Concrete block

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Concrete blocks, concrete masonry units or CMUs have traditionally been used for exterior walls and retaining walls. Concrete blocks are a cost effective construction material frequently used for their variety of finish and colors.

Contents

History

The word “concrete” originated from the Latin word “con- cretus” (which means to be compact or condensed). This material initiated in Europe as far back the Roman era where builders had access to deposits helping them com- pose natural cement. This practice is displayed throughout the eighteenth century in which most of the various man- made cement materials were developed. Hydraulic lime, a similar material, was established in 1756 by an engineer named of John Smeaton. This discovery was achieved as Smeaton was trying to produce a material strong enough to rebuild one of his projects known as the Eddystone lighthouse off the coast of Devon. Though this formula was originally created by the Romans, it seemed to have disappeared with the fall of the Roman Empire in the 15th century until the Smeaton had rediscovered it. Later on in the 19th century, englishmen Joseph Aspdin and Isaac Charles Johnson simultaneously refined this manmade cement composition. Aspdin was known for his synthesis of limestone and clay, recognized today as Portland cement. However, this product did not seem to display as much strength as the altered composition Johnson had created. This product, known as the portland cement we use today, consists of the same formula with the addition of silica, alu- mina, and iron oxide-contain- ing materials. The distinguishing factor in Johnson’s product was the introduction of a natural el- ement; he burned these aggre- gates until they fused together, resulting in an admixture called clinker. This process altered the properties of concrete adding strength, durability and consist- ency in comparison to natural cement. Around the same time, the Americas were experimenting with natural cement. In fact, it was first used in association with the Erie Canal. Due to the growing reputation of Portland cement in Europe, American businesses started using this syn- thesized concrete. In the 1870s, the formula for Portland cement was revealed to the population and resulted in decline of pro- duction of natural concrete. The next major historical evo- lution of concrete occurred when reinforced concrete was invented in the late nineteenth century. Concrete itself was still not considered the most ef- ficient material used to build structures such as bridges as it did not tolerate well to tension in comparison to compression. Hence, reinforced concrete was discovered by embedding metal bars. This allowed struc- tures to respond well to tension and withstand bending result- ing in this technique to be used worldwide in the early twentieth century. In 1888, another type of con- crete was patented; pre- stressed concrete. This innova- tion was mostly recognized for its use in World War II to con- struct structures such as bridg- es. “Rather than reinforcing a highly stressed portion of a con- crete structure with steel, engi- neers could now compress a section of concrete before they subjected it to stress, thereby in- creasing its ability to withstand tension.” Today, many types of concrete mixtures exist in reference to their use, purpose and method of being installed. Some widely known types of concrete ag- gregates used today are as- phalt concrete, decorative concrete, ready-mix concrete, precast concrete, as well as ter- razzo. In the modern world, the evolution of technology has led us to take advantage of con- crete which can now be found ready or pre-mixed. This product is mixed prior to being delivered to the required site. In addition, agitator trucks are now used to carry concrete, which is being mixed as it is in transit to the site. These trucks are widely seen in many construction zones today as they have become one of the most successful ways of us- ing and transporting concrete. This simple mixture of raw ma- terials, composed in a remote area of the world, has evolved into a complex formula now used worldwide.

Material analysis

MasterFormat Number: 04 22 00

Traditional uses

  • Concrete block is traditionally used to build economical structural walls.

Emerging uses / trends

  • Modifications to the manufacturing process increased architectural applications by incorporating color, texture and pattern.
  • Concrete can be used for many applications in the house like making kitchen countertops, sinks, fireplace etc.

Finish and aesthetic qualities

  • Surface textures are classified as fine, medium and coarse and are determined by the mix of portland cement, aggregate, and water.
  • Common finishes include polished face, ground face, split face, split rib, standard, and brick block.
  • Standard colors vary from shades of gray to brown and are dependent on the type of aggregate.
  • Additional colors are created by adding mineral oxide elements to the concrete mix.
  • Most concrete blocks have one or more hollow cavities, and their sides may be cast smooth or with a design. They are stacked one at a time and held together with fresh concrete mortar to form the desired length and height of the structure.
  • Blocks can be plastered, painted or sealed with transparent sealant to expose their original texture and paint.
  • It is strong and durable in compression when hardened and becomes stronger with time. Its tensile strength can be improved by the addition of steel reinforcement.

Surface forms / shapes

  • Blocks can be split to produce a rough finish.
  • Sculptured, pierced, and patterned blocks are created by using raised or recessed forms during manufacturing.
  • Blocks can also be glazed for decorative effect.

Cost analysis

  • Price varies between $0.75 and $1.25 per block depending on the type and size.

Material properties

Primary structural use

  • This material is primarily used in compression.
  • They are predominantly for utilitarian structural work, such as walls and screens, but come in variety of sizes and shapes that can also be used decoratively indoors as well as outdoors. .[1]

Dimensional and opening restriction

  • Typical face dimension of 7 5/8” x 15 5/8” with 3/8” mortar joint. Block units often do not exceed 24” in length.
  • Blocks are available in 4”, 6”, 8”, 10”, and 12” widths.
  • The dimensions of patterned and pierced bricks vary.
  • Openings in concrete block walls must be supported by a lintel.

Acoustic property

  • Blocks with vertical-slotted faces have sound absorbing properties.
  • Blocks cavities can be filled with fibrous material to absorb sound.

Manufacturing process and assembly

  • The concrete commonly used to make concrete blocks is a mixture of powdered portland cement, water, sand, and gravel. A typical concrete block weighs 38-43 lb (17.2-19.5 kg).
  • Lightweight concrete blocks are made by replacing the sand and gravel with expanded clay, shale, or slate. A typical lightweight block weighs 22-28 lb (10.0-12.7 kg) and is used to build non-load-bearing walls and partitions.
  • The production of concrete blocks consists of four basic processes:
    • Mixing - The required amounts of sand, gravel, and cement are put in a stationary mixer where they are blended together for several minutes. After the dry materials are blended, a small amount of water is added to the mixer. Admixture chemicals and coloring pigments may also be added at this time. The concrete is then mixed for six to eight minutes.
    • Molding - The mixture is then forced downward into molds. The compacted blocks are pushed down and out of the molds onto a flat steel pallet. The pallet and blocks are pushed out of the machine and onto a chain conveyor where it passes under a rotating brush which removes loose material from the top of the blocks.
    • Curing - The pallets of blocks are then moved into a curing kiln. The blocks are held in the kiln for one to three hours at room temperature to allow them to harden slightly. Steam is then gradually introduced to raise the temperature. When the curing temperature has been reached, the steam is shut off, and the blocks are allowed to soak in the hot, moist air for 12-18 hours. After soaking, the blocks are dried by exhausting the moist air and further raising the temperature in the kiln. The whole curing cycle takes about 24 hours.
    • Cubing - The blocks pass through a cuber which aligns each block and then stacks them into a cube three blocks across by six blocks deep by three or four blocks high. These cubes are carried outside with a forklift and placed in storage.[2]
  • Blocks are set into place and held together with mortar.
  • In addition to the basic components, the concrete mixture used to make blocks may also contain various chemicals, called admixtures, to alter curing time, increase compressive strength, or improve workability.

Maintenance

Although concrete is a notably strong material, it still has its weaknesses, resulting in the obligation of main- tenance. Many measures can be taken to fix and even prevent common damages such as stains and cracks. It is always recommended to coat concrete after in- stallation in order to prevent attacks to surfaces by ac- ids, alkalis, salt solution, organic chemicals and natu- ral elements. The type and thickness of coatings also vary depending on their uses. For example, thin coat- ings are normally used for damproofing, mild chemi- cal attacks, and decorative purposes while thick ones applied when waterproofing, protecting from sever chemical attacks and physical damage. In case of internal moisture, coatings used should transmit water vapour and be applied on the exterior wall. A variety of solutions are available to meet specific purposes. If concrete is not protected, the chance of stains is most probable. The most common stains found on concrete consist of rust, oil, grease, dirt, mildew and asphalt. Due to its porosity and absorbency, stains are likely to pen- etrate the exposed surface and should be removed immediately to avoid further damage. There are many ways to remove stains including brushing and wash- ing, steam cleaning, water blasting, abrasive blasting, flame cleaning, mechanical cleaning, and chemical cleaning. The removal approach depends on the type of stain and length of time it has been con- tact with the material. Another damage regularly occurring with concrete is cracks. This factor is usually caused by temperature changes or base movements, ultimately allowing water and air to reach the interior of a structure. While most coatings will not bridge larger cracks, there are some elastomeric and polymer coatings that can aid in the prevention of internal damage by sealing minor cracks. For instance, elastomeric coatings will bridge cracks under 0.8 mm while polymer coatings are formulated to seal cracks in horizontal concrete structures using gravi- ty. When it comes to larger cracks, the best solution is to chisel the crack to look like a dovetail joint and fill it with cement that ex- pands as it dries. The dovetail shapes are more effective because they are fixing the crack from underneath instead of the sur- face. As you can see, fixing damage to concrete can be quite complex so sealing concrete and ensuring the proper grade for the required task are essential to pre- vent any problems.

Environmental Impact

  • The production of Portland Cement is energy intensive, therefore concrete masonry units have relatively high embodied energy of 730 to 960 Btu/lb. In addition, the production of Portland Cement contributes to significant amounts of carbon dioxide in the atmosphere. For example, 1 tonne of carbon dioxide is released for every one tonne of cement that is produced.
  • Concrete masonry units are very durable and require little maintenance. Units can be found that are made with fly ash, a fine powder retrieved from the dust controlled systems of electric power plants, or blast surface slag, a by-product of the production of pig iron. The use of recycled industrial waste aggregates in the production of these concrete blocks is a more sustainable option.
  • The use of light weight block also improves the environmental impact and autoclaved aerated lightweight concrete blocks are recommended for their higher insulation value.
  • Using concrete masonry units which include insulation helps to conserve energy, and using units which can be used as both interior and exterior finish surfaces conserves resources.
  • Concrete blocks are safe and easy to use and can be recycled as hardcore or aggregate.
  • Water pollution is considered a major environmental issue associated with concrete because the water used to wash the inside of concrete trucks, tools and related instruments is highly alkaline and toxic to fish and other aquatic life. However, recycled waste water from mixing processes in the production phase can be used to clean demolition concrete for reuse.[3]
  • Hazardous material in wet concrete and mortar include alkaline components such as lime (calcium oxide) that are corrosive to human tissue.
  • Concrete floors and walls with moisture problems may support the growth of mold and mildew.
  • Concrete block is the best material for termite resistance because it is not a source of nutrition for termites.

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Ecological Footprints of Concrete

Since becoming green is evolving into a new trend in design, choos- ing the right materials is crucial. For example, concrete, a commonly used building material, has its multiple positive attributes that assist the environment while presenting a couple of letdowns. Ecological foot- prints aren’t just the footprints of the building. They also include the impact on the surrounding environment. These include the construc- tion and the greenhouse gasses generated from the production of the materials, the building, and from day-to-day usage after completion. These greenhouse gasses are then absorbed by the surrounding vicin- ity, which forms “carbon footprints”. Cement is likely to be a strong contributor to these ecological foot- prints, depending on the material choice in the construction process. In fact, one ton of CO 2 is emitted for every ton of cement produced. This negative aspect is associated with concrete due to its high per- centage of cement. Since concrete is hard to avoid in the process of construction, there are ways to reduce the concentration of cement in concrete. Different forms of concrete could be used to replace the cement concentration in concrete such as recycled concrete, which could be used in non-structural components. Another solution is to use ground-granulated slags in blended concrete, which will reduce the proportion of cement. Therefore, while concrete does present some environmental damage, a reduction in emissions can be made using simple solutions.


Recycling Concrete

Collecting concrete from demolition project and recycle it can save a considerable amount of cost since it saves the transporting cost to the landfill (as much as $0.25 per ton/ mile), the disposal cost which can reach as high as $100 per ton and it also eliminate the expense of aggregate material imports and exports.

The process of recycling concrete is as follows: First, concrete rubble and aggregates are derived from the demolition of concrete structures and partially filtered from other construction materials, such as wood, glass and dirt. Contaminants are then removed and the aggregate is washed with recycled waste water. After being crushed, graded, and screened into appropriate sized fractions, this recycled aggregate can be mixed with cement to create fresh concrete.

Life Safety & Rating Standards

Flammability rating

  • The fire-resistive ratings of concrete block depend on the type of aggregate used and the average thickness of the block.
  • Blocks with pumice aggregate have the highest hourly ratings.
  • Concrete Block is a non-combustible material.

References

Notes

  1. ↑ Interior Mterials & Surfaces The Complete Guide by Helen Bowers
  2. ↑ http://www.madehow.com/Volume-3/Concrete-Block.html
  3. ↑ Material for Interior Environment by Corky Binggeli, ASID
  • Beall, Christine, Complete Construction: Masonry and Concrete for Residential Construction, The McGraw Hill Companies, August 2000.
  • Beall, Christine and Rochelle Jaffe, Concrete & Masonry Databook, The McGraw Hill Companies, September 2002.
  • Kind-Barkauskas, Friedbert, Kauhsen, Bruno, Polonyi, & Stefan,Brandt, Concrete Construction Manual. December 2002.
  • Aitcin, Pierre and Sidney Mindess. "Sustainability of Concrete". London: Spon Press, 2011.

Student contributions

See also

Building references

External links

Additional resources

Leading manufacturers

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