Abstract
Polystyrene injected concrete (PIC) is a type of lightweight concrete that internationally has a number of different applications, ranging from thermal insulation to shallow ground improvement (PIC Raft). Due to the lightweight character of this material, several benefits arise from its use in buildings and infrastructure. The engineering community has significant understanding of the strength characteristics for PIC. However, less is known about the durability characteristics of this material. The intention of this paper is to provide some insight into the durability of PIC in conjunction with its onsite engineering performance under static and dynamic loading when used as a shallow ground improvement method. This paper presents the engineering properties that have been measured on site and in the laboratory. The onsite engineering properties have been assessed by means of static plate load testing (PLT) and light weight deflectometer (LWD) undertaken on a 4m2 trial PIC pad that was constructed at a low soil modulus site in Christchurch. Laboratory results of the engineering and durability properties are also presented which were undertaken with the respective AS/NZS standards. These test methods include: unconfined compressive strength (UCS) testing, breaking load testing, moisture content, ambient density, dry density, cold and boiling water absorption and hydraulic permeability. The testing also included cycles of soaking and drying PIC specimens in sodium chloride and sodium sulphate solutions. This was carried out to model PIC’s resistance to salt attack, as well as degradation from chlorides and sulphates. A discussion of both the onsite and the laboratory performance of PIC is presented with useful observations that were carried out.
1. INTRODUCTION
Lightweight Aggregate Concretes (LWAC) are available in a wide range of densities, strengths and sizes (Chandra and Berntsson, 2002) and are used internationally for a wide variety of applications. Polystyrene Injected Concrete (PIC), a type of LWAC, consists of cement, water and aggregates, including lightweight recycled polystyrene (referred to as EPS). EPS is a low strength but lightweight material that provides lightweight properties to the PIC, while also recycling a material that cannot be disposed of due to its inability to break down. As a result, PIC has a lower compressive strength and modulus than standard concrete but has a lower density and higher modulus than common soils or engineered fills. These properties are considered advantageous for a number of applications.
Lightweight aggregate concretes (LWAC) have been utilised for applications that include:
- Lightweight precast panels for cladding applications; the thermal properties of EPS provide significant insulation performance
- Lightweight backfill for retaining structures; LWAC produce lower lateral earth pressures compared to normal soils
- Void filling applications e.g. decommissioning of underground storage tanks or filling underfloor voids
- Stabilisation of subgrades for roading applications
- Raft type shallow ground improvement under shallow foundations
While PIC has had limited use in New Zealand to date, it’s utilisation as a shallow ground improvement method has begun to gain traction following the recent earthquake events (Canterbury and Kaikoura) that have impacted infrastructure significantly.
The design mixture used for testing utilised an 850 kg/m3 mixture designed by Axis Policon Ltd. The testing carried out was commissioned by Axis Policon Ltd., in order to better understand the performance characteristics of their PIC Raft shallow ground improvement product.
2 Engineering properties – Pic shallow ground improvement triaL
Testing was undertaken at a site in Christchurch to better understand the engineering properties and performance of PIC when utilised as a shallow ground improvement method. Testing was carried out firstly on the untreated subgrade material (Silt, non-plastic), to obtain the performance characteristics of the in-situ soil. Controlled construction of a 4m2 PIC Raft (see Figure 1) was then carried out in order to allow testing of the strength and deformation characteristics of the PIC Raft and to gauge the level of “improvement” that had occurred at the site.
