General Concepts > Input Guide

In the absence of more specific values please use the figures below. More information is available in the topics whose names are highlighted in blue - just click the blue text.

Input Variables

1	Cement content	The cement content normally varies between 300 kg/m3 and 450 kg/m3.
2	Water content	The amount of water in the concrete mix, litres/m3, is normally between 170 l/m3 and 210 l/m3.
3	Flexural strength	Usually between 3.5 MPa and 5.5 MPa. It depends on water content and cement content. If fibres are used this value can be as high as 14 MPa.
4	Layer 1 (slab) thickness h1	For new heavily trafficked pavements between 200 and 300 mm. For new low volume roads between 100 and 180mm. For overlays and inlays between 100 and 250 mm, or 50 - 100 mm for ultra-thin continuously reinforced concrete (UTCRC), depending on support and traffic.
5	Layer 2 (subbase) stiffness E2	Above 20 000 MPa for a concrete overlay on an old concrete pavement. Above 1 000 MPa for stabilised or asphalt subbases. Between 300 MPa and 600 MPa for natural gravel subbases. Above 30 MPa for in situ materials.
6	Layer 2 (subbase) thickness h2	Practically between 100 mm and 150 mm for stabilised or asphalt subbases. Can be as low as 30 mm for an asphalt bedding or interlayer.
7	Layer 3 stiffness E3	Appropriate value to be chosen by the designer based on material availability.
8	Layer3 thickness h3	Appropriate value to be chosen by the designer. If layer does not exist then the value is 0 mm.
9	Layer 4 stiffness E4	Appropriate value to be chosen by the designer based on material availability.
10	Layer 4 thickness h4	Appropriate value to be chosen by the designer. If layer does not exist then the value is 0 mm.
11	Layer 5 stiffness E5	This is the in situ material. Appropriate value to be chosen by the designer is based on material availability.
12	Growth of HV traffic	Typical values are between 1 % and 6 % p.a.
13	Speed of heavy vehicles	Between 5 km/h and 120 km/h depending on the longitudinal gradient.
14	Contact pressure	Between 0,4 MPa and 0,85 MPa depending on the tyre rating.
15	Annual rainfall	The average annual rainfall in mm.
16	Daily temperature cycle	Between 9o C and 20o C depending on the location and its climate.
17	Joint movement	Only applicable for old pavements where value ranges from 0 to 1,0 mm. For new pavements use 0 mm.
18	Void extra	Enter 0 unless the subgrade consists of untreated expansive clays or collapsible sands. When substantial voids can be expected use the value of 2 as a maximum.
19	Initial IRI	If in doubt use the value of 1.5 m/km.

Input Constants

1	Life period	This is the period of anticipated functionality of the road. Values between 30 and 50 years have been used. A typical and often-used value is 40 years.
2	Average daily truck traffic (ADTT)	Heavy vehicles per day per lane. The counts may vary between a few and few thousands. To indicate a high figure: in 2002 the ADTT measured in the busiest outer lane of the N3 Freeway in the vicinity of Pietermaritzburg was 1691 heavy vehicles per day per lane.
3	Axles per heavy vehicle	The number varies from an obvious minimum of 2 to a practical maximum of 9. In the above example, the average number of axles per heavy vehicle was 5.05.
4	Loads on edge	Wheel loads placed closer than 300 mm to the edge of the pavement are considered to be edge loads. The value depends on geometry and layout of the pavement. With lane-specific inlays this figure may be as high as the maximum of 50 %.
5	Damage constant	From a range between 250 000 and 500 000, a value of 330 000 has been used on freeways in SA.
6	Damage exponent	The value of 4.5 is recommended.
7	Ratio k1 = E1/f	Recommended value is 7700 for normal concrete and 5000 for steel fiber reinforced concrete.
8	Ratio k2 = ITS/f	Recommended value is 0.75.
9	Bond	A value between 0 and 10 indicating the cohesion between the slab and the subbase. Zero implies no cohesion; 10 means a perfect bond.
10	Drainage factor	A value between 0 and 10 indicating the permeability of the slab by surface water and its ability to drain water. Zero implies no ability to drain; 10 means a complete absence of free water below the pavement surface.
11	Erosion factor	A value between 0 and 10 that is indicating the susceptibility of the subbase to erosion and pumping. Zero implies no erosion and pumping effect; 10 means a total vulnerability. Please consult the table in Voids topic.
12	Joint spacing	Spacing should be 25 to 30 times the slab thickness, with a maximum value of 4,5 m.
13	Steel diameter	For dowels the value is normally 10 - 15 % of slab thickness, and for CRC the value is approximately 5 - 10 % of slab thickness.
14	Steel spacing	For dowels the spacing is normally 300 mm and for CRC this should be adjusted to achieve a crack spacing of between 1.5 and 2.0 m.
15	ACV	Extreme values are 1 and 30; lower number means stronger aggregate. Typical values range between 15 and 25, depending on the aggregate type.
16	Aggregate type	The type of fine aggregate used in the concrete mix expressed as a factor that varies from 0.7 to 1.3
17	Aggregate size	Usually between 9.5 mm and 37.5 mm but should not exceed 25% of the slab thickness.
18	Cement type	The type of cementitious material used in the concrete mix expressed as a factor that varies from 0.9 to 1.1.
19	Aggregate content	The content of coarse aggregate in the concrete mix which is usually between 0.65 and 0.75.
20	Fibre content	The quantity of fibres introduced into the concrete mix; it normally varies between 40 kg/m3 and 80 kg/m3.
21	Fibre factor	A factor that depicts the type, shape and roughness of fibre. Use 0.7 for cold-drawn, straight, rough high quality steel fibres with hooked ends.
22	Humidity	The annual average relative humidity, in %, at the location of the pavement.
23	Cost of concrete	This is the unit capital cost of the slab (in R/m3) that includes the supply, placing, finishing, curing and transverse jointing of concrete.
24	Cost of steel	The unit cost (in R/t) includes supply of steel, its placing, fixing, and support cages.
25	Cost of subbase	The unit cost (in R/m3) includes supply, placing and construction of subbase.
26	Cost of subgrade	The unit cost (in R/m2) includes supply, placing and construction of subgrade.
27	Structural repair cost	Unit cost, in R/m2 of shattered concrete; it covers the cost of removal and replacement of damaged concrete and includes supply, placing, finishing, curing, formwork, jointing and steel.
28	Surface repair cost	Unit cost, in R per annum/m2 of the total surface area of concrete; it includes joint resealing, sealing of cracks as well as temporary repair of potholes and punchouts.
29	Cost of fibres	The cost, in R/kg, of supplying and introducing fibres into the mix.

Years to first rehabilitation. This is the period after which major repairs will have to be done for the first time. A low figure, such as 7 years will allow the construction of a relatively weak but inexpensive pavement. However, this pavement will have to be attended to frequently. A high figure, such as 25 years, implies a strong pavement with long periods between repairs. However, the capital cost of this pavement may be relatively high. Use the What-If facility to find out the trade-off between the road standard (% shattered concrete, % pumping, % faulting) and the overall pavement cost.

Distribution of axle loads. A collection from which one can select an appropriate distribution is available on the Axle Loads Page. If one cannot choose any of the distributions offered, there is an option to enter one's own figures via the 'Other' radio button. Three aspects of traffic should always be taken into consideration together: the average daily truck traffic (ADTT), the average number of axles per heavy vehicle, and the distribution of axle loads.