ACID ATTACK ON CONCRETE

CONCRETE IS CHEMICALLY BASIC, HAVING PH VALUESRANGING BETWEEN APPROXIMATELY 12.5 AND 13.5,DEPENDING UPON THE MIXTURE PROPORTION. SINCE IT HASHIGH ALKALINITY, IT IS READILY ATTACKED BY VARIOUSACIDS, WHICH HAVE PH VALUES OF LESS THAN 7.

ACID ATTACK ON CONCRETE

IT CAN BE ACCEPTED AS A GENERAL RULE THAT ACIDS AREDAMAGING TO CONCRETE. THEY DO NOT ENTER INTOCOMPLEX CHEMICAL REACTIONS SUCH AS THOSE THAT TAKEPLACE DURING SULFATE ATTACK, RESULTING IN EXPANSIONAND CRACKING OF THE CONCRETE, BUT SIMPLY DISSOLVETHE MORE SOLUBLE CONSTITUENTS OF THE SET CEMENT,DESTROYING ITS CRYSTALLINE STRUCTURE AND LEAVINGONLY AN INCOHERENT RESIDUE. DIFFERENT ACIDS ACTPREFERENTIALLY AND AT DIFFERENT RATES ON THE CEMENTCOMPOUNDS, BUT THE ULTIMATE RESULT OF SUSTAINEDATTACK IS DESTRUCTION OF THE CONCRETE.

ACID ATTACK ON CONCRETE

WHEN CONCRETE COMES IN CONTACT WITH ACIDICSOLUTIONS, CONSTITUENTS OF THE CEMENTFRACTION AND SOMETIMES THE AGGREGATE ARESELECTIVELY DISSOLVED.

THIS LEADS TO AN INCREASE IN POROSITY, WITHVARIOUS IMPLICATIONS IN TERMS OF MECHANICALPROPERTIES (PRINCIPALLY STRENGTH) AND THEPERMEATION PROPERTIES OF THE CONCRETE COVER.

ACID ATTACK ON CONCRETE

CONCRETE CAN COME INTO CONTACT WITH ACIDIC SOLUTIONS FOR AWIDE RANGE OF REASONS.

IN PARTICULAR, HYDROCHLORIC ACID (HCL) IS USED IN THE PICKLINGOF STEEL TO REMOVE SURFACE LAYERS OF RUST.

ORGANIC ACIDS ALSO ARISE IN INDUSTRIAL APPLICATIONS, BUTPARTICULARLY IN THE FOOD AND DRINK INDUSTRY.

ACETIC ACID (CH 3·COOH) AND LACTIC ACID (CH 3·CHOH·COOH) ARECOMMONLY ENCOUNTERED IN THIS TYPE OF ENVIRONMENT, WITHLACTIC ACID BEING PRESENT IN SOUR MILK AND CERTAIN ALCOHOLICBEVERAGES. ACETIC ACID IS ALSO PRODUCED BY SOUR ALCOHOLICBEVERAGES AND IS THE MAIN ACIDIC CONSTITUENT OF VINEGAR.

ACID ATTACK ON CONCRETE

ACID ATTACK ON CONCRETE

ACID ATTACK ON CONCRETE

THE AGGRESSIVE ACTION OF ACIDS ON CONCRETE IS PARTLYDEPENDENT ON THEIR STRENGTH, WHICH IS MEASURED IN TERMS OFTHE EASE WITH WHICH THEY LOSE A PROTON (H+) THROUGHDISSOCIATION IN A SOLVENT. THE DISSOCIATION OF A MONOPROTONICACID, HA, IS GIVEN BY THE EQUATION

ACID ATTACK ON CONCRETE

WHERE CONCRETE COMES IN CONTACT WITH AN ACIDIC SOLUTION, ITWOULD BE EXPECTED THAT THE LOWER THE PH OF A SOLUTION, THEFASTER THE RATE OF DETERIORATION.

WHEN ACID SOLUTIONS COME IN CONTACT WITH HARDENEDCONCRETE, VARIOUS REACTIONS INVOLVING CEMENT HYDRATIONPRODUCTS AND AGGREGATE MINERALS WILL OCCUR. IN THE CASE OFCALCIUM HYDROXIDE, THE GENERAL REACTION IS

WHERE HA IS A MONOPROTONIC ACID. THE REACTION OCCURS AT A PHVALUE OF BELOW 12.6.

ACID ATTACK ON CONCRETE

ETTRINGITE UNDERGOES A REACTION WITH ACID BELOW A PH VALUEOF 10.7:

IN THE CASE OF CSH GEL, THE REACTION (WHICH OCCURS BELOW A PHVALUE OF ~10.5) IS

WHERE SI(OH) 4 IS AN AMORPHOUS SILICA GEL.

ACID ATTACK ON CONCRETE

THE MAIN ENVIRONMENTAL FACTORS THAT INFLUENCE THE RATE OFDETERIORATION OF CONCRETE EXPOSED TO ACIDIC CONDITIONS ARETHE CONCENTRATION OF THE ACIDIC SPECIES, TEMPERATURE ANDWHETHER ANY MECHANICAL ATTRITION IS ALSO ACTING ON THECONCRETE SURFACE.

ACID ATTACK ON CONCRETE

GENERALLY, AN INCREASE IN TEMPERATURE WILL LEAD TO ANINCREASE IN THE RATE OF DETERIORATION AS A RESULT OF ACIDATTACK. THIS IS PRIMARILY DUE TO AN INCREASE IN THE SOLUBILITYOF THE SALTS FORMED. HOWEVER, WHERE SALTS POSSESS A NEGATIVEPARTIAL MOLAR ENTHALPY OF SOLUTION AND THUS BECOME LESSSOLUBLE AT HIGHER TEMPERATURES, GREATER RESISTANCE MAY BEOBSERVED. AN EXAMPLE OF THIS IS CALCIUM ACETATE.

ACID ATTACK ON CONCRETETHE INFLUENCE OF CALCIUM HYDROXIDE ON ACID RESISTANCE IS BESTILLUSTRATED THROUGH EXAMINING THE EFFECT OF THE COMBINATION OFPORTLAND CEMENT WITH OTHER CEMENT COMPONENTS, SUCH AS FLY ASH,SINCE THIS LEADS TO A DILUTION OF PORTLAND CEMENT, RESULTING INLOWER CA(OH) 2 LEVELS. THE INFLUENCE OF FLY ASH CONTENT IS SHOWN INFIGURE. SIMILAR IMPROVEMENT IN ACID RESISTANCE CAN BE ACHIEVEDWITH GGBS, SILICA FUME AND METAKAOLIN.

ACID ATTACK ON CONCRETE

THE EXTENT TO WHICH ATTACK MAY OCCUR IS INFLUENCED BY ANUMBER OF FACTORS, THE MOST IMPORTANT OF WHICH ARE THEPERMEABILITY OF THE CONCRETE, THE CONDITIONS TO WHICH IT ISEXPOSED AND THE TYPE OF CEMENT USED.

PORTLAND CEMENTS ARE THE MOST VULNERABLE TO ATTACK, SINCETHEY CONTAIN A HIGH PROPORTION OF CALCIUM HYDROXIDERELEASED DURING HYDRATION OF THE CALCIUM SILICATES.

ACID ATTACK ON CONCRETE

THE VULNERABILITY OF CONCRETE CONTAINING HIGHER LEVELS OFCA(OH) 2 IS THE RESULT OF THE FACT THAT THIS PHASE IS THE FIRST TODISSOLVE AS CONDITIONS MOVE FROM ALKALINE TO ACIDIC. THUS,CONCRETE WITH A HIGH CA(OH)2 CONTENT WILL DISPLAY ARELATIVELY RAPID INCREASE IN POROSITY – AND LOSS IN STRENGTH –AS ACID ATTACK PROCEEDS.

FOR THE SAME REASON, THE PORTLAND CEMENT CONTENT OF ACONCRETE MIX INFLUENCES ITS RESISTANCE TO ACID ATTACK. AHIGHER PORTLAND CEMENT CONTENT WILL INCREASEVULNERABILITY, SINCE THE PROPORTION OF CA(OH) 2 WILL BE RAISED.

ACID ATTACK ON CONCRETE

IT HAS ALSO BEEN ESTABLISHED THAT A HIGHERAGGREGATE CONTENT HAS THE EFFECT OF PREVENTING THEFORMATION OF LONG CRACKS AND REDUCING THE CRACKDENSITY WITHIN THE CORRODED LAYER, MAKINGDIFFUSION THROUGH THIS ZONE SLOWER. THE FIRST OFTHESE EFFECTS IS THE RESULT OF THE TENDENCY OFCRACKS TO PROPAGATE BETWEEN AGGREGATE PARTICLES,PRODUCING A NETWORK OF SHORT CRACKS WITH A MORETORTUOUS CRACK PATH.

THE SECOND IS THE RESULT OF AGGREGATE PARTICLESRESTRAINING SHRINKAGE OF THE CORRODED LAYER,LEADING TO LESS CRACKING.

ACID ATTACK ON CONCRETE

SIMILAR TO CEMENT HYDRATION PRODUCTS,CARBONACEOUS AGGREGATE SUCH AS LIMESTONE ANDDOLOMITE USED IN CONCRETE PRODUCTION IS ALSOSUSCEPTIBLE TO ACID ATTACK.

ACID ATTACK ON CONCRETE

BECAUSE THE RATE OF INGRESS INTO CONCRETE OF SPECIESIN SOLUTION IS CONTROLLED BY THE CAPILLARY POROSITYOF THE CEMENT MATRIX, THE W/C RATIO PLAYS ANIMPORTANT ROLE IN DEFINING ACID RESISTANCE. FOR THEMAJORITY OF ACIDS, A LOWER W/C RATIO YIELDS GREATERRESISTANCE TO CORROSION, PRINCIPALLY AS A RESULT OF AREDUCED RATE OF INGRESS THROUGH THE CORRODEDLAYER AND A HIGHER STRENGTH IN THIS LAYER, RESULTINGIN REDUCED LOSS OF MASS AS A RESULT OF MECHANICALWEAR.

ACID ATTACK ON CONCRETE

POLYMER-MODIFIED CONCRETE IS PRODUCED BYINCLUDING A POLYMER WITH THE CONVENTIONALCEMENTITIOUS CONSTITUENTS DURING MIXING. THEPRESENCE OF THE POLYMER HAS THE EFFECT OFENHANCING A NUMBER OF PROPERTIES. HOWEVER, IN THECONTEXT OF ACID ATTACK, THE MAIN BENEFIT TO THISAPPROACH IS THAT THE POLYMER COMPONENTS OF THERESULTING CEMENT MATRIX ARE LARGELY UNAFFECTED BYCONTACT WITH ACIDS, THUS REDUCING THE RATE OFDETERIORATION. IN ADDITION, THE POLYMER WILL USUALLYFILL THE CAPILLARY POROSITY BETWEEN CEMENT GRAINSTO SOME EXTENT, REDUCING POROSITY.

ACID ATTACK ON CONCRETE

A RANGE OF POLYMERIC MATERIALS CAN BE USED FORPOLYMER MODIFICATION, INCLUDING POLYMETHYLMETHACRYLATE (PMMA), POLYSTYRENE (PS), POLYESTERS,STYRENE ACRYLIC ESTERS, STYRENE BUTADIENE,POLYVINYL ACETATE (PVA), ACRYLIC POLYMERS,POLYACRYLONITRILE (PAN) AND EPOXY RESINS.

ACID ATTACK ON CONCRETE

PLACING A LAYER OF ACID-RESISTANT AND WATER-TIGHTMATERIAL BETWEEN CONCRETE AND THE EXTERNALENVIRONMENT IS CLEARLY AN APPROACH THAT IS LIKELYTO IMPROVE DURABILITY IN ACIDIC ENVIRONMENTS.

RESIN INJECTION OF CRACKED CONCRETE IS NOW ANESTABLISHED REPAIR TECHNIQUE, WHICH IS LIKELY TOPROVIDE RENEWED ACID PROTECTION OF CONCRETE.

ACID ATTACK ON CONCRETE

SURFACE TREATMENT WITH A WATER-REPELLING AGENTREPRESENTS ONLY A PROVISIONAL REMEDY THAT IS ABLETO DELAY CONCRETE ATTACK. PLASTICISED POLYVINYLCHLORIDE (PVC) LININGS CAN ALSO BE USED TO CONTROLDETERIORATION AND EROSION OF CONCRETE IN ACIDENVIRONMENTS. THIS TECHNIQUE HAS BEEN USEDCOMMERCIALLY FOR MANY YEARS. THE PROCESS USES APVC LINING IN A CONCRETE PIPE, WHICH OFFERS THEADVANTAGES OF SIZE VERSATILITY AND LOWER COST WHENCOMPARED WITH PLASTIC PIPE.