What lies beneath: key considerations for 3G pitch specification

With cost, permeability, and performance all key contributing factors in the construction of 3G pitches, and specification largely governed by carpet manufacturers, choosing the most appropriate sub-base can prove to be a challenging process for contractors and end-users alike. Here, Tim Edwards, Technical Sales Manager Tarmac's Topsport product line, explains the importance of choosing the correct sub-base system for an artificial pitch.

Once solely found at state-of-the-art professional sports clubs, 3G pitches are a synthetic surface option that have rapidly grown in popularity across a variety of applications over the last decades. Now they are just as likely to be installed at a school as they are a Premier League Academy.

This has largely been attributed to the enhanced usability of an artificial installation, as they can be used in most weather conditions. Not only does this lead to year-round playability, but for those sites that hire out their facilities, it can lead to improved revenue streams too.

However, while the playing surface, or carpet, is often the main talking point of an installation, the sub-base is arguably just as - if not more - important given it provides the 'feel' of a pitch and aims to help mirror the playability of a full grass installation. It also determines the ability of a pitch to both hold the optimum amount of water within the surface, and ensures any excess is able to freely drain away. As such, it can be very expensive and time consuming to correct if done wrong.

3G pitches are typically installed over one of two types of base - dynamic or engineered - with budget often being the main factor when commissioning an installation. Dynamic bases offer a more accessible route for the likes of schools and amateur sports clubs, as engineered bases are likely to be more suited to professional installations given their relatively higher cost.

An engineered base tends to consist of a Type 3 stone sub-base, topped by a porous open-textured asphalt layer and shockpad. This ensures long-lasting performance and excellent stability, and is likely to operate effectively for a number of carpet cycles. However, given the cost of installing an engineered asphalt base, it is frequently used by elite sport teams rather than by club teams or schools, simply due to expense.

A dynamic base is a more accessible way for those clubs and schools with smaller budgets to achieve a 3G pitch which replicates a natural grass surface. Typically, a dynamic base consists of a crushed limestone bed which is then topped with a shockpad (although this step is optional) before the carpet is laid. Dynamic bases are also particularly hardwearing and able to withstand significant impact - ideal for training or municipal pitches which will be in regular use throughout the year, or pitches which will double up as both rugby and football pitches.

Getting the right blend of material in a dynamic base is a very specific process and crucial to guaranteeing the permeability and performance of an artificial grass surface. One of the more common errors we see when creating a dynamic base is using a blend that fails to balance the drainage and stability requirements of the pitch.

To give some context, a base consisting of 2-6mm crushed limestone will offer excellent permeability and allow any water to freely drain through the pitch without waterlogging the carpet, yet will fail to deliver the requisite stability to allow the contractor to lay an even carpet. This could lead to a bumpy and uneven finished installation which would certainly be noticeable when playing on it. Conversely, a purely 0-6mm blend would offer improved stability but inferior drainage. To overcome this issue, Tarmac's Topsport team has designed a bespoke blend of 0-6mm Dynamic Layer stone which is both inherently stable, yet offers the necessary permeability to allow water to freely drain from the carpet surface at the optimum rate.

However, even blends which are labelled the same can vary considerably across the country and from quarry to quarry. In particular, the type of limestone used and the way it is crushed can impact the composition of the dynamic layer. For example, a hard or carboniferous limestone is likely to produce a less fine dust, while a softer magnesium limestone is likely produce much more dust.

To combat this, at Tarmac we blend our Topsport dynamic layers in line with BS EN 1097-6:2013 Clause 9, and ensure we keep consistency in our blends across the country by looking at the source components, offering a bespoke blend per supply unit. This is particularly useful in minimising geological issues and keeping logistics down on a project, as the cost of transporting materials can quickly add up.

With minimal industry standard specification routes or legislation to follow, determining the most appropriate product can be a challenge for end-users who must rely on manufacturers and contractors to guide them correctly. Carpet manufacturers often provide a fairly specific sub-specification of what they'd like a contractor to use with their products, to ensure optimum stability and the most effective co-efficient of permeability to prevent the carpet from becoming waterlogged. However, this can vary considerably depending on the turf type and manufacturer.

Ultimately, the sub-base construction can make or break the performance of a 3G pitch. Failing to get the right blend of aggregates can easily result in a surface that is uneven or poorly draining. In the absence of an overarching standard governing dynamic base composition, contractors must commission pitches in line with carpet manufacturer guidelines. However, the ambiguity often associated with such specifications can create room for error. With contractors ultimately bearing the responsibility of pitch construction, taking the extra time to fully understand the intricacies of dynamic base composition can be the defining factor between a happy or disgruntled end-user. For those contractors keen to know more, I'd encourage them to get in touch with a member of the Topsport Technical Sales Team.

For more information on Tarmac's range of Topsport artificial sports surface solutions, please visit www.topsport.co.uk.