Housing Authority Research Fund

Project Title : Developing Innovative Precast Systems, including Application of Structural Lightweight Aggregate and Rapid Hardening PFA Concrete, for Economic Production (ref: CB20030026)

Summary of Report

Research Provider : City University of Hong Kong

Project Coordinator : Dr. Tommy Y. Lo Department of Building and City University of Hong Kong

Disclaimer : The research provider owns all the intellectual property rights of the research and the research report, including any registered patent and registered design as the case may be. Permission to use the report or any part thereof must be obtained from the research provider.

All right reserved. No part of this Summary of Report may be reprinted or reproduced or utilized in any form or by any electronic, mechanical, or other means, now known or hereafter invented, including photocopying and recording, or in any information storage or retrieval system, without permission in writing from the research provider.

2006

Paper: HARF Project by City University of Hong Kong

Project: Developing Innovative Precast Systems, including Application of Structural Lightweight Aggregate and Rapid Hardening PFA Concrete, for Economic Production

1. Executive Summary

In June 2003, the Housing Authority Research Fund commissioned the City University of Hong Kong to undertake a research study on the feasibility of using structural lightweight concrete (LWAC) in precast systems for high-rise housing block construction. The study commenced on 23 June 2003 for a period of two years.

As structural lightweight concrete is typically 25% to 35% lighter than ordinary concrete yet exhibits a compressive strength for the same order. LWAC can offer design and construction flexibility and savings in the overall project cost. Compared with the NWC, LWAC has better thermal and sound insulation, resistance to fire, impact and cyclic loading, and reduced micro-cracking at cement/aggregate interface. However, structural LWAC has not been widely used in Hong Kong.

This study has examined the properties of LWAC, mix design, design standards, relevant codes of practice and the current use of precast concrete construction for high-rise structures in both Hong Kong and overseas. The study has also reviewed the precast techniques being used in the Hong Kong Housing Department’s high-rise domestic housing blocks. Prototype design and test methods were proposed and laboratory testing and full scale trial were carried out for design verification and modification.

In this study, the lightweight expanded aggregate (Leca) was used to produce the lightweight aggregate concrete. 60 trial mixes including PFA were made and tests on the main concrete properties were carried out. According to the strength results, the mixes were categorized into 30 and Grade 35.

This study concluded use of PFA in precast concrete is not recommended for reasons of cost, technical and practical considerations.

With the exception of shrinkage and elastic modulus, the engineering properties of LWAC including strength, workability, bonding strength and water permeability are not obviously different with those of NWC. The flexural and shear behaviors of the reinforced/precast composite slab prototype have been provided in stable and reliable performance and complied with the theoretical calculation. The LWAC can meet the requirements for structural applications.

This report presents the test results and findings of the testing programme, which forms the basis for further application at project level. This report provides recommendations of adopting structural LWAC in precast systems as an alternative to normal weight concrete for high-rise structures.

2. Final Results and Findings

2.1 Properties of lightweight aggregate concrete compared with normal weight concrete

Density of LWAC is some 25% to 35% lighter than normal weight concrete

LWAC does not show significant difference of the engineering properties including strength, workability, bonding strength and water permeability.

Shrinkage of LWAC is about 33% higher than that of NWC.

Elastic modulus of LWAC is about 20% lower than that of NWC.

The effect of cement content on LWAC is not significant. With cement content increases, compressive strength of concrete do not increase significantly.

Though the bond strength of LWAC increases with strength grade increasing, the results do not indicate significant difference between Grade 30 and 35.

Flexural and shear behaviours of the reinforced/precast composite slab prototype have demonstrated a stable and reliable performance and complied with the theoretical calculation.

The gain in strength of LWAC, with or without PFA, is similar to NWC.

LWAC mixes added PFA can reach Grade 30 and 35. Based on LWAC test data fallen within the grades, mechanical properties of LWAC with PFA are similar as those of LWAC without PFA.

Based on limited data of selected PFA mixes, the durability of PFA added LWAC are improved.

Rapid hardening PFA concrete is not recommended to be used in precast concrete for reasons of cost, technical and practical considerations.

Adoption of admixtures in LWAC is similar to NWC when the required concrete slump does not meet the requirements

2.2 Design of structural lightweight aggregate concrete

Though density of LWAC is much lower than NWC, cube compressive strength of LWAC can meet Grade 30 (44MPa) and Grade 35 (49MPa), with the average compressive strength at 28 days exceeded the respective grade strength by at least 14 MPa (QSPSC). It meets the minimum strength of concrete for structural applications in various standards or codes of practices.

The reinforcement cover for durability is the same as for normal weight concrete.

LWAC gives a better performance in fire resistance than NWC and reflects its improved performance in the relevant clauses of BS 8110, which specifies reduced minimum dimensions and reduced cover requirements, particular for higher durations. In addition, more cover for the anti-spalling steel is essential for NWC while LWAC is less onerous.

2.3 Control of LWAC

Pre-wetting of aggregate is necessary. The degree of pre-wetting depends on the water absorption rate of aggregate and prolonged pre-wetting would not be beneficial. For practical consideration, LWA pre-wetted for 30 to 60 minutes before mixing is recommended.

For LWAC the performance is controlled by the slump tests and concrete cube tests, similar to NWC.

3.0 Practicality, applicability and limitations in HD's new/existing buildings and/or for the construction industry as a whole

3.1 Applications of lightweight aggregate concrete in Housing Authority housing blocks

Precast construction using lightweight concrete surely is an innovative idea for providing alternative and speedy means of construction. It is concluded in this study that LWAC can be used for structural applications of the following structural elements of high-rise housing blocks:

Precast composite slab Precast staircase with flight and shaft Precast water tanks Precast facades

Protocol test in the laboratory has (a) verify the anchorage length of the post-tensioning tendon using normal concrete and lightweight concrete and (b) test the water resistance performance of the protocol joint design under the designated water head. Results of the protocol test look positive.

3.2 Uncertainties to be addressed

When applying lightweight concrete to HA’s new/existing buildings and/or for the construction industry as a whole, there are uncertainties required detail investigation. We need full scale test or pilot test to study:

• the production control on quality of the precast segments, in particular the quality of concrete surface of the joint for subsequent grouting • detail lifting design, procedures and maneuvering of the precast segment, in particular at high level • detail design such as tendon fixing of post-tensioned system, procedure, end block design, jacking sequence; monitoring of the performance of the bonding agent and non-shrink grout within the joint for waterproofing capacity; the jacking loss in post-tensioning operation • detail interface design of the precast element onto the existing building frame • the work flow with concrete production supplier, the operation and technique at site condition, training and control of workmanship • uncertainties on site control, necessary amendments and refine of the design to improve reliability

4.0 Equipment, manpower resources and cost implications of the newly established technologies/techniques/methods,

4.1 Resources

The method and manpower resources for placing lightweight concrete are same as normal concrete.

4.2 Cost benefits of using lightweight aggregate concrete

The advantages of the proposed precast composite lightweight/normal weight construction over the existing construction method for Harmony block projects are:-

• Lower concrete density in economic saving in cost; • Larger panel size to reduce the construction joints; • Clear working areas and fast erection; and • Handling cost.

While the cost of LWAC is higher than NWC, the increase in unit material cost can be compensated by the savings in loading to the foundations, e.g. reduction in the number of piles or reduced pile diameter. The overall cost saving can be higher when savings in labour, material handling, construction time, reduced number of construction joints etc. are made for precast concrete.

In the Cost Saving Analysis of Using Prefabricated Structural Lightweight Elements to both H-piles and Bored pile foundation of the Housing’s projects. The analysis as given in the table below shows that the cost saving in foundation is around $200 per cu.m concrete use. Taking the extra cost for using LWAC element per unit LWAC volume is 144.65HK$/m3, the overall cost saving can be around HK$58~71.

Cost Summary for HA block using LWAC slab, stair shaft, façade and water tank Element Ave. Pile Cost Saving Extra cost for Overall Cost Length (m) Per m3 using LWAC (m3) Saving (HK$) Bored Pile 16 215.95 144.65 71.30 H- Steel Pile (a) 40m 202.74 144.65 58.09 (b) 45m 211.37 144.65 66.72 (c) 50m 211.65 144.65 67.00

5.0 Ways leading to further investigations and/or research.

The research team recommends that structural lightweight aggregate concrete can be used in precast staircases, slabs, facades and water tanks for high-rise housing blocks. With the possible approval by ICU, pilot test or full scale test can be carried out by design team/CityU in partnering arrangement to refine the structural lightweight concrete construction suitable for HD's new/existing buildings.

Moreover, HA may have to review the description of concrete/lightweight aggregate of the technical specification when structural lightweight aggregate concrete was adopted.