Early age cracking may be what is commonly known as "plastic shrinkage cracking", which normally is cracking of horizontal surface before and during setting (initial phase), and "thermal cracking" which normally is in the period of cooling following the period of temperature rise due to heat of hydration (thermo phase). In the initial phase, any mix water absorption by the LWA (i.e. like in concretes with relatively dry LWA) may contribute to increased settlement, capillary tension of pore water and shrinkage, and thus, an increased risk of cracking in typically the first hour after finishing. However, in the early hardening age the absorbed water may constitute a reservoir which contributes to swelling of the paste which counteract any plastic shrinkage and/or any contraction due to cooling, and thus, to reduced risk of cracking. In the thermo phase the temperature rise in LWAC is potentially higher due to the lower heat capacity of the LWA, which may result in larger contraction during cooling, which again may generate higher stress, and finally to higher risk of cracking. On the other hand the autogenous shrinkage is significantly reduced or even eliminated, and the E-modulus is lower, which both contributes to lower stress, and thus, to lower risk of cracking. The net result is often reduced risk of early age cracking.

The alkali-silica reaction (ASR) was recognized by Stanton in 1940 and is still today a world-wide problem. The basic reaction mechanisms are reasonably well understood and in many cases a prevention of the ASR is possible. However, there are still severe damages on concrete structures caused by alkali-silica reaction, especially related to slow/late aggregates. This paper presents the experiences with the German Alkali-Guideline during the past decades in Germany and introduces latest approaches in developing forward-looking test methods to assess the durability of concrete regarding ASR due to alkali-reactive slow/late aggregates. The emphasis is particularly placed on assessing the influence of an external alkali supply, i.e. the impact of de-icing solutions, on pavement concretes. For this purpose the cyclic climate storage was used and proved as appropriate method.

This paper reports the results of a study on the resistance of lightweight aggregate concrete to the penetration of chloride ions. Concrete specimens were fabricated with a blended silica fume cement at a water-cementitious materials ratio of W/CM = 0.40 or 0.30 and with combinations of aggregate as follows: (i) limestone coarse aggregate and river sand, (ii) expanded slate coarse aggregate and river sand, or (iii) expanded slate coarse and fine aggregate. A further series of mixes was made using the latter combination of aggregates with the blended cement being partially replaced with 25, 40 or 56% fly ash. Concrete specimens were subjected to a series of tests including “rapid chloride permeability” (ASTM C 1202), and non-steady-state diffusion (bulk diffusion test). Tests were conducted at 28 and 56 days, and 1 and 3 years. The results up to one year clearly show the benefits of incorporating expanded slate in the concrete, with permeability and diffusion coefficients being reduced significantly. The improvements attributed to the presence of the lightweight aggregate appeared to increase with the maturity of the concrete and, after 3 years continuous curing, the reduction in the apparent chloride diffusion coefficient was observed to be as much as 70%. As expected, the addition of fly ash produced further reductions in permeability and diffusion. The data developed in this study were used as input parameters for service life predictions. Although, there are insufficient data to allow firm conclusions to be drawn from these analyses, it is clear that the incorporation of lightweight aggregate will lead to a significant extension of the service of life.

In order to evaluate the distribution of damage related to delayed ettringite formation (DEF) in Maryland bridges, a pilot field study was carried out using both destructive and nondestructive test methods. A sampling design was developed based on the Maryland Bridge Inventory. This was screened for bridges with ratings of 4 or 5 and the term “wet map cracks” in the inspection reports. A sample of 16 bridges was selected to give a uniform geographical distribution across the Maryland State Bridge districts. At each bridge several cores were drilled for subsequent examination of fracture surfaces by Scanning Electron Microscope (SEM) with energy dispersive X-ray fluorescence. At every bridge, ettringite was detected, but alkali-silica reaction (ASR) gel was detected only very rarely. In more heavily damaged locations, the occurrence of ettringite crystals was more frequent, appearing in the rims around aggregates as well as in voids. Also, the morphology of the ettringite crystals appeared to be more lamellar than acicular. The implications are that: DEF is widespread geographically, map cracking is not diagnostic only for ASR, and the onset of DEF may be associated with a change in ettringite crystal morphology.

This paper describes the development and material testing results of high strength lightweight concrete (HSLC) mixture proportions for use in precast, prestressed concrete highway girders. As part of a research project at Georgia Tech, it was necessary to develop concrete mixture proportions using slate lightweight aggregate able to achieve design strengths of 8,000 and 10,000 psi (55.2 and 69.0 MPa) with an equilibrium unit weight of approximately 120 pcf (1922 kg/m3) or less. A methodical approach to proportioning mixture components was developed and used to configure HSLC mixtures. Field testing was conducted to verify the mixture proportions for use in a production environment. An extensive material testing program based on specimens cast during all phases of the research project produced a database of material properties to include strength, modulus of elasticity, rupture modulus, coefficient of thermal expansion, and chloride permeability.