How to Reinforce Concrete Block
Reinforce Concrete Block TIPS:
- Mortar is strong in compression - weak in tension
- Concrete block needs steel reinforcement
- Concrete block should be attached to concrete footer
- WATCH steel & concrete block video below
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DEAR TIM: I'm building a new home and two concrete block piers toppled over when a ladder was leaned against one. The one pier crashed into the second one. Why did this happen and what could have prevented it? I thought concrete block was the smart product to use for my foundation and home, but now wonder if I made a big mistake. Have you built with concrete block successfully? What did you do? Alan T., Charleston, SC
DEAR ALAN: Let's start out by reassuring you that concrete block is a great building material. It's strong, it can be used in any number of ways, and it is used in all sorts of residential, commercial and industrial applications. Stop second guessing yourself. You need to direct that attention to the contractor on your job and any written specifications that were part of your blueprints and plans.
Tension & Compression
The block piers toppled over because the connection between the mortar and the concrete block is not able to resist the force of being pulled apart. The ladder placed against the top of the pier converted the pier to a lever. The seemingly minimal force at the top of the ladder was multiplied at the base of the pier where it fractured.
The mortar in between each course of block is strong if you try to squeeze, or compress it. But if you try to bend or stretch it, that's tension, it's very weak. Most masonry products, including concrete, only have 10% of the strength in tension as they have in compression.
Fill Hollow Cores
The vast majority of concrete block are hollow. When laid on their side and stacked in an offset manner they resemble, to a degree, a honeycomb made by a hive of bees. Bees fill the void spaces of the honeycomb with honey. Your contractor should have filled the hollow spaces in the block with pea gravel concrete and steel bars.
Realize he may have been getting ready to do this just before the accident occurred. Someone may have goofed up and put sideways pressure on the pier before the concrete and steel could be added. Not all contractors work 24/7/365. I only did it on rare occasions myself to hit a mission-critical deadline.
Concrete, mortar, and almost all rock are very strong when you compress them, but these materials usually only have one tenth the strength when you try to bend or stretch them. Engineers and technical folk call those bending or stretching forces tension. You commonly hear them talk about the tensile strength of a building material.
Steel Is The Answer
Steel, on the other hand, has fantastic tensile strength. For example, common one-half-inch reinforcing steel on a residential job site may be rated at 40,000 pounds of tensile strength. That means it fails or tears apart once you apply 40,000 pounds of pulling force to it. WOW!
Steel in Concrete Block Video
The first 20 seconds of this video show you how steel needs to be in the CENTER of the voids of concrete block. Then watch at 1:10 to see how they got the vertical steel in the PERFECT location. You'll see a flat 2x4 up in the air in a straight line. This face of the 2x4 was carefully staked out and represents the CENTER of the wall.
Once your contractor mixes up small pea gravel concrete, not the sandy mortar he used to lay the block, and pours it into the block voids along with the long steel rods, the concrete block then starts to resemble reinforced poured concrete.
Pin Block to Footing / Footer
It's a best practice to use this same reinforcing steel to connect the concrete block walls to the poured concrete footer. I did this on all my jobs making sure the steel rods protruded up from the concrete footer about 2 feet. I then dropped the long steel bars from above down to the top of the footer so the steel pieces overlapped.
The reason you want to use the pea gravel concrete instead of mortar is it's stronger. The rocks used in concrete mixes are responsible for much of the strength concrete achieves once it's cured. You want to use smaller pea gravel because the void spaces in the concrete block wall are not too large once mortar oozes in between the block and the steel is in place.
It's important the steel bars in the block are centered in the void spaces to ensure they're surrounded by concrete. In a retaining wall situation, it's important to place the steel so it's closer to the wall face that's not touching the earth. It's very important to have a structural engineer specify what needs to be done in these situations. Follow her or his recommendations closely.
You can also used thin wire reinforcing for concrete block walls. This unique product is placed on top of a row of block in the horizontal mortar joints. This thin steel, once embedded in the mortar, adds significant strength to long concrete block walls.
Pilasters or wide piers can also be incorporated into long and tall concrete block walls to add more strength. Once again, it's best to have a structural engineer specify where and how these elements should incorporated into your design. The money you spend on getting the advice of a structural engineer is often the best spent money on a job.
Here's all you need to know about concrete block and reinforcing steel: It's a great combination, it's affordable, it doesn't require lots of skill to combine them and you end up with a foundation that's as strong or stronger than a poured concrete foundation.
One of the things I really like about concrete block is the average person can build with them. It takes some skill to lay them plumb and level, but it's a skill that can be achieved with modest practice. You can used decorative concrete block around your home for all sorts of projects, just be sure you reinforce them if you want them to stand the test of time.