Now that we learned about the finish grading prep work, we can now move into our next “finish trade”. Flatwork!

Flatwork is the poured hardscape for all your patios, walkways, driveways, sidewalks, etc. Concrete is typically the main choice material for this due to its ability to form a flat, even surface. It’s also very strong once it’s cured.

Laborers forming a concrete sidewalk as the cement mix runs out of the chute from the back of a concrete mixing truck. — Pouring concrete for the city sidewalk.

In this post we will be discussing:

“Off-site” flatwork:
- Sidewalk & Drive Approaches
“On-site” flatwork:
- Driveways
- Walkways
Subgrade prep
- Moisture & Compaction
- Reference Soils reports
Forming, Pouring, & Finishing
Control Joints

Similar to the finish grading, your FLATWORK sub is also involved on both the production side as well as the land development side. So in this case, we refer to the private side as “on-site”, and the public side as “off-site”.

“OFF-SITE” FLATWORK

The “off-site” flatwork refers to everything WITHIN the city’s “right-of-way” (also abbreviated as “ROW”). This includes the city sidewalk and the drive approaches.

“ON-SITE” FLATWORK

The “on-site” flatwork refers to everything BEHIND the “ROW”. So basically, everything within the private homeowner lots (aka “production”).

It’s important to know the difference between “on-site” and “off-site” costs. It will come into play when you’re building your budget for a project and when negotiating with the seller (which we’ll discuss in more detail in a later post).

SUBGRADE PREP

As we discussed in the last post (Finish Grading Introduction), your finish grader will cut the subgrade for the flatwork. HOWEVER, you must specify a “tolerance” in the grade as this work transitions between the finish grader and the flatwork sub.

For example, a finish grader will cut the grade for a 4” thick sidewalk WITHIN A TOLERANCE OF +/-0.10’ (or 1.2”). Which means that the flatwork sub will accept the subgrade if it’s between 2.8” or 5.2” (4” +/- 1.2”) BELOW the top elevation of the future sidewalk (see illustrative example below).

Illustrative example showing the acceptable grade tolerance range of the grade cut for a future flatwork sidewalk done by the finish grader. — Example of “Grade Tolerance”

(NOTE: This is good practice to ALWAYS specify the grading tolerances for any two subs involved with any sort of grading. i.e. a rough grader and a paving sub).

If this is not specified with both of your subcontractors, then you may have an angry sub looking to hit you with some change orders! Subcontractors may have different default tolerances of their own that they follow, so if your flatwork sub has to do MORE grading and deal with MORE dirt spoils than he was anticipating, then he may be looking for some more money.

MOISTURE & COMPACTION

Once the subgrade is graded properly, your flatwork sub will then add moisture and compact it in order to get “optimum moisture content”. The soil underneath is so vital to the performance of the concrete. And if the soil is not compacted correctly, it can shift and move significantly causing some very large cracks in your concrete.

Roller compactor sitting on the ground in between two wooden forms that frame the future sidewalk in order to compact the subgrade. — Roller compactor needed to compact the subgrade before pouring the concrete.

SOILS REPORT

Your best source for learning about the soil characteristics of your specific project and what is required of your concrete in order to achieve the greatest performance is your soils engineer. (See Watch out for Corrosive Soils post). See below for a small excerpt example from a soils report provided by Geotek. Remember, your soils report will become your project’s Bible during development.

Please note that there are some pretty standard things in the industry for your flatwork (i.e. the thickness, the type of cement, the need for reinforcement or not, etc.). However, if your project has very expansive soil or it’s highly corrosive, your soils report may call for a higher-strength concrete, or maybe a Type “V” cement mix, etc. And what that means for you as a developer is that YOUR COSTS WILL GO UP! So make sure you account and budget for any anomalies ahead of time.

FORMING

The next process is now setting up forms for the concrete. Here your subcontractor will frame the areas of the flatwork using stakes and lumber (2×4’s or 2×6’s usually). In addition, the subcontractor must ensure that the frames are level and that the corners are square. Also, your subcontractor will make sure there is a slight slope in the surface of the flatwork in order for water to run off instead of pooling on top (typically 2% – or in other words, about 1/4″ of fall for every 1’ of run).

Wooden stakes and 2x4's framed in the dirt for a future sidewalk and walkway. — Wood frames for future sidewalk and walkways.

POURING

Once the forms are ready, cement mixer trucks then come in and start pouring the concrete (or “mud”) from the back of their chutes. As a result, the laborers have to work fast to move the concrete mix around and level it using a “screed” (a long metal straight edge or even a wood 2×4).

A cement mixer truck pours concrete from the back of his chutes at a construction job site. — Cement mixer truck.

The driver of the cement mixer stands outside of his truck to ensure the cement gets poured from the chutes on the back of his truck into a small concrete pump. — Cement mixer truck.

Laborers stand in the wet cement mix as they use a long 2x4 screed to level the concrete. The other laborers place it in between the wooden forms using the pump hose. — Laborers place the concrete using the pump hose as others use a screed to level it.

Next, these guys will smooth out the edges using a steel concrete edger and then the main surface area using such tools known as “darbies” and “floats” (similar to a trowel, but with a much thicker base). These tools will smooth out the concrete and correct any imperfections.

FINISHING

For your final finished look, there are multiple styles that can be used. Some of the most common methods are: float finish, broom finish, exposed aggregate finish, and stamped concrete finish. Usually if there is a callout or need for some special finish somewhere in your project, it will be called out by your Landscape Architect on their plans.

a laborer uses a flat tool to smooth out the wet cement of the concrete slab — A laborer uses a float to finish the surface of the cement.

“FLOAT” FINISH

The float finish is a very basic look. It’s left smooth after the use of the floats and darbies as mentioned previously. The downside with this finish is that it can get very slippery when it gets wet.

“BROOM” FINISH

With the broom finish, a laborer will literally drag a broom over the concrete, which will give it a rougher texture, thus helping the concrete not to be so slick when wet.

A broom being dragged over concrete to leave aa rough texture. — Broom finish for concrete

“EXPOSED AGGREGATE” FINISH

We also refer to this as a “Top-Cast” finish. Top-Cast is a water-proof concrete retarding agent used to expose the natural aggregate within the concrete. As a result, this provides some nice texture as well as leaves a very pleasant aesthetic look.

Concrete slab with an exposed aggregate finish (natural gray color) — Exposed Aggregate Finish (Tan)

“STAMPED CONCRETE” FINISH

The subcontractor will literally use large stamps with different patterns and press it into the top of the concrete to achieve a certain desired finished effect. It’s a cheaper alternative to actually installing stone, brick, etc. while still producing a very similar look!

Stamped concrete texture for an apartment driveway alley. — Stamped Concrete

Decorative stamps for concrete — Stamped Concrete

CONTROL JOINTS

One final thing that your flatwork sub is responsible for is the “Control Joints”, which help controls the cracks in the concrete. All concrete is anticipated to crack over time. It’s inevitable. HOWEVER, you can definitely help mitigate the cracking or try and control where it occurs by placing control joints.

The cracks will find the weaker parts of the concrete slab and will crack along these pre-existing cuts. Thus, the concrete can still look aesthetically pleasing. If you think of a graham cracker as an example, the cracker already has some perforated cuts in it. If you were to try and break that graham crack in half it would most likely snap along the weakest point, the pre-existing cut. Similarly, the concrete will pretty much work the same way.