The engineering and durability properties of polystyrene injected concrete (PIC) and its performance as a shallow ground improvement

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The engineering and durability properties of polystyrene injected concrete (PIC) and its performance as a shallow ground improvement

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.

Figure 1: Schematic cross section of the 4m2 trial PIC raft constructed for testing of the engineering properties of PIC when utilised as a shallow ground improvement method

The testing carried out involved the use of:

• Static Plate Load Testing (PLT)
• Light Weight Deflectometer (LWD) testing
• Unconfined Compressive Strength (UCS) testing

These tests were chosen to allow comparison of the pre and post static and dynamic performance of the treated areas and to gauge the increase in strength and stiffness of the PIC Raft over time.

For testing purposes, the stripped ground testing was carried out at 250mm below ground level (bgl) in order to gauge the in-situ soil strength and deformability. The proposed treatment area was then excavated to 600mm bgl before placing the 350mm PIC.

2.1 Static Plate Load Test (PLT)

Static PLT’s were undertaken on both the stripped ground surface (250mm bgl) and the finished level of the PIC Raft following 28 days of curing. These tests were carried out to show the improvement in the static soil modulus (Ev) of the treated area following the addition of a 350mm thick PIC Raft ground improvement.

The testing was carried out using a 300mm diameter static plate load apparatus, the dimensions of which are consistent with foundation sizes utilised in NZS 3604. The test procedure followed the German standard for plate load testing (DIN 18 134), with digital acquisition of data, which involves loading the soil to 500 kPa in 7 increments whilst measuring the settlement of the plate, before unloading the soil in 3 increments. The soil is then reloaded in 5 increments to 450 kPa whilst measuring the settlement of the plate to obtain the reloading characteristics of the soil. The plate was placed having direct and full contact on the surface of the test area.

The use of the German standard for static PLT (DIN 18 134) was chosen to allow the calculation of static soil modulus ratios at the site. This allowed the initial static soil modulus (Ev1) and the compacted static soil modulus (Ev2) to be obtained and then compared. Ev1 essentially gives the in-situ static soil modulus while Ev2 gives the maximum static soil modulus that is able to be obtained by the soil following being loaded under 500 kPa of pressure. The modulus ratio therefore shows how close a soil is to being “acceptably compacted.”

The static PLT’s undertaken at the test site showed that the introduction of a 350mm thick PIC Raft as a shallow ground improvement method reduced the immediate static settlements experienced at the treated area under 500 kPa loading from 8.16mm to 0.20mm (97.5% reduction). As shown in Table 1 below, this was accompanied by an increase in the static soil modulus of the treated area from 14.15MPa to 561.47MPa (3970% increase).

Table 1: Soil modulus (Ev) values from static plate load tests for pre and post PIC Raft placement

Ev1 (MPa) Ev2 (MPa) Ev2/Ev1
Stripped Ground Static Plate Load Test 14.15 37.06 2.62
28 Day Static Plate Load Test 561.47 839.89 1.5
Increase in Static Soil Modulus Following Placement of PIC Raft 3970% 2270%

By substituting the 14.15MPa silt with the 350mm PIC Raft ground improvement, the modulus ratio of the site reduced from 2.62 to 1.50. The introduction of the PIC Raft at the site therefore significantly reduced the level of settlement that could be expected at the treated area under static loading.

2.2 Light Weight Deflectometer (LWD)

LWD tests, like the static PLT, were undertaken on both the stripped ground surface (250mm bgl) and the finished level of the PIC Raft following 28 days of curing. These tests were carried out to show the improvement in the dynamic soil modulus (Evd) of the treated area following the addition of a 350mm thick PIC Raft.

The testing was carried out using a 300mm diameter plate and LWD apparatus. The test procedure followed the ASTM E2835-11 standard which involves dynamically loading the soil to 100 kPa whilst measuring the settlement of the plate. This dynamic test is carried out three times before taking the average settlement of the plate under 100 kPa of dynamic loading. This then allows the dynamic soil modulus to be obtained from the results measured. Figure 2 below shows an example of the LWD apparatus used for the dynamic soil modulus testing.

The LWD testing undertaken showed that the introduction of a 350mm thick PIC Raft increased the average dynamic soil modulus from approximately 26MPa to 185MPa (see Table 2). This is an increase of approximately 700% between pre and post improvement.

Table 2: Dynamic soil modulus (Evd) values from LWD tests for pre and post PIC Raft shallow ground improvement

Evd (MPa) Evd (MPa)
Stripped Ground Light Weight Deflectometer Test 27.21 25.45
28 Day Light Weight Deflectometer Test 190.68 178.57
Increase in Dynamic Modulus 701% 702%

C:\Users\Tom Smith\AppData\Local\Microsoft\Windows\Temporary Internet Files\Content.Word\FWD Pic 1.jpg

Figure 2: LWD equipment in use on PIC Raft shallow ground improvement

2.3 Unconfined Compressive Strength (UCS)

UCS tests were undertaken on samples obtained from the PIC Raft at the test site. These tests were carried out at regular intervals during the curing of the PIC Raft to show the improvement in the UCS with time. The testing was limited to 28 days as this is the accepted curing time where concrete is expected to reach its maximum strength (>99%).

The testing was carried out using 100mm diameter and 200mm long core samples drilled from the test site after 7, 14 and 28 days of curing. The test procedure followed NZS 4402:1986, Test 6.3.1 and consisted of measuring the strain (change in length) of the core sample as it is loaded in the UCS apparatus. The samples are continually loaded until they “fail” (rupture) and the maximum stress exerted on the samples is recorded as the peak strength. Figure 3 below shows the peak UCS values for the different samples collected during the curing process.

As shown in Figure 3, the strength of the PIC raft is over 1.0 MPa after 7 days of curing. The strength of the PIC raft continues to increase until a peak measured strength of nearly 3.5 MPa after 28 days of curing. This shows a significant increase in strength of the PIC raft over 28 days to a maximum strength that is approximately 40% greater than the design strength of 2.5 MPa. The Young’s modulus measured during the UCS testing also showed an increase from 912.6MPa after 7 days to 1516.5MPa after 28 days (66% increase).

Figure 3: Development in unconfined compressive strength of PIC Raft over 28 days of curing

3 DURABILITY PROPERTIES OF PICT RAFT

For PIC Raft to comply with the requirements addressed in the New Zealand Building Code (NZBC), the design of the PIC Raft needs to consider:

  • New Zealand Building code B1/VM4-Foundations
    • The strength of PIC must be such that it provides the required stability to a building
  • New Zealand Building Code B2-Durability
    • Laboratory testing of PIC Raft properties and durability performance
    • Performance of materials similar to that of PIC Raft (i.e. concrete)
    • Any limitations to the use of PIC Raft that may cause it not to comply with the requirements of the NZBC

As the strength requirements were obtained during the onsite strength and stiffness testing regime, the durability assessment focused on the durability requirements as set out in the B2-Durability document of the NZBC. As outlined in B2 of the Building Code, the requirement to ensure structural stability of the building and the difficulty of access and replacement of the building element (PIC Raft) require PIC Raft to be durable enough for a 50 year design life.

The durability of standard concrete for structural use (i.e. concrete having a strength between 20MPa and 50MPa) is well covered and addressed in the NZS 3101 standard (2006). In this document, the durability depends on the exposure environment at the given geographic location within New Zealand. Following this, the required concrete mix is then designed accordingly. However, the durability of PIC is outside the scope of NZS 3101.

The intention of the durability assessment was to demonstrate that PIC complies with the NZBC requirements by proof of performance according to Verification Method B2/VM1 based on comparisons to the performance of similar materials (B2/VM1, 1.3 Similar Materials) and successful performance in a set of different laboratory tests (B2/VM1, 1.2 Laboratory Testing).

3.1 Durability Characteristics of Similar Materials

As per B2/VM1 (1.3, Similar materials), an assessment of PIC Raft was undertaken to draw comparisons to concrete. Concrete (as defined by NZS 3101-2006) is “a mixture of Portland cement or any other hydraulic cement, sand, coarse aggregate and water”. PIC Raft is a lightweight concrete design, with the only change to the design of the PIC Raft material from standard concrete being the removal of some heavyweight aggregates and replacement with EPS. PIC Raft is therefore almost identical to concrete (as described by NZS 3101, 2006), with the only exception being the substitution of some coarse aggregates for lightweight EPS aggregates. Given this similarity, the performance of current concrete materials in New Zealand can be used as a guideline for the performance of PIC Raft.

The concrete mix used in the PIC Raft is supplied by well-established concrete suppliers in New Zealand who produce concrete mixes that comply with all the necessary standards.

Some of these standards include:

  • NZS 3101:2006; Concrete Structures Standard
  • NZS 3104:1991; Specification for Concrete Production
  • NZS 3109:1997; Concrete Construction
  • NZS 3111:1986; Methods of Test for Water and Aggregate for Concrete
  • NZS 3112.1:1986; Specification for Methods of Test for Concrete; Tests relating to fresh concrete
  • NZS 3112:1986; Specification for Methods of Test for Concrete

Due to this, the material can be assumed to have similar durability performance characteristics to that of standard concrete and other cement based materials (cement paste, grout etc.).

3.2 Laboratory Testing of PIC Raft Durability Properties

An assessment of PIC Raft’s likely placement conditions found that it is likely to be subject to a number of degradation mechanisms throughout the design life of the material.

The degradation mechanisms that PIC Raft could be subjected to include:

  • Sulphate and chloride attack (corrosive soils, chemical contamination, salt water etc.). This type of degradation would cause breakdown of the cementitious structure of the material over time.
  • Salt attack (coastal/marine environments). This would cause cracking and spalling of the material under repeated cycles of saturation by a salt solution.
  • Freeze/thaw degradation (cold environments, subjected to numerous freeze/thaw cycles). As with salt attack, this mechanism would cause cracking and spalling of the material under repeated freezing cycles.
  • Petroleum based degradation of EPS. EPS is likely to undergo breakdown of the material if subjected to direct contact with petroleum based products. Durability from petroleum is not covered in this report as it is well documented internationally.

The test methods utilised were aimed at assessing the performance of PIC Raft when subjected to the likely mechanisms of degradation mentioned above. The test methods also assessed the properties of PIC Raft, to gauge the materials resistance to these types of degradation.

The type of tests undertaken, the results of the testing and the respective standards followed are presented in Figure 4.

Figure 4: Summary of durability testing of PIC Raft samples

The resistance to salt attack test was chosen utilising both the sodium sulphate and sodium chloride methods in order to model the performance of the PIC Raft under sulphate, chloride and salt attack as well as freeze-thaw degradation mechanisms.

When comparing the results collected during the laboratory testing to the respective standard, it was found that the PIC Raft material performed well. All testing passed the requirements of the relevant standards and results indicated that PIC Raft can be assumed to meet the durability requirements for 50 year design life under ideal conditions, provided the limitations of the material are considered.

The limitations to application of PIC Raft can be considered as follows:

  • Due the lightweight properties of the PIC Raft and the bulk density of the material (850 kg/m3) being less than that of water (1000 kg/m3), there is the potential for buoyancy effects to occur if PIC Raft is used under the water table or at a level where potential water table rise could impact the PIC Raft:
    • PIC Raft should be placed above the water table to remove the potential for buoyancy forces acting on the PIC Raft.
  • Although PIC Raft performed well in the resistance to salt attack testing, one of the test samples showed minor spalling from the sodium sulphate solution testing. Given this:
    • It is recommended that pH testing be undertaken on suspicious sites to confirm that a low pH (acidic) soil is not present. If a low pH is encountered, sulphate testing of the soil should then be undertaken to assess the potential of sulphate attack at the site. If excessive sulphates are encountered, a Damp Proof Membrane (DPM) or similar material should be placed at the base of the excavation in order to isolate PIC Raft from contacting sulphate rich soils.
  • Given EPS’s poor durability when contacted with petroleum products or subjected to excessive and prolonged heat exposure (causing breakdown of the EPS material):
    • PIC Raft should not be utilised where there is potential for the material to come in contact with petroleum based products or be subjected to excessive and prolonged heat (i.e. fire).

4 CONCLUSIONS

Polystyrene Injected Concrete (PIC) Raft is used as a shallow ground improvement material under foundations where unfavourable soil conditions are encountered. PIC Raft essentially consists of cement, water and aggregates including lightweight recycled polystyrene (EPS).

By conducting in situ and laboratory tests, the engineering and durability properties of PIC have been assessed in order to better understand its performance as a shallow ground improvement method.

In situ static Plate Load Testing (PLT) and dynamic Lightweight Deflectometer (LWD) indicated that the introduction of a 350mm thick PIC raft under a 300mm wide plate (designed to replicate a typical NZS 3604 type foundation) producing a static stress of 500kPa and a dynamic stress of 100kPa:

  • Reduced the immediate static settlement of the plate at the test site by 97.5%
  • Increased the static soil modulus of the treated area from 14.15MPa to 561.47MPa
  • Increased the average dynamic soil modulus of the treated area from approximately 26MPa to 185MPa

Laboratory UCS testing showed that the strength of the PIC raft continued to increase until a peak measured strength of nearly 3.5 MPa after 28 days of curing. The Young’s modulus measured during the UCS testing also showed an increase from 912.6MPa after 7 days to 1516.5MPa after 28 days.

PIC Raft provides structural stability to a building and, being placed under a building’s foundation, it is also difficult to replace. As per section B2 of the NZBC, this requires PIC Raft to be durable (i.e. not break down, degrade or lose strength etc.) for a period of 50 years.

Following assessment of PIC Raft’s durability including laboratory testing, an assessment of similar materials and interpretation of the likely performance characteristics, PIC Raft was found to meet the durability requirements for 50 year design life under ideal conditions, subject to the following conditions of use:

  • PIC Raft should be placed above the water table
  • It is recommended that pH testing be undertaken on sites suspected to contain a low pH (acidic) soil, sulphate rich soil or any other suspicious soil chemistry
  • PIC Raft should not be utilised where there is potential for the material to come in contact with petroleum based products or be subjected to excessive and prolonged heat (i.e. fire)

REFERENCES

American Society of Testing and Materials (ASTM) E2835-11 (2015). Standard Test Method for Measuring Deflections using a Portable Impulse Plate Load Test Device. West Conshohocken, PA, U.S.A.

Chandra, S. and Berntsson, L. (2002). Lightweight Aggregate Concrete, Science, Technology and Applications. Noyes Publications, New York, U.S.A.

DIN 18134 (2012). Soil Testing Procedures and Testing Equipment –Plate load test. English translation of DIN 18134:2012-04.

New Zealand Standard (2006). Concrete Structures Standard NZS 3101:Part1:2006. Standards Council, Wellington, New Zealand.

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