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.


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.


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.


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.


Managing Concrete Test Data

Thanks to Kenneth for this great post!

Get better results by taking care of technicians and cylinders.


Kenneth C. Hover


On some jobs, concrete test cylinders “don’t get no respect.”

Sometimes we forget to let the testing company know we are pouring. Sometimes between the truck chute, the pump, and stacks of materials there isn’t much space for the testing technician to work. Sometimes the technician’s work area isn’t level, and sometimes it is just plain unsafe. Sometimes the technician has to carry the freshly cast cylinders a good distance to store them for the first 24 hours onsite, and we rarely use a curing box to physically protect the cylinders and maintain the specified air temperature. We argue that the test cylinders are not a fair representation of the actual concrete in the structure: Differences can include water and air adjustments, consolidation, concrete and air temperature, and moisture control. And we like to complain that the cylinders are not handled gently enough between the site and the lab.

Although many of these concerns are justifiable, and while properly making and handling cylinders is not easy, we have to face the facts: By the time the cylinders are broken, the dust settles, and the results printed and distributed to the owner, designers, contractor, and concrete producer, those test results generally are considered to legally indicate the actual strength of the concrete. Test results are the stand-ins for actual concrete performance, especially in the early stages of the project before the concrete in place has had an opportunity to speak for itself. It is during these early days in the life of the concrete test results are accepted as the primary indicators of concrete quality, and in these same early days, the owner is making the decision to pay or not pay, or to release or withhold the retainer. When the cash flow stops, the test cylinders suddenly get a lot of respect and attention!

Credit: Portland Cement Association

The safer and more comfortable the technician, the more he or she can concentrate on your tests.


Credit: Portland Cement Association

The safer and more comfortable the technician, the more he or she can concentrate on your tests.

The importance of tests

As an analogy, consider the task of hiring someone fresh out of school or a training program. If that job candidate has no actual experience to date, you might place a lot of importance on things like grades or standardized test scores (even though we might question the relationship between test scores and real-world, productive capability). When evaluating a job applicant with several years of relevant professional experience, however, their actual performance counts far more than standardized test scores.

Similarly with concrete, until the structure has been in service for awhile and experienced a few seasons of freezing or thawing, or been loaded to a significant fraction of its design load, the actual long-term performance of the concrete is unknown—we need test scores such as air content, unit weight, and compressive strength to give us confidence. Once that same structure has performed its desired function, carried its intended load, and survived its expected environment for awhile, those test scores are no longer looked upon as the primary evidence of acceptability.

10 Things on Testing to Cover in the Prepour Conference

– Why certified technicians are necessary

– The routine for communicating placing schedules, start times, frequency, and numbers of tests required, as well as who gets the notifications

– The desired ages of concrete when tested

– The need for testing to meet contractor needs: Do you need early-age tests to provide information for removing forms or shores, or for stressing PT tendons, or for applying construction loads?

– The locations for sampling concrete and conducting tests (Note: Samples taken from other than the chute of the ready-mix truck are nonstandard, and a method and protocol for obtaining such nonstandard samples needs to be defined in detail to limit variability.)

– The requirements for the level and safe testing locations

– Cleanup requirements

– The location of the curing boxes and who is providing the curing boxes

– How to get access for transporting cylinders back to the lab

– The procedures and contact info for disseminating test results

Of course, we can always fall back on the ACI 318 Building Code provision that core tests can be authorized “if the likelihood of low-strength concrete is confirmed,” and this often gets us out of trouble, especially given the acceptance criteria that “the average of three cores is equal to at least 85% of f´c and if no single core is less than 75% of f´c.” But even if the cores turn out OK, how much time and money was lost in the process? It costs a lot more to extract a core than to make a cylinder—wet-coring is messy, and all of this takes time to organize, drill, cure, test, report, and then wait for the green light to get back on schedule. Wouldn’t it pay to encourage reliable compression tests in the first place?

Accounting for the technician

Let’s start by planning a convenient work zone for the test technician. Everybody wins when it’s easier for that person to properly sample concrete, perform air and slump tests, make cylinders, and store them in a safe, temperature-controlled environment. Uneven, unlevel surfaces are miserable for slump and air tests, and bad conditions usually increase the apparent slump. The same bad ground makes it harder to consolidate a cylinder. If certification programs wanted to test applicants under real-life conditions, they would place the candidate between the truck and pump, on wet rocky ground, balancing on a nasty piece of plywood on the edge of the excavation.

Of course the actual conduct of the tests is in the hands of the testing technician, which is why most of our standard specifications require certification. Top-grade testing companies do a great job of making sure their people are well trained and certified, but it does not hurt the contractor to reinforce this need and even to verify certification.

Credit: Portland Cement Association

According to the commonly required ASTM C172, Standard Practice for Sampling Fresh Concrete, testing starts by collecting the sample from the truck chute.


Credit: Portland Cement Association

According to the commonly required ASTM C172, Standard Practice for Sampling Fresh Concrete, testing starts by collecting the sample from the truck chute.

With so much going on during a major concrete placing operation, it is easy to overlook the post-pour fate of concrete test cylinders, which are to remain onsite for up to 48 hours after casting, and then transported to the testing lab. If those cylinders were made near the point of concrete discharge, it is likely they will be in the way after the trucks and pump depart. So they often are moved to temporary storage, and if this happens a few hours after casting, the concrete might be at its most fragile. It is difficult to lift and transport a concrete cylinder to minimize damage. Plastic caps are not only handy for reducing drying of the top surface, but they also reinforce the mold so it is stays circular during handling.

But even when the cylinders are gently moved to a safer place, ASTM C31-09, “Standard Test Method for Making and Curing Concrete Test Specimens in the Field,” requires “Immediately after molding and finishing, the specimens shall be stored for a period up to 48 hours in a temperature range from 60° F and 80° F [16° C and 27° C] and in an environment preventing moisture loss from the specimens. For concrete mixes with a specified strength of 6000 psi [40 MPa] or greater, the initial curing temperature shall be between 68° F and 78° F [20° C and 26° C].” Although there are a few jobsites in which the air temperature will not dip below 60° F nor rise above 80° F for a couple of days after casting (Waikiki in January comes to mind, but a field trip is required for verification), such limited temperature swings cannot generally be relied upon. Curing boxes therefore are required most of the time, winter and summer.

Credit: Portland Cement Association

A curing box equipped with a heater and thermostat is useful whenever ambient air temperature will drop below the minimum temperature specified in ASTM C31, (68° F for fc ¥ 6000 psi, 60° F otherwise).


Credit: Portland Cement Association

A curing box equipped with a heater and thermostat is useful whenever ambient air temperature will drop below the minimum temperature specified in ASTM C31, (68° F for fc ¥ 6000 psi, 60° F otherwise).

Using curing boxes

For some reason, few in the design-build-supply-test chain like to provide or take responsibility for curing boxes. Part of the problem might be confusion between standard-cured and field-cured cylinders. We have been talking about initial standard curing conditions in the field which are followed by standard lab-curing conditions until the specimen is tested. The purpose of standard curing is to control thermal and moisture conditions and to isolate the inherent properties of the concrete as-delivered from the variable conditions of the jobsite. Sometimes we want to get a handle on the effects of the actual site environment on concrete properties, so we intentionally expose test cylinders to field conditions. However, this doesn’t always work as well as it seems like it ought to, owing to the fact that test cylinders heat up and cool down toward ambient air temperature far faster than the actual structure they are supposed to represent. Cylinder specimens are far more likely to cook in the summer and freeze in the winter than the structure being tested. So unless field-cured cylinders are specified or required by the contractor, the initial curing temperature limits pertain, and a cure box is the best way to stay in compliance.

Credit: Kenneth C. Hover

With this highly effective curing box used by the Washington DOT, the cooler is partially filled with water to increase thermal mass and slow temperature changes.


Credit: Kenneth C. Hover

With this highly effective curing box used by the Washington DOT, the cooler is partially filled with water to increase thermal mass and slow temperature changes.

Another point of confusion is the weird effect of temperature on concrete strength gain. Although it is true that higher concrete temperature accelerates hydration of cement, faster hydration can lead to poorer quality of hydration products and a reduction in 28-day strength. High temperature curing generally increases strength at an age of up to about three days, but decreases 28-day strength. It is not unusual for reported concrete strengths to drop in the summer, due largely to hot concrete cylinders baking in the sun. I have observed a surface temperature of 124° F for 6×12 concrete cylinders in black plastic molds after a few hours in the afternoon sun in June in upstate New York. The concrete was an ordinary sidewalk mix, and 28 days later no one would have thought to lay the blame for low strength on whoever chose to place the specimens at the base of a west-facing stone wall (an unintentional solar oven). On an earlier project in the same location, I monitored a 4×8 cylinder that froze solid overnight while the well-protected structure stayed toasty warm. Given a curing box helps achieve specified 28-day compressive strength in both summer and winter, it is just silly to not make one a standing part of concrete placing operations.

It makes great sense, therefore, to include testing procedures and logistics as part of your prepour conferences (see 10 Things on Testing to Cover in the Prepour Conference). By increasing the chances of getting acceptable concrete strengths, the retainer you save may be your own.

Concrete Curling


Curling is the phenomenon when the surface of the concrete shrinks greater than the middle and bottom of the slab. Mud drying does the same thing, see picture.


The bowl like affect it causes within the jointed areas creates raised areas along those joints. In warehouse applications it creates speedbumps and wear for forklifts and wheeled vehicles.  It makes flooring a real challenge.

A few things to drastically decrease curling:

  • Use a well graded mix. All Duke City mixes use combined gradation technology.
    • So called GAP grading requires more water to fill the voids left by large aggregate next to small aggregate. Intermediate aggregates established by combined gradations fill that void.
  • DO NOT ADD ACCESS WATER TO THE MIX- the more water in the mix the more it shrinks.
    • Use water reducers to increase slump for workability
  • Since the cause is uneven evaporation, top drying out faster than the bottom; create similar conditions for the surface as well as the subgrade.
    • Don’t over wet the subgrade unless you can drown the surface
    • VERY IMPORTANT—Keep the surface wet with curing methods
      • Wet burlap and keep wet
      • Dike and flood surface area
      • Cover in plastic to seal in moisture
    • Hot-Windy days are worse conditions than rainy days

Curling can be reduced using the right tools and techniques.


Flyash and Concrete

There has been a lot of talk about flyash in New Mexico lately.  If you listen to the talk you have probably heard our market has been put on a Flyash allocation by the sole supplier, Salt River Materials Group. So what does that mean to you the concrete customer short term and long term?


If you have been around this market for a while you know the evolution of flyash here in Albuquerque. If not, a quick story.  Flyash, a byproduct of coal burning power plants, was introduced to us in the early 80’s.  At that time we replaced up to 20 percent with flyash. At that time it was called “Hamburger Helper” among other things since it allowed the supplier to use less cement but still achieve strengths, although it took a little longer. It is much finer than cement but it is spherical rather than crystal shaped.  This acts as ball bearings and helps workability and pump ability.  The complaints at the time were that it was sticky and hard to finish with the increase of super fines.

About this time, middle to late 80’s, the interstate concrete in the area was starting to fall apart after only 15 or so years. Upon further investigation ASR, Alkali Silica Reactivity, was found to be the culprit.  The Alkali in the cements were interacting with the Silica in the aggregates of this area causing a gel in the concrete attaching itself to particles in the mix.  When water migrated into the concrete after final set it would cause this gel to expand and break up the concrete from the inside.

Testing with another Class of Flyash , F, with lower Alkali, proved to mitigate the ASR. The Class F flyash would soak up great amounts of the Alkali in the cement thus reducing half of the equation.  Today the City of Albuquerque and NMDOT both require 25% Class F flyash in their approved mixes just for this reason.  We here at Duke City have been a big proponent of using Class F flyash in all exterior applications to mitigate ASR influencing architects and engineers not familiar with the ASR issue in our area to consider specifying Class F flyash.

Fast forward to today. Because of maintenance issues, lower natural gas prices, lower usage of electric power among other factors, the supplier of area has allocated flyash to its customers based on prior usage. Four Corners PNM coal burning power plant is the main provider for Class F flyash for Colorado, New Mexico, Arizona and California.  We credit Salt River Materials Group for classifying a consistent flyash out of this plant.  The other sources, not so much.

This allocation actually has an end date, April 10, 2016, but the writing maybe on the wall for the future supply of Class F flyash. You have all heard the attack on coal from the environmental sector and if clean natural gas pricing remains low flyash maybe be a thing of the past.

What then do we do to mitigate ASR?

Since the ASR “Gel” requires water, interior mixes wouldn’t have the same risks as exterior mixes exposed to moisture, therefore those mixes could go to straight cement. The risks would be greater for subgrade applications, but not as severe as exposed concrete to the elements.

As you may or may not know, Duke City Redi-Mix coarse aggregate is Basalt. The other coarse aggregate used in our market is the natural river rock in the area which is considered very reactive, with a  high silica content. Basalt is considered non-reactive.  We do use natural sand, as do all producers in the area, which is also very reactive. Duke City is half way to resolving the issue due to the fact our coarse aggregate is not reactive.

The current alternatives to mitigate ASR are through chemical admixes, namely Lithium, using alternate sources of aggregates or other pozzolans. Duke City is currently testing a lot of the available alternatives.  The availability, workability of the mixes and cost of these alternatives will make this transition a challenge for all of us.


  • Lithium cost is in the $25 per gallon range. The dosage averages one gallon per yard depending on the severity of the reaction. AND Lithium is known to accelerate set times.
  • Since all natural sands in this state are silica based we must crush a non-reactive aggregate down to use as a fine aggregate. Natural sand is spherical, great for pumping and finishing. Crushed fines are angular. Not really pump able and very tough to finish. AND, get this, it will cost $10 to $12 more per yard.
  • Other pozzolans are being considered including pumice. Testing is underway.


In light of all of this I believe Duke City will be able to get through this initial shortage of flyash without resubmitting mixes or losing any integrity in any of our mixes. Looking to the future we will be able to keep the increase in costs to a minimum using the alternatives mentioned earlier due to the fact that we use a non-reactive coarse aggregate and must just deal with the natural fine aggregate.  Please be assured Duke City Redi-Mix will address these issues with the highest integrity and commitment to quality you have grown to expect.