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.

Methods to Measure Sustainability

The growing demand for green buildings has been invigorated by green building programs such as the LEED rating system.  With new construction projects, owners and builders alike are looking for materials that offer reduced operating costs, improved health benefits, and perform better environmentally throughout the life-cycle of the products. 

Although this can result in some confusion in assessing the impact of such materials on all of these aspects, there are many good resources being developed to assist in making informed decisions.  Experts working in the field of sustainable design are improving the methods that are used to more accurately measure the value of the return on the green investment. 

One of these tools is called a Life-Cycle Assessment (LCA) and is used in determining the actual performance of green building materials.  This cradle-to-grave assessment starts with a product evaluated by the impact of extracting the raw material, to the manufacturing process and distribution, to the durability and performance.  Additionally the ease of reusing or recycling the material is judged and taken into account. 

When considered early in the planning and design stages, sustainable criteria offer the greatest cost-efficiency as well as the best ability to influence design. Therefore, it is important that architects and owners have the most up-to-date information available right from the start of a project. 

Green products are typically made from reused or recycled-content or from renewable resources.  The EPA says the goal should be to promote clean air and reduce the emissions of VOC (volatile organic compounds) and formaldehyde.  They should not contain CFCs, HCFCs or other toxic substances and need to promote practices that conserve energy and water.  Furthermore they should offer good durability and ease of maintenance, and aid in reducing a building’s overall operating cost.   

The EPA concludes that green buildings should be designed to reduce the environmental impact on human health and the natural environment by:

·         “Efficiently using energy, water, and other resources

·         Protecting occupant health and improving employee productivity

·         Reducing waste, pollution and environment degradation”

 

 

 

 

Conco is a leading supplier of concrete services for the Western United States.  As one of the leading commercial concrete contractors in the San Jose area, we have been involved with large-scale projects in the area including the Mineta Airport.  Our concrete services include commercial, educational, parking and other construction development as well as public works projects.

 

 

 

Tallest Buildings in the World

Now that Conco is doing the concrete work for the new Wilshire Grand Center that will reshape the Los Angeles skyline as the tallest building west of the Mississippi, we got curious about other “tall building.”  Throughout the world, proposed or newly constructed projects are pushing the envelope as skyscrapers get taller and taller.  We will do a series of ten blog posts describing some of that construction. 

The Burj Khalifa, located in Dubai, United Arab Emirates, shatters several records for not only the tallest building (by a significant amount), but also the tallest man-made structure and free-standing structure.  Previously the KVLY-TV mast in Blanchard, North Dakota had been the tallest structure, and the CN Tower in Toronto, the tallest free-standing structure. 

According to the Council on Tall Buildings and Urban Habitat, which ranks the tallest buildings in the world, measurements are “from the level of the lowest, significant, open-air, pedestrian entrance to the architectural top of the building. This includes spires, but does not include antennae, signage, flagpoles or other functional-technical equipment.”  The Burj Khalifa stands at 2,716.5 ft. tall with a record 160 stories and the highest outdoor observation deck in the world.  It also wins in the category of highest continuously occupied floor and an elevator with the longest travel distance in the world. 

Construction began on the Burj Khalifa in September, 2004 and the building was formally opened in January, 2010.  The architecture and engineer for the project was Skidmore, Owings and Merrill of Chicago.  Adrian Smith was the chief architect.  The chief structural engineer was Bill Baker and the primary contractor was Samsung C&T of South Korea.

Conco is a leading supplier of concrete services for the Western United States and is once again involved in doing concrete work for a major landmark project. The new Wilshire Grand Center will reshape the Los Angeles skyline as the tallest building west of the Mississippi.  As a part of the project, Conco successfully poured the largest continuous mat foundation ever done in the U.S. on February 15, 2014.

 

Skyscrapers Keep Pushing the Limits

Early this year, Conco will provide concrete services for a record breaking skyscraper being built in downtown L.A.  Not only will Conco execute the largest continuous mat pour ever done over the course of three days, but the new Wilshire Grand Center will become the tallest building in the U.S. west of the Mississippi.   

According to a global forecast from the Council of Tall Buildings and Urban Habitat  (CTBUH), 2013 was a robust year for new skyscrapers and 2014 is looking to be even better.  In 2013 the One World Trade Center, after much controversy, was designated as the tallest building in the U.S. at 1,776 feet.  The new tower is scheduled to open in 2014 and has replaced the Willis Tower in Chicago as the tallest building in the U.S. 

The CTBUH’s job is to document the progress and trends of skyscrapers built throughout the world.  Their publication estimates that 75 skyscrapers at least 650 ft. tall or higher were completed in 2013, and that there will be approximately 100 buildings at least that height or taller that will be finished by the end of 2014. 

The editor of CTBUH, Daniel Safarik, cites their reasons for the increased activity in skyscrapers as, “The advancement in high-strength concrete technology, combined with land scarcity and plenty of billionaires willing to spend a great deal of money for single-or double floor unity high in the sky has made this a new reality.”

 

One of the newer trends in skyscrapers is to build extremely narrow, very tall structures that carry a small footprint.  Several examples are currently being built in New York City.  Another new construction trend is prefabrication, which is helping to speed up the building process.  Furthermore there is a concerted effort to develop fully sustainable skyscrapers referred to as zero net energy buildings that have zero net consumption and zero carbon emissions annually. 

Mumbai and China will be the strongest markets for new skyscrapers in 2014 with continued growth in skyscraper construction throughout the Middle East.  

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 and highways.

Great Progress Achieved Thus Far on the 49ers Levi Stadium

The new San Francisco 49ers Levi Stadium construction project is progressing at the fast pace needed in order to meet the approaching August 2014 opening date.  When the Levi was awarded the 2016 NFL Super Bowl game, it required the stadium to be operational for at least two years before the big game.  Hosting the Super Bowl is an honor for the stadium and will be the celebration of the 50th anniversary of the championship game.

Conco was awarded the concrete package for the $1.2 billion project that broke ground in April of 2012.  The new stadium is being built by the construction/design team of Turner Construction and Devon Construction.  Due to the aggressive timeline, construction is being handled differently.  In most cases, stadiums are built like a wedding cake with one tier on top of another.  For the new Levi, the stadium was divided into four sections and each section is concurrently being constructed as separate projects. 

Conco had to jump right into the project with the enormous task of excavating 60,000 yards of dirt, reinforce and pump 18,000 cubic yards of concrete, backfill and build a half-mile of perimeter wall – all in 40 calendar days.  According to an article in the Concrete Pumping, Fall 2013 publication, the design specifications “made the level of difficulty extremely high for what is the largest structural steel building under construction in the U.S.”  To demonstrate how large the project is, the foundation took twenty-three semi truckloads of anchor bolts to complete.  Crews worked basically day and night starting at 5 a.m. and finished up at 1 a.m. 

Concrete pumping was constant throughout the construction of the foundation and was skillfully accomplished with the help of Conco’s recently acquired, Schwing S 58 SX boom trucks, equipped with a 188-foot reach and easy set-up.  The Schwing KVM 52 and S 47 SX were also utilized.  The foundation portion was done as a series of smaller pours rather than a large slab pour.  On a typically day, 500 yards of concrete were pumped with most of the concrete a standard 4,000 psi mix, and 8,000 psi at shear lugs.  

The Schwing 58 meter boom trucks were a valuable asset on the project.   Pumps were located both inside and outside the structure and the narrow-front spread and curved outriggers provided the crew the ability to shoot around any obstacles that were in the way.  There were times that a pump ran almost 24 hours a day with one operator quickly replacing the prior operator to start pumping again.  Conco successfully met the Phase 1 foundation deadline in July 2012.  The whole team pulled together for the difficult project and has done an excellent job!       

Conco is a leading supplier of concrete services for the Western United States. We got our start in the San Francisco area in 1959 by offering clients the best value on a wide range of concrete services and products.  Since that time, we have steadily taken a leading role as one of the Bay Area’s top concrete contractors. Our concrete services include commercial, educational, parking and other construction development as well as public works projects and highways.

Berkeley Bowl West Filled a Need in the Community

 

 

Conco has provided concrete services for an array of construction projects including that of helping to build a large grocery/produce store in Berkeley, CA.  The $30 million Berkeley Bowl West was built as a sister store following the popularity of the original Berkeley Bowl.  The new store is more spacious than the original one and at 140,000 sq. ft. became the largest grocery operation in the East Bay Area. 

The Berkeley Bowl West was built with both surface and underground parking, and provides a total of 211 parking spots that is twice the amount of parking at the original store.  The careful attention to parking was to ensure that the crowded parking situation at the Berkeley Bowl would not occur at the new store.   

The contractor for the project was Oliver & Company and the architect was Kava Massih Architects.  The engineer was Rutherford & Chekene and Conco provided the concrete work. The building was constructed with many sustainable features and was equipped by the Sun Light & Power company with 636 solar panels that powers much of the building’s operations.    

The interior of the Berkeley Bowl West is designed to have wider aisles to make for easier navigation than the original store and the produce and bulk food sections are both larger to meet the demand.  The store does not just offer groceries but has a wide range of prepared foods as well as a large café and kitchen area.  The building is spacious enough to house the offices for both of the stores, a warehouse, and a 3,000 sq. ft. community room.  A portion of the produce and meats sold are organic and much of it comes from local farms.

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 San Jose area.  With state-of-the-art equipment and modern facilities, we work diligently to meet goals and stay within budget.

 

Why Concrete Cracks

It is a fact that concrete is the best and most widely used building material.  It is far more durable that wood or asphalt and generally requires a lot less maintenance during its lifespan than other types of material.  While we understand the hardness and durability of concrete, we are also aware that it has the tendency to crack. 

Concrete contractors know this and work to mitigate many of the controllable conditions that create the cracks.  They use preventative methods when laying the concrete that can reduce and control the amount and type of cracks.  According to the Portland Cement Association, the most common type of cracks is caused by tensile stresses that develop with the slight change of volume as concrete dries.  This is the reasons that joints are placed in concrete pavement which allows for the shrinkage in a controlled manner, thus reducing cracking.

In most cases, cracks are not structurally damaging and are to be expected in foundations and slabs.  The three basic reasons for cracking are thermal temperatures difference, plastic cracking, and shrinkage cracking.

When cracks occur before the hardening is complete, it is most likely a result of settling or shrinkage in the volume.  The Concrete Foundation Association says that shrinkage of the surface (plastic-shrinkage cracks) is caused by loss of water while the concrete is still plastic.   The types of cracks that occur after the hardening process is completed are generally the result of drying shrinkage, thermal contraction, or subgrade settlement. 

Experienced concrete contractors know all of this and much more.  They understand the proper mix and type of concrete to help eliminate cracking and to prepare the concrete to withstand all the weather conditions it will face over the years.  

Conco is a leading supplier of concrete services for the Western United States. We got our start in the Bay Area in 1959 by offering clients the best value on a wide range of concrete services and products.  Since that time, we have steadily taken a leading role as one of the Bay Area’s top concrete contractors.

The Earliest Cranes

A crane is a type of machine that creates a mechanical advantage for moving heavy objects that are beyond the normal capacity of a man.  It can lift and lower objects and move them horizontally to transports heavy materials from one place to another using a hoist or winder, wire chains, and sheaves (wheels or rollers).  A crane is an essential piece of equipment in the construction industry, the transportation industry, and the manufacturing industry.

The invention of the crane has had important implications on the advancement of our civilization.  It is thought that the most powerful hand crane in history could multiply the force it imparted to the operator by 632 times.  When looking at what the Egyptians accomplished with the building of the Pyramid, 50 men were utilized to move a 2.5 ton stone block, which translates to about 50 kg per person.  During the Roman Empire, a Roman crane increased the lifting capacity of a person to about 3,000 kg or 60% higher. 

Historians believe that the Ancient Greeks were the first to invent cranes in the late 600 BC and were powered by men or animals.  Archeologists have discovered distinctive cutting marks on stones from early Greek temples that prove they were lifted by cranes.  These early cranes were made of wood and enabled the Greeks to construct taller buildings. 

The Romans further advanced the development of the crane that allowed for larger and heavier weights to be lifted.  They invented cranes that were powered by groups of men or animals and used extensive pulley and rope systems.  The Roman Polyspastos crane consisted of 3 ropes and 5 pulleys which were worked by 4 men to move 3,000 kg.  

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 and highways.

The Early History of Concrete

The earliest evidence of concrete is from a naturally occurring event caused by a spontaneous combustion between limestone and oil shale in Israel more than 12 million years ago.  These deposits still exist today.  More recently and manmade, concrete was first used by the Egyptians around 3,000 BC.  They mixed mud with straw to bind dried bricks and later used lime and gypsum mortar for the binding agent when building the Pyramids. 

Through time, concrete was used to build many of the ancient structures of which some still exist today.  The Greeks first used lime mortars around 800 BC, which were much harder than what the Romans would later use.  There is even evidence that the Assyrians used a fully waterproof concrete to construct the Jerwan Aqueduct around 700 BC. 

Modern concrete is derived from the developments made during the Roman Empire.  The Romans widely used concrete from 300 BC - 476 AD, which made possible their revolutionary architectural designs that were structurally very complex and dimensional.  Some of the cement used was from a slaked lime found in volcanic ash called pozzuolana.

The Romans used the concrete to create buildings with arches, vaults and domes.  The most famous examples of building projects include the Roman baths, the Coliseum, the Pantheon in Rome, and the Pont du Gard aqueduct in southern France.  After the fall of the Roman Empire, the art of concrete was lost for many hundreds of years. 

An interesting fact is that modern tests have proven that Roman cement (opus caementicium) had as much compressive strength as modern Portland-cement concrete.  Of course, its tensile strength was much lower without the reinforcement of today’s concrete.

Conco is a leading supplier of concrete services for the Western United States. We got our start in the Bay Area  in 1959 by offering clients the best value on a wide range of concrete services and products.  Since that time, we have steadily taken a leading role as one of the Bay Area’s top concrete contractors. Our concrete services include commercial, educational, parking and other construction development as well as public works projects and highways.