Laboratory Testing: Old-new concrete slabs

Case study: SOFO System

Old-new concrete interaction
Early age monitoring
Refurbishment of structures
Widening of structures
Hardening time

Project Description

Summary: Old-new concrete interaction in bridge slabs

Clients: MSC-EPFL

Place: Lausanne (Switzerland)

Period: 1997 - N/A

Installation: MCS/IMAC-EPFL

Structure's short description: Old-new concrete specimens have consisted of two concretes with different ages, as shown in Figure 1. The first, called old concrete, was poured at least two months before the second, called new concrete. The quantities of cement, water and aggregate are especially calculated for old concrete in order to reproduce the mechanical properties corresponding to 40 years old concrete. The initial thickness of specimen was 17cm, but to increase the interaction between the two concrete layers, the upper surface of old concrete was treated by hydroblasting two days before the pouring of new concrete. The new concrete has consisted of 300kg/m3 of cement, type CEM I 52.5R, cement water ratio of 0.5 and aggregate from lake of Neuch?el with maximal diameter of 16 mm. The new concrete is reinforced with bars 6f14. Two hours after the pouring the new concrete layer was covered for 105 hours with an isolating mat.

Aim of monitoring: Refurbishment or widening of an existing concrete structure gives as a result a new hybrid structure consisting of new and old concrete. The long-term behavior of such a structure and notably the mechanical interaction between two concretes with different ages are still not well known. That?s why this project was performed with the aim to increase the knowledge and to evaluate mechanical interaction between an old and new concrete. For this purpose the hybrid old-new concrete specimens were built and tested at the Swiss Federal Institute of Technology in Lausanne (EPFL).

Number of sensor: 70

Main Results: Total of 14 specimens was fabricated and tested. To simulate long-term behavior, the specimens were aged in controlled conditions. Each specimen was equipped with SOFO sensors and thermocouples in order to monitor the deformation in each concrete layer. The position of sensors is presented in Figure 1. The monitoring was performed over the whole life of specimens, from the pouring of concrete to the final, crash test. Some of interesting results related to early age deformation taken from this application are presented here. The deformations of sensors registered during the early and very early age of the new concrete of the specimen AA3 are presented in Figure 2. Since the sensors at different positions measure different deformations, the distortion of the cross-section of specimen is observed and in more details presented in Figure 3. The distortion of the cross-section caused the bending of the specimen and generated stresses that could provoke an early cracking. Hardening time of new concrete could also be determined from the very early age measurements.

Early and very early deformation of new concrete and deformation of old concrete

Evolution of the cross-section of the specimen AA3

Bibliography:

Monitoring of concrete at very early age using stiff SOFO? sensor , B. Glisic, N. Simon, Cement and Concrete Composite, Elsevier, Vol 22, No 2, p 115-119, 2000

Special sensors for deformation measurements of different construction materials and structures , B. Glisic, D. Inaudi, P. Kronenberg, S. Lloret, S. Vurpillot , SPIE, 6th International Symposium on Smart Structures and Materials, Newport Beach, USA, Vol 3670, p 505-513, 1999

Packaging interferometric sensors for civil structural monitoring , D. Inaudi, B. Glisic, S. Vurpillot, SPIE, OFS-13, 13th International Conference on Optical Fiber Sensors, Kyongju, Korea, Vol 3746, p 120-123, 1999

Deformationsmessungen mit glasfasersensoren , N. Casanova, D. Inaudi, TFB Veranstaltung 4371/72, Wildegg, Switzerland, Ref 5, p 1-22, 1999

Early Age and Long-term Monitoring of Concrete Structures Using Fibre Optic Sensors , Branko Glisic, Daniele Inaudi, Conspectus, Journal of HDB, Singapore, p 93-102, 2001