Concrete strength is typically taken as the compressive strength of a group of standard cubes at 28 days that are representative of one concrete mix of which not more than 5 per cent of test results are expected to fall. This also referred to as characteristic strength or fck and gives us the compressive strength classes in EN 206. An example of such a strength class is C40/50, where 40 refers to the minimum cylinder strength and 50 refers to the minimum cube strength in MPa and is commonly referred to as 50N concrete. Testing a construction material 28 days after placing could be seen as a leap of faith as a building could be advanced two-to-three storeys within that time frame. However, the strength development of concrete over time is well known and confidence in the material allows for testing after 28 days. Some reports suggest that concrete is tested at 28 days as it is a lunar cycle, but this is a somewhat of a romantic idea. The real reason for testing at 28 days is that the concrete strength development curve using Portland cement at standard conditions is well established and is trusted. More than 95 per cent of its design strength is achieved at 28 days and at a confidence interval of 5 per cent specifying authorities throughout the industry choose to standardise on 28-day testing to establish consistency for testing procedures. An early indicator of 28-day strength of Portland cement is testing at seven days where 70-80 per cent of the characteristic strength is usual. [caption id="attachment_33695" align="alignright" width="300"] Figure 1 – Typical strength development of GGBS Concrete[/caption] The adaption of ground granulated blast-furnace slag (GGBS) into the Irish construction industry over the past 15 years has had a massive impact in reducing CO2 and enhancing concrete performance in terms of higher concrete strength and increased durability. The strength development of concrete with GGBS is different to the well-known concrete strength development curve based on Portland cement and is shown in Figure 1. Typically, concrete with 50 per cent GGBS will have lower early-age strength than Portland cement-based concrete, but should achieve equal 28-day strength development and higher long-term strength development. Long-term strength is significantly higher, with a 10-20 per cent increase at 56 days from 28 days, with strength continuing to develop over several years. The long-term strength development of concrete with 50 per cent GGBS is acknowledged in the Irish Concrete standard I.S. EN206-1, where it is stated in Table NA 2, 3, 5 and 6: “For any given cover, while lower compressive strength classes are specified for concrete mixes using CEM III/A*, CEM III/B**, CEM II/B-V cements or equivalent combinations, the strength class equal to that for mixes using CEM I or CEM II/A cement types can be specified at a later age (56 days), since concrete mixes using the former cements will give increased strength gain beyond 28 days (compared to the latter cement types).” It should be noted that this is only true when it is specified and it is not a general rule.  *CEM III/A is equivalent to 36-65 per cent GGBS **CEM III/B is equivalent to 66-80% GGBS with a limit of 70 per cent according to I.S. EN 206-1

Specifying characteristic concrete strength

Specifying characteristic concrete strength with a minimum of 50 per cent at 56 days is a progressive step that the industry can now take with trust and confidence to fit with the concepts of lean thinking and design. Instead of the usual three cubes for compliance assessment, one cube at seven days and two cubes at 28 days, an additional fourth cube at 56 days could be included in specifications. It will allow for more durable and sustainable concrete-mix design and possible material reduction from over-yielding characteristic strength. It could also ease the debate, excitement, litigation and cost around borderline 28-day cube compliance. But why stop at 56 days; why not go out to 84 days or beyond when GGBS concrete has reached its characteristic strength and go for extra lean design? Concerns are often raised about low early-age strength development of GGBS concrete in terms of striking formwork, but the real consideration is how this early-age strength is assessed. Standard cured concrete cubes are made as a compliance check against the concrete mix produced. They are limited to their dimensions, 100mm x 100mm x 100 mm or 150mm x 150mm x 150mm and the standard curing conditions (20oC in water) in which they are stored. These cubes are not representative of in-situ concrete strength over early stages as they are not influenced by site conditions such as ambient temperature, climatic conditions, section size and formwork type. Placing cubes on elements cast for the same purpose is also not representative. Concrete-strength assessment methods such as temperature matched curing (TMC), the LOK test, maturity functions and maturity metres give accurate measures of in-situ strength at early ages. These in-situ strength assessment methods can play a role in fast track construction and accelerating construction programmes. In most instances, criteria for the removal of formwork to vertical elements or soffits to slabs is specified in terms of time, usually hours or days. Striking times for the vertical shuttering is typically 16 to 24 hours and striking times on slabs can be three-to-seven days, depending on the project. Alternatively, strength requirements can be used to determine when it is safe to remove formwork or load an element. The false work designer can calculate the required in-situ strength to remove shuttering so that this strength can be assessed, rather than waiting on time to pass. In the case of walls and columns, the requirement ranges from 2 MPa to 5 MPa and typical slab strengths are in the region of 15-25 MPa. Specifying the required strength then allows for measurements to be taken ahead of given times to see if the required strength is reached to most likely improve turnaround times on pours.

Temperature matched curing

Ecocem has used temperature matched curing (TMC) extensively in the industry to support the specification and use of GGBS and to accurately measure the in-situ strength of concrete in real time. It offers opportunities to make construction execution decisions such as when to remove formwork and props, load elements and cut strands on post-tensioned slabs ahead of the specified time. The TMC method is simple and effective for site or precast use. A thermocouple/temperature probe is tied to the reinforcing steel and this is linked to a control unit on a curing tank. The unit heats the water in the curing tank to ‘match’ the in-situ temperature of the concrete. Cube specimens are prepared into the curing tank immediately after casting. These cubes are subjected to the same temperature profile as the element cast and when tested at agreed intervals given accurate measures of in-situ strength. [caption id="attachment_33696" align="alignright" width="300"]figure-2 Figure 2 – Schematic of Temperature Matched Curing (TMC)[/caption] A schematic of the TMC set up can be viewed in Figure 2. The temperature matched curing system is easily set up on site, requiring water, continuous 110V power and a small amount of space. Case Study – Indaver Incinerator Ecocem, together with FSA Construction, John Sisk and Sons and Kilsaran Build, used the system of TMC on the Indaver Incinerator project in Duleek in 2009. An in-situ strength of 35 MPa was specified by PM Group for the bunker slab wall to be poured on the 1.1m deep foundation slab. The concrete was specified as C40/50 with 70 per cent GGBS required for sulphate resistance and for low heat properties. Concerns were raised over early-age strength development of this mix to meet the 35 MPa requirement in four days. The results in Figure 3 show how the standard cured cube result of 28 MPa is below the requirement whereas the TMC result of 47 MPa at day four shows strength exceeding the 35 MPa requirement. [caption id="attachment_33697" align="alignright" width="300"]figure-3 Figure 3 – Standard cured and TMC cube results from Indaver[/caption] The TMC cubes take into consideration the heat of hydration from the section size, the concrete mix design and the ambient conditions and give accurate measurements of in-situ strength. The use of TMC on this project allowed the contractor to pour the bunker wall after four days. If standard cured cubes were relied upon, it may have been the fifth or possibly the sixth day before the continuing works could proceed. The over-yielding TMC strength suggests that perhaps a lower grade of concrete could have been used leading to savings on materials. This shows the importance of how concrete strength is considered. Case Study – DLR Lexicon The system of temperature matched curing was also applied on the DLR Lexicon project in conjunction with Kwik Structures, John Sisk and Sons and Kilsaran Build. It was used to determine the optimal striking time of vertical formwork using C40/50 with 50 per cent GGBS for an architectural finish in January 2013. [caption id="attachment_33698" align="alignright" width="300"]figure-4 Figure 4 – TMC results from DLR Lexicon[/caption] The walls were thin sections, 180mm, and ambient conditions hovered around 5oC leading to concerns of early age strength development using 50 per cent GGBS in the concrete. Striking time to the walls was given as 24 hours in the specification by Kwik Structures. Early-age strength results in Figure 4 show that the TMC cube met the mechanical strength requirements for the striking of formwork after 16 hours. This was eight hours inside the recommended 24 hours. Case study – Royal College of Surgeons Scheduling the crane erection is critical in most construction programmes, particularly in a congested and busy city. Temperature matched curing was carried out with Bennett’s Construction on the Royal College of Surgeons (RCSI) site in Dublin. There was concern over the ability of the concrete mix containing 70 per cent GGBS to achieve the early-strength requirement of 37MPa in the crane-base pour and no risk could be taken not to achieve the strength requirement. The crane base measured 7.2 x 7.2 x 1.75m deep and the use of 70 per cent GGBS was agreed with OSCS engineers, Bennett’s and Kilsaran Build to mitigate the risk of thermal cracking. Bennett’s had scheduled to load the crane base seven days after the base pour had taken place. It was agreed that cube testing would take place at three, four, five and six days. This would ensure both reliability and confidence in the in-situ strength development and when the crane could be erected. In-situ strengths are given in Table 1.
Concrete Mix Age (Days) TMC Strength (N/mm²)
C40/50 70% GGBS 3 40.7
C40/50 70% GGBS 4 42.5
Table 1: In-Situ strengths from the RCSI Crane Foundation The TMC cube results indicated that the requirement of 37N was reached at three days. The system remained in place for an extra day with a four-day cube being tested to confirm that the required strength had been achieved. Testing was not required at the agreed fifth and sixth day. The crane was erected after seven days, but the results indicated that this could have occurred earlier. [caption id="attachment_33699" align="alignright" width="300"]figure-5 Figure 5 - In-situ temperature of Crane Base at the RCSI[/caption] The in-situ temperatures of the crane base, Figure 5, peaked at 50.9°C. This is significantly higher than standard 20°C that cubes are usually cured at. This differential in temperature shows how poorly standard cured cubes represent in-situ early-age strength.

Future of concrete-strength assessment

The next generation of concrete-strength assessment methods are concrete maturity systems that give accurate real-time assessments of in-situ strength using wireless and smart technology. The Concretemote is a system developed by BAS in the Netherlands and now licenced by Doka that can send concrete strength assessments to a mobile phone. Another such system is the START system developed by OTB Concrete. This was used by Byrne Brothers to measure the in-situ strength for post tensioning in the construction of the Shard in London. Technology is developing and innovative thinking is taking place. Solving longstanding concrete-strength considerations and making ideas become reality needs new and appropriate assessment methods. Ecocem’s technical service offering includes TMC testing and assistance at all stages of a project from concrete mix design and environmental assessment to on-site assistance. Ecocem is a registered training provider with Engineers Ireland and can deliver a registered CPD to your company. Authors: John Reddy, technical development manager at Ecocem, chartered civil engineer, member of the Institute of Concrete Technology and committee member of the Engineers Ireland Structures and Construction Division.  Aidan Fogarty, quality and technical services engineer at Ecocem, civil engineer from DIT Bolton St and concrete technologist.