Laboratory mixture design procedure for HBM
The strength designation for HBMs is the Rc strength class in (MPa). In the example “C3/4”:
- the first number after C is the Rc (in MPa) at 28 days for cylinders with a slenderness ratio of 2. In this example, it is 3 MPa.
- the second number is the Rc for cylinders with a slenderness ratio of 1 and cubes. In this example, it is 4 MPa.
- the slenderness ratio is defined as the height to diameter ratio of a cylindrical specimen.
The maximum strength class for A10 HBMs of C9/12 is included to mitigate potential issues related to reflective cracking through thin asphalt overlays.
In case of defective works or conflict, the in-situ wet density may be used to verify the strength of the HBM.
For testing freshly compacted material, nuclear density testing to BS 1924-2 using a calibrated nuclear density gauge is the definitive method. The gauge must be used in the direct transmission mode of operation. The source rod must be lowered to within 25 mm of the bottom surface of the layer.
Each test at the location must consist of at least 3 measurements at 120 degrees to each other using the same source rod hole and the density taken as the average of the higher 2 results. Readings must be taken within two hours of completing compaction.
The in-situ density of HBM must be taken as the average of the wet density at five positions determined by the authority, and must achieve no less than 95% of the nominal wet density at the design strength specified.
For laboratory testing purposes, HBM samples can be cast in cubes or cylinders. Where agreed, coring may be used to obtain cylindrical specimens. However, low strength HBMs (i.e. anything less than strength class C6/8) or mixtures using slow hydraulic binders, standard unconfined compressive strength tests tend not to give reliable results.
In-situ testing can also be used to determine stiffness (e.g. via Light Weight Deflectometer) and density (e.g. via Nuclear Density Meter). Tests for HBMs are defined in the various parts of BS EN 13286. Test methods for density of HBM are also detailed in BS 1924-2.
