Guide to Redi Mix Concrete

Strong communication is a quality that any ready mix supplier and buyer should value when planning out a project. This is because there is no “one size fits all” when it comes to concrete mixes. Creating the perfect mix depends on a multitude of factors generated by the performance requirements of the job, in both the plastic/pliable form, as well as the final and cured structure. Find out why finding the perfect supplier is so important. 

The Mix

Each build that requires a concrete mixture will have different requirements generated from an engineer. These will determine the structural integrity and longevity of the project taking into consideration factors such as setting times, anticipated climate exposure, durability, texture, and strength. All very important, and very different for each project! 

The mix itself is composed of aggregates – the most abundant piece – cement, and water. In addition to the structural aspects, some other ingredients you might consider including in a concrete ready mix are fiber reinforcements (to eliminate the need for rolled welded wire mesh saving material and labor costs), decorative aggregates, or any additional requests submitted by the engineer. Each situation will require specifications tailored to that project. 

Transportation and Pouring

After discussing how the mix is to be prepared, the next steps to consider and discuss are transportation (time and climate) and pouring the mix at the destination. As soon as you have the ingredients in a mix – water, cement, and aggregates – a chemical reaction begins to take place. This process is called hydration and is what forms the end result known as concrete. The time it takes to begin hardening will depend on the mix, so it is essential your concrete supplier has experience and knowledge to ensure a pliable pour that won’t harden before it is finished. 

The Final Product

After the mix recipe is created, the hydration process takes place, and concrete is formed, the final step in the process is curing. Curing also plays a big role in the strength and durability of concrete. “The length of adequate curing time is dependent on the following factors: Mixture proportions, Specified strength, Size and shape of concrete member, Ambient weather conditions, Future exposure conditions.” Zemajtis, Jerzy. Role of Concrete Curing, http://www.cement.org/learn/concrete-technology/concrete-construction/curing-in-construction.

Curing methods include hydrating the concrete, adding special curing compounds, or sealing the surface. Properly cured concrete can add strength, resistance to freezing and thawing, the prevention of crazing (fine cracks), and resistance to abrasion. It’s very important to take the correct steps in the final stage of the concrete pouring process.

In Conclusion

The process of concrete mixing is a very detailed, unique, and critical process for the longevity and durability of a structure. When choosing a concrete mix supplier, experience and education in the process is a big characteristic to take into consideration. Time, money, and safety are all reliant on this decision. 

Guide to Gravel

Are you in the planning phases of building your dream yard? Maybe you’re looking to add some gravel to a driveway or pathway?  Erosion control? 

Whatever gravel needs you have, we can help!

Although gravel can add a unique and decorative touch to create a beautiful design in your yard, it also serves many practical and environmental purposes. Make sure to learn about the type of gravel you plan to implement, as well as the structure of the rock, before you make a purchase. Some gravel provides a rigid and strong platform, while others, of similar shape and color, can be brittle and easily shift with weight. Once you choose the type of rock, then there are many color and size options available to make your yard stand out in the beautiful way you imagined. 

At Duke City Redi Mix, there are typically 2 types of gravel for sale – Round Rock and Crushed Rock. Although they are both appealing with colors and shape, they serve two very different purposes.

Round Rock

Round Rock is also commonly known as pea rock. This type of rock is very smooth with rounded edges making it easy to move and shift around each other with little resistance. Without rigid edges, piles of round rock have many voids that allow them to be shifted in to. This makes round rock great for drainage and roofing applications. 

Round rock is beautiful in yards, but recommended to be places in areas you don’t plan on walking on. 

Crushed Rock

Crushed rock is the opposite of round rock. Crushed rock is composed of tough, rigid edges, that easily interlock with each other when formed into a pile. Because of this interlock, crushed rock creates a tight bond making it hard to shift even as weight is applied. This makes it great for applications such as driveway structures, walkways, erosion control, and base course for road construction. Let’s take a further look into why crushed rock is commonly used for road construction.

Underneath the pavement you are accustomed to seeing in your everyday life, there are many layers of structure hidden from sight. Base course is made up of a mixture of fine dust and crushed rock, and then placed upon the subgrade, or native soil. This provides a foundation for the pavement that is placed on top. The structure it provides eliminates the shifting of material that could potentially be created from the weight and movement of moving vehicles. It is a crucial piece of road construction that most people don’t even know exists!

It’s easy to assume gravel serves as exclusively visual purposes, but now you know there is so much more! Make the most of your dream yard and choose the gravel that suits your practical and designer needs. With so many color options available, the possibilities are endless.

What do concrete and ham have in common?

Last weekend we celebrated Easter Sunday and, much like a lot of you, I enjoyed some excellent ham for the main dish.  As a person who is almost constantly thinking about concrete and how we can make it better I was thinking about what ham and concrete might have in common.  I was reminded of this sign. The best concrete and the best ham both have this in common: they are properly cured.

Concrete is made up of many different ingredients but the key ingredient is cement.  Concrete and cement are NOT the same thing. You wouldn’t pour a cement sidewalk anymore than you would bake a loaf of flour.  

So what is cement and why is it essential? 

Cement is the essential binder that makes concrete a solid thing after the chemical reaction is complete.  For the sake of simplicity (it is actually far more complicated) cement is made by taking limestone and super heating it to remove all of the hydrogen and oxygen from it.  When we, as concrete suppliers, get it in powder form it is starved for those two elements. We then add those two elements back to it in the form of water (H2O), form it into the shape we desire and let it get hard as rock again.  

This chemical reaction is called hydration.  Controlling hydration by adding the correct amount of water throughout the entire reaction process is key to making good concrete.  Adding too much water will dilute the mixture, creating weakness, while not enough water will leave un-hydrated cement which also causes weakness along with a serious lack of economic value.  

Back to ham:

This post is not about water to cement ratios in concrete mixes.  That’s for a different day. This post is about what can be done later in the hydration process which continues for a very long time after this step.  Just like good ham, proper curing is essential to utilizing this chemical reaction to create the best final product.  

When the concrete is placed and finished to the desired shape and look it will reach initial set, meaning it is hard to the touch but does not have any structural integrity.  From this point on the hydration process is continuing but the strength can no longer be impacted by adding water. Proper concrete curing means providing enough water to the placement to keep feeding hydrogen and oxygen to the chemical reaction and make the most of each grain of cement.

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There are many concrete curing methods.  I will cover some of the best I have seen but this is most definitely not an exhaustive list of concrete curing options. 

Internal Curing

All different types of concrete aggregate have some level of absorption.  When proper aggregate handling is happening at the batch plant, the aggregate brings water to the hydration party.  As the cement particles inside the placement are trying to reach equilibrium (think of your high school chemistry class) and use up all of the available hydrogen and oxygen the water is available to the cement from inside the aggregate.  This often takes a long time. If done properly, the compressive strength gain from internal curing will be seen between 7 and 28 days after placement. Although aggregates with high absorption can be difficult to handle for a concrete supplier the benefits from internal curing far outweigh the extra work.

Flooding

Whenever it is possible to setup a dam or a dike and flood the placement this should be done.  This will provide constant water to the placement and allow the cement to pull all of that is necessary to completely hydrate it.  The Hoover Dam was built between 1931 and 1936. Many people believe it is still hydrating. Due to its thickness and constant supply of water from the lake, the cement is still pulling hydrogen and oxygen.  

Fogging

Fogging can happen a little bit earlier in the curing process.  Vaporized water is presented over the top of the placement to combat evaporation from heat and wind.  It allows the right amount of water to enter to surface of concrete without adding to much water to degrade the surface.  

Hoses and Felt Backed Sheets

  This is one of the most effective curing methods I have seen.  A hose with many small holes put on top of the slab then a felt backed vinyl sheet is placed over the entirety of the placement.  The hose provides a slow and even release of water while the sheets prevent it from evaporating from sun and wind.  

Concrete curing is an essential part of any concrete placement.  The picture below (and some good Easter ham) have sparked this blog post.  This sign placed by the city to prevent people from walking or driving on recently placed concrete is actually quite ironic.  There is some truth to it but there is no external curing method being implemented. As a result, when I see this sign I ask “Is it? Is it, REALLY?” I think not.  

Just like your honey cured Easter ham, make sure and properly cure your concrete!

To Our Community

To Our Customers, Partners, and Community,

We want you to know that Duke City Redi-Mix stands strongly behind our city and have always aimed to ensure the highest quality product, exceptional service, and a deep care for our local community. Today, service and community are our top priorities more than ever. 

In times like these, it’s important to come together to link ideas, services, products, and people, creating a structure that cannot be broken. It is our goal to collaborate within the construction community to create a clear path of communication between engineers, advisors, clients, and employees. 

We are doing our best to maintain the economic momentum our city had seen in recent times. We are working harder than ever to ensure project timelines are being continued or shifted in the most appropriate manner to best serve everyone involved. Thank you for extending patience and understanding through changes. 

The term “local” has an abundance of meaning behind it, and we would like to take this time to thank our team and clients for their continued support. Thank you for allowing us to be your local concrete and aggregate company. To serve the city of Albuquerque is a responsibility that we do not take lightly. We are grateful.

If you have any questions, we are happy to answer them as promptly as we can. Thank you, Albuquerque.

Sincerely, 

Duke City Redi-Mix

Benefits of Concrete Applications

Concrete as a construction material provides many opportunities for builds such as availability, affordability, durability, and longevity. It is an option that many companies require and utilize often, as it is a common material found among structures. However, why is concrete such a viable option?

Durability

Concrete is significantly more durable than other building materials and is highly resistant to damage from common weather and natural disasters such as heat, wind, humidity, moisture, and impact. Concrete can also be mixed with specific materials that allow it to be catered to the climate of the environment in which a concrete plant may reside. This also makes it a very safe option because through abrasion and rough actions, concrete is able to maintain its desired engineering properties.

Fire Resistant

The second biggest performance factor that concrete can provide is being fire resistant. Concrete is fireproof and often meets fire codes. “While concrete walls can generally withstand up to four hours of extreme fire pressure, most wood-framed walls would fall in less than an hour.” (https://www.nachi.org/concrete-exterior-walls.htm). This reason alone is why a builder may desire concrete over other materials, especially in locations that wildfires may be of concern.

Cost

The final, and possibly most influential piece of concrete, is cost. With increased workability, labor costs are cut and a better finish is provided. Concrete prices also tend to remain more stable than other building materials such as steel. Concrete can be crushed and used again in future mixes, allowing is to be a very sustainable and green product.

Conclusion

At Duke City Redi Mix we are proud to supply a product that so many people use every day.  Just like any other product, concrete can be done right and it can be done very poorly.  Our raw materials, processes, equipment and (most importantly) people are the best in the business.  Not all concrete suppliers are the same but you can trust that you are getting the best from Duke City.

Our expertise has been used to develop a proprietary product that includes a pre-determined slump, strength, and fiber already included for ease and consistency of ordering as well as a bottom line price without hidden charges. Those factors, along with the combination of a double dose of fiber, replacing welded wire mesh, water reducers to give you the workability you desire for most applications, our high quality aggregates and using the most technological advanced cement in our area gives you, the customer, a concrete you can trust all in one mix.  It is called DC Zia Everlast.  Please visit our website or call 505-877-5777 for more info.

 

Not All Rocks Are The Same

The basic properties of concrete consist of aggregate, cement, water and admixtures.  There are many subcategories in each of those basic ingredients, but that gives an idea of what we use to make concrete.  Aggregates make up most of a yard of concrete, making up about 75% of the volume and when added to a binder, like cement, can (or cannot) make a very durable and strong concrete.  The following is a high level look at how important aggregates are when looking at concrete suppliers.  There are books written on the topic that don’t begin to scratch the surface of aggregates in concrete, but I will attempt to keep it concise and focus on the important information.

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The rock and sand used in concrete is considered aggregate.  Because it consists of such a high percentage of the volume of concrete, it is essential that the quality of those aggregates is very good.  But what makes high quality aggregate?  I won’t bore you with the many tests that rock and sand go through before they are used but here is a list of things we look for in a high quality aggregate:

Cleanliness – rock that is mined from an area that is high in clay or silts often has issues with “cleanliness”. Silt and clay is very bad for concrete because those materials tend to soak up a lot of water making that fluidity unavailable to make the concrete workable.  To maintain workable concrete water is added creating diluted cement, or a low water to cement ratio.  Ultimately concrete strength and durability are impacted.  Another problem with “dirty” aggregates is the bond between the cement mortar and the aggregate.  If this bond is not strong, strength and durability are severely compromised.

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Quality – aggregate quality testing is an important part of any mix design. The abrasiveness, strength, absorption, specific gravity, and many other properties are tested before an aggregate can be used in concrete. Each of these is important and has many effects on the chemistry of the concrete and how easy it is to achieve repeatable and consistent results.

pingpongGradation – aggregate gradation is a term used to quantify the percentage of each size of rock there may be in a stockpile of aggregate. Not all rocks are the same size, and in fact, we don’t want them to be the same size.  A variety of different sizes fit together and a make a stronger matrix than all one size of rock.  As an example, if put ping pong balls into a glass there would be a lot of open spaces in between each ball.  If marbles, BB’s, and shot gun pellets are all added to the glass the air would be displaced much better.  It works the same way in concrete.  Large gaps in the sizes of rock in each mix cause a high demand for cement mortar significantly increasing the cost of the concrete, among other negative effects.

Aggregate quality is one of the most important factors to consider when choosing a concrete supplier.  All other ingredients of concrete are pretty much universal and are readily used by all suppliers.  Aggregate is the main difference between suppliers in any given market.  Aggregate cleanliness, quality and gradation directly impact your ability to place, finish, and cure concrete.  Most importantly, your choice in aggregate (and subsequently the concrete supplier) directly impacts your ability to make the owner happy and your ability to get the next job.

 

 

 

The cost of freight, what’s the big deal?

Duke City Redi-Mix (DCRM) can basically be split into a few categories regarding our freight costs. Here at DCRM we haul sand & gravel products that we deliver to customers and for stocking purposes, we haul admixes and other aggregates to make concrete, and we deliver redi-mix concrete. At first glance these different forms of hauling material may seem like kindred spirits, and to an extent they certainly are, at least in the sense that in each case something is being delivered, but that’s where the similarities start to differ and express themselves in the form of dollar signs. Yes, dollar signs! Something we can all agree is at least marginally important to us if not the only reason we get up in the morning! Personally, I get up for waffles! YUM!! Anyway, in a completely professional sense, let’s discriminate between the various types of delivery/freight options we encounter on a daily basis at DCRM. The following account will aim to discern the differences and similarities between them and how they affect company and consumer costs.

Let us consider the variety of freight options that exist for our company’s purposes. First, we not only manage our own fleet of trucks, but we also hire other companies and independent haulers depending on our needs. Secondly, we have both incoming and outgoing freight. Tertiarily (I’ve always wanted to use this word in a sentence 😊), we use a variety of different single and combination-trucks to accomplish our goals. As you can imagine, all of these delivery/freight variables also influence the bottom line, determining company and consumer costs.

Yes, even a company as awesome as DCRM uses other trucking companies & independent haulers to meet our shipping needs. Truth is, when it comes to our shipping needs, we are certainly capable of being self-reliant, but let’s be honest, sometimes the most efficient way to be effective is to bring together under one management other quality companies and independents needed to turn out our product. We are fortunate at DCRM to have quality help in supplying our facilities with the products we need in order to make and deliver the products you need! Notice how I said YOU NEED our products!!

Incoming & outgoing freight – Just like it sounds, we haul materials in and ship products out. We haul a variety of different materials to our different facilities not only for the production of our redi-mix concrete but also for our sand & gravel sales and distribution. To make redi-mix concrete we must freight in materials like sand, different gravel products, cement, flyash, color pigment, and a variety of different concrete admixtures. With these materials we produce and ship redi-mix concrete to our customers. Our sand & gravel division freights in a large variety of different decorative landscape gravel products as well as gravel products more suited for commercial & industrial use. From our distribution hubs we deliver materials throughout the region and occasionally out of state.

In order to accomplish our goals, we use a variety of different specialized trucks suited for stocking materials and delivering products to customers. In general, these different trucks can be classified as single-unit and combination trucks. The single-unit trucks we use to haul freight are represented by our tandem-axle dump trucks and our concrete mixer trucks. The combination trucks we use to haul freight are our end-dump tractor-trailers, bulker tractor-trailers & flatbed tractor-trailers. Each of these delivery vehicles comes with its own challenges and variables to consider. For example, some trucks are larger than others and take longer to complete a delivery. However, larger trucks can also be more cost effective due to larger quantities of material they can deliver per load compared to a smaller truck. Concrete mixer trucks, bulker tractor-trailers & flatbed tractor-trailers all require significantly longer delivery times due to the nature of off-loading these vehicles, and as we know, time is money. Larger trucks can also have higher maintenance costs due to their size and mixer trucks due to their number of moving parts.

Higher maintenance costs and extended delivery times mean higher overall company costs & consumer costs per delivery. For example, our concrete mixer trucks are our highest maintenance cost vehicles and take an average of about 2 hours per delivery, and our tandem dump trucks are our lowest maintenance cost vehicle and take an average of 1 hour per delivery. As you can probably see, there’s a significant difference in company freight cost that gets passed on to the consumer per delivery depending on which method of delivery is required. Here’s a “short list” of some other things to consider regarding company freight costs; truck payment, ongoing maintenance (tires, oil, part replacement), badging-labeling-paint, truck washing, fuel, drivers wages, insurance (also workman’s comp insurance), registration, shop-mechanic expenses, oversight, etc. Throw a little profit on top of that & you have your consumer cost.

So, what are our company freight costs for our different trucking units? Well, that’s proprietary, so I’m not going to tell you! What I can tell you is the consumer costs per type of delivery truck, which are also a reflection of our overall company cost per vehicle (including company profits). We deliver products to customers with three different types of trucks or truck & trailer combinations and we are competitive across the board with standard regional trucking rates. First, our concrete mixer truck consumer cost can be as high as $120 per hour truck time. Our tandem axle dump truck, in which we use to do the majority of our residential sand & gravel deliveries, consumer costs are based on a $75 per hour round trip calculation. Our tractor-trailer (18-Wheeler) deliveries are based on a $90 – $100 per hour round trip calculation for local deliveries. Our over the road consumer costs for tractor trailer are calculated at $3.50 to $4.00 per loaded ton mile (based on a full load of at least 23 tons). Our tractor-trailers costs can vary depending on what they are hauling.

This is just a snapshot of our freight costs and options at DCRM. There are obviously several more contributing factors and minor details to be accounted for in calculating overall costs. For a more holistic description on the subject of freight costs, options and profitability, I can be contacted at DCRM main office (My consultation fee is $50 an hour 😊). Seriously though, In a culture that has come to expect free delivery and government hand-outs, and has forgotten that very few things in life are truly fee, I’ve had to explain the very fine details of our freight costs more times than I can remember and I don’t mind breaking it down once again for the curious inquirer.

 

Josh Barela

The DCRM Sand & Gravel Guy

Hot and Windy?

The Hot, dry, windy weather of our area this time of year is not the most favorable environment to place concrete. It causes rapid drying and shrinkage of the surface causing early surface cracking, crazing and a false set that induces more substantial cracks. This is all caused by the surface drying out sooner, sometimes much sooner than the rest of the slab. So, what is done to help this unfavorable situation? Here are a few suggestions:

    • • Do not over wet the subgrade. If we are attempting to even out the evaporation of the entire depth of the slab, it doesn’t make sense to give the bottom more water while the surface is fighting to keep its water. Give the free water a chance to escape down ward as well as up. If it is a dry hot day, you may not want to apply any water to that subgrade.
  • • Consider adding micro fiber to your order. Each dose per yard include over a million polypropylene fibers that are distributed throughout the mix. These fibers act as a micro crack inhibitor. Surface cracking can be drastically reduced by adding fiber to your concrete.
  • • Don’t add excess water to the mix. This only exasperates the problem. That water will eventually leave the concrete and what is left behind are voids that enable shrinkage thus cracking to happen. Have you ever seen the bottom of a dried up lake bed? As the water leaves the surface It has cracked and shrinks making these squares that are all curled up. This is a drastic picture of the same thing that happens to the surface of the concrete. The slump, or wetness or workability of the concrete, is increased with water and is very tempting if you have ever had to work low slump or stiff concrete, but this can also be achieved by using mid and high range water reducers. These water reducers have become the best friends of the professionals. They get the workability without jeopardizing the integrity of the mix and increasing shrinkage and cracking. Every gallon of water added to a yard of concrete over the design amount reduces the compressive strength by up to 500 psi. Ask a Duke City representative to guide you on which water reducer is best for your application.

 

  • • That free water must be able to leave the entire depth of the slab, therefore do not seal the surface too early, i.e., put a steel trowel on it. Air entrained mixes, used for exterior concrete, will blister even more if the surface is closed too early. Use a fiber or magnesium float to flatten the surface. This rough application leaves the surface open to allow free water to escape. Sealing too early causes blistering and scaling by trapping water under the surface. I have seen it to many times, people want to get to work and start troweling way too early. Patience is the best approach. This sealing also produces a false set on the surface (the surface is hard but the concrete under that surface is still not at final set). The concrete seems spongy and will eventually shrink enough to pull the immovable surface apart. Most exterior concrete requires a broom finish; this will also keep open the surface for escape of that free moisture.

 

  • • Curing the concrete after the finish is so important. After the FREEwater is gone and we have final set, we must allow the water intended for the cement particles to stick around. That surface is shrinking and cracking because it is starving for water. Any way you can provide that water is a plus. A membrane curing compound is a start. It seals the surface and doesn’t allow moisture to leave the concrete. The cement will use that water to hydrate, but this membrane can be torn or rubbed off by foot traffic, making it ineffective. The best way to cure is to keep water on the surface by either diking and flooding or using a felt backed vapor barrier on the surface. This felt backing provides a reservoir of water for that surface. Another approach is applying water to the concrete for the first 4 to 7 days will help.
  • Plan your placement for the most favorable conditions. Early cool mornings are much better than HOT windy afternoons.

Duke City Redi-Mix has developed and continue to refine our mixes to help meet our southwest sometimes harsh environment, but there are safeguards that must be used to place that concrete to complete the job of the concrete.

We have used our many years of combined experience in this Valley to develop a line of mixes that are designed for the hot, dry summers and the cold freezing winters. We call them DC Zia Everlast.

DC Zia 1 Final (004)

The combination of a double dose of fiber, replacing welded wire mesh, water reducers to give you the workability you desire for flat surfaces, our high quality aggregates and using the most technological advanced cement in our area gives you the customer a concrete you can trust all in one mix. Ask for more information when you call.

 

Concrete Shrinkage…the struggle is real

caveman-fireThe Problem

Once upon a time, on a cold night, a caveman built a nice hot fire on a solid piece of limestone and fell asleep next to it.  Many hours later, after his fire had burned out and had cooled, he woke to rain drops on his face.  He noticed on the edge of his fire there was a suspicious powder that he had never seen before.  As the rain drops fell into this powder it began to steam and then quickly got hard as rock.  The rain drops continued and eventually all that was once powder was now solid.  Several hours later he came back to the fire and all of the hardened powder had cracked.  As he picked up a piece of the cracked concrete to investigate, his wife walked up and said, “Shrinkage sucks, doesn’t it?”

Ever since the invention of using super-heated limestone powder (cement) Slab-curling-1-e1546988014973mixed with water and other ingredients to make concrete, shrinkage has been an issue.  Regardless of what you do to it, the fact remains that concrete shrinks.  Depending on the other ingredients you use along with cement and water, concrete shrinkage can be reduced to a negligible amount resulting in reduced cracking and curling.

During the chemical reaction between cement and water, called hydration, cement particles are drawn to each other creating a very strong bond.  As that process continues through initial set and the available water gets reduced, the now hard concrete continues to shrink causing stress and tension within itself.  That tension ultimately results in cracking, curling, or other unwanted results.

How do we know how much our concrete is going to shrink? I’m glad you asked.  ASTM C157 is the standard test method for measuring concrete shrinkage.  In this test concrete mortar bars are cast, measured and placed in a water solution for 28 days.  At the end of this period the bars are then measured again for a change in their length.  The total maximum length change depends upon the specifications for a specific job but anything around 0.05% length change is good.

The Solutions – In Design

                There are two different schools of thought on how to reduce shrinkage.  The first and oldest is to increase the cement content in order to increase the strength of the hardened paste in the concrete.  This theory is based on the idea that if you make the concrete strong enough quickly enough it won’t shrink because it is too hard to shrink.  Many engineers specify high psi, high cement content concrete in order to reduce shrinkage cracking.

The other theory, and the one Duke City subscribes to, is to reduce the amount of cement in the mix to reduce the mechanism that causes shrinkage in the first place.  This reduces the strength and concrete, of course, and you open yourself up to greater risk of not meeting other specification requirements, such as compressive strength.  This method of shrinkage reduction requires greater control and consistency in aggregate gradation and proportions in the mix.  Gap grading, especially with lower cement content, will be detrimental to reducing shrinkage and compressive strength.

The other concrete design element to reducing shrinkage is admixtures.  There are many products on the market that reduce shrinkage, and even some claim to eliminate it.  SRA (Shrinkage Reducing Admixture) is a product that we have on hand which has proven to be very helpful in meeting some of the tighter specifications.  There are different dosages depending on what you are trying to achieve.  Additionally, there are products out there that will reduce shrinkage enough to extended control joint spacing by multiples of three or four.

The Solutions – In Practice

Curling is a problem most flatwork concrete contractors deal with on a daily basis.  This occurs when a slab shrinks at a different rate on the top than it does on the bottom.  The top layer of the concrete typically dries and shrinks faster than the bottom causing it to pull the top edges together creating a bowl effect.  If you have ever driven down a concrete highway and it seems like you are hitting a small speed bump every 10 feet, you have experienced concrete curling.  The solution for this type of shrinkage is even and consistent concrete curing; ie: keeping the moisture content of the entire placement even and consistent.  (Stay tuned for future blog post on curing)

Increasing the water content in the concrete can cause more shrinkage.  When that additional water evaporates, the hydrating cement tries to fill that void and as it does it causes shrinkage, not to mention reduced compressive strength.  Instead of using water to increase slump and workability, use a water reducing admixture.  These don’t add water to the mix and still give you the workability you are looking for.

Fibers can significantly reduce cracking caused by concrete shrinkage in the first 24 to 48 hours, and beyond.  Adding fiber to the concrete creates a binder that holds everything together and while shrinkage is at its worst will significantly reduce early age cracks.  Duke City offers fiber and it can be used in any concrete mix to help further combat the effects of shrinkage.

Conclusion

All concrete shrinks.  Some mixes shrink a lot, while others don’t.  Duke City mixbezfugovi-podove2 designs are all maximized to meet a very broad range of specifications while at the same time keeping shrinkage to a minimum, even when it isn’t part of specific spec.  Our raw material processing and inspections are constantly being evaluated and updated to ensure we provide the highest quality product.  Much can also be done on the jobsite to combat the effects of concrete shrinkage.  If you have further questions regarding our process or what you can do on the jobsite, please give us a call at 505-877-5777.

 

Miles Shiver IV

General Manager

Duke City Redi-Mix

Cold Weather Concrete

by Miles Shiver IV, General Manager

New Mexico’s mountain climate affords us the great benefit of seeing four distinct seasons.  We often see temperatures over one hundred degrees in the summer as well as freezing cold in the winter.  These seasonal swings require concrete producers and contractors to stay on their toes in regards to concrete temperature, set times, travel times and a variety of other issues caused by cold or heat.

Ideal concrete temperature is a figment of the imagination.  Don’t let anyone tell you otherwise.  Every type of placement has an ideal situation and that situation rarely seems to repeat itself.  Regardless of the season, our target concrete temperature is about 65 degrees as this applies well to most applications.  In the summer this poses quite an issue with raw materials, concrete plants and mixer trucks all being super heated by the desert sun.  This problem is exasperated by the nature of the cement/water chemical reaction.  This scientific wonder is exothermic (releases heat) through its natural process which in turn speeds up the chemical reaction in the adjoining particles and before you know it you have a “hot load” that is turning into rock before your eyes.  For this reason we use ice in our water to get the concrete temperature down to a manageable level.  More on this in the spring.

In the winter, where morning temperatures are well below freezing almost every day, we have the opposite problem.  To achieve 65 degree concrete in the winter we have to use every bit of heat we can get to keep the concrete from falling asleep on us.  Concrete will “go to sleep” around 50 degrees and does not “wake up” again until there is some external heat source to kick off the chemical reaction again.  The most common way we do this is to heat the mix water to a level that will keep the heat building by using science to our advantage.  Friction from the mixing process is our friend in the winter because this adds heat as well.  You may see a truck driving down the road with the drum spinning hard to add heat to the load on a cold morning.  This is a good thing.

Everything I have said so far is the producer’s problem, but fear not, we have been doing this long enough to know what it takes to do our job well.  All of these changes make the contractor’s job a little more difficult as well.  Most importantly, any placement needs proper planning.  Keeping an eye on ambient temperatures is a wise practice, to say the least.  Pouring before the sun comes up is common in the summer but in the winter it is wise to allow the heat of sun to help your placement get the heat it needs.  When your truck arrives on the job site you should see steam coming out of the hopper, water tank and water hoses (depending on ambient temperature).  This will give you confidence that your concrete is being delivered in the best possible condition.

When you begin to unload and place the concrete keep in mind that it could be hot to the touch.  Place and level the concrete as you would in the summer, but in the winter you get the added benefit of a slightly slower set time.  Your steel trowel finish probably won’t start until a little later than you are used to.  Pay special attention to the areas that will not get any direct sunlight because these will setup later.  If your entire placement is in the shade ask about using accelerators when placing your order.  Remember that heat is your friend in winter and is required to keep the chemical reaction going.

Once your placement is down and finished you will need to keep it from freezing.  Concrete blankets are a necessity if the temperature will drop below freezing for the 5 to 7 days following the pour.  The concrete will be generating enough heat on its own, your  goal is to keep the heat from leaving the scene.  Make sure your blankets are secure and cannot be blown off by one of those pesky winter breezes.  It is also a good idea to take the blankets off during the day to allow the sun to continue heating the concrete and then returning the blankets for the cold overnight temperatures.

As with anything, there is much more to the cold weather concrete, but I hope this information helps you start thinking about your upcoming projects.  When placing your order please ask our knowledgeable dispatchers for more information.  We are here to partner with you in providing the best quality project whether it is big, small or anything in between because Quality Matters Here.