The use of High Alumina Cement [HAC] in building construction was banned in 1976. Strength assessments were then undertaken on many buildings however survey reports were in time either lost or required updating. In recent years the importance of chemical attack and reinforcement corrosion has been recognised.
HAC is now in its most vulnerable condition. Conversion may have reached its limit with the consequent loss of strength and increased likelihood of durability problems.
HAC investigations are now more relevant than ever before.
The following sections provide background information on the use of HAC and the various stages of investigation.
General Background to the Use of HAC
In the post war period High Alumina Cement [HAC] was used extensively in the UK to produce precast prestressed concrete roof and floor beam units. HAC was used to obtain a high strength very quickly. This enabled the concrete elements which were manufactured to be removed quickly from the moulds and subsequently re-used.
High Alumina Cement has been produced from the 1920's by Lafarge at their Thurrock Works. They were and still are the sole source of HAC in this country. During the 1950's and 60's the use of High Alumina Cement in the production of precast elements was substantial.
In 1974 a school swimming pool roof collapsed. It was found that in high temperatures and damp conditions and, especially in the presence of sulphates and chlorides, the cement changed its chemical properties and the concrete partially lost its strength. This process was known as conversion.
Following this failure a great deal of investigation of existing buildings and research into the properties of the material was undertaken. Rules were developed which enabled engineers to check existing buildings and to report upon the condition and safety. These rules were detailed in a report produced by the Building Regulation Advisory Committee Sub-Committee P which was set up to investigate and report on the use of HAC concrete. The production of HAC ceased in the mid 1970's and its use is now banned from structural elements.
Identification of HAC
HAC was mainly used in the manufacture of precast concrete elements. A wide range of section types and sizes were manufactured using HAC. In particular however the 'X Joist' precast beam was the most popular element used both in floor and roof structures.
Click HERE to see a cross section diagram of an 'X- Joist'
For each serial size of 'X Joist' the number of reinforcing wires varied. This affects the strength and it is therefore essential to know the wire strand configuration for strength assessments.
Problems associated with HAC Construction
The presence of HAC in the concrete matrix can give rise to internal stresses within the concrete which can lead to cracking and ultimately to failure of the concrete element. These internal stresses are the result of an alteration in the chemical structure of the aluminate hydrates within the concrete matrix. This alteration in the chemical structure is known as the process of conversion.
Conversion is associated with a slight volume reduction in the concrete matrix leading to a reduction in strength. There is no direct correlation between conversion and loss of strength. The actual loss of strength will depend primarily on the original water/cement ratio of the concrete. In general the lower the water/cement ratio, the lower the loss of strength for a given degree of conversion.
If the concrete becomes wet, chemical attack may occur which under certain circumstances could result in almost total loss of strength. There are two forms of chemical attack which may occur, namely sulphate attack and alkaline hydrolysis. Sulphate attack can occur if sulphates present in screed materials or plaster coverings are transferred to the HAC. Alkaline hydrolysis can occur if sodium or potassium hydroxide present in Portland cement screeds or concrete is transferred to the HAC. For both types of chemical attack persistently wet conditions are required.
In recent years it has been noted that the depth of carbonation to precast concrete HAC members may reach the depth of reinforcement, thus reducing the alkalinity of the concrete surrounding the reinforcement. Indications of surface corrosion to the reinforcement have been reported with HAC concrete elements in an internal environment.
Assessment of HAC Construction
Sub-Committee P Requirements
In 1975 the Building Regulations Advisory Committee set up a Sub-Committee to investigate the use of HAC concrete. Its report recommended the design criteria to be used in checking the adequacy of buildings containing HAC structural members and advised on which categories of buildings, if any, containing HAC members need not be appraised.
Buildings were exempt from appraisal if:
The buildings did not consist of more than 4 storeys
Floor spans did not exceed 6.5m
There was no persistent leakage or heavy condensation
The buildings contained residential accommodation
There are two main aspects to the investigation. Firstly, a strength assessment and, secondly, a durability assessment. These are now described in the following sections.
A structural analysis is required to determine if the precast concrete members have sufficient structural capacity, even at the significantly reduced fully converted strength, to safely withstand the applied loadings.
Sub-Committee P set out guidelines for the concrete strength and partial safety factors to be used in the structural analysis. The strength of converted HAC concrete has been defined at 21N/mm2 for the purposes of the assessment. This is in contrast to an original design figure of approximately 50N/mm2. The structural capacity of the member is back-calculated using the reduced strength of 21N/mm2. The capacity of the beam to carry the existing loading can therefore be assessed.
The long term durability of HAC reinforced concrete members is affected by chemical attack and reinforcement corrosion.
Testing can be undertaken to determine the presence of alkalis and sulphates which in the presence of persistently wet conditions, are likely to lead to chemical attack. Laboratory testing would be supplemented by a detailed visual inspection of the HAC members to check the existing and predicted environmental conditions in which the beams exist.
The likelihood of reinforcement corrosion would be assessed following further testing to determine the depth of carbonation and visual inspection of the reinforcement in selected areas.
The 3 Stage Investigation
The competence with which the long-term behaviour of HAC concrete elements can be assessed is a direct function of the quality and extent of testing and appraisal undertaken. The investigation of pre-cast concrete floor beams containing HAC may be sub-divided into three sections as follows.
Stage 1 - Identification
Identification of HAC from proof-negative chemical tests to provide 95% confidence of the presence of HAC.
A brief visual inspection to the soffit of suspected HAC concrete elements in selected areas for signs of cracking, excessive deflection and chemical attack.
Stage 2 - Structural Assessment
Undertake proof positive Differential Thermal Analysis [DTA] testing to confirm the presence of HAC.
Determine exactly the form of HAC construction including dimensions and reinforcement details. Unless drawings of the construction are available this will involve 'opening up' of the structure in a small number of selected locations.
Undertake by calculation a structural loading assessment in accordance with the BRAC requirements to essentially determine if the converted HAC has sufficient capacity to withstand the applied loading.
A detailed visual inspection to all readily visible HAC elements to determine the presence of cracking, excessive deflection, moisture ingress and chemical attack.
An initial assessment to determine if conditions exist for chemical attack to occur including sampling and testing of dust samples removed from the HAC elements to determine the alkali/alumina ratio.
Stage 3 - Durability Assessment
Remove lump samples from the HAC elements in selected locations and physically inspect the condition of the reinforcement with regard to corrosion.
Petrographic laboratory examination of thin slides prepared from the lump samples previously removed to determine the depth of carbonation and the actual presence of chemical attack (rather than merely determining if conditions exist for chemical attack to occur).
Assess the short and long term durability with regard to reinforcement corrosion and chemical attack.
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