Mitigate Urban Heat Island Effect with Light Color Pavements
Light Color Pavers and Sustainability
For hardscape owners and designers color is a very important consideration for aesthetics and function, but heres another factor: color can also impact sustainability. Choosing lighter colors can help mitigate the urban heat island effect.
Urban Heat Islands
Heat islands can result in cities when built structures and paved surfaces radiate energy from the sun to a greater extend than farmland or natural areas. In this way, cities create their own microclimates that can be up to 7° F (4° C) warmer than the surroundings.
Some examples of how heat islands negatively impact our environment include more air conditioning use, increased air pollution and green house gas emissions from power plants meeting air conditioning energy demands, and lower human health and wellbeing from excessive heat.
Landscape architects can play a role in cooling cities by specifying increased tree and vegetation cover, adding living or green roofs to structures, and selecting light color surfaces also known as cool pavements.
Research was done in New York City which found that planting trees and vegetation would be greatly beneficial to cool surfaces. However, they encountered a common urban problem - in many New York City neighborhoods there is no space.
Many large cities just don’t have the space required to plant enough new trees and greenery for an effective heat island reduction strategy.
Therefore, the most attainable approach is often to redevelop the large areas of dark, paved surfaces with lighter surface materials.
LEED® Credits for Heat Island Reduction
Heat Island Reduction - Non-Roof: LEED v4.1
LEED points can be awarded for paving materials with an initial solar reflectance (SR) value of at least 0.33.
The Solar Reflectance Index (SRI) is a criterion used by US Green Building Council (USGBC) that measures values of sunlight and radiation bouncing from built surfaces.
SRI is used to indicate how hot a material is likely to become when its surface comes into contact with solar radiation. On a scale of 0 to 100, standard black is 0, and standard white is 100. According to this scale, testing indicates that absorbent materials have lower numbers while reflective materials have higher numbers.
Applying this to the hardscape environment, it follows that dark pavements have low SRI values, whereas light pavements typically have higher SRI values.
In other words, light colored surfaces absorb less heat and make the immediate area more comfortable – think playgrounds or pools where bathers have bare feet. Lighter surfaces also reduce the need for nighttime lighting and make areas safer.
Combining light colors with permeable pavers can provide even more cooling benefits because permeable interlocking concrete pavers (PICP) are designed and constructed to lower surface temperatures through evaporative cooling as well. The Interlocking Concrete Paver Institute (ICPI) has many useful resources for designers at www.icpi.org.
Light Color Paver Protection
New surface treatments from ACM Chemistries protect paver surfaces from fading and stains. Lighter colors no longer have to appear washed out or marred by food or dirt stains. Colors and patterns stay vibrant and stains and dirt can be easily removed.
For more information on surface treatments, check out https://www.acmchem.com/dry-cast-paver-surface-treatments/
One of the more recent developments in dry cast or zero slump concrete paver production is “face mix”. The difference between face mix and conventional thru-mix pavers is that thru-mix paver mix design and color is the same throughout, whereas with face mix pavers there is a pigmented surface layer with a finer aggregate blend. Face mix pavers concentrate expensive pigment, white cement and finer aggregate in a surface layer where they have the most impact. The base contains larger aggregates for higher compressive and flexural strength, and improved durability. This method is well established in the US and has been used in Europe for decades.
In the picture we see examples of face mix pavers with concentrated color on the surface layer.
Face Mix Analogy
How are Dry Cast Concrete Pavers Made?
Most traditional dry cast or zero slump pavers will follow the steps below. Bear in mind that producers are coming up with new and interesting production methods and paver finishes every day!
Mixing raw materials until a homogenous mix is obtained
Feeding mix into a mold
Compression into mold
Inline surface treatment (if applicable)
Dry cast or zero slump concrete holds its shape immediately after a mold is removed, similar to packing sea sand into a bucket (mold) to make a sand castle on your beach vacation.
In dry cast concrete production the raw materials are fed into a mixer which combines them until the mix is homogenous. The mixing is important to get the cementitious materials and water in contact so that a chemical reaction called cement hydration can occur. During the hydration reaction, cement and water interact to form cement paste which hardens and becomes the “glue” that holds the aggregates together.
Dry cast paver production is a highly automated process. The business model of dry cast manufactured concrete products depends on highly efficient, mass production of concrete units that are also efficient to install: at a minimum concrete paver units must be sufficiently strong, dimensionally correct and dimensionally stable when they are installed.
Large, sophisticated plants can cost millions of dollars with new options for molds and finishes becoming available every year.
Dry cast concrete units are mixed and molded into shape in minutes. The freshly compacted units are able to hold their shape immediately after mold is removed including during transportation to the curing station, which is often a kiln. A kiln is a controlled environment where temperature and humidity are optimized to maximize cement hydration, strength gain and color development by the concrete. Dry cast concrete must be cured, usually for at least a few days, so that it can gain sufficient strength to withstand handling, installation, traffic loads and weathering over time.
Inline surface treatments are spray applied to the paver surface before the units are cured, and are bonded to the paver surface during curing. Inline treatments are used to enrich color and protect paver surfaces from staining and fading.
For comparison purposes, see the dry cast vs wet cast production and output summaries below:
Example of dry cast step units and segmental retaining wall
Example of wet cast step units and segmental retaining wall
Concrete Pavers - a Brief Explainer
Concrete pavers are made out of … concrete. Sounds obvious, but there are actually at least two different types of concrete used to make pavers and slabs – dry cast (zero slump) and wet cast concrete. Dry cast concrete is the most common, so lets deal with that first.
The word “concrete” comes from the Latin concretus which means to grow together. This is appropriate because the magic of concrete is that it starts off as loose solid materials, when water is added, the water and cement become a paste that binds the aggregates into a formable mass which fills the shape of the container that it is in. The cement and water chemically react, harden and strongly bind the aggregates together – but this time in the exact shape that we want.
Fundamentally concrete consists of aggregates (sand and stone), cementitious materials and water.
Aggregates are the largest component, typically around 70% by volume. This means that aggregates have a big impact on concrete performance. Because aggregates are heavy, they are usually a locally sourced raw material. Many parts of North America have high quality natural aggregates like limestone and granite. Other areas, Florida for example, have less optimal choices.
In modern times, Portland cement is often supplemented with pozzolans which also fall into the category of cementitious materials. Cementitious materials include regular grey Portland cement, blended cements and pozzolanic materials such as slag cement and fly ash.
Slag cement and fly ash are made from waste products from steel manufacturing and coal burning industries. Pozzolans used to be cheaper than Portland cement, but this is no longer always the case. LEED and other environmental credits can be obtained when recycled pozzolanic materials are substituted for Portland cement.
How does cement work?
Cement hydration is a chemical reaction where cementitious materials and water interact to form a new compound that sets up, hardens and gains strength over time.
Most concrete producers today use a combination of cement and pozzolans. Using combinations of materials is often a win-win-win as it improves concrete performance while lowering cost, and is good for the environment.
In the past 50 years, as concrete has improved performance and decorative appeal, admixtures and pigments have become routine ingredients in the mix
Dry Cast Concrete paver proportions:
Introduction to Dry Cast Concrete Pavers
We tend to think of public transportation as a modern invention – but the Romans used segmented paving stones for public highways that sped troops, trade goods, tax collectors and administrators that ran and funded the empire.
Roman roads were paved with locally available stone materials, but the Romans knew and applied sound, common engineering practices:
A bedding layer of cementitious materials
Several layers of graded rubble topped with paving stones to ensure mechanical durability and proper drainage
Today, 2500 years later, we still follow the same principles.
In modern times, concrete pavers took off as a building material in Europe after World War II.
Rebuilding efforts after World War II faced major shortages of building materials. German and Dutch road designers and contractors were forced to find replacements for clay brick which was needed for houses They developed cost effective, uniform size concrete-based pavers made from readily available materials. The new concrete pavers were tolerant of unstable sub-base and traffic loads, and could be installed by relatively unskilled labor.
In the 1960s a German Engineer Fritz von Langsdorff licensed a shaped interlocking concrete paver (ICP) and developed shapes and colors that tremendously increased design choices
Other German manufacturers followed suit with improved manufacturing and installation methods
Interlocking concrete paver advantages included dimensional consistency, pavement strength and stability, and moisture tolerance. Designers also had access to shapes, colors and textures not available in concrete or clay brick.
In the 1970s the interlocking paver concept was imported into North America, starting in Canada and working south into the US.
Over 50 years many innovations have been introduced, including