces to control the flow of
rainwater. Generally the range of finishes and quality
control offered by pre-casting techniques is wider than
that available for in-situ work, but frequently
construction may involve both techniques. The use of
external renderings offers an alternative range of
finishes for concrete and other substrates.

42. Concrete components
 In addition to the use of concrete for the production of
large in-situ and precast units, concrete bricks and
concrete blocks , the material is widely used in the
manufacture of small components, particularly concrete
tiles, slates and paving slabs.

43. PRECAST CONCRETE
 Precast concrete units may be cast vertically or
horizontally, but most factory operations use either
face-up or face-down, as better quality control can be
achieved by this method.
 Moulds are usually manufactured from plywood or steel.
 Moulds are designed to be dismantled for the removal of
the cast unit and must be manufactured to tight
tolerances to ensure quality control on the finished
product.
 As high costs are involved in the initial production of
the moulds, economies of construction can be achieved
by limiting the number of variations.

44. IN-SITU CONCRETE
 The quality of in-situ visual concrete is heavily
dependent upon the formwork as any defects will be
mirrored in the concrete surface.
 The formwork must be strong enough to withstand,
without distortion, the pressure of the fresh concrete,
and the joints must be tight enough to prevent leakage,
which can cause honeycombing of the surface.
 A wide range of timber products, metals and plastics
are used as formwork, depending upon the surface
finish required.

45. Reinforced concrete
 Concrete is strong in compression, with crushing
strengths typically in the range 20–40 MPa, and up to
100 MPa for high-strength concretes. However, the
tensile strength of concrete is usually only 10% of the
compressive strength.
 Steel is the universally accepted reinforcing material as
it is strong in tension, forms a good bond and has a
similar coefficient of thermal expansion to concrete.
 The longitudinal bars carry the tensile forces while the
links or stirrups combat the shear forces and also locate
the steel during the casting of the concrete. Links are
therefore more concentrated around locations of high
shear although inclined bars may also be used to resist
the shear forces.

46. CONCRETE FINISHES
 Smooth finishes
 Ribbed and profiled finishes
 Abraded, acid etched and polished finishes
 Exposed aggregate finishes
 Textured finishes
 Weathering of concrete finishes
 Tooled concrete finishes

47. EXTERNAL RENDERING
 Renders are used to provide a durable and visually
acceptable skin to sound but unattractive construction.
Renders can reduce rain penetration and maintain the
thermal insulation of walls.

48. Roughcast render

49. Dry-dash render

50. Scraped finish

51. Textured finishes

52. Tyrolean finish

53. END
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