Concrete Design & Production Concrete
Technology Home > Concrete Design & Production Concrete
Basics
Concrete is a mixture of two components: aggregates
and paste. The paste, comprised of cement and water, binds the aggregates
(usually sand and gravel or crushed stone) into a rocklike mass
as the paste hardens because of the chemical reaction of the cement
and water. Supplementary cementitious materials and chemical admixtures
may also be included in the paste.
For more on concrete basics, click
here.
 |
|
PCA's Fundamentals
of Concrete is an excellent resource on concrete basics,
covering the properties of freshly mixed and hardened concrete,
including workability, setting times, strength development,
and drying shrinkage. It also discusses important durability
issues, such as freeze-thaw resistance and corrosion of reinforcement.
|
Materials for Use in Concrete
|
 |
Cement
Cements
set and harden by reacting chemically with water. During this reaction,
called hydration, cement combines with water to form a stonelike
mass, called paste. When the paste (cement and water) is added to
aggregates (sand and gravel, crushed stone, or other granular material)
it acts as an adhesive and binds the aggregates together to form
concrete, the world’s most versatile and most widely used
construction material. More.
Supplementary Cementitious
Materials (SCM)
Supplementary
cementitious materials are generally divided into:
- Pozzolans (fly ash, silica fume, and natural
pozzolans, such as calcined shale, calcined clay or metakaolin)
- Slag
These materials when used in conjunction with portland or blended
cement, contribute to the properties of the hardened concrete through
hydraulic or pozzolanic activity or both.
Additional references:
Supplementary
Cementing Materials For Use in Concrete CD
Fly Ash, Slag,
Silica Fume, and Natural Pozzolans
Design and Control
of Concrete Mixtures
Supplementary Cementing
Materials for Use in Blended Cements
Benefits of Ternary Mixtures
Aggregates
Aggregates
are classified by ASTM C 33 (AASHTO M 6/M 80) as fine or coarse.
Fine aggregate consists of natural sand, manufactured sand, or a
combination thereof with particles that are typically smaller than
5 mm (0.2 in.). Coarse aggregate consists of either (or a combination
of) gravel, crushed gravel, crushed stone, air-cooled blast furnace
slag, or crushed concrete,
with particles generally larger than 5 mm (0.2 in.). The maximum
size of the coarse aggregates is generally in the range of 9.5 to
37.5 mm (? to 1 ½ in.).
More on recycled aggregates,
click here.
Water
W ater
Content: Why Less Is More
The quality of hardened concrete is greatly influenced by the amount
of water used in relation to the amount of cement. Higher water
contents dilute the cement paste (the glue of concrete). Here are
some advantages of reducing water content:
- Increased compressive and flexural strength
- Lower permeability, thus increased watertightness and lower
absorption
- Increased resistance to weathering
- Better bond between concrete and reinforcement
- Less volume change from wetting and drying
- Reduced shrinkage and cracking
- Aggregates
New ASTM Specification for Mixing Water. More.
Chemical
Admixtures
Admixtures
are those ingredients in concrete other than portland cement, water,
and aggregates that are added to the mixture immediately before
or during mixing.
For basics on chemical admixtures, click
here.
A wide range of admixtures are available. The table below provides
a list of common types of admixtures. The effectiveness of an admixture
in concrete depends upon many factors including cementitious materials
properties, water content, aggregate properties, concrete materials
proportions, mixing time and intensity, and temperature.
Type of Admixture |
Standard Specifications
|
Desired Effect |
Air-entraining admixture
(AEA)—
More.
|
ASTM C 260 and C 233 (AASHTO M 154 and T 157). |
To stabilize microscopic bubbles in concrete, which can provide
freeze-thaw resistance and improve resistance to deicer salt
scaling. |
| Water reducing admixture (WR) |
ASTM C 494 (AASHTO M 194) |
Reduce the water content by 5 to 10%, while maintaining slump
characteristics. |
| Mid-range water reducer (MRWR) |
ASTM C 494 (AASHTO M 194) |
Reduce the water content by 6% to 12%, while maintaining slump
and avoiding retardation. |
High-range water reducer (HRWR)
(also called superplasticizer)
|
ASTM C 494 (AASHTO M 194),
ASTM C 1017
|
Reduce the water content by 12% to 30%, while maintaining
slump. |
| Retarding admixture |
ASTM C 494 (AASHTO M 194) |
To decrease the rate of hydration of cement. |
| Accelerating admixture |
ASTM C 494 (AASHTO M 194) |
To increase the rate of hydration of cement. |
| Shrinkage-reducing admixtures |
|
Reduce drying shrinkage (and related cracking) in concrete |
| ASR-inhibiting admixtures |
|
Reduce or eliminate deleterious expansion due to alkali-silica
reaction |
| Corrosion inhibitors |
ASTM C 1582 |
Minimize steel reinforcement corrosion |
Self-Consolidating Concrete
Flow with Show: Self-Consolidating Concrete
Offers New Opportunities for Architectural Concrete
What is Self-Consolidating Concrete (SCC)
and how is it tested?
Mass Concrete
Mass concrete is a hot topic. Owners desire long
service lives so engineers design concrete mixes for low permeability.
These mixes typically have high cementitious materials contents,
which results in high temperatures within the concrete. To avoid
cracking and other temperature related damage to the concrete, contractors
must control the temperature and temperature difference in the concrete.
This can pit the schedule against the service life.
When all involved parties work
together, appropriate changes can be made to achieve the desired
service life with minimal impacts to the schedule. The key is an
understanding of mass concrete. Selection of an appropriate concrete
mix is the first step. More.
Standards
Noteworthy ACI and ASTM documents published
in 2005.
Significance of Tests and Properties of
Concrete and Concrete-Making Materials (STP169D) (LT205)
In
a very real sense, specifications are the “letter of the law.”
But for the professional that needs to know the “how and why”
behind the development, this new encyclopedic reference is the place
to turn. Ever wonder how much strength a test cylinder will lose
from rolling and bumping around in the back of a pickup truck or
from being dropped from waist level? How about the effect of bearing
strips on test cylinders used for splitting tensile strength? Which
DOT performed the original research for strength determination using
maturity? How much heat contribution should I expect from each of
the four major cement compounds? This is the type of information
that can help avoid headaches and avert disasters. This peer-reviewed
work is a must have for the cement and concrete professional producing,
using, or testing materials in conformance with ASTM specifications.
Read a review of the
book.
More information or
to purchase.
|
|

|