Quality concrete is a mixture of cement, water, and aggregates that will ultimately meet the requirements for which it is designed. Generally, the most consistent characteristic of concrete is its aptitude to continually change, creating a need for troubleshooting. Thanks to constant variability in raw ingredients and the science of advancing a mix from design stages to a desired finish, dialing in the perfect product can be extremely frustrating and time consuming. In addition, over-analyzing the problems can sometime create more headaches and leave you in worse shape than you were to begin with. Therefore, carefully diagnosing each individual aspect of the issue through trial and error is a good start- but you have to be practical about it. Nobody wants to spend an entire day batching concrete that can’t even meet specifications to be used in a product, especially if the state is a customer. Ultimately, bad concrete leads to loss of time, loss of money, and loss of business- leaving you stranded in last place. So- where do you start?
I’m sure that all of you have heard that designing a concrete mix is like “baking a cake.” Although this is somewhat true in regards to bringing ingredients together to make a desired product, it’s not quite that easy. In some instances, it can be more like brain surgery, but that is the price you pay for something as valuable and versatile as concrete. Whether aiming to create a state-of-the-art decorative pool deck for a west coast mansion or a standard grey concrete block, problems will arise that cannot simply be ignored. That is where troubleshooting comes into play. In reality, it is impossible to map out every unique situation or combination of concrete problems and exactly how to go about fixing them. Therefore, I am going to break down several common problems with concrete and give some helpful tips/pointers to make life a little easier for concrete producers.
Strength and durability are some of the most important aspects of a concrete mix. Concrete has two different types of strength rating- compressive and flexural. The most commonly referenced, compressive strength, is measured as the resistance of concrete to axial loading at a given rate. Flexural strength is the measure of the tensile strength of concrete, which usually tends to be anywhere from 10-20% of the compressive strength. Both are extremely dependant on several other concrete factors, including the water-to-cement ratio, age of concrete, air content, handling methods, curing techniques, and aggregate properties.
So what do you do if you have low strength?
Here are a few areas to review before getting too far into diagnosing your issues. Most of the time, strength related problems lie within one of the following:
As long as concrete hydrates, it will gain strength. The prime factor that directly affects strength is the water-to-cement ratio (w/c). In theory, the lower the w/c ratio, the higher the strength. However, the w/c ratio has to be high enough to allow for complete hydration of the cement compounds- which is usually around a .28 at the minimum. Obviously, admixtures can play a great role in strength acceleration and retarding, but should not be used unless researched first. W/c ratio should be designed carefully based on the specified 28 day strength.
To be practical, low strengths are not always the result of a w/c ratio. Plenty of producers run into issues that they blame on the air and other ingredients, but sometimes the problem can lie in the batching process itself. When troubleshooting for low strengths or inconsistency, make sure the batching process is what it should be. Personally, I deal with more cases of something going wrong in the batch process than in the mix itself. A concrete mix is a never constant because materials are always changing, but mixes can be dialed in to be extremely consistent in monitored correctly. During the batching process, there are several practices that can help keep your mix as consistent as possible. Correcting batch water for moisture contents and implementing the correct timing sequences for adding materials and admixtures is extremely important. etc. are big players in the outcome of the mix. If not performed properly, the mix can turn into a disaster.
Scales play a strong role in the accuracy of a mix. Batch scales must be accurate if you want an accurate mix. Calibration every 6 months is recommended. If your scales are reading wrong, your mix will typically come out larger or smaller than expected, causing you to over/under yield. For ready mix producers, this can be a huge problem. If you consistently under-yield because your scales are off, then your customer isn’t getting what he/she pays for.
Always make sure your batch machine is mixing properly. A shorter mixing time will lead to clumps in the mix and segregation, while over mixing will knock out desired air and create larger problems of its own.
Make sure and research if your batch machine reads off an OD (oven-dry) weight or a SSD (saturated surface dry) weight. Confusing these 2 can lead to extensive over/under yielding and water content problems.
Personal errors can be a culprit of something going wrong in a mix, leading to low strengths. You may not think it’s possible that your cement delivery person accidentally filled up your fly ash bin with cement, but it happens all the time. In addition, sometimes something goes wrong on the conveyor belt or the loader operator isn’t paying attention and sand or rock might end up in the wrong bin. It is always safe to do routine inspections to make sure everything is where it should be. Make sure your batch operators are trained and certified for the job they are doing, and continue to implement refresher courses whenever possible.
Fine aggregates have the greatest influence on water demand in a mix. Because of the high absorption % in fine aggregates, moisture corrections in batch water are detrimental. If you are using sand with 4.5% total moisture, and the absorption % of that particular sand is 1.5%, then that leaves you with 3% moisture on the surface (free moisture.) This free moisture has to be accounted for since it is extra water in the mix and should be subtracted out of the batch water and substituted with sand. If the sand moisture is less that SSD (saturated surface dry), then you will have to add the corrected amount of water to the mix and remove sand. Please visit the Mix Design section of this website for a more detailed explanation. Fine aggregate particle shape, surface texture, clay, silt, deleterious materials, and gradation all play a role in overall strength of a mix.
To always be on the safe side, a good rule of thumb about water is:
“If you can drink it, then you can use it for concrete.”
Contaminated water is bad for concrete, and it will directly affect strength. Too much water will increase the water/cement ratio, which will also cause a decrease in strength.
If your concrete is coming out too wet, too dry, or just separating like crazy, I would put my money on a water quantity issue any day of the week. About 90% of issues with concrete are from incorrect or inadequate water content.
In a nut shell, admixtures are very beneficial IF USED CORRECTLY. There are several different issues that can arise from improper utilization of admixtures. The easiest and most reliable way to troubleshoot any admixture issue is to contact your admixture provider for the proper methods/techniques. One common admixture problem is cement/admixture interaction. Almost all admixtures are dosed in ounces/cwt. Over/under dosing will only cause problems, which will ultimately affect strength. Another common issue is admixtures not doing what they are suppose to do. Before you call up your local admixture representative and yell at them for a bunk product, take the time to make sure you are following the proper procedures for implementing that particular admixture into your design. Admixtures do go bad, but most likely your problem is something going wrong during the batching/mixing process. All admixtures are designed to be added to the mix at a very specific time- before, during, or after the water. If this simple rule isn’t followed, it can completely eliminate the admixtures ability to do its job. For instance, if a water reducer makes contact with dry cement before it hits the mix, then the water reducer will most likely be in-effective.
Air is probably the hardest factor to control in a concrete mixture. However, it can be the most important. There are so many different factors that can affect air, and the problem isn’t always easy to pinpoint. The main purpose of entrained air in a concrete mix is to resist stress from freeze/thaw. The air voids serve as safety pockets in the mix to allow the rest of the concrete to expand/contract during normal and severe weather without causing the concrete to crack. However, the higher the air content, the lower the concrete strength. Typically, for every 1 % increase in air, strength will decrease by about 4-5%. Different types of cement, supplementary cementitious materials, admixtures, and aggregates all affect air in a unique way. There are several techniques and tricks that can be implemented to help stay in control of these variable factors.
If you are using high alkali cement, you might want to consider a lower air entraining dose. The higher the alkali level of cement, the higher the natural air content of the mix will be. Therefore, reducing the air entraining dosage is crucial. PCA recommends a decrease by as much as 40%.
In addition, the fineness of cement or other cementitious material particles (particularly ground granulated blast-furnace slag and silica fume) can have an adverse affect on air. An increase in cement fineness is likely to decrease the amount of air in the mix due to smaller particles filling voids. In this case, doubling up on air entraining admixtures for the finest cements (such as Type III) can help restore air to the desired content. This is also necessary when designing for higher cement contents.
When using fly ash, make sure to analyze the LOI (carbon content) when designing a mix. If you recently switched suppliers or received a new shipment of fly ash, it is not uncommon for the LOI in the material to change. With the change in LOI, the air must be adjusted in the mix. High carbon contents (LOI) will cause the air to drop out of mixes.
To keep it simple, larger aggregate sizes and dirty aggregate will decrease air content. When the aggregates sizes are smaller, there are more surface voids and pockets in the mix, causing a higher air content. As the concrete becomes more workable, the air content tends to rise (up to a certain point). If an aggregate is extremely dirty, the excessive dust particles will knock the air out of the mix. Typically the mix will end up with a higher entrapped air content and lower entrained air content.
Some admixtures will have an affect on air and others will not. Typically, water reducing admixtures and retarders will increase the air content of a mix, creating a need for adjustment. High range water reducers and plasticizers will increase air content. Please contact your admixture sales representative for specific information needed pertaining to your particular admixtures.
One of the only factors besides weather that isn’t mix related and directly affects concrete strength is the handling and curing of concrete test specimens. It is possible that your concrete tests perfect and meets specifications, but if the test specimens disagree- it could be costly. ASTM and AASHTO both have standard specification procedures for making and curing concrete test specimens to insure that test results are accurate representations of the actual product.
The following results (except those in italics) that can affect concrete strength are from the NRMCA Publication No. 179- “Review of Variables that Influence Measured Concrete Compressive Strength:”
o Insufficient consolidation – Up to 61%
o Leaving rod holes in cylinder
o Flat particle vertical orientation- Up to 40%
o Reuse of plastic molds- Up to 22% ( I am guilty)
o Use of cardboard molds- Up to 21%
o Use of plastic molds- Up to 14%
o Out of round diameter- Up to 10%
o Rodding with rebar- up to 2%
o Vibration while cylinders are still in plastic
o Moving cylinders too soon
o Immediate freezing of specimens for 24 hours- Up to 56%
On this note, I recently did a study on the effects of freezing concrete cylinders immediately for 24 hours, and letting them cure normally for 48 hours after the freeze. The study was on a SCC mix with accelerating admixtures that usually resulted in 3500 psi 24 hour breaks. In my results, I found that the 3 cylinders that I allowed to freeze during the first 24 hours tested at only 3400 psi average after 3 days, and the 3 cylinders that I allowed to cure in a controlled environment of 70 degrees tested at 6100 psi in 3 days.
o Cylinders drying out
o Cylinders left in hot sun
o Cylinders left in field for 7 days in warm temperature- up to 26%
o Cylinders left in field for 7 days @ 73° F with no added moisture- up to 18%
o Transporting cylinders to lab
In my experience, transportation is extremely important. One of my first jobs in the industry was a quality control technician for a ready mix company. The quality control manager was a very stressed man, to say the least. For some reason, he would follow very poor guidelines for transporting specimens, and then he would blame the low strengths on the interns and lab technicians when they had nothing to do with it. I remember riding with him to pick up cylinders that he had made the day before. He would pick them up and set them in the back of the pickup truck, and as soon as driving continued, the cylinders would start rolling around and banging the sides. NO WONDER THE BREAKS WERE LOW. Even a young rookie in my position could piece the low breaks and transportation methods together.
o Cylinders hit with steam curing before reaching initial set
o Curing tank not lime saturated
o Curing room not keeping cylinders wet at all times
o Not maintaining standard temperatures
o Cylinders sticking up out of the water tank
o Convex ends -up to 75%
o Weak, soft capping compound- up to 43%
o Concave ends- up to 30%
o Rough end before capping- Up to 27%
o Rough end, air gaps under cap- Up to 12%
o Convex end, capped- Up to 12%
o Sloped end, leveled by cap- Up to 5%
o Rubber cap, no restraint- up to 53%
o Rough end before capping- up to 27%
o Convex end, capped – up to 12%
o End not perpendicular to axis- up to 12%
o Worn out pads
To add on, when using a compressive testing machine, there are several variables that can influence the test results. Make sure the compression machine has been calibrated within the last year, and make sure all the parts are still functioning properly. When placing the cylinder on the machine, make sure it is flat, plane, and lined up perfectly to the center of the axial load that is being applied.
Now that we have reviewed strength, air, batching, and testing related issues, let’s move onto some of the more common post-placement issues. When troubleshooting concrete problems, it is extremely important to compare and relate symptoms and causes of distress and faults in the concrete. The following are characteristics and defects in hardened concrete and what causes them:
When a surface is delaminated or scaling, the top 1-4” -1/8” or so of the surface is separated from the rest of the product by a thin layer of air, water, or both. The delamination is a separation along a plane parallel to a surface. The areas affected can vary from a couple inches to an entire slab. The most common cause of delamination is finishing the surface too early. If a surface is finished with a trowel before the underlying concrete is through bleeding or releasing air, then the air and water get trapped below the surface, causing the delamination. Scaling is caused from lack of air-entrainment, over-vibration, improper dry-shake methods, freeze-thaw, inadequate mix design, improper use of vapor barriers, and premature surface finishing. Salts, fertilizers and improper curing techniques all directly relate to scaling.
Blisters are bubbles or air pockets that form irregularly raised areas on the surface of the concrete. Blisters are most likely to occur with early finishing, excessive finishing, or high quantities of trapped air under the surface.
There are many different factors that can affect color in concrete. If we are talking about just standard grey concrete without any color pigments or colored cements, slump is the major player in color change. Also, the color tends vary depending on finishing time, how long it takes to finish, and what type of trowel is being used. Sprinkling dry cement on the surface of a concrete to soak up bleed water (which is never a good idea) also tends to make the color of concrete much lighter than usual. Some admixtures can have drastic affects on the color of concrete, and others won’t change it one bit.
Although dusting does not affect strength, it causes a much undesired coat to a concrete surface. If a concrete surface is experiencing dusting, it means the surface will erode easily under traffic and produces a powdery, chalky like appearance. Usually, the surface is very soft and will scratch off with a fingernail or other small object. There are many factors that can contribute to dusting, including excessive water in the surface, early/over troweling, premature finishing, hot humid weather over cool concrete (rapid drying), and lack of early curing.
Crazing, or map cracking, is the characteristic of small hairline cracks in the surface of hardened concrete. The main cause of crazing is rapid surface drying (which can result from high winds and low humidity), but it can also be caused by late curing, excessive bleeding, premature troweling, and high slump concrete.
Spalling is the flaking off of fragments from the concrete, most often caused by expansive forces within the concrete or stress from outside objects. These forces can be caused from corrosion within the concrete due to reinforcement or other embedded objects, or they can be caused from moistures trapped by sealants. The more obvious and physical causes are mechanical blows from operating equipment or accidental pressures.
Flash set or quick set is directly related to rapid and early loss of workability in concrete paste or mortar. Flash setting usually occurs in hot, dry environments, and can easily be controlled with surface retarding admixtures.
False setting occurs when there is a significant loss of plasticity or workability in the surface shortly after mixing, without much heat. Adding additional time to the mixing process will ultimately increase workability and get rid of flash setting.
Plastic shrinkage cracks occur in the surface of concrete during or soon after placement. These cracks usually appear on horizontal surfaces and are perpendicular to the direction that the wind is blowing. It is caused from rapid evaporation of moisture from the concrete surface (usually hot weather applications.) High wind speed and temperatures combined with low humidity increase the possibility of plastic shrinkage. Some methods to stop plastic shrinkage include windbreaks to block wind, aggregates that have free moisture on the surface, using ice in the concrete, keeping the sub grade damp, erected sunshades, fogging, misting, and implementing fibers in the mix.
Honeycombing, also known as grout leak, is the characteristic of voids left in the concrete surface due to the failure of the mortar to effectively fill the spaces within and around the coarse aggregate particles. A few causes of honeycombing include forms leaking at joints, dry mix, segregation, under vibration, and large aggregates. This is very common in the precast industry.
Most outdoor concrete goes through stages of freezing and thawing, which potentially leads to cracks throughout the product. When concrete freezes, it retracts like anything else. When it thaws, it expands again. Just as anything else, when something contracts and expands, properties change, and in the case of concrete- it cracks. There are several different types of freeze thaw damage including internal paste cracking, surface scaling, d-cracking, and pop-outs. Internal paste cracks are throughout the concrete and sometimes appear as large chunks breaking off of exposed aggregate. Surface scaling due to freeze/thaw usually occurs due to a high concentration of salts on the top layer of the concrete- made vulnerable by improper proportioning, improper curing, and careless placement. D-cracking is when the aggregate absorbs too much water and cannot handle freeze/thawpressure.
The best and most obvious method to prevent freeze thaw damage is air entrainment. This is essentially the main reason that air entrainment is used in concrete. The air pockets work as a cushion for the concrete’s expansion during the freeze-thaw process, leaving the structure crack free. Other methods include using high quality concrete with a low w/c ratio, supplementary cementitious materials
Adequate vibration might remove 1-1.5% entrapped air without disturbing the entrained air void system. Inadequate, or excessive vibration, will cause all entrained and entrapped air to drop out of the mix, leading to segregation. Vibration effort for particular products depends on the size of the product as well as the reinforcement within. For instance, a small product with less rebar requires less vibration than a large product with plenty of rebar.
Alkali comes from cement, and silica comes from aggregate (quartz, gravel, etc.) The siliceous aggregate components react with alkali hydroxide in the pore solution to generate a siliceous gel. This gel is very active and expands when in contact with moisture, creating pressure points in concrete and causes it to crack. When the aggregate has a reaction, concrete will crack frequently in the interior and extend to the surface, leaving a whitish and dusty like appearance. Some easy prevention’s would be to use non-reactive aggregates, limit the alkali loading, use blended cements, and use lithium based admixtures.
ACR is the same as ASR, except it is a reaction between dolomite aggregates the alkalis in cement. Some specific preventions include avoiding reactive aggregates, blending aggregates, & using small aggregates.
Visible lines on the surfaces of formed concrete indicating the presence of joints where once layer of concrete had hardened before subsequent concrete was placed.
This is just the beginning of what concretehelper.com will have to offer in the future. This troubleshooting list will be constantly updates as time goes by, so check back for more information on a routine basis!