Jan 13 2014

Concrete Mix Designs

For work requiring more than one cubic yard of material, concrete is usually ordered from a ready mix supplier for delivery to the job site. The supplier will need to know the minimum compressive strength, the maximum aggregate size, and any special requirements such as air entrainment for added freeze-thaw durability. The supplier will then select a mix design that is appropriate for your needs. If you are mix-ing small batches of concrete on site, you will need to understand the basic principles of concrete mix design yourself. The proportion of dry ingredients and the ratio of water to cement are the two most important factors.

Cement and aggregates provide strength, durability, and volume stability in concrete, but too much or too little of one in relation to the other reduces quality.

Lean or oversanded mixes with low cement content and high aggregate proportions are harsh and have poor workability.
Fat or undersanded mixes with high cement content and low aggregate proportions are sticky and expensive.

Within the range of normal concrete strengths, compressive strength is inversely related to water content. That is, the more water you use, the lower the concrete strength. But increasing water content increases fluidity and workability. Since water is required for workability, and since workability is required for high quality concrete, the low water requirements for strength and high water requirements for workability must be balanced. The ratio of water to cement is the weight of water divided by the weight of cement. Water cement ratio affects the consistency of a concrete mix. The consistency, in turn, affects how easily the concrete can be poured, moved around in the forms, compacted, and finished. Up to a point, a mix with more water is easier to work with than one that has less water and is therefore stiffer. Too much water, though, will cause the ingredients to separate during the pouring, placing, and handling and will destroy the integrity of the concrete. Too much water also lowers strength, increases the porosity and water permeability of the cured concrete, and makes it more prone to shrinkage cracking. The trick is to use enough water to make the fresh concrete workable, but not so much that it creates weak or porous structures.

Jan 12 2014

Concrete Ingredients

The basic ingredients in concrete are cement, aggregates, and water. The type, quality, and proportioning of these ingredients affect the curing rate, compressive strength, and durability of the concrete. Chemical admixtures can be used to enhance one or more properties of the concrete or to improve its handling and placing characteristics.

Cement

Cement is not the same thing as concrete. Many people mistakenly refer to “cement” sidewalks or “cement” driveways and the like, but cement is only one of the ingredients in concrete. It is also an ingredient in masonry mortar, stucco, and other materials.

Cement + water = cement paste
Cement + water + sand =cement mortar
Cement + water + sand + lime = masonry mortar
Cement + water + sand + coarse aggregate = concrete

Aggregates

The aggregates most commonly used in concrete are sand, gravel, crushed stone, crushed slag, and pumice. Cement and water are mixed with aggregates to produce concrete. Concrete contains both fine and coarse aggregates. When cement is mixed only with fine aggregate, it is called cement mortar, which is used typically for patching and small repairs, or for coating a concrete surface to provide a smooth, even finish. Masonry mortar is different from a simple cement mortar because it contains other ingredients as well.

Water

As a rule of thumb, water used for mixing concrete should be drinkable. Any water that is drinkable is generally free of harmful impurities. In urban areas where municipal water supplies are available, contaminated water is usually not a problem.

Admixtures

Admixtures are substances other than cement, water, or aggregates which are added to concrete mixes for the purpose of altering properties of the fresh or hardened concrete. Admixtures are not generally required to produce high quality, low cost concrete, but they may sometimes be necessary or desirable to alter specific properties of the concrete for specific conditions or circumstances.

Chemical Admixtures

Set accelerators speed up the setting time and early strength development of concrete. This can be helpful in winter weather to reduce the length of time required for curing and protection and to compensate for the effects of low temperatures on strength development.

May 11 2013

Frost and Freeze/Thaw resistant Concrete

Frost and freeze/thaw resistant concrete must always be used when concrete surfaces are exposed to weather (wet) and the surface temperature can fall below freezing.

Fair-faced concrete façades
Bridge structures
Tunnel portal areas
Traffic areas
Retaining walls

By adding air entrainers, small, spherical, closed air voids are generated during the mixing process in the ultra-fine mortar area (cement, finest grain, water) of the concrete. The aim is to ensure that the hardened concrete is frost and freeze/thaw resistant (by creating room for expansion o any water during freezing conditions).

Type, size and distribution of air voids

Air voids contained in a standard concrete are generally too large (>0.3mm) to increase the frost and freeze/thaw resistance. Effective air voids are introduced through special air entrainers. The air voids are generated physically during the mixing period. To develop their full effect, they must not be too far from each other. The “effective spacing” is defined by the so-called spacing factor SF.

Production/mixing time

To ensure high frost and freeze/thaw resistance, the wet mixing time must be longer than for a standard concrete and continue after the air entrainer is added. Increasing the mixing time from 60 to 90 seconds improves the content of the air voids by up to 100%.

Quantity of air voids required

To obtain high frost resistance, the cement matrix must contain about 15% of suitable air voids. Long experience confirms that there are enough effective air voids in a concrete if the results of the test (air pot) show the following air contents:

– Concrete with 32 mm maximum particle size 3% to 5%

– Concrete with 16 mm maximum particle size 4% to 6%

Fresh concrete with an air void content of 7% or over should only be installed after detailed investigation and testing.

Apr 13 2013

Concrete Admixtures

Concrete admixtures are liquids or powders which are added to the concrete during mixing in small quantities, normally based on the cement content. They influence the properties of the fresh and/or hardened concrete chemically and/or physically.

The standard includes the following under “Special Terms” (slightly abbreviated):

Admixtures – definitions and effects

Water reducer

Enables the water content of a given concrete mix to be reduced without affecting the consistence, or increases the workability without changing the water content, or achieves both effects.

Super plasticizer

Enables the water content of a given concrete mix to be greatly reduced without affecting the consistence, or greatly increases the workability without changing the water content, or achieves both effects.

Stabilizer

Reduces mixing water bleeding in the fresh concrete.

Air entrainer

Introduces a specific quantity of small, evenly distributed air voids during the mixing process which remain in the concrete after it hardens.

Set accelerator

Reduces the time to initial set, with an increase in initial strength.

Hardening accelerator

Accelerates the initial strength with or without an effect on the setting time.

Retarder

Retards the time to initial set and prolongs the consistence.

Water proofer

Reduces the capillary water absorption of the hardened concrete.

Retarder/water reducer

Has the combined effects of a water reducer (main effect) and a retarder (additional effect).

Retarder/super plasticizer

Has the combined effects of a super plasticizer (main effect) and a retarder

(additional effect).

Set accelerator/water reducer

Has the combined effects of a water reducer (main effect) and a set accelerator (additional effect).

Apr 1 2013

Concrete Aggregates

Gravels, stone and sands form the granular structure, which must have its voids filled as completely as possible by the binder glue. They make up approximately 80% of the weight and 70–75% of the volume. Optimum use of the aggregate size and quality improves the concrete quality. Aggregates can occur naturally (fluvial or glacial); for high quality concrete they are cleaned and graded in industrial facilities by mechanical processes such as mixing together, crushing, screening and washing (mechanical preparation).

Suitable as concrete aggregates are materials which do not interfere with the cement hardening, have a strong enough bond with the hardened cement paste and do not put the resistance of the concrete at risk.

Standard and special aggregates

Standard aggregates	Density2.2–3 kg/dm³	From natural deposits, e.g. river gravel, moraine gravel etc. Material rounded or crushed(e.g. excavated tunnel)
Heavyweight aggregates	Density> 3.0 kg/dm³	Such as barytes, iron ore, steel granulate. For the production of heavy concrete (e.g. radiation shielding concrete)
Lightweight aggregates	Density< 2.0 kg/dm³	Such as expanded clay, pumice, polystyrene. For lightweight concrete, insulating concretes
Hard aggregates	Density> 2.0 kg/dm³	Such as quartz, carborundum; e.g. for the production of granolithic concrete surfacing
Recycled granulates	Density approx. 2.4 kg/dm³	From crushed old concrete etc.

Mar 18 2013

Precast Concrete Applications

Precast concrete has been used extensively in recent years. We have already discussed the advantages to this type of repair process. Here are some of the types of concrete structures where precast concrete can be used:

■ Navigation locks

■ Dams

■ Channels

■ Floodwalls

■ Levees

■ Coastal structures

■ Marine structures

■ Bridges

■ Culverts

■ Tunnels

■ Retaining walls

Mar 9 2013

Design and Construction: Building in Value

The question is sometimes asked: “What is the difference between architecture and building?” One facetious answer is: ‘If the roof leaks, it’s architecture.’ The basis of this response lies in the architect’s quest for new and different ways of satisfying humanity’s basic need for shelter and security. Innovation always carries some risks and the risks attached to the pursuit of innovative approaches in the design and construction of buildings are exacerbated by the very nature both of the products (i.e., buildings) and of the industry that produces them.

The leaky roof symbolizes the problems that architects and builders face as they try to explore new designs and utilize new materials and construction methods – neither the designers nor the contractors are completely familiar with the new material or system and there are no previously completed installations to look at for guidance. The result is that new methods are only truly tested when they are used in ‘live’ projects and it may not be until a number of projects have been completed that any new method, material, component or system is completely satisfactory in use.

Mar 7 2013

The nature of buildings

There are many forces that help to determine why individual buildings come to be the way they are and they affect a variety of aspects of any one building: materials, shape, color, size, form, style (or lack of it), choice of engineering systems, structural system, and more. Function or purpose explains much about the nature of many buildings and availability of resources (particularly money and materials) is often the primary determining factor.

Many specific parts of a building’s nature are, however, determined by less obvious influences: a desire to build in an environmentally responsible manner, or a client’s wish to say something about image or status, perhaps by constructing a building larger or taller than that of a competitor. Much emphasis is now placed on building ‘intelligence’ and on streamlining construction to allow for faster completion. Integrated design, with multidisciplinary teams working closely together, is producing buildings quite unlike any before, with building fabric and services functioning together to minimize energy use and improve the health and productivity of the occupants.