International Concrete Abstracts Portal

In the prospect of reducing CO2 emissions and landfilling of waste materials, the preparation of sustainable mortars by alkali activation was studied. According to EN 1504-3:2005, geopolymeric and cementitious mortars belonging to different strength classes (R1 ≥ 10 MPa (1450 psi), R2 ≥ 15 MPa (2175 psi) and R3 ≥ 25 MPa (3625 psi)) were tested and compared. Geopolymers were obtained with fly ash or metakaolin and a blend of sodium silicate and NaOH (or KOH). Mortars were tested in terms of workability, dynamic modulus of elasticity, drying and restrained shrinkage and porosimetry. Durability was also investigated in terms of water vapour permeability, capillary water absorption and corrosion of possible embedded rebars during the curing period and wet-dry cycles in 3.5% NaCl solution. Results showed that geopolymers are subjected to higher drying shrinkage but lower restrained shrinkage than cementitious mortars. Water vapour permeability was higher in geopolymers and capillary water absorption was lower especially in fly ash geopolymers than those of cementitious mortars. During the first month, the high alkalinity of geopolymers extends the active state of both black and galvanized steel bars. However, when exposed to chlorides, fly ash geopolymers offer a higher protection to reinforcements than cementitious mortars.

A novel self-healing system for cement composites is proposed in this study. It is based on the use of extruded cementitious hollow tubes filled with a liquid healing agent to be added in cement composites during mixing. These tubular capsules were characterized in terms of flexural strength, liquid storage capability, mixing survival effectiveness and releasing ability upon crack formation. The suitability of a specific mono-component liquid healing agent – a sodium silicate solution – was also assessed. Finally, the self-healing effectiveness of the proposed system was verified using a three-point-bending procedure to induce crack formation on laboratory scale specimens and to evaluate their mechanical recovery after self-healing. Positive results were achieved, though further research is needed to reach a final optimization.

Simply-supported one-way R/C slabs are commonly used in the covers of small and medium underground facilities, where durability is the main issue face with rather limited service loads and short spans (2-4 m [6.5-13.0 ft]). The structural performance, however, should not be underrated, as being the slab in a roundabout does not prevent a heavy truck from straying off the right lane! To have fresh information on durability and cracking (working loads), and on the bearing capacity and failure mode (ultimate loads), displacement-controlled tests have been recently performed in Milan on four typical rectangular R/C slabs suspended along their short sides via corbels (dapped ends; size: 1.3x2.2x0.15 m [51x87x6 in.]). A transversely-distributed or concentrated load was applied either at mid-span (in the bending tests), or at 1/10 of the span (in the shear tests). The two slabs Type A are provided with longitudinal bent-up bars in the main body and hooks in the corbels. On the contrary, the slabs type B are reinforced via two continuous layers of longitudinal straight bars. Under the working loads, cracking never occurred, neither in bending nor in shear – to the advantage of durability – while above the working loads rather complex crack patterns formed in the D zones close to the corbels, particularly under the concentrated load, which brought in 3-D effects, with a limited reduction in the bearing capacity. Refining the reinforcement layout is shown – once more - to markedly improve slab performance, with little or no extra cost.

Advances in concrete technology have led to a widespread use of High Performance Concretes (HPC) with a low water/binder ratio. Those concretes are prone to early age cracking because of the increased autogenous shrinkage, which is normally insignificant for w/b greater than 0.4 and appears mostly in the first days after setting, when the concrete has not reached its full tensile strength, and so it’s one of the principal causes of early age cracking impairing the structure durability. This study aims at quantifying the efficiency of Internal Curing with pre-saturated Light Weight Aggregates (LWA) on the reduction of autogenous shrinkage in HPC. A standard mixture (w/c = 0.3) was tested together with an Internal Cured one, in which a fraction of the normal weight aggregate was replaced by a pre-wetted LWA, to evaluate the differences in the mechanical properties (compressive and tensile strength, elastic modulus) and shrinkage behavior (plastic, autogenous, drying free and restrained shrinkage). In face of a slight decrease of the strength (about 9%) which did not compromise the structural use of the concrete, the pre-wetted LWA led to a 30% decrease of autogenous shrinkage, and a roughly 50% reduction in cracking potential.

Self-healing cementitious composites are a broad category of smart construction materials to which strong and highly qualified research efforts are currently being devoted worldwide, with the aim of providing a sound scientific background to their consistent, and – design-wise – “consciously safe”, use in the engineering practice. Tailored additions can be employed to enhance the self-healing capacity, among which the so-called crystalline admixtures, play a prominent role. Crystalline admixtures consist of proprietary active chemicals, which, because of their hydrophilic nature, react with water and cement particles in the concrete to form calcium silicate hydrates, increasing the density of the CSH phase, and/or pore-blocking precipitates in the existing micro-cracks. The mechanism is analogous to the formation of CSH and the resulting crystalline deposits become integrally bound with the hydrated cement paste, thus contributing not only to a significantly increased resistance to water penetration but also to the healing of the existing damages and cracks. This paper summarizes the results of a wide experimental investigation jointly performed by Politecnico di Milano (Italy), Indian Institute of Technology Madras, Chennai (India) and Universitat Politecnica de Valencia (Spain) to assess the effectiveness of different commercially available crystalline admixtures on the self-healing capacity of cement based materials.