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.