Introduction on Concrete

Concrete is the most common building material used in today’s construction industry. It can be cast in any desired
shape and fashion and is therefore applicable for most building purposes. Its long life and relatively low maintenance
requirements add to its popularity.

Concrete does not rot, rust or decay and is resistant to wind, water, rodents and insects. It is a non-combustible
material, making it fire resistant and able to withstand high temperatures. In the road sector, concrete is used for a
number of purposes, including pavements, bridges, culverts, retaining walls and other structures.

The material obtained immediately upon mixing of the various concrete ingredients is called fresh concrete, while
hardened concrete results when the cement hydration process has advanced sufficiently to give the material
mechanical strength.

Concrete that is batched and mixed in a plant and then transported by truck in its fresh, or plastic, state to the
construction site for final placement is called ready-mixed concrete. If the resulting structure or highway pavement,
for example, remains in place after placement, the concrete is referred to as cast-in-place concrete, whether mixed on-
site or off-site. Precast concrete refers to any structure or component that is produced at one site, typically in a pre-
casting plant, and then transported in its hardened state to its final destination. The controlled environment of a pre-
casting plant generally permits higher quality control of the product than is possible with cast-in-place concrete
produced at a construction site.

Code-writing organizations, such as the American Society for Testing and Materials, the American Concrete Institute
(ACI), and the American Association of State Highway and Transportation Officials (AASHTO), have published
detailed specifications and recommendations for measuring, mixing, transporting, placing, curing, and testing
concrete. A proper mix design assures that the concrete mix is well proportioned. The mixing time should be sufficient
to assure a uniform mixture. When placing the concrete, care should be taken to avoid segregation. For example, if
dropped too far, the heavy or big aggregate particles can settle and lighter mix components, such as water, tend to rise.
The concrete is conveyed from the mixing truck to its final destination in dump buckets by cableways or cranes or by
pumping through pipelines. In modern high-rise building construction, concrete has been pumped as high as a
thousand feet (330 m).

During placement, large amounts of air are entrapped in the mix, which lowers the strength of the hardened concrete.
Much of the air is removed by compaction, which is achieved by either immersing high-frequency vibrators into the
fresh concrete or attaching them to the outside faces of the formwork. Care must be taken to avoid excessive vibration;
otherwise the heavy aggregate particles settle down and the light mixing water rises to the surface.

For underwater construction, the concrete is placed in a large metal tube, called a tremie, with a hopper at the top and
a valve arrangement at the submerged end.

For so-called shotcrete applications such as tunnel linings and swimming pools, the concrete mixture is blown under
high pressure through a nozzle directly into place to form the desired surface.

Types of Concrete:
Reinforced concrete is defined by ACI 116R as a structural concrete reinforced with no less than the minimum amount
of prestressing tendons or non-prestressed reinforcement as specified by ACI 318.

Pre-stressed concrete is a structural concrete in which internal stresses have been introduced to reduce potential tensile
stresses in concrete resulting from load. This introduction of internal stresses is called “prestressing” and is usually
accomplished through the use of tendons that are tensioned or pulled tight prior to being anchored to the concrete.

Mass concrete any volume of concrete with dimensions large enough to require that measures be taken to cope with
generation of heat from hydration of the cement and attendant volume change to minimize cracking.

Mass concrete is normally placed in dams, bridge foundations, bridge piers, mat foundations, pile caps, thick walls,
and tunnel linings. Mass concrete may or may not be reinforced depending upon the intended purpose of the
structure.Most of the mass concrete is generally placed in gravity dams which doesn’t require reinforcements but even
in these gravity dams, piers for spillway gates and spillway chutes are also heavily reinforced mass concrete.

Portland cement concrete (PCC) Pavements (or rigid pavements) consist of a PCC slab that is usually supported by a
granular or stabilized base, and a subbase. In some cases the PCC slab may be overlaid with a layer of asphalt
concrete.
1.1. Components of Concrete

The American Concrete Institute ‘‘Building Code Requirements for Structural Concrete,’’ ACI 318,
contains the following basic definitions:
Concrete is a mixture of Portland cement or any other hydraulic cement, fine aggregate, coarse
aggregate, and water, with or without admixtures.
The aggregate consists of a mixture of various sizes of gravel and sand. When water is added to
cement, a chemical reaction takes place causing the mix to harden.
Components of Concrete

1. Paste (binder) is a mixture of cement (10%), air (5%), and water (15%). An unhardened
mixture of finely divided hydraulic cementitious material and water.
2. Mineral Aggregate (70%) are the coarse and fine aggregates
3. The volume of the whole mixture is equal to the solid volume of the cement, water, and
aggregate, plus the volume of the entrained and/or entrapped air. The concrete may or may
not contain one or more mixtures.
Requirements of Hardened Concrete as specified by its use:

• It should have the required strength;

• It should be uniform, watertight and resistant to wear, weather and other destructive
materials to which it might be exposed;
• Its constituents be evenly dispersed;
• It may be required to have high resistance to abrasion, fire, aggressive chemicals, or high
impermeability to water or other liquids; and
• It should not shrink excessively on cooling or drying nor expand excessively on wetting.
Factors affecting production of high quality concrete

• Quality of Paste
• Quality of Aggregates
• Strength of the bond developed between paste and aggregate
• Proper Batching Handling and Placing
1.2. Quality Paste
Supervising Concrete Works 1. Fundamentals of Concrete 1.2. Quality Paste
IN PROGRESS

The quality of concrete is largely dependent upon the quality of the paste which is water and
cement that binds the aggregate particles into a solid mass. Therefore, a proper proportion of
water to cement known as the water-cement ratio is essential for successful results.
With the paste in between, aggregate interlocking does not contribute appreciably to the
compressive strength of hardened concrete. It does contribute to flexural strength, which is
important in pavements.
The strength and density of the paste depend primarily on the water-cement ratio and on the
extent to which the cement become hydrated.
Cement
Cement shall conform to one of the following specifications:

1. Specification for Portland Cement (PNS 07:1983)

2. Specification for Portland Cement (ASTM C 150)
3. Specification for “Blended Hydraulic Cements” (ASTM C 595) excluding Type S and SA
which are not intended as principal cementing constituents of structural concrete.
Imported cements shall be certified quality cement which satisfies the requirements by means of
the test specified in PNS 07:1983 or in comparison with the specification of imported cement
from a certified exporting country
Cement will retain its quality indefinitely if it does not come in contact with moisture.
Proper storage of cement at jobsite must satisfy the following:

1. Warehouse or shed should be airtight as possible. No opening between walls and roof
should be tolerated.
2. Flooring of shed should be well above ground.
3. Cement sacks should be stacked close together to reduce circulation of air but should not
be stacked against outside walls.
4. Warehouse doors and windows should be kept closed.
There are various types of cement for different usages; for example, high early strength, sulfate-
resistant or low heat. The cement type to be used should be specified and shall conform to the
applicable specifications.

• Ordinary Portland cement (OPC) is the most widely used type of cement which is suitable
for all general concrete construction. A hydraulic cement produced by pulverizing clinker
consisting essentially of hydraulic calcium silicates, and usually containing calcium sulfate.
• Portland Pozzolana Cement (PPC) is prepared by grinding pozzolanic clinker with Portland
cement. It is also produced by adding pozzolana with the addition of gypsum or calcium
sulfate or by intimately and uniformly blending portland cement and fine pozzolana. It is
used in marine structures, sewage works, sewage works and for laying concrete under
water such as bridges, piers, dams and mass concrete works etc.
• Rapid hardening cement attains high strength in early days it is used in concrete where
formworks are removed at an early stage and is similar to ordinary portland cement (OPC).
This cement has increased lime content and contains higher c3s content and finer grinding
 which gives greater strength development than OPC at an early stage. Rapid hardening
cement is used in prefabricated concrete construction, road works, etc.
• Quick setting cement is used where works is to be completed in very short period and for
concreting in static or running water. The difference between the quick setting cement and
rapid hardening cement is that quick setting cement sets earlier while rate of gain of
strength is similar to Ordinary Portland Cement, while rapid hardening cement gains
strength quickly. Formworks in both cases can be removed earlier.
• Low heat cement is prepared by maintaining the percentage of tricalcium aluminate below
6% by increasing the proportion of C2S. This makes the concrete to produce low heat of
hydration and thus is used in mass concrete construction like gravity dams, as the low heat
of hydration prevents the cracking of concrete due to heat. This cement has increased
power against sulphates and is less reactive and initial setting time is greater than OPC.
• Air entraining cement is produced by adding indigenous air entraining agents such as
resins, glues, sodium salts of sulphates etc. during the grinding of clinker. This type of
cement is especially suited to improve the workability with smaller water cement ratio and
to improve frost resistance of concrete.
Water
ACI 3.4.1 – Water used in mixing concrete shall conform to ASTM C1602

• Water used in mixing shall be clean and free from injurious amounts of oils, acids, alkalis,
organic materials, or other substances that may be deleterious to concrete or
reinforcement.
• Non potable water shall not be used in concrete unless the following are satisfied:
o Selection of concrete proportions shall be based on concrete mixes using water
from the source.
o Mortar test cubes made with non potable mixing water shall have a 7-day and a 28-
day strengths equal to at least 90% of strengths of similar specimens made with
potable water.
1.3. Quality Aggregates
Supervising Concrete Works 1. Fundamentals of Concrete 1.3. Quality Aggregates
IN PROGRESS

Mineral Aggregates shall conform to one of the following specifications.

• Specification for Concrete Aggregates (PNS 18).

• Specification for Lightweight Aggregate for Structural Concrete (PNS/ASTM C330).
• Aggregates which have been shown by special test or actual service to produce concrete of
adequate strength and durability and approved by the authorized representative.
• ACI 3.3.2 – Nominal maximum size of coarse aggregate shall be not larger than;
• 1/5 the narrowest dimension between sides of forms, nor
• 1 /3 the depth of slabs or
• 3/4 the minimum clear spacing between individual reinforcing bars or wires, bundles of
bars, or pre-stressing tendons or ducts.
These limitations shall not apply if, in the judgment of the Engineer, workability and methods or
consolidation are such that concrete can be placed without honeycomb or void.
Quality of Aggregates – Since the aggregate constitute a large part of the concrete equal
importance should be undertaken as that of the quality of the paste.
Utmost care in their selection concerning qualities such as:

• good quality;
• strength;
• durability and;
• freedom from injurious materials.
Although the fine and coarse aggregates represent the major volume of concrete, about 70%, the
important role they serve as the principal ingredient is often overlooked because their cost is
much less than that of cement.
Quality control of aggregates consists of examining and testing for acceptability, performance of
necessary control tests, and checking that they are properly stored and handled.
In general, it is required that the aggregate shall be clean, hard, sound and durable, and that sizes
of particles shall be within specification limits.
In selecting an aggregate, it is economical to require only those properties pertinent to its use in a
particular project. The following criteria should be considered:

• Regardless of use, the grading of the aggregate should be uniform throughout its period of
use and should conform to specifications.
• An aggregate with unfavorable particle shape should not necessarily be rejected, if other
alternatives are very costly.
• An aggregate that contains appreciable amount of organic materials which may interfere
materially with the setting of cement should not be used.
• An aggregate that will not produce concrete of the required strength should not be used.
• An aggregate to be used in concrete exposed to severe weathering should be essentially
free of particles that are soft or friable, or highly absorptive.
• An aggregate containing substances that could react with alkalies in the cement to cause
excessive expansion should not be used in concrete exposed to wetting unless it is
required that low-alkali cement is used.
1.4. Admixture
Supervising Concrete Works 1. Fundamentals of Concrete 1.4. Admixture
IN PROGRESS

Admixture is a material other than hydraulic cement, aggregate, or water, used as an ingredient of
concrete and added to concrete before or during its mixing to modify its properties.

1. Admixtures to be used in concrete shall be subject to prior approval by the authorized

representative.
2. An admixture shall be shown capable of maintaining essentially the same composition and
performance throughout the work as the product used in establishing concrete proportions.
3. Calcium chloride or admixtures containing chloride from other than impurities from
admixture ingredients shall not be used in pre-stressed concrete, in concrete containing
embedded aluminum, or in concrete cast against stay-in-place galvanized metal form.
4. Air entraining admixtures shall conform to “Specification for Air-Entraining Admixtures for
Concrete” (ASTM C 1017).
5. Water-reducing admixtures, retarding admixtures, accelerating admixtures, water reducing
and retarding admixtures, and water-reducing and accelerating admixtures shall conform to
“Specification for Chemical Admixtures for Concrete” (ASTM C 494) or “Specification for
Chemical Admixtures for Use in Producing Flowing Concrete” (ASTM C 1017).
6. Fly ash or pozzolans used as admixtures shall conform to Specification for Fly Ash and
Raw or Calcimined Natural Pozzolan for Use as a Mineral Admixture in Portland Cement
Concrete” (ASTM C 618).
7. Ground granulated blast-furnace slab used as an admixture shall conform to “Specification
for Ground Iron Blast Furnace Slag for Use in Concrete and Mortars” (ASTM C989).
1.5. Strength of the bond developed
between paste and aggregate
Supervising Concrete Works 1. Fundamentals of Concrete 1.5. Strength of the bond developed between paste and aggregate
IN PROGRESS

With the paste in between, aggregate interlocking does not contribute appreciably to the
compressive strength of hardened concrete. It does contribute to flexural strength, which is
important in pavements. With satisfactory aggregates. percolating water or weathering agencies
must act on or pass through the paste. Some of the constituents of the hardened paste are water
soluble, and the rate of leaching of these soluble constituents is greatly diminished with denser
paste.
Hence, it is desirable that the paste be dense and have a low water-cement ratio when the
concrete will have prolonged contact with soft water or with water that contains chlorides,
sulfates, acids, or other aggressive chemicals. The strength and density of the paste depend
primarily on the water-cement ratio and on the extent to which the cement becomes hydrated.