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Concrete Pavement Research

Introduction

Two HVS experimental concrete sections were constructed near Hilton, approximately mid-way between Pietermaritzburg and Howick, adjacent to the N3.

The scope of the study is to investigate the effect of the following variables in the concrete on load transfer at a crack over time:

•  Joint type:
a) Dowels;
b) Plain Jointed with aggregate interlock

•  Aggregate type:
a) Dolerite 26 mm stone size
b) Quartzite 26 mm stone size

To concrete test sections were constructed as follows:

•  A PCC section containing 26 mm dolerite stone, with 6 plain/aggregate interlock joints and 6 dowelled joints; and
•  A PCC section containing 26 mm quartzite stone , with 6 plain/aggregate interlock joints and 6 dowelled joints.

Figure 1: Cross-section of pavement structure

Pavement Structure of Trial Sections

A cross-section of the pavement structure is given in Figure 1 above. The selected and subbase layers were constructed with weathered granite material of G5 quality to increase the stress at the joints and thus to enhance early failures.

Layout of Concrete Trial Sections

Each HVS test is done on an 8 m section. This means that if a standard 4 m-panel length is used; two joints are tested. By appropriately positioning the HVS, the full length of the mid panel (4m in length) together with half of the lengths of the adjacent panels are tested in each HVS test. Thus, to conduct two HVS tests per variable, a minimum of six 4 m-panels were required. In addition, panels had to be provided at the beginning and the end of each test section to serve as a uniform foundation on which to position the HVS (see Figure 2).

Crack inducers (45 mm x 3 mm steel plates) were cast into the concrete at 4 m intervals at the bottom of the slabs to form aggregate interlock cracks at the predetermined positions. For the doweled joints R20 dowels at 300 mm spacing supported on 65 mm x 3 mm steel plates were placed at the bottom of the slab.

Along the plain aggregate interlock joints, the dolerite aggregate concrete slab has cracked in only two joint positions and along the doweled joint section, it has cracked in only one joint position. Along the plain aggregate interlock joints, the quartzite aggregate concrete slab has cracked in one place, approximately 300 mm off a joint and along the doweled joint section; it has cracked in only one joint position.

Figure 2: Schematic layout of trial sections

Construction Techniques

The layer works for the concrete slabs were constructed with standard construction equipment. No special techniques were required. The subbase had to be constructed to strict level tolerances in order to create a sound working platform for the concrete. The shuttering and joint formers were put up as can be seen from Photo 1, with special strain gauges on custom made brackets for measuring shrinkage at the cracks (Photo 2).

The concrete was delivered to site in rotating drum mixers and poured into the shuttering with a chute (Photo 3). The concrete was then levelled by hand (Photo 4), compacted with poker vibrators and levelled with a wooden float (Photo 5). The concrete surface was treated with a white-coloured resin based (Resincure HW) curing compound, after finishing (Photo 6).

Photo 1: Shuttering for dolerite concrete section complete with joint formers for aggregate interlock joints – dowel cages in background

Photo 2: Aggregate interlock joint former and shrinkage measuring devices

Photo 3: First concrete pour

Photo 4: Hand-levelling of concrete

Photo 5: Levelling the concrete

Photo 6: Completed concrete slabs

Deflection Testing

Falling Weight Deflectometer (FWD) testing was conducted during October 2003. On both concrete test sections, tests were done across each joint position in both wheel tracks. The relative movement calculated across each joint position, is indicated in Figure 3.

Figure 3: FWD relative movement across joint positions

In the direction east to west, the concrete cracked close to joint number 3 and at joint number 7 on the quartzite concrete section. Similarly, the dolerite concrete section cracked close to joint number 2 and at joints number 4 and 7. From Figure 3 it is obvious that increased relative movements occurred where the concrete was cracked.

Preliminary Results

HVS testing of the concrete pavement trial sections commenced on 2 November 2003 across an aggregate interlock joint in the quartzite aggregate concrete slab. Pumping of the subbase material through the joint was visible after 7 700 load repetitions and to the side of the slab after 40 400 load repetitions of a 40 kN dual wheel (see Photos 7 and 8).

Photo 7: Pumping of subbase material through aggregate interlock joint after 7 700 load repetitions

Photo 8: Pumping of subbase material at side of aggregate interlock joint after 40 400 load repetitions

Preliminary results of this first test indicate a strong linear relationship exists between the loads applied and the relative movement between the two slabs at the joint; with an increase in relative movement with increasing load repetitions (see Figure 4).

Figure 4: Relative movement across aggregate interlock joint in quartzite aggregate concrete slab