This document discusses materials used for pre-stressed concrete, including high strength concrete and high tensile steel. It notes that pre-stressed concrete requires high compressive strength concrete with higher tensile strength than ordinary concrete. It also discusses that high strength concrete can achieve compressive strengths ranging from 70-100 N/mm2 without unusual materials. Regarding high tensile steel, it states that steel with slightly increased carbon content is generally used, and discusses strength requirements and permissible stresses in the steel. Proper protection of prestressing steel is also important to prevent corrosion.

1. Materials for Pre-stressed
Concrete
Chapter 2

2. 2.1 High Strength Concrete
2.1.1 High Strength Concrete Mixes
Prestressed concrete requires concrete which has a high
compressive strength at a reasonably early age, with
comparatively higher tensile strength than ordinary concrete.
Many desirable properties, such as
durability,
impermeability and
abrasion resistance,
are highly influenced by the strength of concrete.

3. • Also known as early gain in strength of
cement. This cement contains more %age of
C3S(tri-calcium silicate) and less %age of
C2S(di-calcium silicate ), high proportion of
C3S (tri-calcium silicate) will impart quicker
hydration
The strength obtained by this cement in 03
days is same as obtained by O.P.C in 7 days.
• Initial and final setting times are same as OPC.ie.
30mins and 10 hrs.And soundness test by Le-
Chatielier is 10mm and Autoclave is 0.8%.

4. • With the development of vibration techniques
in 1930, it became possible to produce,
without much difficulty, high strength concrete
having 28 day cube compressive strength in
the range of 30-70N/mm².

5. Recent developments in the field of concrete mix design
have indicated that it is now possible to produce even ultra
high strength concrete, of any desired 28 day cube
compressive strength ranging from 70 to 100N/mm²,
without taking recourse to unusual materials or processing
and without facing any significant technical difficulties.

6. 2.1.2 Strength Requirements
The minimum 28-day cube compressive strength prescribed in
the Indian standard code IS:1343-1980 is 40 N/mm² for pre-
tensioned members and 30 N/mm² for posttensioned members.
A minimum cement content of 300 to 360kg/m³ is prescribed
mainly to provide to the durability requirements. In high strength
concrete mixes, the water content should be as low as possible
with due regard to adequate workability, and the concrete should
be suitable for compaction by the means available at the site.

7. To safeguard against excessive shrinkage, the code
prescribes that the cement content in the mix should
preferably not exceed 530`kg/m³. the specified works
cube strength of 40 N/mm² required for prestressed
members can easily be achieved even at the age of
seven days using rapid hardening Portland cement.

8. 2.1.3 Permissible Stresses in Concrete
The permissible compressive and tensile stress in
concrete at the stage of transfer and service loads
are defined in terms of corresponding compressive
strength of concrete at each stage.
In the Indian standard code, the reduction
coefficient applied to compute the design maximum
permissible compressive stress in flexure varies
from a value of 0.41 for M-30 grade concrete to
value of 0.35 for M-60 grade concrete.

11. 2.1.4 Shrinkage of Concrete
The shrinkage of concrete in pre-stress members is due
to the gradual loss of moisture which results in changes
in volume. The drying shrinkage depends on the
aggregate type and quantity, relative humidity,
water/cement ratio in the mix, and the time.

12. 2.1.5 Deformation Characteristics of Concrete
The complete stress-strain characteristics of concrete in
compression is not linear, but for loads not exceeding 30
percent of the crushing strength, the load deformation
behavior may be assumed to be linear.

14. 2.1.6 Design of the High Strength Concrete Mixes
The properties of high strength concrete mix with a
compressive strength of more than 40N/mm² is greatly
influenced by the properties of aggregate in addition to
that of the water/cement ratio. To achieve high strength,
it is necessary to use the lowest possible water/cement
ratio , which always affects the workability of the mix.

22. 2.2 High Tensile Steel
2.2.1 Types of High Tensile Steel
For prestressed concrete members, the high tensile steel used
generally consists of wire, bars or strands. The higher tensile
strength is generally achieved by slightly increasing the carbon
content in steel in comparison with mild steel. High tensile steel
usually contains 0.6 to 0.85 percent carbon 0.7 to 1 percent
manganese 0.05 percent of sulphur and phosphorus with hints of
silicon. The high carbon steel slabs are hot-rolled into rods and
cold drawn through series of dies to reduce the diameter and
increase then tensile strength.

23. 2.2.2 Strength Requirement
The ultimate tensile strength of plain hard drawn steel
wire varies with its diameter, the tensile strength
decreases with increase in the diameter of wires. The
ultimate tensile strength of different sizes of wires, bars
and strand, as specified in relevant Indian standard
codes are compiled in the tables 2.24 to 2.27

26. 2.2.3 Permissible Stresses in Steel
Tensile stress in steel at the time of tensioning
behind the anchorage and after allowing for all
possible losses are generally expressed as fraction of
the ultimate tensile strength or proof stress. The
recommendations of the various national code vary
marginally with regard to the allowable stresses n
prestressed members at different stages. The
permissible stress value specified in the Indian,
American and British code and F.I.P³¹ are compered
in table 2.28 .

27. 2.2.4 Relaxation of Stress in Steel
When a high tensile steel wire is stretched and maintained
at a constant strain, the initial force in the wire does not
remain constant but decreases with time. The decrease of
stress in steel at constant strain is termed relaxation . In a
prestressed member. The high tensile wire between the
anchorages are more or less in state of constant strain.
However, the actual state of relaxation will be less then
that indicated by test of a wire at constant length, as there
will be shortening of the member due to other causes.

29. 2.2.5 Stress Corrosion
The phenomenon of stress corrosion in steel is particularly
dangerous , as it results in sudden brittle fracture. Stress
corrosion cracking results from the combined action of
corrosion and static tensile stress, which may be either
residual or externally applied. This type of attack in alloys in
due to the internal metallurgical structure, which influenced
by composition, heat treatment and mechanical processing .
The causes of the susceptibility of high tensile steels to
stress corrosion are manifold.

30. 2.2.6 Durability, Fire Resistance and Cover
Requirements for P.S.C. Members
The alkaline environment of Portland cement
concrete generally protects embedded tendons
and other supplementary reinforcements against
corrosion form various environmental agencies.
However, the carbonation of hydrated cement
gradually progresses from the surface to the
interior of concrete, thus reducing the effective
protection provided by the concrete against
rusting of steel tendons.

31. • Many code have provided for minimum cover
requirements in this regard. It is pertinent to
note that not only the thickness of cover but
also the density of concrete in the cover is
important to provide effective protection to
steel.

32. Fire resistance is measure of the ability of the
structural member to withstand the effect of fire
without reaching any of the limit state. It is
expressed in terms of time by standard fire tests
outline in BS:476(part 8) 1972 and ASTME 119-
1979.fire resistance of structural concrete
elements is influenced by following parameters:

33. a) Size and shape of the element
b) Detailing, type and quality of reinforcement or
prestressing tendons.
c) The level of load supported and pattern of loading
d) Type of concrete and aggregate
e) Condition at end bearing
f) Protective cover to reinforcement

34. 2.2.8 Protection of Prestressing Steel, Sheathing and
Anchorage
To prevent deterioration due to corrosion unbounded
tendons should be coated by non-reactive materials like
epoxy or Zinc aluminum. Non corroding sheathing
material like density polyethylene (HDPE) is beneficial.
The space between sheathing and duct can be filled
with corrosion inhibiting materials like grease, wax or
petroleum jelly.