FRESH CONCRETE PERFORMANCE GUIDELINES FOR CONSTRUCTION PROJECTS IN NEW ZEALAND JAMES MACKECHNIE Allied Concrete/University of Canterbury ABSTRACT Fresh concrete properties are poorly defined in most construction projects and this can be problematic when these specification requirements affect productivity and hardened properties of concrete. This paper discusses five main fresh concrete properties that have a strong influence on the workability and hardened concrete performance; slump, air content, segregation, bleeding and setting time. Each property is discussed in terms of it overall role in influencing fresh concrete properties and also how these affect hardened properties such as strength, dimensional stability and durability. Examples from case studies are also used to illustrate the role of these fresh properties have on concrete performance supplied to projects around New Zealand. Slump and air content are thought to be well understood but yet are sometimes specified in an impractical manner both in terms of limits and tolerances. Bleed rate, segregation and setting are generally assumed to be normal until there is an issue on site and then resolution becomes difficult without clear methods and benchmarks to assess these properties objectively. The issue is further complicated by adopting performance limits from other countries without any acknowledgement that local materials might differ significantly, which affects the perceived in situ performance. Outcomes from this paper are presented graphically to illustrate that these fresh properties are strongly linked and cannot be viewed or specified in isolation. Findings also include new research into fresh concrete properties including measurement of hydraulic bleeding in deep piles, setting time monitoring using calorimetry and rheological comparisons for special concrete mixes. Recommendations are made that will allow a better framework to be developed by concrete suppliers in order that fresh properties may be more accurately predicted and measured. This paper represents the first part of an overall specification guideline, the second part on hardened properties of concrete will be published in 2017. James Mackechnie is the South Island plant engineer for Allied Concrete and is also an adjunct research fellow at the University of Canterbury. He is a chartered professional engineer and has a PhD in civil engineering from the University of Cape Town. INTRODUCTION Fresh properties of concrete are often not well defined in construction projects with these properties being either ignored or specified in an impractical manner that unnecessarily limits flexibility and/or productivity. Clearer guidance is required about these properties including slump air content, segregation, bleeding and setting time if this situation is to be improved. The most important fresh properties of concrete are illustrated in Figure 1 where these initial properties are related to practical outcomes such as workability, strength, dimensional stability and durability. This graphical representation shows how fresh and hardened properties are closely related and changes may be difficult to achieve. Figure 1: Map showing the main fresh properties of concrete and influences Fresh concrete specifications may include all sorts of additional requirements, which while having some relevance to concrete, generally do not need to be rigidly applied. This is particularly important when constituents and proportions such as water contents and mix design are prescriptively specified. Figure 2 shows some of the fresh properties that are sometimes specified and the main factors affecting each of these. Figure 2: Diagram showing possible fresh properties of concrete that can be specified SPECIFICATIONS Specifications for the supply of concrete to construction projects tend to be a combination of prescriptive and performance-based criteria. Prescriptive elements in specifications are generally simple to achieve but sometimes these are in direct conflict with the stated performance required for the project [1]. Internationally there is a movement towards performance-based specifications whereas locally some structural designers appear to favour prescriptive-based approach where recipes need to be followed or specific materials included in concrete mixes such as chemical admixtures or additives [2]. This type of approach allows less flexibility and is often not effective in achieving the performance intended. Figure 3 shows the difference between performance and prescriptive specifications used to control fresh properties. The prescriptive properties often have limited relationship with fresh properties and the rationale for their use is not clear. In reality, many construction specifications do not provide much guidance or limits on any fresh properties except for consistence level, which is usually measured with the slump test. Self-compacting concrete has the advantage that fresh property performance can be triangulated to some degree using spread testing, T500 time and visual stability index rating. Figure 3: Performance versus prescriptive specifications for fresh concrete Prescriptive approaches are not without some scientific basis but generally the stated limits are not critical since high quality concrete can still be produced when outside these recommendations. For example, limiting the minimum temperature of concrete to 15 0C is impractical in New Zealand and setting is not an issue at lower temperatures of as low as 5 0C when using chemical admixtures that accelerate set times [3]. The fresh properties of concrete need to be clearly stated in construction projects in order that performance can be controlled and understood by all construction parties. This paper considers the following fresh properties in more detail; consistence as measured by slump, air content, bleed and settlement, segregation of high flow mixes and setting time. SLUMP TESTING Slump testing measures the consistence of concrete from which the workability of a specific mix can be inferred but not accurately compared across a range of other concrete mixes. Slump targets have been increasing over the last few decades since water reducing admixtures have decoupled the relationship between slump and compressive strength. Unfortunately many specifications have ignored these developments while some have seen slump as the property that designers can use to compensate for a lack of site supervision or understanding existing standards [4]. This has led to several myths persisting in the structural engineering fraternity that are compared with generally accepted metrics shown in Figure 4 below. Figure 4: Myths and metrics associated with slump testing of concrete Performance Specifications should avoid prescriptive slump limits and allow contractors to nominate an appropriate slump, which then becomes the performance target for that contract. Engineers who believe that no form of retempering is permissible on site should consider when last they have been held up in traffic on route to a meeting. The use of super-plasticisers to retemper special concretes has been done successfully for many years without affecting the hardened properties of concrete. Innovation Rheological testing of concrete is a powerful method to understand fresh concrete properties, especially when higher performance is required [5]. Rheology is the science of deformation and flow of material and may define the fresh properties in terms of yield shear stress (boundary between liquid and solid behaviour) and plastic viscosity (resistance to flow once liquid). These principles of rheology are used in modern concrete mixes in Europe that allow automation of the batching process and better control than visual assessment using dry batching plants. Slump meters installed on concrete trucks are a rough approximation since the meter is unable to differentiate between the differing rheology of concrete mixes, load size effects and are unable to be continuously calibrated using actual slump testing. AIR CONTENT The durability requirements for air entrainment of concrete in the New Zealand standards are poorly understood and needs to be clearly stated in order to avoid any confusion. - Air entrainment does not provide any antifreeze benefit to fresh or setting concrete since all concretes are extremely vulnerable to frost damage during the first few nights after casting - Lower grades of concrete (typically 17.5-25 MPa and 17.5-30 MPa in the south of the South Island) require air entrainment since these concrete have relatively high water/cement ratios and are more vulnerable to freeze-thaw action - Lower strength concrete is generally air entrained regardless of exposure since there are other benefits to these mixes such as improved workability and material savings - Higher strength concrete may require air entrainment if the surface is exposed to multiple freezing cycles per annum (structures at higher altitudes or freezer rooms entrances) The effect of external freezing is not immediately felt internally in concrete due to thermal mass effects and as water freezes below 0 0C when enclosed in capillaries and voids [6]. This process is illustrated in Figure 5, which shows how water initially freezes in more saturated capillaries and then flows towards partially saturated voids, allowing pressure relief caused by ice expanding by 9%. Figure 5: Concrete microstructure response to freezing internally Performance Specifications for larger infrastructure projects sometimes require high