A REVIEW OF SHOTCRETE MATERIALS, MIX DESIGN AND APPLICATIONS Department of Civil Engineering, University of Cape Town Shotcrete for Africa Conference 2 – 3 March 2009 M G Alexander and R Heiyantuduwa University of Cape TownConcrete Materials and Structural Integrity Research Group

Overview Introduction Shotcrete processes Applications of shotcrete Shotcrete materials and mix design Properties of shotcrete Conclusions

Introduction Shotcrete is a form of concrete commonly made with small sized aggregate, pneumatically projected at high velocity from a nozzle onto a suitable substrate to produce a dense homogeneous mass. Advantages of Shotcrete: Economical Formwork not required Can achieve high compressive strengths Good bond with various substrates

Shotcrete processes – dry mix process Transport pneumatically Aggregate Cementitious material Water Admixture: powder Admixture: liquid Fibres Batch Mix Mix in nozzleGauge by eye Put into mechanical feeder Project onto surface Gauge by flowmeter

Mix Shotcrete processes – wet mix process Aggregate Cementitiou s material Water Admixture: conventional Admixture: special Fibres Batch Pump Project onto surface Inject Batch Mix Aggregate Cementitious material Water Aggregate Cementitious material Water Aggregate Cementitious material Batch Water Aggregate Cementitious material Batch Water Aggregate Cementitious material Batch Water Aggregate Cementitious material Mix Batch Water Aggregate Cementitious material Mix Batch Water Aggregate Cementitious material Mix Batch Water Aggregate Cementitious material Put into delivery equipment/pump Mix Batch Water Aggregate Cementitious material Inject compressed air Put into delivery equipment/pump Mix Batch Water Aggregate Cementitious material Pump Inject compressed air Put into delivery equipment/pump Mix Batch Water Aggregate Cementitious material Pass through nozzle Pump Inject compressed air Put into delivery equipment/pump Mix Batch Water Aggregate Cementitious material Project onto surface Pass through nozzle Pump Inject compressed air Put into delivery equipment/pump Mix Batch Water Aggregate Cementitious material Aggregate Cementitious material Water Aggregate Cementitious material Batch Water Aggregate Cementitious material Batch Water Aggregate Cementitious material Batch Water Aggregate Cementitious material Aggregate Cementitious material Aggregate Water Cementitious material Aggregate Cementitious material Aggregate Water Cementitious material AggregateBatch Water Cementitious material Aggregate Batch Water Cementitious material Aggregate Batch Water Cementitious material Aggregate Mix Batch Water Cementitious material Aggregate Put into delivery equipment/pump Mix Batch Water Cementitious material Aggregate Pump Put into delivery equipment/pump Mix Batch Water Cementitious material Aggregate Pass through nozzle Pump Put into delivery equipment/pump Mix Batch Water Cementitious material Aggregate Inject compressed air Pass through nozzle Pump Put into delivery equipment/pump Batch Water Cementitious material Aggregate Project onto surface Inject compressed air Pass through nozzle Pump Put into delivery equipment/pump Batch Water Cementitious material Aggregate Water Cementitious material Batch Water Cementitious material Batch Water Cementitious material Batch Water Cementitious material Put into delivery equipment/pump Batch Water Cementitious material Inject compressed air Put into delivery equipment/pump Batch Water Cementitious material Pump Inject compressed air Put into delivery equipment/pump Batch Water Cementitious material Pass through nozzle Pump Inject compressed air Put into delivery equipment/pump Batch Water Cementitious material Project onto surface Pass through nozzle Pump Inject compressed air Put into delivery equip. / pump Batch Water Cementitious material

Comparison of dry and wet-mix processes Dry-mix processWet-mix process Instantaneous control over mixing water and consistence of mix at the nozzle. Better suited for mixes containing low-density aggregates, refractory materials and shotcrete needing early strength. Batches of mixed material are capable of being transported longer distances. Mixing water is controlled at the point of manufacture and can be measured. Special mixing and transporting equipment needed. Less dust generation and cement loss.

Comparison of dry and wet-mix processes (cont‘d) Dry-mix processWet-mix process Minimal waste and greater placement flexibility. Low water/cement ratio results in higher strengths. Very operator sensitive. Better assurance that the mixing water is thoroughly mixed with other ingredients. Higher wastage. Lower rebound. Capable of greater production. Higher cost of placed material.

Applications of shotcrete New structures Linings and coatings Repair Strengthening and reinforcing Support of underground openings Refractory shotcrete Special shotcrete

Example of shotcrete support and walls in a tunnel project tunnelling

Materials and mix design Cementitious materials oSABS EN common cements of Type I and Type II oType III cements should only be used in the 42.5 strength oclass (Above cements often contain mineral additives in the blend) oCondensed Silica Fume (CSF) oCalcium Aluminate Cement Aggregates oAggregate used for shotcrete should comply with requirements for use in conventional concrete i.e. SANS oRounded aggregates are preferable (but are frequently crushed in SA, and therefore angular) oWell graded aggregate is important with no fraction constituting more than 30% of the total

Materials and mix design (cont‘d) Mixing water The requirements for mixing water are the same as for conventional concrete i.e. clean and potable. Admixtures Accelerators, air-entraining admixtures, pozzolanic additives, plasticisers, retarders, activators Reinforcement Weldmesh, fibre reinforcement: polymeric or steel

Materials and mix design Typical shotcrete mix proportions Mass (per m 3 ) Cementitious content - dry mix350 – 450 kg - wet mix400 – 500 kg Silica Fume (5 – 12 % of cementitious mass)30 – 50 kg Water/cementitious ratio0.35 – 0.5 Aggregate/cementitious ratio3 – 5 Steel fibre (when present)30 – 50 kg

Typical properties of shotcrete vs. cast concrete PropertyHigh quality shotcrete Cast in situ concrete Compress. strength, 1 d (MPa)206 (estim.) Compress. strength, 28 d ( MPa) Elastic modulus, 28 d (GPa)3431 (estim.) Poisson ’ s ratio, 28 d – 0.22 Tensile strength, 28 d (MPa)> 2 (estim.)3.8 (estim.) Initial setting time (start-end) (min)3 – 545 – 145 (estim.)

Typical properties of shotcrete vs. cast concrete (Cont’d) PropertyHigh quality shotcrete Cast in situ concrete Shrinkage after 100 d (%)0.06 – – 0.08 Specific creep - 60 d (%/MPa)0.01 – Density (kg/m 3 )2140 – – 2450 Total porosity (%)15 – 2012 – 18 Permeability (m/s) to to Microcracking, 28 d (cracks/m) Coefft. of thermal expan. (K -1 )8 – 15 x – 12 x10 -6 Slump (estimate) (mm)0 to to 120

Durability of shotcrete Particularly for linings in tunnels: oCarbonation oInfiltration through the lining of pure waters or waters carrying aggressive agents, resulting in leaching and/or degradation Durability testing Can be done on mock-up panels Should also preferably be done on in-situ samples  Boiled Absorption (BA)  Volume of Permeable Voids  Durability Index tests (SA tests currently being developed for conventional concrete)

Conclusions The properties and performance of shotcrete are largely dependent on the conditions under which it is placed (i.e. curing, compaction) as well as on the characteristics of the particular equipment, and ultimately on the competence and experience of the application crew. Based on improvements in material quality and versatility, which provide enhanced physical properties, shotcrete techniques could further replace more conventional construction procedures.

Thank you for your attention!