SUB BASE DESIGN

Cement (Hydraulically) Bound Materials

This group refers to those materials that use cement as a binder, ie, a 'glue' to stick together the individual particles. 

Mass Concrete
This is only normally used in applications where:- 
exceptional loads are anticipated 
the paving units are to be directly bedded, such as setts, cobbles or duckstones 
bad ground may be a problem 
It is more expensive than either of the above methods. A 'slab' of mass concrete, usually 100mm or 150mm thick is spread and compacted, then allowed to set. A few days later (2-7 days, depending on time of year and weather conditions), the paving is laid on a bedding material over the now solid concrete. 

A medium strength concrete is used in most applications. Concrete is best supplied ready-mixed to ensure thorough mixing and a correct ratio of cement to aggregates. Strength C20 is the usual grade, and a 25mm slump is reasonably stiff and not too 'runny' to handle. If your application warrants a concrete sub-base, it would be wise to consult a paving /concrete contractor, or civil engineer for accurate specification. 1m³ concrete covers 10m² at 100mm thick and 6.5 m² at 150mm thick. Concrete is ordered and delivered by volume, not by weight. 

CBM - Grades 1 - 5
There is a range of cement bound materials that are not as inherently strong as the mass concrete described above, but are considerably stronger and stiffer that unbound crushed rock sub-bases. These have limited applications for small paving projects such as driveways and patios, but are used on larger commercial applications, such as pedestrian schemes and car parks, as well as roadways and filling station forecourts. 

The Specification for Highway Works (SHW) arranges CBMs into 5 categories described below... 

CBM 1 - clause 1036 of SHW
This is the 'weakest' of the CBMs and was previously known as 'soil cement'. The aggregate, which can be almost any granular material, including recycled materials, is mixed with cement and water and then compacted in place. The quantity of added cement has to be sufficient to achieve a 7-day crushing strength of 4.5N/mm². It can be batch mixed, which offers control over weight/volume of aggregates and cement, or mixed in-situ by, effectively, rotavating cement into an existing layer of suitable material. 
There is a slightly stronger version, CBM1a, which has a 7-day strength of 10N/mm² 
It is most commonly used as a capping layer or low-grade sub-base beneath flexible pavements. 

CBM 2 - clause 1037 of SHW
This is a coarser and stronger material than CBM1, being based on a 40mm down granular aggregate or blast furnace slag, and sufficient cement to reach a 7-day strength of 7N/mm² CBM2 can be batch mixed or mixed in-situ. Again, there is an 'enhanced' version, with the stunningly original title of CBM2a, and this would have a 7-day strength of 10N/mm² 
This, too, is used as a capping or sub-base layer beneath flexible pavements. 

CBM 3, 4, and 5 - clauses 1038 and 1039 of SHW
These materials are closer to what is commonly thought of as concrete, and are what used to be called 'lean mix'. They are mixed in batch plants, using approved natural aggregates (BS812 and BS882) although they can incorporate suitable recycled aggregates, such as crushed concrete and ggbfs (ground granular blast furnace slag). Typical size of aggregate is either 40mm or 20mm, and 7-day strength is a minimum 10N/mm² for CBM3 and 15N/mm² for CBM4. 
These materials can be used as road bases beneath heavy-duty paving, such as major highways, as well as being used as rigid sub-bases or high strength capping layers. Because they form an integral part of the final structure, quality control is paramount and so these materials must be batch mixed and they are typically laid by a slipform paving machine. 
 
Typical Sub-base Designs
Here are four different types of sub-bases. Note that these are not 'official' classifications, in that they are not recognised by national building and civil engineering authorities. They are a simple generalisation, to aid comprehension of the various specifications for sub-bases discussed within this web-site. 

In most cases, one of the following types should be suitable for residential projects. The notes on each paving type will refer to one of the following sub-bases. In extreme conditions, e.g. very heavy loads expected on paving, or bad ground conditions, a special sub-base may need to specified for your particular situation.