Cracks Happen

Concrete's gonna crack.

Since concrete is weak in tension ... even a little bit of ...

1. shrinkage (contraction) ...

2. flexure ...

3. settlement ...

... will cause it (concrete) to crack.

So ...

1. We can lessen shrinkage by demanding low water content (low slump values) ...

... low water/cement ratio ...

... low overall cement (less `paste' means less shrinkage)

... larger aggregate size (less relative surface area for paste to cover)

But less cement also means less strength.

2. We can allow for shrinkage (and contraction) with contraction joints.

Concrete is gonna wanna shrink after placement (due to moisture loss). Once cured, it is also gonna wanna expand and contract (due to temperature changes).

So, let's let it.

We will let walls move, to some extent, ... and slabs float.

For example, a large slab ... around the boundaries we may want to detail an `expansion/contraction' joint. (See sketch.) Such a joint will allow the slab to shrink or contract (or expand) without getting `hung up' on the wall.

Often such joints are formed using a `filler' that expands and contracts with the joint (so there isn't just a `space').

Note: some foundation details show slabs poured monolithic with foundation walls (e.g. Frost Protected Shallow Foundations). Scary. Expect a crack. Expect a big crack. If the slab wants to contract, it will crack. If there is some differential settlement, it will crack.

Sometimes I will detail a slab tied to a foundation (wall or footing) ... but only with a lot of reinforcement.

So, some details show reinforcement tying the slab to the foundation. Fine. Expect cracks somewhere else.

Here is a number ...

Overall shrinkage can be as much as 600 x 10^-6 in./in. in structural concrete. (And even more in junk-quality.)

What does this number mean?

It means that for every linear inch of concrete - it will overall shrink 600 millionths of an inch.

So? That's `nothin'.

Exactly.

But let's look at a 50 ft long slab, or wall, or sidewalk. It will shrink 600 millionths of an inch x 600 inches ... equals ... 0.36 in.

(That's 3/8ths of an inch.)

That is significant.

So, that's why, every 50 ft or so, ... in a sidewalk, slab, or wall, ... put in ½ in. contraction joints.

(Whether or not there's reinforcement.)

An exception to this might include a basement slab.

3. We can lessen the size of cracks with reinforcement.

It takes huge amounts of reinforcement to keep concrete from cracking at all. So, what we will do - is put in enough reinforcement so that the cracks are ...

many,

distributed, and

small.

So, if I am going to tie a slab to a wall, I will use reinforcement for the tie - and I will specify at least temperature-shrinkage (t/s) amounts of reinforcement throughout the slab. Because I know that the slab is gonna crack - especially if I have constrained it (at the perimeters).

Other note: if I am using the slab to restrain the bottom of the wall - for example - a basement retaining wall (discussed earlier in the course) - I will spec the slab cast against the wall - and I will NOT allow contraction joints throughout the slab ... otherwise the wall will push the slab and close the gaps. And we don't want walls to move.

So my concrete is gonna crack - but with reinforcement - the cracks are small (hopefully `hairline'), dispersed, hardly noticeable.

Some designers and contractors like to use `fibers' for reinforcement.

Assignment (not graded): do a search on fiber reinforcement for concrete slabs and put a summary or summary article in your notebooks.

4. Concrete is gonna crack ... so we will control where it cracks with control joints.

A control joint is something we put in our concrete to tell it where to crack.

For example - in slabs - as the Contractor is `finishing' - we have him/her/it `knife' a grove into slab. (Or cut (saw) it in later.)

This groove should be at least one-third the depth to be effective. (See sketch.)

What it does is it forms a local weakness in the slab - and so theoretically it (the slab) will crack there first.

If the control joint is not effective - it will crack somewhere else.

Here is the thing - if our control joint is straight - the crack will be straight (and hidden in the `groove').

If we let the slab crack on it's own - you guessed it - it/they (the cracks) will likely be ragged, probably ugly looking.

For things like walls, where we don't have direct access to the surface we want to control (due to the forms), we will cast features into the concrete that will leave planes of weakness ... (see sketch).

Here is a cool rule of thumb ...

5. Concrete cracks IN SQUARES.

So, if you have a 5 foot wide sidewalk ... expect cracks every five feet. If you let the sidewalk crack on it's own - then expect ragged, ugly cracks. If you tool control joints into the sidewalk - it will crack in the joints (and look beautiful).

So, if you have a 24 x 36 garage slab ... specify control joints every 12 ft ... (allowing the slab to crack into (six) perfect squares).

(I generally specify control joints every 12 - 15 ft.)

So, if you have an 8 ft tall wall ... specify control joints every ... you guessed it ... 8 ft along the wall. (Though some spec them farther apart than this ... at their peril in my opinion.)

6. Deal with potential cracking at discontinuities (`sharp corners') in your concrete ... such as ...

steps in walls

openings in walls

(inside) corners

Some designers detail 45 deg pieces of rebar placed across the corner at the bottoms of wall steps. (See sketch.)

Code calls for (2) # 5 rebar at concrete wall openings (windows and doors) extended 24 in. each way past the corner of the opening. (See sketch.)

Corners are also good places for control or contraction joints.

So ... in summary,

... when I detail concrete ... I know it's gonna crack ... I let it crack with contraction joints ... say every 50 ft, or more. I control where it cracks with control joints ... say every 12 - 15 ft, ... in `squares'. I deal with potential cracking at corners. And I use lots of reinforcement.

Additional notes ...

Sometimes I will let the Owner and Contractor decide about reinforcement in slabs, if the slabs are not `structural' to other parts of the building.

Sometimes I will let the Contractor decide the specifics about crack control, control and contraction joints - but make it sure it's clear that the end product and it's `look', durability, and performance is the Contractor's responsibility.

Drying (shrinkage) occurs much slower than curing (hydration). At 28 days the almost all of the curing has taken place ... but we may only be 1/3^rd of the way through the shrinking process. Overall shrinking takes years.

Construction issue ...

* Don't let the concrete dry out too fast, as differential drying plus the concrete not curing will cause what I call `drying' cracking.*