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  • Writer's pictureCory Grant

What is the Pinto Effect & Can It Be Avoided?

Updated: May 26, 2023



A pretty common occurrence in exterior concrete the so called “Pinto Effect” is an unsightly patchy look to the surface to your concrete that seems to be a mystery. Not just to Concrete Contractors, but also to suppliers and experts. Having had it on tens of projects of my own and having tried many different ways to either avoid it or remedy the patchiness after the fact, I have put together what I have tried and what many others have also tried across the web. I hope this is helpful for either concrete contractors who are having to explain to clients that this is out of their control, but also for customers, so they can understand what the hell is going on with their concrete!






The Pinto Effect, named after the Pinto Horse (which is actually a great name, because it looks a shit load like it), is a really unfortunate and heart breaking result to what could be a massive investment for a customer with their driveway, patio or pathway. It is incredibly frustrating also for the poor concrete contractor who has constructed the area, as they have no doubt placed and finished the concrete in the same way that they have done with hundreds, if not thousands of slabs in the same way. After doing a bit of digging, there seems to be a few theories as to why it occurs.





Causes


These seem to be varied and some have no link to the other. Here is what has been suggested.






  • Calcium Chloride Accelerator

It is suggested from many articles (one particularly that everyone seems to quote, that was written in the 1960’s) that I have read that this is a major cause. The Calcium Chloride is used to speed up the set time of the concrete. I agree, this may have some bearing on much discolouring in concrete, I do not believe that it does in this case. Why? Because we as a company have not used Calcium Chloride as an accelerator for over a decade. We stopped using it as it does discolour the finish of the concrete bit does so in the form of a white film on the concrete surface. We, along with countless other companies, use NCA (Non Chloride Accelerator). This eliminates Calcium Chloride.


  • Rain on The Slab

Rain on the concrete slab after it has been poured and finished. This points towards one scenario that the concrete is finished and then later that evening, while the concrete is curing, that the water on the surfaces causes some sort of effect on the concrete curing evenly and creates the discolouration. This sounds to me like something that somebody has made up without thinking it through! If this was the case, wouldn’t every slab that has been rained on in the evening have this discolouration?


The other scenario is where there has been rain on the surface after the concrete has been laid on the ground but it is at somewhere along the finishing stage. This falls into the next potential cause…


  • Bleed Water

Concrete bleeds. Sometimes a lot. This next cause does make sense in some ways. Water standing on the concrete either from bleed water or from rain, then worked into the surface is a big no no as far as concreting best practices go. Working water back into the concrete creates a weak surface that will be far weaker than the rest of the slab. It creates a substandard surface that will wear very fast and more often than not, have a chalky fell to the surfaces. The top of the slabs will deteriorate vary fast, leaving the surface looking like exposed aggregate!


Do I think that this causes the Pinto Effect? No I don’t. If this was the case then you would clearly see more of pattern to the discolouring. As concrete is finished with trowels, you would see some sort of indication of a wiping affect. Exterior residential concrete is typically finished with a combination of hand trowels, walking (funny) trowels, Fresno trowels and depending on the contractor and size of the job, a power float.


  • Residual Fertilizer

This one I thought was just horse shit (possibly, literally!). I can’t say I have ever prepared the sub-grade underneath my new slab to a point that it was ready to pour, then go and throw a whole lot of fertiliser on the ground. This one was just stupid.




  • Differential Curing

There are a few different things that could cause this to happen. Some are under the contractors control, to a point. Others are not, which includes environmental factors and contaminants in the concrete prior to it arriving at the job (maybe a little horse poo in the concrete). This theory says that as the concrete is curing, certain areas of the slab are curing faster than the rest. This also in theory makes some areas cure a different colour. This does make a lot of sense, as some slabs with exactly the same mix design, finished in the same way by the same workers, and no excess water added to the concrete, will look slightly different in colour the poured on different days. To me, this points to environmental factors such as the weather and temperature.



One obvious example of inconsistent curing is when plastic is laid on a slab to help it cure. If the plastic is laid completely flat then there is not necessarily a problem. But if the concrete has bubbles in some areas between the plastic and the concrete then you will get some very patchy looking concrete once the plastic is removed. As concrete warms as the concrete cures, sweating occurs and these bubbles will cure at different rate to the spots that are covered. If you look at some images, these patches look very similar to Pinto.



Adding excess water to the concrete, either at the plant or at the job. There is no doubt that this is not ideal but I don’t think this alone is a major reason for differential curing. The inconsistent patchiness in a concrete area effected by Pinto has what seems like random patches throughout a slab and sometimes just in one or two spots. Unless someone is filling up a wheelbarrow half with concrete and half with water and placing those in random spots then this theory should be debunked. Many jobs I have laid and have seen have one side of a driveway covered in Pinto and the other isn’t. These were all laid from the same truck uniformly in one, without stopping.


Sub base consistency is another one that is pointed to often. A couple of theories say that with a hard-fill sub base under a driveway will draw moisture out of the concrete evenly which leads to the surface curing uneven. This makes a lot of sense. Standard procedure for the bulk of contractors is to lay concrete on a compacted hard-fill base. Prior to pouring it is commonplace to wet the sub grade to both slow down the set time of the concrete to give more work time, but also to help the slab cure evenly. It has been said that as the sub grade is wet down that some of the fines are washed away in some areas revealing only aggregates. The areas where there is only sand cures different to the rest. I would be all about this theory except for the fact that many slabs have polythene underneath them but still have Pinto!


What the hell could it be then?


Personally, I think it is environmental, with a touch of these other factors. After it having happened on so many of my own projects and looking at many others, here is what I think is happening…


Excess water added to the concrete does not help. Watering down the mix design to your concrete plays with the curing. Concrete plants generally design their mixes based on the local area and conditions. For example, a 25mpa mix design (which I believe should be standard for exterior concrete, especially driveways) will have a certain amount of cement, aggregates, sand and admixtures added to each cubic metre of concrete, in order for the concrete to cure in a certain amount of time based on the local conditions. In most cities, these mix designs with be different based on the seasons. Adding water inconsistently truck to truck (especially at the job site), creates inconsistency. This definitely plays a part.


The biggest factor I can see though, is the weather and temperature. What do I mean exactly? As the concrete is laid, certain areas of the slab will be a certain temperature. This will be mostly consistent when the concrete first gets placed on the ground. As time progresses throughout the day, this is when the factors above come together to create the perfect storm for inconsistent curing.


During the later stages of curing, when the weather cools, certain areas that may very well look even to the eye, are actually holding more moisture under the surface. When the temperature drops, these spots will take longer to cure and these spots will cure while at the lower temperature. For example, where we do see Pinto happen frequently is along fence lines that are in the shade or under an eave line on a house. Most of the slab, especially those that receive sun, will cure to a point at 20 degrees celsius, where the areas that are holding water under the surface (often areas in the shade) will not achieve the sea curing stage until the temperature has dropped by another 10 degrees. This is a big reason why most of the Pinto Effected jobs happen in the colder months.


These differences in both the water content and the temperature differential are a tag team that makes your concrete look like a stupid horse!



Prevention

There are some stupid suggestions that I am not even going to mention here (like excess fertiliser), that will not help. Here are some actionable steps you vacant take though to at least lessen the chances of it happening.




  • Higher strength concrete

As stated earlier, concrete driveways, patios and pathways should enemy opinion be laid at 25mpa. This not only provides a better and harder product for your client, but also helps the concrete get to a certain strength faster. If you get you final finish on your concrete earlier in the day, it gives the surface more time to dry and for that excess water within the slab to evaporate, lessening the chance of differential curing.



  • Non Chloride Accelerators

This just adds to the suggestion above. You may need to make sure you can finish quickly and be on top of your game by adding accelerators, but it will close that curing windows down.


  • Limit Additional Water

Excess water is the enemy. It is quite common practice to add water to the truck at then job site, especially on residential jobs. Some will raise the concrete strength (say from 20mpa to 25mpa) to somewhat counter this. But you argent really doing yourself any favours, clearly. If you can make the adjustment to using super plasticisers in lieu of water, you will not only mostly eliminate the Pinto problem but also provide a far better product for your clients.


  • Avoid Excess Ground Water

- Lightly moisten the ground under your pour, don’t flood it. More water equals more bleed water, equals more inconsistency.


  • Pour Early

Pouring earlier in the day will create more time also for the concrete to cure. The bulk of our projects that we had affected were poured late in the day. This obviously can’t always be avoided. If that is the case, raise all of the above remedies!


Can Pinto Be Fixed?


Not really. Not without adding a new surface to the concrete. Here are all of the suggestions that I have read and I have tried all of them.


  • Vinegar & Water

Apparently, this will help bleach the concrete and hide the dark patches. I have tried it and it most certainly did not work when I tried it. I could see how it may work with efflorescence, which is a different kettle of fish.


  • Light Acid Wash

Nope. The Pinto is on the very surface of the slab, it is deeper than. That. An acid wash will burn the very top of the slab off. On the jobs we have done this on, it made the areas stand out even more and also gave the concrete a yellow tinged to it. As you can imagine, the clients were not what I would have called happy!


  • Time

You gotta love this one. Just tell your client… It will fade over time. Yeah a lot of time! Basically what you are telling them is that they are going to have to live with it.


Real Solutions


  • A Tinted Sealer

Not ideal I guess, but better than looking at that patchy concrete. The periodic re-sealing of the concrete is a discussion that will need to be had with the client.


  • Overlay The Concrete

An overlay applied to the concrete is really the best long term solution. There are many brands and systems in the market. These can be applied in a number of different textures and looks. With this remedy, the client will receive a hard wearing uniform surface. Some overlays will need re-sealing, some won’t.


So Who Is To Blame? Contractors or Concrete Suppliers or Both?

It really is tough to point the finger at anyone. The fact of the matter is that you cannot control the weather. Granted, you can try and pour your concrete in the perfect conditions every time, but in the real world, the construction industry doesn’t always work like that. Concrete is a vicious and unforgiving beast and it is a very hard and high risk trade. Concrete costs hundreds, if not thousands of dollars per truck load and you have a small window to get the desired finish. You have to appreciate that the contractor only has a limited window where it is isn’t raining. On top of that, you have main contractors working above them that are forcing them to get the work completed by a certain date. Other trades have to work around them to get their work completed. Here is the thing… Even if they do follow all of what I have outlined above, who is to say that the concrete supplied to the job is consistent!


Concrete Suppliers are far from perfect. They are dealing with natural products which have inconsistencies and they are trying to make a perfectly inconsistent product from it. Suppliers have what is sometimes very inconsistent sand and aggregates that could be coming from different quarries, depending on what is available. Sometimes sand and aggregates can be different even coming from different parts of the same quarry! I personally had many things come out the back of a concrete truck on my jobs from pieces or wood and bricks through to rats and headless rabbits! If you read the fine print of from most concrete suppliers they will have made it very clear that they do not guarantee consistency truck load to truck load, let alone e for Pinto Effect.


I guess what I am getting at is that neither the concrete contractor nor the concrete supplier is squarely to blame and to be fair, shouldn’t be. Whether the Pinto Effected concrete is classed as defective, is going to be something that should be discussed before the work begins for the client, otherwise a quite long, awkward conversation is going to be had with potentially any or all three parties involved having a loss.


Good Luck.












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