Chemical Admixtures in Concrete

Chemical admixtures are natural or manufactured chemicals that are added to concrete along with portland cement, aggregate, and water.  They can be a liquid or powder and are added to the concrete immediately before or during the mixing process.  The chemical admixtures in a mix will account for less than 5% of the total concrete.   

The purpose of chemical admixtures is to give the concrete additional properties to handle circumstances such as pumping requirements or when placing concrete during extreme weather conditions.   According to the Portland Cement Association, “Admixtures are classed according to function. There are five distinct classes of chemical admixtures: air-entraining, water-reducing, retarding, accelerating, and plasticizers (superplasticizers).”

The National Ready Mix Concrete Association (NRMCA) says that the most common chemical admixtures used are air-entraining agents, water-reducers, and water-reducing retarders and accelerators.  They state that admixtures are added to concrete to increase “durability workability or strength characteristics of a given concrete mixture.”

Accelerators are used to increase the early strength of concrete by causing it to harden quicker and are used to protect concrete from extreme cold or when there is a tight project deadline.  Chemical accelerators are chloride based or non-chloride based and calcium chloride is one type commonly used as it is inexpensive.

Retardants are used to slow down or delay the hardening process such as in situations where the job demands a slower pour due to the scope of the project or when in very hot temperatures.  It provides a crew additional time to place and finish the concrete.   Common retardants are lignin, sugars, and citric and tartaric acids.

Air-entraining is the process of deliberately adding microscopic air bubbles into the concrete.  The purpose of doing this is to increase the concrete’s durability when faced with cycles of freezing and thawing conditions and to improve the concrete’s workability.    Furthermore segregation and bleeding is diminished or completely eliminated.  The only negative is that the concrete experiences a 5% reduction in strength for each 1% of air added.

1. http://www.cement.org/cement-concrete-basics/concrete-materials/admixtures
2. http://www.nrmca.org/aboutconcrete/cips/15p.pdf

Conco is one of the leading concrete contractors in San Francisco offering a range of innovative, quality services.  We have been honored to work on many high-profile projects and were recently awarded the concrete package for the San Francisco 49ers Stadium.  Our concrete services include commercial, educational, parking and other construction development as well as public works projects.

High-Strength Concrete

The design of a project determines the performance requirements of the concrete in terms of the compressive strength needed and level of durability to withstand weather conditions.   High-strength concrete is defined as having a compressive strength of 6,000 psi (40 MPa) or greater.  The last couple of decades have seen a demand for stronger concrete. The Portland Cement Association reports that two buildings in Seattle, Washington were constructed using concrete that has a compressive strength of 19,000 psi.

In order to design high-strength concrete, research and time is put into optimizing and controlling each component of the mix.  The concrete needs to have a lower water-cementitious material (W/C) ratio that is somewhere in the range of 0.23 to 0.35, and usually requires a superplasticizer admixture. 

Moreover the high-strength concrete requires an aggregate that is durable and strong and is the optimal size and coarseness to ensure the ideal bond with the cement paste.  Generally the mix will contain a mineral admixture such as silica fume or fly ash (Class C or F) to prevent free calcium hydroxide crystals from forming that can affect the strength of the concrete.     

There are many kinds of projects that require high-strength concrete such as the construction of long-span bridges and decks or projects that have unique architectural features.  Without high-strength concrete, it would not be possible to construct the super tall skyscrapers of today.  Another advantage to the stronger concrete is that columns can be made smaller but still be as supportive while offering more usable interior space.

Conco is taking a more active role in the Portland area with our expansion of high-quality commercial concrete services.  We bring our years of experience delivering dependable services since 1959, and a vast portfolio of noteworthy and large-scale projects from throughout the Western U.S.  Our work includes commercial, industrial, educational and multi-residential construction as well as public works projects.

Why Concrete Shrinks and How it is Addressed

All concrete experiences some degree of shrinkage, which can result in cracks in the concrete.  Shrinkage may also affect the long-term strength and durability of the concrete.  Even though concrete contractors know that shrinkage is caused by a loss of volume that occurs in different stages of the process, it is still challenging to prevent or alleviate shrinkage, and thereby the cracking.  It is especially an important consideration when constructing industrial floors or pavement.

Since shrinking is one of the inherent properties of concrete, it is impossible to totally eliminate cracking but it can be controlled.  This is why contractors and engineers work to understand and address the volume change in concrete to reduce the amount of shrinkage.

According to the Portland Cement Association, “The shortening of concrete slabs can be caused by temperature decreases or moisture loss. These two causes are also related to curling and warping of slabs, respectively. Curling is the deformation of the slab due to a difference in temperature between the surface and the bottom of the slab (temperature gradient).  Slab “warping” is the deformation of the slab surface profile due to a difference in moisture between the surface and bottom of the slab (moisture gradient).”* 

To quickly move and place concrete, it is mixed with more water than is absolutely necessary for hydrating the cement materials.  Once the concrete is placed, the excess water separates from the hardening concrete, which results in a loss of volume that causes the concrete to shrink.  The concrete’s reinforcement or base friction works to restrain it, which in turn can create tensile stresses in the slab causing cracks. 

In an effort to prevent the shrinkage, one of the things that contractors watch out for is fluctuating temperatures during the first 24 hours after placement.  Also considerable care is taken to use the correct concrete mixture design so that the least amount of extra water is added and the largest size of aggregate is used.  All of this works toward reducing shrinkage.

*http://www.cement.org/for-concrete-books-learning/concrete-technology/focus-on-floors/concrete-shrinkage

 

Conco is one of the leading commercial concrete contractors in Sacramento and has been delivering first-rate services in the area since 1959.  We are experts at creating cost-effective solutions that take advantage of the most up-to-date techniques.  Our concrete services include commercial, educational, parking and other construction development as well as public works projects.

 

Blended Cements

The American Concrete Institute’s (ACI) Cement and Concrete Terminology, defines blended cement as “hydraulic cement that is produced by intergrinding portland cement clinker with the other materials or by blending portland cement with the other materials or a combination of intergrinding and blending.”  As recently as 2012, there were changes made in blended cements that have been approved by the ASTM for both special applications and general use.   

 

From the ASTM’s C595 abstract on hydraulic blended cements, they describe Type IS as Portland Blast-furnace Slag cement and Type IP as Portland-pozzolan cement.  They state that, “They can also be described according to air-entraining, moderate sulfate resistance, moderate heat of hydration, high sulfate resistance, or low heat of hydration properties.”  Type I (SM) Slag-modified Portland cement has a standard that contains less than 25% slag by mass of the cement.  The ASTM’s C595 permits Type IS Portland Blast-furnace Slag cement to contain between 25 and 70% by mass of the finished cement. 

Blended cements are used to prolong the life span of concrete as it reduces the concrete’s permeability to water.  Whereas concrete mixes made of portland cement are more porous and are susceptible to cracking during freeze/thaw cycles.  It can even be a cause of deterioration in the rebar as moisture can seep in over time.  Another benefit to blended cements is they reduce the water-cement ratio (w/c), which is one way to produce high-strength concrete. 

Blends can be customized by varying the proportions of the mix and be produced to meet the demands of a specific project.   Most importantly the ASTM dictates that, “cement shall undergo different tests to determine the following properties: chemical analysis, fineness by sieving, fineness by air-permeability, autoclave expansion, time of setting, air content of mortar, compressive strength, heat of hydration, normal consistency, specific gravity, water requirement, mortar expansions of blended cement and pozzolan, drying shrinkage, activity index with Portland cement, and sulfate resistance.”

* ACI 116R-00, Cement and Concrete Terminology, American Concrete Institute, Farmington Hills, Michigan, 2000

* http://precast.org/2012/12/new-astm-blended-cement-types/

* http://www.astm.org/Standards/C595.htm

 

When your project demands exceptional services and the best value, Conco can deliver.  We have been offering premium concrete services throughout the Western U.S. since 1959, and have a regional office in the Seattle area.  With state-of-the-art equipment and modern facilities, we work diligently to meet goals and stay within budget.

Exciting Career Opportunities for Carpenters

 

If you are a skilled carpenter looking to make a career move, you may want to consider the Conco Companies.  As one of the premier commercial concrete contractors in the country, we are currently offering great career opportunities for the right candidates.  You will have the chance to work with some of the finest professionals in the field and to be a part of a company that continues to bring a standard of excellence to each job we undertake.  We have locations in Northern California, Southern California, Washington State and Colorado.      

Our carpenters are expected to perform all of the carpentry work on a construction project including, but not limited to: erection, alteration, repair, modification, demolition and addition or improvement of or to a building or any other structure or construction.  These are union positions with the exception of those in Colorado.  Furthermore we pay competitive wages as well as offering employees great medical and retirement benefits.

Conco is an exciting place to work as we continue to be involved in some of the most high-profile projects in the country.  We are finishing up the concrete work for the San Francisco 49ers new Levi Stadium, which is scheduled to open in August, 2014.  In 2013, we successfully assisted on repairs of the Bay Bridge in San Francisco.

Currently we are providing concrete services on the new Wilshire Grand Center in Los Angeles where we recently broke the Guinness World Record for the largest continuous mat pour.  We are proud to report that the 21,200 cubic yard pour was completed without a single injury to a worker. 

We know you will be impressed with our credentials as well as our impeccable reputation.

To apply to a Conco job opening, please refer to our website for contact information.

Conco is a leading supplier of concrete services for the Western United States. We have built our reputation with superior concrete formwork, reinforcing, place and finish, shotcrete and other ancillary services that benefit from our vast experience.  Our concrete services include commercial, educational, parking and other construction development as well as public works projects.

Great Job Opportunities for Backhoe and Crane Operators

The Conco Companies is currently seeking Operating Engineers for backhoes to excavate and load loose materials.  All applicants must have prior experience with both cranes and hoisting equipment.  The Conco Operating Engineers are responsible for operating the hoist and swing equipment that is used to move machinery, materials and other large objects.

We have positions open at all of our locations that include Northern California, Southern California, Washington State and Colorado.  These are union positions (with the exception of Colorado), and Conco offers competitive wages along with medical and retirement benefits.

As one of the leading commercial concrete contractors in the country, Conco is an exciting place to work.  Our extensive portfolio includes providing concrete services on many notable projects throughout the Western United States.  We recently did the extensive concrete work for the new 49ers Levi Stadium and assisted on the repairs for the Bay Bridge in San Francisco.  We are currently working on the Wilshire Grand Center in Los Angeles where we just completed the world’s largest continuous mat pour!     

For anyone who may be starting a career or seeking a new direction, the construction industry has a lot to offer and is currently experiencing an increase in jobs.  The United States Bureau of Statistics reports that employment for construction equipment operators is projected to grow 19% during the period from 2012 to 2022.  The demand for workers is a result of the construction industry rebounding now that the worst of the recession is behind us.  Operators that are trained on multiple types of heavy machinery will have the best opportunities open to them.

If you are interested in a job operating a crane or backhoe, contact your local trade union to find out how to become an apprentice or get information about trade schools in your area.  A heavy equipment training school can teach you everything from operating the equipment to emergency response training, hazardous waste operations, and OSHA regulations.

To apply for a job opening with Conco, please see our website for more information.

Discover the experience of working with one of the top professional commercial concrete contractors in the Western United States.  Conco has been in business since 1959 delivering first-rate concrete services and has all the experience and expertise to handle even the most challenging projects.

What is Building Information Modeling?

According to the National Institute of Building Sciences’ National BIM Standard Committee, Building Information Modeling (BIM) is defined as, “a digital representation of physical and functional characteristics of a facility.   BIM is a shared knowledge resource for information about a facility forming a reliable basis for decisions during its life-cycle; defined as existing from earliest conception to demolition.”*

The concept behind BIM is to have an integrated digital file that is created or gathered on a structure that contains coordinated, reliable information about the project from design and construction through to operations.  While the information may be proprietary, it can be shared with others who are responsible for planning, designing, constructing, and maintaining similar facilities.  BIM in itself is not propriety software but a standardized format used for storing digital files.  Although the term dates back earlier, it was widely introduced into the industry when Autodesk, a company that specializes in 3D design and engineering software, published a white paper on “Building Information Modeling” in 2002.

Wikipedia states that BIM is more than 3D visualization, but also “covers spatial relationships, light analysis, geographic information, and quantities and properties of building components (for example, manufacturers' details).”  As such, BIM is a tool for not only architects but also for engineers and contractors as it helps to connect design, analysis, and documentation together.  In the early design stages, it aids in increasing performance and efficiency of a structure and allows changes to be made that are less costly than further along in the construction process. 

Since creating the BIM on a project starts from the earliest planning stages to completion of the facility, it is a time-consuming job and an expert may be brought in to manage the task.  This person is in charge of coordinating all of the processes and information that takes place throughout construction to fully represent a horizontally integrated building.

1.     http://academics.triton.edu/faculty/fheitzman/NBIMSv1_ConsolidatedBody_11Mar07_4.pdf

2.     http://en.wikipedia.org/wiki/Building_information_modeling

Conco is a leading supplier of concrete services for the Western United States. We have built our reputation in the Los Angeles area with superior concrete formwork, reinforcing, place and finish, shotcrete and other ancillary services that benefit from our vast experience.  Our concrete services include commercial, educational, parking and other construction development as well as public works projects.