Q&A /

Concrete Block Crumbling

Beloved TIM: I own a diner where decorative physical block forms the lower role of the exterior walls. At the front of my building, a wing wall extends away from the building. Some of the solid concrete block are crumbling abroad to null while others adjacent to them appear to be in perfect status. The cap block at the top of the wall are showing the worst deterioration. These cake are less than 15 years sometime and the builder assured me they'd last at to the lowest degree 100 years. Why are they declining, what'south the best style to repair them and how can I forestall failure in the future? Marty Southward., Concord, NH

Honey MARTY: Wow! The photo you sent really shows the damage. Isn't it incredible that the mortar joints between the cake are still in dandy shape while the cake is crumbling? That'due south a significant clue every bit to why the block failed.

Some of the concrete blocks in this wall are beyond repair. The mystery is why only some failed. Photo Credit: Marty S.

Some of the concrete blocks in this wall are beyond repair. The mystery is why only some failed. Photo Credit: Marty Due south.

Physical block is a great fabric when it'south made correctly. Information technology's artificial rock when you lot call back about it. Your architect'southward statement about it lasting at least 100 years is not far off when you look effectually you. With minimal effort, I can show you physical walls congenital by railroad companies that accept been exposed to harsh winter conditions and the walls await much like the day they were poured.

To empathise why some concrete can terminal so long and other crumbles in a curt amount of time y'all but need to think about how concrete is made. The well-nigh bones concrete has just four ingredients: rock, sand, water and Portland cement. All the ingredients are important, only only one is responsible for the long-term immovability of the concrete and that'southward the Portland cement.

In the example of the railroad span abutments that are over 100 years quondam and in great shape, I'thousand certain the reason is considering the builders added actress Portland cement to make information technology extremely strong. The current minimum requirement for exterior physical is to make it and then information technology'southward a vi-bag mix or tests out to 4,000 pounds per foursquare inch (PSI) of compression strength. Nothing says you can't add more than cement to get to 5,000 or 6,000 PSI!

There are many possible reasons why the physical block crumbled and it'southward virtually incommunicable to bespeak to the smoking gun. It could be that when the physical block were made, that portion of the mix didn't get thoroughly blended. It's possible that the batch was mixed correctly but extra water was introduced into the block that are deteriorating. That extra water can dilute the amount of Portland cement in the mix.

It's also possible the failed block were not cured properly and the water in the block evaporated too quickly. This h2o is required to fuel the standing hydration chemical reaction that causes the billions of microscopic cement crystals to abound and interlock with the sand and rock. If concrete is non cured correctly information technology never attains its design strength. I could proceed and on with other possible causes, but those are the elevation ones I feel might be the cause.

The fact that the mortar in between the block is still there and in cracking shape tells you that it'southward not just a elementary matter of the wall getting wet and then repeated freeze / thaw cycles blew information technology apart. If that was the instance, the mortar would probably exist rotten equally well.

The repair process is non besides hard at all. It's just time consuming. Whatsoever seasoned mason can practise the repair. The key is obtaining concrete block that match in color so the repair doesn't stand out like a sore thumb. This is a perfect case of why I always tried to order a few hundred extra brick or cake for my jobs and leave them behind for the owner. Wouldn't information technology exist dainty if you could just go grab the extra ones the architect left backside?

Y'all'll want to pay attending to the existing mortar in the wall near where the failed block are located. Expect at the mortar very closely, even with a magnifying glass. You should be able to see individual grains of sand. Annotation the size and color of the sand particles.

When mortar is fresh and new, the cement and lime paste coat all the sand and mortar looks uniform in colour. Only after a few years Mother Nature erodes and washes off the thin grey mortar paste revealing the sand. This sand makes up close to seventy percent of the matrix of what yous run across, and then the color and size of the sand particles must exist matched if at all possible.

I'thou sure the sand came from a nearby sand and gravel pit, so go go some brick sand from the pits, allow it to dry and run across if it looks like the sand you see in your mortar joints.

In one case the mason repairs and lays the new block, continue the mortar damp for at least a week if possible. Subsequently that let the repaired area to sit for some other iv weeks. At the stop of the expect menstruum, wash the wall with a dilute solution of muriatic acid and h2o. This will assist wash off the mortar paste revealing the sand in the repaired joints.

Yous and so want to coat the entire wall with a silane-siloxane water repellent. These are readily available both online and from specialty stores that sell products to brick and concrete masons. Beware some of the highly advertised products you see on TV as these are not silane-siloxane products.

Pay attending to the label on the water repellent yous purchase and follow the instructions to the alphabetic character. Some require ii coats be practical within so many minutes of each other. To go fantastic penetration of the sealer into the wall, take a helper with you. As yous apply the sealer with a hand-pump garden sprayer, the helper uses a powerful foliage blower to blast the wall right behind you. This air force per unit area drives the sealer deep into the mortar joints and the cake. Don't skip this important step.

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