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The Art and Science of Concrete
by Susan Jasan - posted 06/20/17

 

 

 

 

 

 

Remember when creating your design, that plants can also stain concrete. Holly berries make an interesting contrast here, though many times even the colored blooms of plants can create unwanted stains in concrete.


The hardscape of a landscape provides the “bones” of an overall design. Typical hardscape features are patios, pathways, stairways and various garden structures.

Concrete is often used for paved areas, and the number of options available to homeowners is only limited by one’s imagination. The bright white of new concrete is highly reflective and sometimes isn’t the first choice of a finish. Given a little bit of time, any concrete will begin to age and the bright-white tones will mute to a tan, earthen color simply through normal weathering.

It sure looks simple when the concrete truck pulls up and the mixture is poured into forms. Expert laborers work the concrete, and before you know it, there’s a new walkway or patio. Seems simple, but it requires both art and science to do it correctly and do it well.

Just as with planting your favorite new nursery plant, ground preparation is the key. This is true for the installation of any concrete material. Depending on the soil type, compaction of a sub-base may be required, as well as installation of material for subsurface drainage. The goal is to keep even soil-moisture content under your concrete. In some cases, a moisture barrier should be placed between the base material and the concrete itself to prevent moisture wicking up through the concrete to the surface.


Contrasting concrete borders can tie a site together. Here the color of the coping is repeated on the stairs. The contrast is a visual reminder for guests that there are stairs transitioning between the two levels of the backyard.
 

Equally important is temperature. The ideal temperature range for pouring concrete, and it to cure, is 50 to 85 F. Pouring concrete in higher temperatures is often necessary, but special steps must be taken to guarantee a quality finished product. The pros know!

Consider incorporating natural stone in large concrete patio areas to add definition. Here, a flagstone border is inset within the concrete of the patio. Note the joints are set on a 45-degree angle for the interior pattern of this sitting area. Simple details add the finishing touch to a project.

Concrete poured on grade should have steel reinforcing to reduce cracking as much as possible. Rebar, a steel reinforcing bar, is often used. Sometimes a welded wire mesh of 6-inch-square grids can be used. Typically the reinforcement should be at least 2 inches below the surface, but it should also be pulled up into the body of the slab during the concrete installation, not left at the ground level beneath the concrete. Your specific project will determine the amount of steel and the size required.

Forms provide the framework for the concrete pour. Usually coarse sand or limestone “screenings” will be put in place before pouring the concrete. This helps keep the concrete at a uniform depth. This base and the forms should be sprayed with water shortly before pouring the concrete. Proper moisture in the surroundings, as well as in the concrete itself, is critical for proper curing.

As the concrete is worked into place, it is important that there are no air pockets or voids in the concrete. Working the pour with a shovel, hand tamping or a mechanical vibrator is the first step. Then a screed (long board) is drawn back and forth across the forms, leveling the concrete in the form. After screeding, the surface is typically tamped using a mechanical trowel or a float to give it a smooth finish.

Next comes the finishing with a steel trowel, wood float or a broom. The broom finish is the most common. It provides a good footing when wet, has a slight textural quality and is still relatively easy to clean.

As important as the overall design of the concrete feature and its proper installation, there is one additional step that must not be overlooked: the joints. Three types of joints are used to control cracking.

First, expansion joints are placed between separate pours of concrete and at regular intervals. They are also used to join differently shaped areas of concrete: curved sections to straight sections, or between slabs – such as buildings, retaining walls, steps and posts. It’s also the simple things that are often overlooked, such as using an expansion joint between an air-conditioning pad and a structure in order to reduce any vibration transfer from the AC unit to the house.

Wood or bituminous impregnated fiberboard is often used for expansion joints, which allows the natural expansion and contraction of concrete. In areas with expansive soils, such as clay, very often the joints should also be doweled together with steel to address the vertical changes that can occur between various sections of concrete.

 

Left: Stamped concrete can mimic natural stone, but at a much lower cost than stone veneered on a concrete base. Here the concrete walkway ties together multiple areas. Remember that the rise (height of a step) outdoors should be 6 inches, with a tread of 14-15 inches. Middle: This concrete pattern mimics a cut flagstone in color, texture and form. The sealer and grout color add to the look. Right: This concrete patio ties together a cooking and serving area (foreground), a dining area (middle ground) and a fire pit in the background. The countertops, as well as the seat walls, are all poured concrete. The color works well with the stone used in the bar area as well as the tones of the patio.


The second type is actually a series of control joints. Control joints do just that: They are meant to control where cracks occur in the concrete. The reality is that concrete will crack. It’s just a matter of where and when, so using control joints helps direct the crack along the joint without adversely affecting either the soundness of the concrete or its aesthetic value. Typically, a control joint is sawn or tooled to a depth of at least one-fourth or one-third the thickness of the concrete and is completed within 12-24 hours after finishing. Construction joints are located where the casting of the concrete can be stopped and separate large areas or structures.

Left: A single step is never desirable, unless a contrasting border on the upper portion of the landing announces the transition, as shown here. Shadows between surfaces are also good visual clues before transitions.

Right: Concrete can be used to edge flowerbeds.

If you’re thinking about pouring your own concrete patio, think about the joints as you lay out the design. Locating the joints in advance will help create a visual rhythm that can add to the overall aesthetics of your finished project. If the joint pattern isn’t well thought out, it becomes haphazard and you’ll end up with poor results despite your best efforts.

Remember to periodically install “sleeves” below your concrete. These are usually schedule-40 PVC pipes or similar, that allow for installation of future lighting, irrigation or utilities under your concrete walkway, patio or driveway, without having to disturb the concrete itself.

The timing of your concrete is also important. Ideally, ready-mixed concrete should be delivered and placed within 90 minutes after cement has been added to the mixture.

Stamped concrete has become very popular in recent years and can almost be considered an art form. The combination of patterns, colors and finishes is almost endless. It also takes a trained craftsman for the best results. In some cases, resealing stamped or colored concrete is important, but be sure to use the proper sealant. Some sealers can be very slippery when wet, so be sure to ask before selecting a sealer.

No matter your choice of concrete or any of the various staining, coloring or finishes, maintenance is still important. Sealing joints can prolong the life of the joint material. There are also a variety of joint fillers: from fiber, to plastics, to cork, and more that can add a special look to your finished project.

When planning any home improvement, consider not only the installation costs, but also the long-term benefits. Always consider the long-term maintenance, or the ease of maintenance, when choosing your materials.

Reputable concrete contractors can work magic in the landscape. Hardscape features are a significant investment and can add to your property value. They are also long-lasting features and add tremendous enjoyment to outdoor living areas.

 

NOTE: Thank you to Brian Cook, owner of Ozark Patterned Concrete, for sharing his expertise, as well as his designs and patterns, for this article.

 

A version of this article appeared in a July/August 2014 print edition of State-by-State Gardening.
Photography courtesy of Susan Jasan.

 


Susan Jasan is a landscape designer and owner of Landscape Creations by S Jasan, a landscape-design firm specializing in site planning and custom residential landscape design. She can be reached at susan@LCbySJ.com.