Basildon Test Results

Onsite Structural Testing and Assessment of a Cavity Lock Anchor Installation at Basildon, Essex.

Background

During the course of an investigation into the condition of High Speed System Build (HSSB) dwellings in Basildon, Scott Wilson Kirkpatrick & Partners (SWK) were asked to assess the vulnerability of these concrete panel buildings to accidental loadings. It was concluded, as a result, that strengthening was required to increase the robustness to be equivalent to that of conventional domestic properties (BS8103: Part 1: 1986 Cl 4.1). An important part of the strengthening system would be to make the floors and roofs, which are constructed from precast concrete planks, act as a monolithic slab or plate. If this could be achieved, forces applied to the ends of the planks from the party walls of the dwellings could be transmitted to the shear walls which lie parallel to the direction of span of the planks and stability could be assured.

To achieve this SWK in collaboration with Cavity Lock Systems Ltd (CLS) developed an anchor arrangement which involved forming holes by dry diamond drilling horizontally through the floors and roofs at right angles to the direction of span of the planks and grouting in a CINTEC anchor to lock the planks together and provide lateral continuity.

As the system was new and untried a block of three, two storey dwellings was selected for installation trials to establish the practicability of the system.

Testing Conducted by BRE on behalf of SWK

BRE was initially commissioned by SWK to establish if the trial installation of the anchors induced effective lateral connectivity between the planks.

To establish this BRE conducted dynamic tests on two sections of floors in areas A and B (Fig. 1) since this is the most economic and quickest form of full scale testing available.

Fig. 1

Plan at first floor level of terrace of 3 dwellings.

Before the anchors were installed dynamic impact tests were conducted in rooms in areas A and B to identify the most suitable sections to test. Then a controlled forced vibration test was conducted on floors in areas A and B before the anchors were installed and the same tests repeated after installation of the anchors.

These tests established the following:-

1. The floors in end B were considerably stiffer than in end A, both before and after the anchors were installed. This may have been due to initial curvature induced in the floors in area B as a result of clay heave acting on the walls of the structure.
2. The stiffness of the floors in both areas A and B was increased by the addition of the anchors.
3. Before the installation of the anchors the deflected shape of the floors in both ends A and B was consistent with a system in which little lateral connectivity between planks was present (Fig. 2).
4. With the anchor system installed the deflected shape of the floors in areas A and B was consistent with a system with lateral continuity such as a plate (Fig. 3).

Fig. 2

Floor spanning in one direction.

Fig. 3

Floor spanning in two directions.

The dynamic tests therefore demonstrated that the anchors induced plate action in the plank floors which supported the presence of the lateral connectivity assumed by SWK in justifying the stability of the dwellings with the anchors installed.

As a result of these tests, and trials conducted by SWK to assess the reliability of the installation method, the whole estate of HSSB housing was strengthened using the anchor system without the need to decant the occupants.